Dissertations / Theses on the topic 'Spray coatings'

Des couches minces hybrides ont été préparé par une version modifiée de la méthode classique de déposition « couches-par-couches ». Les couches inorganiques de ces films hybrides sont déposés en utilisant la méthode de pulvérisation consécutive (qui est la méthode modifiée) pour laquelle la pulvérisation de solutions aqueuses se fait de manière alterné avec des temps de pulvérisation très courts (moins de 5 secondes). Nombreuses couches inorganique sont obtenues par cette technique telles que des couches de phosphate de calcium, oxalate de calcium, fluorure de calcium, Blue de Prusse, chlorure d’argent, etc. L’application principale des couches hybrides contenant de phosphate de calcium et Collagène est la fabrication de nouveaux biomatériaux pour la régénération/réparation du cartilage articulaire (le domaine d’ingénierie tissulaire). La structure de type « sandwich » de ces components permet la mimique du cartilage articulaire naturel. Pour la préparation de ces couches hybrides on a utilisé un film mince de polyélectrolytes de (PAHPSS). Le dépôt de couche mince inorganique peut se faire aussi en utilisant la méthode de pulvérisation simultanée des deux solutions contenant les cations et les anions. Des couches minces homogènes sur des surfaces d’ordre des cm2 ont été obtenues et pour lesquelles l’épaisseur peut être contrôlée par la variation de paramètres de pulvérisation tels que le temps de pulvérisation, la concentration de la solution, la nature du substrat
Hybrid thin films were prepared by a modified variant of the classical Layer-by-Layer (LbL) deposition method. Inorganic coatings are deposited using the consecutive spray method, in which aqueous solutions are alternatingly sprayed for short times (less than 5 seconds). Numerous inorganic coatings were prepared this way such as calcium phosphate, calcium fluoride, calcium oxalate, silver chloride, Prussian Blue. The main target of the hybrid films comprised of calcium phosphate and Collagen is for application in the cartilage repair. “Sandwich”-like structures of these components mimic the natural articular cartilage structure. In order to obtain the hybrid film containing Collagen and calcium phosphate, we used a (PAHPSS)barrier film. The deposition of inorganic coating can also be realized using the simultaneous spraying method where both solutions (cationic and anionic solution) are sprayed simultaneously. Homogeneous on several cm2 of surface thin films are obtained in which the film thickness is controlled by spray parameters such as spraying time, solution concentration, nature of substrate to be coated

This work investigates the feasibility of the use of plasma spray deposition as a method of producing high performance polymer coatings. The work concentrates on the understanding of the processing of the plasma spraying of polymers, the behaviour of polymeric materials during deposition, and the study of process-structure-properties relationships. Processing modelling for the three stages of the evolution of a polymer deposit (droplet-splat-coating) has been carried out using heat transfer theory. A theoretical model is proposed which consists of three parts: the first part predicts the temperature profile of in-flight particles within plasma jet, the second part predicts the cooling of isolated splats impacting on a substrate and the third part, the heat transfer through the coating thickness. The heat transfer analysis predicts that the development of large temperature gradients within the particle is a general characteristics of polymers during plasma spraying. This causes difficulties for polymer particles to be effectively molten within the plasma jet without decomposition. The theoretical calculations have predicted the effect of processing parameters on the temperature, the degree of melting and decomposition of in-flight polymer particles. With the aid of the model, the conditions for the preparation of high integrity thermoplastic deposits have been established by the control of the plasma arc power, plasma spraying distance, feedstock powder injection, torch traverse speed and feedstock particle size. The optimal deposition conditions are designed to produce effective particle melting in the plasma, extensive flow on impact, and minimal thermal degradation. The experimental work on optimizing processing parameters has confirmed the theoretical predictions. Examination of polymer coating structures reveals that the major defects are unmelted particles, cracks and pores. Five major categories of pores have been classified. It also revealed a significant loss in crystallinity and the presence of a minor metastable phase in the plasma deposited polyamide coatings due to rapid solidification. The study has indicated that the molecular weight of a polymer plays an important role on the splat flow and coating structure. Under non-optimal deposition condition, substantial thermal degradation occurred for which a chain scission mechanism is proposed for plasma deposited polyamide coatings. There are difficulties in achieving cross-linking during plasma spray deposition of thermosets. The theoretical calculations predict that adequate cross-linking is unlikely in a coating deposited under normal conditions, but preheating the substrate to above the cross-linking temperature improves the degree of cross-linking of the coatings substantially. In addition, the coating thickness has a major effect on the degree of cross-linking of thermosets. The calculations also predict that lowering the thermal conductivity by applying a thermal barrier undercoat and using a faster curing agent to reduce time required for the cross-linking reaction can improve the degree of cross-linking of thermoset deposits. The experimental results for the degree of cross-linking and wear resistance confirmed these predictions.

Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.
"December 2006." Title from PDF title page (viewed on Oct. 29, 2007). Thesis adviser: Ikramuddin Ahmed. Includes bibliographic references (leaves 68-71).

Ti2AlC coatings have been fabricated by cold-spray deposition. The microstructure evolution as a function of basic spray parameters temperature and pressure onto AA6060 aluminium alloy and 1.0037 steel substrates has been studied. Adherent and dense 50–80 μm thick Ti2AlC coatings were deposited on soft AA6060 substrates under gas temperature and pressure of 600 °C and 3.4 MPa, respectively, whilst comparable results were obtained on harder 1.0037 steel by using higher temperature (800 °C) and pressure (3.9 MPa).

The practical advantages of thermal spray coatings like high deposition rates, low cost and tribological properties of high wear resistance have enabled these coatings to become an integral part of aircraft and automobile industry. Recent advancements in thermal spraying techniques like high particle speed and temperature call for new applications for these coatings. This experimental study addresses the Rolling Contact Fatigue performance of thermal spray coatings deposited by a variety of techniques like High Velocity Oxy-Fuel (HVOF), Detonation Gun (D-Gun) and Plasma spraying. RCF tests were conducted using a modified four ball machine in conventional steel ball bearing and hybrid ceramic bearing configurations. Tribological conditions during the RCF tests were varied by changing the test lubricant and the lubrication mechanism, contact load and shape of the drive coated rolling element to vary the roll/slip ratio. RCF tests were analyzed on the basis of the performance, coating failures using surface and subsurface observations, and residual stress studies. Experimental and theoretical studies of the ball kinematics have also been included. These tests revealed that the performance of the coated rolling elements was dependent upon the coating and the substrate properties. The coating thickness, substrate hardness, tribological conditions during the test, coating and substrate material as well as the coating process and the substrate preparation significantly affect the coating performance and the failure modes. Three different failure modes of these coatings have been discussed along with the changes in the near surface residual stress behaviour of the coated rolling elements.

The Cold Spray (CS) is a novelty technique for the production of coatings within the field of Thermal Spray. The use of low work temperatures (under the melting point of the material) is what makes it different from the other conventional spraying techniques. CS is an ideal technique to spray materials that are sensitive to temperature, such as nanostructured and amorphous materials, or sensitive to the oxygen, as could be titanium. These characteristics may be specially promising within the field of biomedical coatings. Compared to the conventional processes, it would allow to produce coatings in a more cost-effect and environmental friendly way. The research is mostly focused on the obtaining of highly rough Commercial Pure Titanium (CP-Ti) and Calcium Phosphate (CP) coatings by CS for joint prosthesis application. An exhaustive development of coating parameter optimization has been developed and the different deposition mechanisms for each feedstock powder were analyzed as well. The mechanical and biological properties of the optimal coatings were evaluated consequently. The mechanical testing (e.g. Tensile and shear strength test and Taber test), was performed on the optimized CP-Ti coating overcoming the ASTM standard specifications for joint, shoulder and knee prosthesis; CP-Ti coating showed good bonding between particles as well with the substrate. The used of coarse titanium particles provided high roughness (Ra=40±12 µm and Rz=235±44 µm) that shows an increase in cell viability, proliferation and differentiation as well as mineralization, in comparison with a low-roughness titanium surface obtained by Sand Blasted (SB).Afterwards, additional surface modification procedures were investigated to obtain nanotextured surfaces on the as-sprayed CS titanium coatings to evaluate their in-vitro response for comparison. The anodic oxidation treatment leads to a TiO2 Nanotube (NT) layer with a pore diameter between 50-100 nm and a roughness of Ra=36.8±4.6 nm, whereas the alkaline treatment leads to a nanosurface of Ra=1.2±0.2nm, composed by very fine porosity <100nm. The nanotextured treatments lead to an increase of cell viability in comparison with the as-sprayed CP-Ti coating as well as high cell differentiation due to its nanofeatures. On the other hand, Hydroxyapatite (HA) coatings were well-deposited by CS without amorphization. Two different HA powders were used as feedstock, a sintered HA powder with a crystalline microstructure, and agglomerate HA feedstock powder with a nanocrystalline microstructure. Both powders show different deposition by CS according to their microstructure; particle deposition of sintered HA powder occurred by slight void collapse and dynamic fragmentation followed by cracking and crushing and reduction in crystal size by plastic deformation mechanisms, whereas agglomerate HA powder a consolidation between particles is caused by the tamping effect produced by the continuous impact of incoming particles onto the already adhered ones, leading to coating build up. Although sintered HA coatings show high cell differentiation, agglomerated HA coatings result in higher cell viability and proliferation, as well as a more homogeneous cell deposition along the coating; agglomerate HA coatings were compared to HA coatings obtained by conventional techniques (e.g. Plasma Spray), and an increase of cell viability and proliferation were observed according to the crystalline HA percentage (CS>PS). To sum up, CS has shown to be suitable to produce metallic and ceramic coatings for joint applications, especially those dealing with materials that are sensitive to the temperature and oxygen. The non-microstructural changes from the feedstock powder to the coatings leads to a big advantage in CS to obtain customized coatings.
La Proyección Fría o también conocida por el nombre de Cold Spray (CS), es una técnica muy novedosa para la obtención de recubrimientos en el ámbito de la proyección térmica. El uso de bajas temperaturas de trabajo (siempre por debajo del punto de fusión del material) es lo que la hace diferente respecto al resto de técnicas convencionales, ya que la hace ideal para depositar materiales sensibles a la temperatura (ej. Materiales nanoestructurados y amorfos) o reactivos al oxígeno (ej. Titanio). Además tiene un gran potencial a nivel económico y ambiental por ser un proceso eficiente y respetuoso con el medio ambiente. La investigación se basa principalmente en la obtención de recubrimientos de titanio rugoso y fosfato cálcico mediante proyección fría, donde se han estudiado los diferentes mecanismos de deposición de las materias primas y se ha elaborado un estudio a nivel mecánico y biológico. Además, se han realizado tratamientos adicionales sobre los recubrimientos de titanio para obtener superficies nanotexturizadas y poderlas comparar in vitro. El uso de partículas gruesas de titanio facilita la obtención de grandes rugosidades superficiales en los recubrimientos de titanio, mejorando así la viabilidad y proliferación celular, al igual que la diferenciación y mineralización celular, en comparación a superficies con bajas rugosidades (como las superficies de titanio granalladas). Aun así, los tratamientos de nanotexturización, tales como el anodizado (donde se obtuvieron nanotubos de TiO2 con un diámetro entre 50-100nm) y el tratamiento alcalino (en el cual se obtuvieron porosidades <100nm), promovieron aún más la respuesta celular de los recubrimientos de titanio en los aspectos previamente mencionados. Por otra parte, los recubrimientos de hidroxiapatita (HA), se depositaron por CS sin ninguna amorfización. También se observó que el proceso de obtención de la HA influía en la deposición de la partícula. La deposición de una HA sinterizada ocurre mediante el colapso de poros y fragmentación dinámica de la partícula, reduciendo así el tamaño de cristal por mecanismos de deformación plástica. Por el otro lado, el polvo de HA obtenido mediante aglomeración, se deposita mediante la compactación y consolidación entre partículas, pudiendo hacer crecer la capa. In vitro, la HA sinterizada da una mayor diferenciación celular, mientras que la HA aglomerada da una mayor viabilidad y proliferación celular. Los recubrimientos de HA aglomerada fueron comparados con los que actualmente se producen por proyección de plasma, y se observó una mejor respuesta celular en los recubrimientos obtenidos por CS debido a que no ofrece cambios microestructurales en el polvo de partida. Para resumir, el CS es una técnica idónea para la producción de recubrimientos metálicos y cerámicos para prótesis articulares, especialmente con materiales sensibles a la temperatura y al oxígeno. Al no producirse un cambio microestructural en el recubrimiento durante el proceso de su obtención, permite la elaboración de recubrimientos personalizados.

Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.
"December 2006." Title from PDF title page (viewed on Sept. 18, 2007). Thesis adviser: Ikram Ahmed. Includes bibliographic references (leaves 79-81).

The overall aim of this research project was to expand the understanding of the deposition of titanium and the nickel-based superalloy Inconel 718 by spray deposition methods. The spray processes employed were cold spraying and high velocity oxy-fuel (HVOF) thermal spraying. The first part of the work was undertaken to expand the understanding of the deposition of titanium by cold spraying; the HVOF process is unsuitable for Ti because of the metal's high reactivity. The deposits were produced from commercially pure titanium using cold spray equipment designed in the University. Using helium gas, the effects of different powder particle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on deposit formation were investigated. Using a simple one-dimensional model of compressible gas flow and particle acceleration, particle velocity distributions were calculated to aid interpretation of experimental data. Results show that titanium can be successfully cold sprayed onto substrates of Ti6AI4V and mild steel, with the critical velocity for deposition of this powder type of approximately 690 m s-1. The level of porosity was generally in the range of 13-23% and the adhesive bond strength was dependent on surface preparation but independent of gas pressure with values ranging from 22 MPa to 10 MPa for ground and grit blasted substrates respectively. This compares with a value of around 80 MPa which is typical for well adhered HVOF sprayed coatings. The second part of the study was concerned with comparing the deposition of Inconel 718 by cold spraying and HVOF thermal spraying; the latter employed a JP5000 liquid fuel gun. A Tecnar DPV-2000 instrument was used to systematically investigate the effect of changes in spray parameters (spraying stand-off distance, oxygen/fuel ratio, total mass flow rate, combustion pressure), on particle velocity and temperature during HVOF spraying. It was found that generally the particle velocity was more strongly affected by the stand-off distance and combustion pressure of the spraying gun whereas the particle temperature was mostly influenced by the particle size and combustion pressure. The microstructures of coatings sprayed under 4 different well controlled conditions were investigated and changes in the morphology of splats and partially melted particles in the coating were related to the particle temperature and velocity at impact. The HVOF had high bond strength and low oxygen level of typically 0.45 wt% (corresponding to an oxide content of less than 1.6 wt.%). By contrast, in the cold sprayed coatings, the bonding was considerably low (-14 MPa), independently from the process conditions. It was found that the process parameter that mainly affected the properties of the cold sprayed deposits was the gas pressure. More specifically, the microhardness of the coatings increased with the pressure whereas the relative porosity decreased.

Nanocomposite thin films with gold nanoparticles embedded in a host metal oxide prepared by spray pyrolysis deposition have been investigated. A single-step process has been developed using a one-pot solution containing precursors for both gold nanoparticles and host metal oxides. The films obtained display combined features of colouration, electrical conductivity and solar control. In this study two precursors for gold nanoparticles were used: preformed gold colloids and HAuCl₄. Three metal oxide host materials, TiO₂, SnO₂ and ZnO, were investigated. These films were deposited at a substrate temperature of 200-600 °C. Powder X-ray diffraction analysis reveals the presence of metallic gold. SEM inspection typically showed particulate gold of 5-20 nm in diameter, distributed at the surface or within the host matrix. Optical spectroscopy showed an intense absorption in the visible region due to the characteristic surface plasmon resonance (SPR) effects of gold nanoparticles. The wavelength of the SPR peaks varies depending on the refractive index of surrounding host material which is significantly influenced by the substrate deposition temperature. On the other hand, SnO₂ and ZnO, together with the introduction of dopants, were further investigated as suitable materials for transparent conducting oxides (TCO). SnO₂:F films were found to attain very low electrical resistivity, while ZnO films exhibit higher transparency in the visible. A double layered structure with a TCO layer of SnO₂:F on top of a layer embedded with gold nanoparticles has been employed to achieve the combined functionalities of conductivity and colouration. The electrical conductivity is significantly enhanced compared to a nanocomposite single layer film due to the introduction of the TCO top layer. In this thesis, spray pyrolysis deposition has demonstrated a simple and rapid approach to the production of a variety of thin films. It can be immediately integrated with current industrial coating equipment and scaled up for large-scale production process.

Metal-ceramic (Cermets) materials that combine properties of both: high hardness, high wear resistance, and high working temperatures of ceramics and the ductility, toughness, and heat conductance of metals. Cold gas dynamic spraying, or simply cold spray, is a solid state thermal spray process that has been in development for the last 25 years. In the cold spray process, ductile materials are accelerated in a supersonic flow. These particles impact a substrate and adhere by plastic deformation. The continuous accumulation of these particles covers the substrate and creates a dense coating. The cold spray process is beginning to become a popular method to consolidate some select cermet materials into coatings. This technique can be advantageous when an erosion and wear resistant coating is required. During the deposition of these coatings, researchers have shown that the ceramic particles have a dramatic influence on the deposition behavior by causing an increase in deposition efficiency and coating adhesion. These effects have been presented in several experiments but have yet to be thoroughly explained. The goal of this investigation is to increase the knowledge, on a fundamental level, with regards to the deposition behavior of metal-ceramic blending and cermet powders. Ultimately, the focus is to prove the feasibility of these coatings for the requirements needed in the engineering industry. The first part of the investigation is a fundamental study on the deposition behavior of metal-ceramic blends with different compositions. Three theories that aim to explain the increase in deposition efficiency were proposed in the literature and further investigated in this study. One proposed mechanism for the increase in deposition efficiency was established by probability analysis to be too unlikely to contribute to the increment in deposition efficiency. The other two proposed mechanism, the presence of asperities caused by ceramic particles, and the oxide removal produced by the impact of ceramic particles, shown to play a major role in increasing the deposition efficiency. The effect of the ceramic particle morphology on the deposition behavior of metal-ceramic blending was studied in the second part of the investigation. This study greatly complements the previous one adding more depth to the investigation and confirming results. The increment in deposition efficiency normally seen with the addition of small amounts of angular alumina was not seen when spherical alumina was added instead. The creation of asperities during deposition was explored for the two morphologies and was determined that spherical alumina does not produce the same asperities at the surface. In addition, the coating sprayed with spherical alumina showed very little ceramic retention compared with the ones sprayed with angular alumina. These results have a direct impact on the mechanical properties of the coatings. Wear resistance for coatings sprayed with spherical alumina showed no improvement compared with pure aluminum coating due to the low ceramic content. Hardness was lower in coatings sprayed with spherical alumina for the same feedstock powder composition but was harder when the final coating composition was considered. Adhesion strength significantly increases with the addition of ceramic content in the feedstock powder; this increase was greater for coating sprayed with spherical alumina. The third part of the investigation focuses on understanding the mechanism of deposition for cermet particles with various morphologies. Six commercially available CrC-NiCr powders were studied, varying in morphology and metal/ceramic ratio. Spherical powders led to the erosion of the substrate and no coating was formed. Porous agglomerated and sintered powder lead to severely cracked coatings. For dense agglomerated and sintered powders, the outcome of powder depended on the initial metal/ceramic ratio, powders with 25%wt.NiCr led to erosion while 35%wt.NiCr powders led to a dense coating. Finally blended ceramic metal powders also lead to a successful coatings. All coatings obtained had lower ceramic content than the initial feedstock powder. Interrupted deposition tests, FEA analysis, and SEM observation were used to draw conclusions on the deposition behavior and explain the results. Finally, the last part of this investigation aims to apply the knowledge learned to an applied engineering problem. The problem that is targeted is the replacement of chrome plating for the aerospace industry. A commercially available cermet powder CrC-NiCr (65/35) was proposed as a replacement of chromium plating as well as a restoration for this coating and its alternatives (electroless nickel-plating, and WC-Co-Cr HVOF). The coatings and restoration were analyzed by SEM and tested by strip rupture rest, neutral salt spray fog, and fluid immersion testing. The adhesion strength, porosity, and hardness of the cold spray coating was also tested. The deposition and restoration of coatings were successful; a hard and dense coating was obtained with good adhesion strength. The process of restoration chromium-plating and its alternatives was also developed with a clean interface was achieved in each case. Coatings and restoration passed strip to rupture rest as well as fluid immersion test in two selected industry fluids. Neutral salt spray fog test revealed that the cold spray coating and repairs may have a path that allows the solution to penetrate the substrate and start the corrosion process. This behavior was found in a few select spots and should be further investigated. Overall, the coating proved to have potential as an alternative of chromium-plating or to restore damaged hard coatings.

Thermal barrier coatings (TBC) with MCrAlY (M=Co and/or Ni) bond coats have been widely used in hot sections of gas turbines to protect underlying superalloys from high temperatures, oxidation, and hot corrosion. Deposition of MCrAlY bond coats using atmospheric plasma spray (APS), as oppose to conventionally employed vacuum/low-pressure plasma spray and high velocity oxy-fuel deposition, allows greater flexibility in ability to coat economically and rapidly for parts with complex geometry including internal surfaces. There were three objectives of this study. First, relationships between APS spray parameters and coating microstructure was examined to determine optimum spray parameters to deposit APS CoNiCrAlY bond coats. Second, free-standing APS CoNiCrAlY coatings were isothermally oxidized at 1124ºC for various periods to examine the evolving microstructure of internal oxidation. Third, as a function of time of isothermal oxidation (i.e., internal oxidation), thermal conductivity and coefficient of thermal expansion were measured for free-standing APS CoNiCrAlY bond coats. Thirteen CoNiCrAlY coatings were deposited on steel substrates by APS using the F4-MB plasma torch. APS CoNiCrAlY bond coats were produced by incremental variation in the flow rate of primary (argon) gas from 85 to 165 SCFH and the flow rate of secondary (hydrogen) gas from 9 to 29 SCFH. Optimum coating microstructure was produced by simultaneously increasing the flow rate of both primary and secondary gas, so that the particle temperature is high enough for sufficient melting and the particle velocity is rapid enough for minimum in-flight oxidation. Optimum spray parameters found in this study were employed to deposit free-standing APS CoNiCrAlY coatings that were isothermally oxidized at 1124ºC for 1, 6, 50,100, and 300 hours. Extent of internal oxidation was examined by scanning electron microscopy and image analysis. Internal oxidation occurred by a thickening of oxide scales segregated at the splat boundaries oriented parallel to the coating surfaces. Thermal conductivity and coefficient of thermal expansion (CTE) of the free-standing APS CoNiCrAlY coatings were measured as a function of internal oxidation (i.e., time of oxidation or extent of internal oxidation). Thermal conductivity of free-standing APS CoNiCrAlY was found to decrease with increasing internal oxidation from 28 to 25 W/m-K. This decrease is due to an increase in the amount of internal oxides with lower thermal conductivity (e.g., Al2O3). CTE of free-standing APS CoNiCrAlY, measured in temperature range of 100°~500°C, was also found to decrease with increasing internal oxidation. Internal oxides have lower CTE than metallic CoNiCrAlY coatings. These evolving properties of APS CoNiCrAlY should be beneficial to the overall performance of TBCs in gas turbine applications.
M.S.M.S.E.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Materials Science & Engr MSMSE

The main objective of this research work was to produce environmentally friendly cermets coatings, Ni and Co free matrix, alternative to conventional WC- Co, by thermal spray processes. WC-Co cermets has always been one of the most on demand coatings in anti-wear and anti-corrosion applications in industry but environmental problems caused by heavy metals matrix (Co and Ni) force the redesign of many conventional processes. These elements are not harmful in their fundamental state, but processing generate changes in oxidation states that make them carcinogenic and mutagenic. For this reason, this research has focused on either replacing the existing processes or use raw materials that are less harmful. Therefore depositing the green carbides cermets onto the different substrates by conventional or novel (CGS) techniques are the main motivation points of this thesis. No one has previously successfully deposited such material by CGS or HVOF method with the same properties as the conventional WC-Co which was also one of the main motivation points. In this work we could produce Ti-TiC coatings with different percentage of Carbide phase by CGS, TiC-based with FeCrAlTi metal matrix by HVOF method, SiC-based cermets with Ti and TiCr metal matrixes by both CGS and HVOF technique and WC-based cermet with Ti metal matrix by CGS, HVOF and APS techniques. For this reason the main objectives of this Thesis entitled “Deposition of green carbide cermets coatings by thermal spray techniques”. GLOBAL TREND FOR TIC SYSTEM: For all three Ti-TiC powders by increasing the SOD from 20 to 40, hardness has been increased which can be related to the decreasing the porosity of produced coatings by increasing the carbide phase which can be observed at SEM micrographs. The Higher adhesion strength of coating related to Ti-65%TiC can be explained by higher amount of hard carbide phase. When powder impact the substrate, powder with higher amount of hard phase deforms the substrate more and immerge the substrate deeper and as a result stronger mechanical bonding between the coating and substrate will be happened. From the wear rate comparison of obtained coatings regarding different Ti-TiC cermets can be resulted that Ti-65%TiC showed better wear resistance properties because of higher hardness and lower porosity. The optimal coating for Ti-TiC system was obtained at medium pressure and medium-high temperature related to Ti- 65%TiC and by increasing the temperature, there was an erosion and the coating was brittle and fully cracked. At high pressure and high temperature there was no deposition at all. Obtained coating related to TiC-FeCrAlTi by HVOF showed relatively good microstructural and mechanical properties. GLOBAL TREND FOR SIC SYSTEM: For this system there was no significant difference between microstructural properties of obtained coatings related to both powders by CGS method, both coatings had a hardness of about 500 HV. Though in order to improve the microstructural properties of coating related to Ti-SiC especially the hardness, coating went through the thermal treatment (annealing at 750°C). The microstructure of the coating was significantly changed because the interfaces between the deposited powder particles tended to disappear and a stronger metallurgical bond was formed [61–63]. HVOF coating of Ti-SiC presented higher hardness value than CGS mainly because of decomposition of Ti-SiC powder during the process and therefore hardening effect of SiO2 phase. Besides a higher hardness, HVOF coating was more brittle (less fracture toughness value) than CGS coating due to the lower content in elemental ductile Ti matrix and the presence of fragile and hard SiO2 phase formed during the HVOF spraying process. The lower value of fracture toughness in HVOF coating can be also attributed to the fact that during CGS the powder is not subjected to phase changes and no draining of the ductile free metallic matrix in the microstructure happens. The Ti present in the coating is kept and acts as a ductile element hence improving the fracture toughness of the CGS coating. Wear rate comparison of formed coatings related to Ti-SiC by CGS and HVOF using Propylene was shown in previous chapter, significantly lower wear rate of cold sprayed coating of Ti- SiC than HVOF can be explained by higher fracture toughness value of obtained coatings by CGS despite of higher hardness value of obtained coating by HVOF. Wear rate comparison of formed coating related to TiCr-SiC by HVOF and using H2 and Ti-SiC by HVOF and using Propylene showed that coating related to TiCr- SiC had better wear resistance than Ti-SiC because of less formed porosity in coating especially on top of the coating which was in contact with rubber wheel also from the results of fracture toughness of coatings can be explained that coating from TiCr-SiC had more fracture toughness value than the one from Ti- SiC by HVOF. GLOBAL TREND FOR WC SYSTEM: Ti-WC (400HV) was deposited onto the carbon steel substrate by HVOF and a coating with hardness of almost 900 HV was obtained. The produced coating by using Propylene had higher hardness than the one produced by using H2 though the thickness of coating for both coatings was not thick enough. This powder was sprayed onto the carbon steel substrate with a bond coat layer of Titanium by CGS in order to increase the adhesion between the powder particles and the substrate surface and after obtaining the coating the sample went through the post thermal treatment (HT Temperature: 650°C, HT isotherm: 60 mins, Ramp: 5 (°C/min), Atmosphere: Vacuum) in order to increase the hardness of the coating. After HT, hardness of coating reached a value of 1200 HV from 750 which was considerable. The higher hardness of HVOF Ti-WC than CGS can be explained because of decarburization of WC particles during HVOF, as the particles are exposed to hot and usually Oxygen rich environment. As a result, W2C and depending on the processing conditions even W is formed as well as volatile CO and CO2. From Ti-WC (650HV) and (1500 HV) powders the coatings were produced by APS technique though they were so porous. CONCLUSIONS: From this study following findings can be resulted: 1. Green cermets coatings with different metallic matrix (Ti, FeCrAlTi) and different carbides (TiC, SiC, WC) have been produced successfully by different thermal spray techniques and can be used as alternative to WC- Co or Cr3C2-NiCr for anti-wear applications. Because of the high density of WC-Co and Cr3C2-NiCr coatings in applications where command reduction in weight is needed and its high relative cost as compared to TiC based hard metals and hazardous nature of Cobalt and Nickel matrix, green cermet coatings can be good alternatives in the bench market. 2. In CGS method has been observed a trend which by increasing the temperature to the max and increasing the pressure, visible cracks in deposited particles, weak bonding in central area of splats-substrate interface, less penetration and less plastic deformation have been observed. This trend can be explained by impact velocity of the particles which has been increased by increasing the spraying temperature and pressure till the critical value (the optimal condition) and afterward the excessive particle velocities has been resulted erosion and avoiding a proper deposition process. Spraying pressure and temperature both had the direct relation with the impact velocity of particles though increasing the pressure affected the impact velocity of particles more than the temperature. This phenomena has led to a decrease in hardness value and fracture toughness of produced coatings. The optimal parameters have been set at intermediate temperatures and pressures. 3. In HVOF there was no significant difference between mechanical properties of the coatings by using hydrogen and propylene as fuel gases. Though for each system, produced coatings by using Propylene as fuel gas had slightly higher hardness value and lower wear rate which was considered as the optimum coating by HVOF for each system. 4. In general HVOF has produced coatings with presence of continues oxide phases network and lower content in elemental ductile metal matrix which as a result higher hardness and lower fracture toughness has been achieved. In contrast CGS has produced coatings with absence of fragile oxide phases which has led to the higher fracture toughness but lower hardness value. In CGS the hard carbide phase was not melted and was not distributed in metallic matrix homogenously and that is why it has lower value of hardness against HVOF coatings. The optimum coating of this study which can be considered as the alternative to WC-Co or Cr3C2-NiCr for anti-wear applications was Ti-WC by CGS after a post heat treatment. This coating reached the significantly enhanced microstructural properties due to the release of residual stress during the thermal treatment and hardness value of 1200 HV with significantly low wear rate.
En primer lugar, el objetivo principal de este trabajo de investigación fue producir recubrimientos cermets de respetuosos del medio ambiente, matriz libre de Ni y Co, alternativa a WC-Co convencional, por la proyecciones térmicas. WC-Co cermets siempre ha sido uno de los recubrimientos más demandados en aplicaciones anti desgaste y anticorrosión en la industria, pero los problemas ambientales de la matriz de metales (Co y Ni) se han debido a cambios en los procesos convencionales. Estos elementos no son dañinos en su estado fundamental, pero el procesamiento genera cambios en los estados de oxidación que los hacen cancerígenos. Por eso, esta tesis se ha centrado en reemplazar los procesos existentes o utilizar materias primas que son menos nocivas. Por lo tanto, depositar los nuevos cermets en los diferentes sustratos por proyecciones térmicas convencionales o novedosas (CGS) son los principales puntos de motivación de esta tesis. Nadie ha depositado previamente con éxito dicho material por el método CGS o HVOF con las mismas propiedades que el WC-Co convencional, que también fue uno de los principales puntos de motivación. En este trabajo podríamos producir recubrimientos de Ti-TiC con diferentes porcentajes de fase de Carburo por CGS, TiC con aleación metálica de FeCrAlTi por método HVOF, SiC con matrices metálicas de Ti y TiCr por ambas técnicas CGS y HVOF y WC cermet con matriz de metal Ti por técnicas CGS, HVOF y APS. Por eso, los objetivos principales de esta Tesis se titulan "Deposición de respetuoso con el medio ambiente recubrimientos de los cermets por el proyecciones térmicas".

This thesis investigates the mechanisms of erosion-corrosion of Cr3C2-NiCr thermal spray coatings under high temperature, high erodent velocity, turbine conditions. Erosion-corrosion is a generalised wear phenomenon where the combined effect of each degradation mechanism generates more extensive mass loss than the sum of each mechanism acting independently. Previous research has highlighted several theoretical mechanisms under this generalized process, ranging from the erosion induced breakdown of oxide scales in corrosive environments, through to the development of oxide layers in highly erosive environments. Prior to this current work experimental simulation of these mechanisms has focused on bulk alloy materials with well characterised oxidation responses, under conditions of low temperature, low erodent impact velocity and high erodent flux, conditions which are readily generated within laboratory scale rigs and which tend towards the low impact energy conditions encountered within fluidised bed combustors. Few works have addressed erosion-corrosion under simulated turbine conditions of high temperature, high erodent impact velocity and low erodent flux. While comparative trials have been run under such conditions on a purely mass loss basis, little has been presented regarding the microstructural analysis of such degradation, particularly for materials that rely on the industrially relevant, slow growing oxide scales Cr2O3 and Al2O3. Thermally sprayed Cr3C2-NiCr coatings are routinely applied to combat wear at high temperature due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr2O3 oxide formed over both phases. However, most published work characterising the erosion-corrosion response of these coatings has been conducted on a comparative basis by contrasting coatings of various composition ratios, deposited by various techniques, with the response of well characterised bulk materials. Little has been presented on the microstructural mechanism of erosioncorrosion of Cr3C2-NiCr coatings, a point highlighted by the limited understanding of the oxidation mechanism of the Cr3C2 phase, the oxidation mechanisms of the combined composite Cr3C2-NiCr and the influence of the coating splat structure on the oxidation response. While the erosion response of thermal spray coatings and bulk cermets is more widely understood, most works have been conducted under milder conditions than used in the current work. In addition previous works have been conducted primarily on assprayed coatings with few works taking into account the effect of heat treatment induced changes in the coating composition and microstructure that occur with extended in-service exposure at elevated temperature. In addressing the short comings in the current state of knowledge, the aim of this work was to characterise the mechanism of erosion-corrosion of high velocity sprayed Cr3C2-NiCr thermal spray coatings under turbine conditions, incorporating the effect of variation in the composition and carbide distribution with inflight degradation, variations in starting powder morphology, heat treatment, erosion conditions and exposure temperature. 75 Cr3C2-25(Ni20Cr) coatings were deposited by Aerospray HVAF, GMA Microjet HVOF, Stellite Jet Kote HVOF and TAFA JP-5000 HVOF spraying under optimised conditions using agglomerated/sintered and blended powders. The prealloyed powder based coatings, characterised in terms of microhardness, porosity content and phase degradation, were found to exceed the average values of coating quality presented in the literature. The blended powder based coating of this work was comparable with the coating attributes presented in the literature for plasma and HVOF coatings based on this powder morphology. Based on these results the coatings were considered representative of those sprayed industrially and therefore the responses of the samples in this work to oxidation and erosion were considered indicative of the response of industrially applied coatings of this composition in service. Heat treatment trials were conducted on the Aerospray HVAF and Microjet HVOF coatings at 900ºC in air and argon for up to 60 days to simulate the compositional and microstructural development of these coatings under elevated temperature conditions in service. In the prealloyed powder based coatings, rapid carbide precipitation occurred within the first two days in both coatings to reach the steady state composition of 75-80vol%. Minimal in-flight carbide dissolution in the HVAF coating led to preferential carbide precipitation on the retained carbide grains. In the Microjet HVOF coatings, which suffered extensive inflight carbide dissolution, carbide precipitation occurred as fine precipitates in the carbide-free zones, forming large sponge-like agglomerates. With extended exposure Ostwald ripening led to coarsening of the individual carbide grain size and widespread agglomeration of the carbide grains into an extensive three dimensional network after 30 days exposure, with minimal development out to 60 days. Compositionally, heat treatment led to a dramatic reduction in the supersaturated matrix phase Cr content, with the steady state Cr composition of the Microjet HVOF coating exceeding that of the Aerospray HVAF coating based on XRD analysis. Cr3C2 was the only carbide detected with heat treatment. Heat treatment of the blended powder based coating led to sintering of the single phase splats. Diffusion of the carbide elements into the matrix phase splats occurred, allowing fingers and nodules of the carbide to develop into this phase as well as increasing the matrix phase Cr concentration. Oxidation of Cr3C2 by hot stage XRD analysis at 600ºC, combined with TGA analysis at 600-850ºC, supported the mechanism of stepwise decarburisation prior to Cr2O3 formation, presented as one possible mechanism in the literature. Oxidation of the Cr3C2-NiCr coatings over the range 700-850ºC was dependent on the starting powder morphology and the extent of dissolved carbide in the matrix phase. Oxidation of the as-sprayed prealloyed powder based coatings was dictated by the matrix phase, the high Cr concentration resulting from carbide dissolution leading to rapid growth of the Cr2O3 phase over the oxidising carbide grains. Growth stresses induced by such overgrowth lead to the formation of interfacial voids over the carbide grains at high temperature. Heat treatment reduced the matrix phase Cr concentration, resulting in the coating phases oxidising independently with a reduced magnitude of lateral matrix based scale growth over the carbide phase. In the blended powder based coating, bulbous Ni oxides dominated the scale topography. With extended exposure a continuous Cr2O3 scale formed below the faster growing Ni oxides, which enabled lateral growth of the scale over the carbide based Cr2O3 scales. Following heat treatment the matrix phase Cr concentration increased, minimising the development of Ni oxides on this phase. Erosion studies were carried out in a custom built high temperature erosion apparatus. Ambient temperature trials were conducted using Al2O3 erodent at velocities of 150m/s. The as-sprayed prealloyed powder based coatings exhibited a brittle impact response, which was accentuated in the Microjet HVOF coating by the increased extent of in-flight carbide dissolution and the splat structure which made this sample more susceptible to brittle erosion mechanisms. Heat treatment of these coatings led to sintering of the splats and a more ductile impact response due to the increased ductility of the matrix phase. The as-sprayed blended powder based coatings exhibited a range of impact responses from brittle erosion of the carbide through to ductile erosion of the matrix based splats. Mass loss was accentuated by the poor intersplat adhesive strength. Heat treatment led to sintering of the splats, resulting in a more microstructural based erosion response. The two prealloyed powder based coatings generated similar erosion rates under the aggressive conditions, distinctly more erosion resistant than the blended powder based coatings. Heat treatment improved the erosion resistance of all the coatings, however, the duration of heat treatment had a negligible effect on the magnitude of erosive mass loss. Erosion at 800ºC, with an impact velocity of 235m/s, lead to significantly deeper erodent penetration into the coating than noted at ambient temperature. The significant increase in the matrix phase ductility at elevated temperature minimized the impact of carbide dissolution on the matrix impact response in the prealloyed powder based coatings. The primary effect of carbide dissolution was to reduce the carbide concentration, allowing deformation of the matrix to dictate the coating response. Carbide development with heat treatment significantly reduced the ability of the matrix to deform in this manner. The increased matrix phase ductility in the blended powder based coating reduced the concentration of impact energy on the splat boundaries, leading to a more microstructural based erosion response. Heat treatment had a negligible effect on the coating response, given the reduced significance of the splat boundary adhesion. Erosion at 700ºC generated similar erosion responses in the prealloyed powder based coatings to those noted at 800ºC, the lower matrix phase ductility reflected in the more brittle response evident as brittle cracking and fracture. The effect of carbide development with heat treatment was not as dramatic as at 800ºC due to the reduced matrix phase ductility at this temperature. Erosion of the blended powder based coating at 700ºC generated the same spectrum of erosion response as noted at 800ºC in both the as-sprayed and heat treated states, with the variation in matrix phase ductility with temperature overshadowed by the heterogeneous coating impact response resulting from the heterogeneous phase distribution. The steady state erosion rate at 700ºC was comparable across all of the coatings in both the as-sprayed and heat treated conditions. At 800ºC, heat treatment had a negative impact on the prealloyed powder based coatings, but no definitive effect on the blended powder based coatings. The Microjet HVOF coatings were more erosion resistant than the Aerospray HVAF coatings under these conditions. These results pointed to a reduction in the significance of the coating splat structure on the magnitude of erosion, in favour of a more microstructural based response at high temperature. In both the ambient and elevated temperature trials the coating microhardness values proved to be a poor indicator in predicting the magnitude or relative ranking of the erosion response of the different coatings under these aggressive erosion conditions. Erosion-corrosion under turbine conditions of high temperature, high erodent velocity and low erodent flux, was simulated by oxidizing samples at 900ºC and subjecting them to one second of erosion every 48 hours over a period of 60 days. The degradation testing was assessed in accelerated testing in additional trials by polishing the oxide scale formed at 900ºC from the sample surface every 48 hours over the same time period. Under these conditions the coatings formed thick oxide scales that penetrated into the coating. Preferential internal oxidation of the Cr3C2 phase occurred in the coating, consuming the grains in the near surface zone through the formation of Cr2O3 to a depth dependent upon the test temperature. Oxygen ingress occurred along the carbide-matrix interface and was accentuated in the regions of impact damage surrounding the erodent indentations. Internal oxidation of the carbide phase sealed off the pockets of matrix phase which were eventually consumed by oxidation once they were no longer able to maintain a protective Cr2O3 oxide. The extent of internal attack was consistent in the 20, 40 and 60 day samples, suggesting that the internal oxidation front proceeded at a constant rate in front of the erosion front into the coating. The prealloyed powder based coatings were more resistant to such internal degradation relative to the blended powder based coatings, with an internally oxidised zone of 6ìm relative to the 10ìm thick internally oxidized band in the blended powder based coating. While each erodent impact event may be classified as oxidation affected erosion, the low erodent flux effectively led to a long-term response more accurately described as erosion affected oxidation.

Thermal barrier coatings (TBCs) are a simple and proven method to protect hot section components. Suspension Plasma Spray (SPS), an emerging process technology to generate TBCs, compared with traditional Atmospheric Plasma Spray APS, can deposit thinner coat-ings with finer microstructure. Operating parameters play an important role in developing certain properties of coating. In this thesis work, power level, gas flow rate, number of spray-ing strokes, spray gun's nozzle size i.e. internal diameter and suspension rate were controlled to produce coatings with different microstructures and porosity levels. According to the ex-perimental results, the power level of plasma gun play an essential role on coating micro-structure, for instance, the density of vertical cracks increased with growing the power level. The number of spraying strokes showed also an impact on coating porosity. However, due to different nozzle sizes i.e. diameter, the same coating property were controlled by different operating parameters. For coatings deposited by small and large nozzles, their coating thick-ness and roughness mainly relied on power level and gas flow rate. In contrary, it seems that the coating roughness was not influenced by the same parameters when it was deposited by medium nozzle. Also, gas flow rate do not have as big as influence on coating thickness

The majority of coated structural components are subjected to fluctuating internal and/or applied stress because of oscillating mechanical loads. The fatigue behavior of coatings and the overall cyclic failure response of coated structures have remained relatively unexplored. This study was an effort to investigate the fatigue behavior of plasma spray coatings on polymer matrix composite materials. Since no ASTM standard is available, we designed our own experiment to determine coatings suitability under cyclic loading, response in dynamic loading conditions, fatigue failure modes and fatigue life. Coatings were tested at different stress levels and frequencies. The stresses versus number of cycles (S-N) curves for the coatings were generated. The results indicate that the plasma spay coatings on polymer matrix composite materials are suitable for dynamic loading conditions.
Wichita State University, College of Engineering, Dept. of Mechanical Engineering
Includes bibliographic references (leaves 42-43)

There is a rapid growth in demand for improved surface performance in many priority industrial sectors. There have also been rapid developments in methods of surface engineering and in tribological understanding. There is accordingly an exciting opportunity for cost effective industrial exploitation of materials with desirable properties. Among these materials, titanium and its alloys occupy an important place: their use in sectors such as aerospace and biomedical has already started and they are very promising candidates for an increasing number of industrial applications, provided some weak points are solved.

The majority of the structural weight of many common commercial aircrafts is composed of high strength aluminum alloys. The properties of high performance aluminum alloys such as a high strength to weight ratio (specific strength), ease of recycling, crash energy absorption capacity, and corrosion resistance make them ideal for use in the aerospace field. As a result of the high performance nature of the parts and specific properties of the materials, manufacturing requires intricate casting, precision machining, and specific heat treatments – which results in expensive components. As a result of its excellent corrosion resistance properties, pure aluminum coatings are commonly used in the aerospace field for corrosion protection of steel, aluminum alloy components, and titanium alloy components. The common method to deposit these coatings is called ion vapour deposition (IVD). These IVD aluminum coatings provide the coating adhesion, coverage, thickness, and corrosion resistance required to protect the part. The present study was motivated by the potential use of the cold gas dynamic spray (CGDS) process to repair a) damaged aluminum alloy aerospace parts and b) damaged pure aluminum IVD coatings. The primary research objective was to successfully produce these repairs using commercially available aluminum alloy feedstock powders deposited with commercially available CGDS equipment. This work was treated as prequalification work for The Boeing Company to commercialize this process and therefore the repairs aim to meet the same standards (military and industrial) required of the original aluminum alloy parts and IVD aluminum coatings. The use of CGDS was shown in this research to be a very promising as a process for the restoration of aluminum alloy aerospace components. The adhesion strength of the repaired aluminum components was found to be well above the accepted range for thermally sprayed repairs according to industrial standards. The repairs were subjected to a highly corrosive environment and showed only minor pitting. These sites could be reduced in the future with improved machining techniques and attention to surface detail prior to exposure to the salt fog. The only requirement that the repaired components did not meet was for the wear properties of the anodized layer, measured thought Taber abrasion testing. The results of this test, at times, approached the desired values, and it is believed that, in the future, the quality and consistency of the coatings could be improved and the test would meet industrial standards. The results of this research show that the use of CGDS as a process for the restoration of damaged aluminum IVD coatings is possible and is a promising alternative to conventional methods. The CGDS coatings were scrutinized to the same level as required of IVD coatings when they replaced toxic cadmium coatings in the late 1980s. The coating adhesion, demonstrated through glass bead abrasion and strip rupture testing, was shown to meet the current industrial standards. The corrosion testing of the repairs resulted in no visible red rust of the steel components, even when the steel was exposed.

Des revêtements anticorrosion de zinc et d'aluminium ont été développés respectivement par des techniques de pulvérisation à froid à haute pression et à basse pression. Pour les revêtements de zinc, la dépendance de la température de pulvérisation sur l'épaisseur a été analysée et la température critique de dépôt a été trouvée à 230°C. Des variations de pression de 2 MPa, 2,5 MPa et 3 MPa à température constante 290 °C ont montré la valeur d'épaisseur de couche plus élevée à 2 MPa. Il a été également confirmé que l'épaisseur du revêtement à tendance à diminuer avec la pression. Le taux d'alimentation en poudre ainsi que la distance de pulvérisation ont également été considérés comme des paramètres influençant l'épaisseur. Les conditions optimales de projection ont été trouvées pour 3 rps et 30 mm, respectivement. Enfin, la température et la pression du gaz ont été optimisées par le plan d’experience dit de Doehlert. Leurs influences sur la qualité du revêtement de zinc ont été discutées en termes de microstructure, de porosité, d'épaisseur et de résistance à la corrosion. Une porosité maximale de 4,2% a été atteinte avec la pression la plus élevée et avec une température modérée (260 ° C < T < 300 ° C). Ces conditions favorisait l'érosion du substrat et la faible déformation des particules lors de l'impact. Deux conditions optimales ont ainsi été trouvées: 320 ° C – 2,5 MPa et 260 ° C – 2,5 MPa. Des expériences électrochimiques macroscopiques et locales ont été ensuite réalisées. Une résistance à la corrosion plus élevée a été détectée pour la condition 320 ° C – 2,5 MPa. Les revêtements étaient alors suffisamment épais pour protéger le substrat et le mécanisme de corrosion était liée au comportement des couches d'hydroxyde et d'oxyde de Zn. Il est a noter que la rugosité du revêtement devra être pour réduire l'amorçage de la corrosion. Pour les revêtements d'aluminium, les paramètres de déposition optimaux ont été trouvés à 400 ° C / 0,65 MPa. Des particules de céramique ont été ajoutées pour densifier le revêtement permettant une réduction de porosité de 8% à 6,4%. Un traitement de surface par laser a été ensuite effectué. Dans ce travail, la puissance du laser s’est révelée insuffisante pour atteindre le point de fusion de l’aluminium, cependant, la dureté des revêtement a pu être modifée. Les résultats ont montré une augmentation de la microdureté des revêtements de 5% avec l'ajout de particules céramiques tandis qu’une réduction de dureté de 39% et 35% a été mesurée sur le revêtement en aluminium pur et composite respecitvement. La diminution de dureté lors le traitement au laser a été attribuée au recuit de surface, à la libération de contraintes internes et à une possible recristallisation locale. Enfin, les caractérisations électrochimiques ont montré une résistance à la corrosion plus élevée pour les revêtements composites céramiques que l'aluminium pur et les revêtements traités au laser
Anticorrosion coatings of Zinc and Aluminium were developed by high pressure and low-pressure Cold Spray techniques, respectively. For Zinc coatings, the dependence of spraying temperature on thickness has been analyzed and the critical temperature of deposition was found at 230 oC. For lower temperatures, the coating was considerably thinner. Dependence of thickness on pressure variation 2 MPa, 2,5 MPa and 3 MPa at constant temperature 290 oC has shown the highest thickness value at 2 MPa. It was confirmed that the coating thickness tends to decrease with the pressure rise. The powder feeding rate as well as the spraying distance were also considered to influence the thickness. The optimal conditions were found for 3ps and 30 mm, respectively. Finally, the gas temperature and pressure were optimized by a Doehlert uniform shell design. Their influences on the zinc coating quality were discussed in terms of microstructure, porosity, thickness, and corrosion resistance. A maximum porosity of 4.2% was reached with the highest pressure and with a moderate temperature (260 °C < T < 300 °C). These conditions promoted erosion of the substrate and a lower accommodation of particles at the impact. Thicker coatings were obtained at higher temperatures because of better particle straining. Two optimal conditions were then identified: 320 °C–2.5 MPa and 260 °C–2.5 MPa. Macroscopic and local electrochemical experiments were performed. Higher corrosion resistance was detected for the condition 320 °C–2.5 MPa. Coatings were enough thick to protect the substrate and the corrosion mechanism was driven by the classical Zn hydroxide and oxide layers. Note that the coating roughness may be optimized later to reduce the corrosion initiation. For aluminum coatings deposited by a low-pressure cold spray method, the optimal spraying parameters according to deposition efficiency were found at 400 °C /0.65 MPa. Ceramic particles were added to densify the coating and allowed to reduce porosity from 8% to 6.4%. Instead of ceramic particle addition, laser surface treatment was performed after coating design. Laser power was not enough high to reach the surface melting, however, the coating microhardness was modified. Results showed a microhardness increase of coatings of 5% with the addition of hard particles whereas the microhardness decreased after the post-heat treatment (pure aluminum coating reduction of 39% and for composite coating 35%). The hardness reduction during the laser treatment was attributed to surface annealing and the release of internal stresses and possible recrystallization with the subsequent grain growth. Finally, the results of the electrochemical investigations showed higher corrosion resistance of ceramic composite coatings than both pure aluminum and laser-treated coatings

Plasma spray coatings are vital to the capabilities of jet engines. They allow engines to operate at combustion temperatures that would otherwise melt the superalloy components. Coatings tighten clearance between rotating components, increasing engine compression. They prevent chemical attack and physical erosion. Plasma spray coatings are imperative to the durability and efficient operation of the modern jet engine. In this application coating material property variation has a significant cost. In addition to the variation inherent in the process, some of the biggest contributors to coating property variation have been traced to spray gun nozzle wear and powder feed variation[3, 4]. Presented here are multiple methods utilizing flow induced acoustic signals to quantify noise parameters, measure component wear, diagnose the plasma spray process and detect coating property deviation. Methods have been developed for offline and online analysis of components in addition to online process analysis. These include characterization of nozzle wear by throat roughness measurements and nozzle casting, offline detection of nozzle wear by attenuation of discrete tone generation and broadband signal variation, and offline measurement of powder port wear by jet screech frequency variation. Online methods include pre-ignition nozzle degree of wear measurement by discrete frequency changes; online parameter change detection, process deviation detection with potential source identification, as well as variation in coating property detection by broadband acoustic signal changes. Offline methods allow for 100% accurate new nozzle manufacturer identification. By the same test nozzle wear state can be predicted with over 95% accuracy with the potential for a degree of wear determination. Internal diameter changes of less than 10 microns can similarly be detected. Analysis of online plasma spray acoustic signals as described here can distinguish nozzle state and powder feed variation with over 90% accuracy. The capabilities developed here will aid in plasma spray process variation detection and contribute to identifying the source of this variation. This will improve coating quality and consistency, reduce failures, lower operational costs and ultimately make jet engines more economical, safer, and more fuel efficient with significant environmental and financial cost reduction.
Ph. D.

Plasma spray coatings are vital to the capabilities of jet engines. They allow engines to operate at combustion temperatures that would otherwise melt the superalloy components. Coatings tighten clearance between rotating components, increasing engine compression. They prevent chemical attack and physical erosion. Plasma spray coatings are imperative to the durability and efficient operation of the modern jet engine. In this application coating material property variation has a significant cost. In addition to the variation inherent in the process, some of the biggest contributors to coating property variation have been traced to spray gun nozzle wear and powder feed variation[3, 4]. Presented here are multiple methods utilizing flow induced acoustic signals to quantify noise parameters, measure component wear, diagnose the plasma spray process and detect coating property deviation. Methods have been developed for offline and online analysis of components in addition to online process analysis. These include characterization of nozzle wear by throat roughness measurements and nozzle casting, offline detection of nozzle wear by attenuation of discrete tone generation and broadband signal variation, and offline measurement of powder port wear by jet screech frequency variation. Online methods include pre-ignition nozzle degree of wear measurement by discrete frequency changes; online parameter change detection, process deviation detection with potential source identification, as well as variation in coating property detection by broadband acoustic signal changes. Offline methods allow for 100% accurate new nozzle manufacturer identification. By the same test nozzle wear state can be predicted with over 95% accuracy with the potential for a degree of wear determination. Internal diameter changes of less than 10 microns can similarly be detected. Analysis of online plasma spray acoustic signals as described here can distinguish nozzle state and powder feed variation with over 90% accuracy. The capabilities developed here will aid in plasma spray process variation detection and contribute to identifying the source of this variation. This will improve coating quality and consistency, reduce failures, lower operational costs and ultimately make jet engines more economical, safer, and more fuel efficient with significant environmental and financial cost reduction.
Ph. D.

Commercial airplanes are still using aluminum alloys as their primary structural material. Even if the used carbon fiber reinforced polymers is becoming more popular due to their extremely high strength to weight ratio, the majority of the existing flying fleet is still made out of aluminum alloys. This material was primarily used due to its high strength to weight ratio, ease to machine, excellent corrosion resistance properties and its high crash energy absorption. Aircraft components made of aluminum alloys are subjected to high stresses and harsh environments during flight, potentially leading them to crack and/or corrode. Presently, there is no industrial approved method to repair these components. Recycling damaged aircraft parts by repairing them would result in large cost savings for the industry. The present study was motivated by the potential use of the cold gas dynamic spray (CGDS) process to repair damaged aluminum 7075-T6 aircraft components. Two feedstock materials were used to repair this alloy in this research: pure aluminum and aluminum 7075. Pure aluminum is used in the aircraft industry on non-bearing components due to its extremely high corrosion resistance properties. Aluminum 7075 is the material of choice for structural applications due to its high strength. The results of this study show that CGDS could be potentially used to repair aluminum components on aircrafts. However, this research demonstrated that new commercially available equipments need to be further developed to successfully produce repaired components that meet the industry standards.

The Copper Development Association (CDA) has identified over 450 copper alloys registered with the U.S. Environmental Protection Agency (EPA) as antimicrobial. With growing antibiotic resistance, there is a need for copper coatings with increased antimicrobial capability. Cold spray is a high velocity, high deposition rate process that forms dense coatings with little to no oxides or inclusions. It is possible that this process contributes to the increased antimicrobial capability of copper cold spray coatings as compared to other additive processes. The focus of this effort is to understand the effects of powder production and cold spray process parameters on copper cold spray coatings in order to optimize antimicrobial properties. Specifically, this work looks at the differences in conventional and nanomaterial copper cold spray coatings. Materials characterization and test methods show differences in adhesion, microstructure, corrosion, mechanical properties, and surface topography. Materials data is compared against Abaqus FEA software model outputs, and antimicrobial efficacy test data, based on the EPA approved procedure, is used to support materials observations and modelling outputs.

The main subject of this thesis is the fabrication of functional titanium dioxide coatings by means of Atmospheric Plasma Spray (APS) and Cold Gas Spray (CGS). Functional role may be understood as the capacity of TiO2 surfaces to respond in a determined way under certain conditions. Firstly, conventional coating processes, sensing mechanisms and overall efficiencies were deeply studied. As regards to experimental results, it was observed that H2 contained in the plasma mixture could reduce TiO2 towards non stoichiometric or stoichiometric compounds such as titanium sub-oxides (TiO2-x) or Magnéli Phases (TinO2n-1) respectively during the in-flight of the particles. Large accumulation of oxygen vacancies in the crystal lattice of rutile led to a donor level to the conduction band. Therefore, a corrosion-resistant ceramic material with a low electrical resistivity was obtained on ceramic tiles. This unexpected procedure led to deposit APS TiO2-x / TinO2n-1 coatings on stainless steel and apply them in electrochemical bi-polar batteries. Then, from the created feedback thin stainless steel and aluminium films, carbon-polymer composites or nickel foams as common standard electrode materials were selected and coated. Produce the active layer of a metal oxide gas sensor using APS fed by TiO2 was still a target to be accomplished. With the aim of offering more innovation to conventional metal oxide sensors, it was determined to build-up the sensing layer on a thin polymeric flexible substrate. It was possible to reach certain spraying conditions that avoided thermal degradation of the polymer. Furthermore, heterogeneous disposition of the coating, where some areas were coated and certain spots uncoated provided electric contact between the electrodes and structure that eased elastic deformation of the film. Satisfactory performances were obtained testing the response of the device in front of a target gas and radiation. Thenceforth, transition to thermally less-aggressive technologies was carried out. It was decided to focus the efforts on CGS, which does not require melting the material for being deposited. Subsequently, nanostructured anatase was used as feedstock in order to achieve photocatalytic layers with large specific surfaces for applying them in the degradation of different contaminants. It was used a powder able to create chemical bonds with the substrate and among the particles at the impact. Unfortunately, feeding system was repeatedly clogged because of the high agglomerating capacity of the powder. Blends were prepared with copper and microstructured TiO2 that flowed appropriately so as to avoid the obstruction of the pipelines. First, Cu/nano-TiO2 coatings were deposited using spraying conditions that favoured the deposition of nanostructured anatase at the top surface, which assured the development of the photocatalytic process. Samples successfully degraded toluene in gaseous phase. On the other hand, micro-TiO2/nano-TiO2 blend was not suitably deposited onto steel. Ceramic particles may not deform plastically. Thus, chemical bonds with the substrate and among particles had to be boosted for building-up the coatings. Substrate surface based on APS TiO2-x with controlled roughness provided composition, hardness and required geometry for adhering nano-TiO2 particles. In this way, CGS nano-TiO2 coatings were tested for degrading phenol and formic acid in liquid phase. The obtained results equalized or even improved the performance of sol-gel coatings. Metallic Ti coatings were previously deposited onto the polymer by CGS for afterwards spraying nano-TiO2, following the know-how gained in CGS nano-TiO2 photocatalysts. Again, lower layer acted as a bond coat between the original substrate and nanostructured anatase. Osteblast cultures were tested on PEEK, CGS Ti on PEEK and CGS nano-TiO2 deposited on CGS Ti layer. Higher cell adhesion, proliferation and differentiation were obtained as long as CGS coatings were applied, which leads to an improved bioactivity of polymeric implants.
El principal objetivo de esta tesis es la fabricación de recubrimientos funcionales de óxido de titanio obtenidos por Atmospheric Plasma Spray (APS) y Cold Gas Spray (CGS). El rol funcional debe ser entendido cómo la capacidad de las superfícies de TiO2 de responder de una manera determinada ante ciertas condiciones. El H2 contenido en el plasma podía reducir el TiO2 hacia compuestos no estequiométricos o estequiométricos como los sub-óxidos de titanio (TiO2-x) o las fases de Magnéli (TinO2n-1). Una gran acumulación de vacantes de oxígeno en la estructura cristalina del rutilo llevó a la formación de un nivel dador hacia la banda de conducción. Este inesperado procedimiento llevó a producir recubrimientos APS TiO2-x / TinO2n-1 sobre acero inoxidable y aplicarlos como electrodos en baterías bi-polares. Posteriormente, se decidió recubrir con este material electrodos típicamente utilizados como láminas finas de acero inoxidable y aluminio, compuestos de carbono-polímero y espumas de níquel. Con la intención de ofrecer más innovación a los sensores convencionales de óxido metálico, se decidió fabricar la capa activa sobre un sustrato polimérico flexible. Fue posible alcanzar ciertas condiciones experimentales que evitaron la degradación térmica del polímero. Se centraron esfuerzos en CGS, que no necesita fundir el material para producir el recubrimiento. De esta forma, anatasa nanoestructurada se utilizó como materia prima con el objetivo de lograr capas fotocatalíticas con gran superfície específica, capaces de degradar diferentes contaminantes. Se utilizó un polvo capaz de crear enlaces químicos con el sustrato. Se prepararon mezclas con otros polvos con el objetivo de mejorar su fluidez y evitar la obstrucción de las tuberías. Primero, recubrimientos Cu/nano-TiO2 fueron depositados utilizando condiciones que favorecieron el anclaje de las partículas de anatasa en la superfície del recubrimiento. Las muestras degradaron tolueno en fase gaseosa con éxito. Por otro lado, la mezcla micro-TiO2/nano-TiO2 no se depositó sobre acero. Se utilizó un sustrato préviamente recubierto con APS TiO2-x. Estos recubrimientos degradaron con éxito fenol y ácido fórmico en fase líquida. Se decidió incrementar la bioactividad del PEEK (polyetheretherketone). Sin embargo, fue posible anclar partículas de TiO2 sobre el polímero previamente recubierto por Ti mediante CGS, obteniendo recubrimientos gruesos con una buena adherencia. Cultivos de osteoblastos fueron analizados sobre PEEK, Ti en PEEK y nano-TiO2 en PEEK. Se obtuvo una mayor adhesión, proliferación y diferenciación celular a medida que los recubrimientos CGS fueron aplicados.

A thermal barrier coating (TBC) is used as thermal protection of metallic components exposed to hot gas streams in e.g. gas turbine engines. Due to a high thermal expansion coefficient, low thermal conductivity, chemical- and thermal stability, yttria stabilized zirconia (YSZ) is the most widely used material for TBCs today. In the work presented in this master thesis an aqueous nitrate precursor solution was prepared and deposited on stainless steel substrates by spray pyrolysis to produce 8YSZ coatings (8 mol% of Y2O3 in ZrO2). The precursor solution concentration and deposition parameters, including set-point temperature and volume sprayed, were optimized to produce continuous and crack-free green coatings.The deposited green coatings were characterized by scanning electron microscopy, thermogravimetry and Fourier transform infrared spectroscopy to study the influence of substrate temperature on the microstructure of the green coatings. A substantial change in microstructure was observed for the green coatings in a certain temperature range indicating that a minimum deposition temperature was necessary to obtain crack-free green coatings.Heat treatment was necessary to decompose the nitrate species in the deposited film. During heat treatment, vertical cracks were introduced into the coatings due to the nitrate decomposition. The cracking behavior of the coatings was studied for different drying times and conditions, and it was found that the crack propagation can be controlled to obtain the preferred size and geometry of the cracks. Due to built-up stresses in the coating, which can exceed the fracture toughness of the material, it was found that there was a maximum film thickness achievable before spallation of the coating for a given precursor solution. Therefore, the possibility of spraying multi-layered coatings was investigated. The introduction of a second layer showed that it was possible to double the thickness of the coating.

To reduce the environmental impact, there is a trend for the thermal power generation industry to increase the usage of the biomass fuel. However, the severe corrosion of boiler materials that is brought about by the high chlorine and alkaline content in the biomass limits the application of biomass fuel and the operation temperature of the biomass boiler. NiCr based alloys that have high Cr content are an ideal solution to combat this problem; they can protect the boiler steel substrate from the chlorine-induced corrosion. There are quite a lot of coating deposition techniques, but the most suitable one for the deposition of Ni50Cr alloy on the boiler material and thereby provides satisfactory protection is still in debate. The major aims of this research included: determining the most suitable coating deposition technique for high temperature oxidation/corrosion applications of NiCr alloys in four candidates (high velocity oxygen liquid/gas fuel (HVOLF/HVOGF) thermal, cold spray and laser cladding); describing and explaining the high temperature oxidation/corrosion performance of coatings deposited using shortlisted deposition routine. The key objectives of this research were: a literature review that can identify the knowledge gap existing in the current research of high temperature oxidation/corrosion NiCr-based coatings; successful deposition of NiCr-based coatings using techniques include in this study, and the deposition product should be of satisfactory quality; high temperature oxidation/corrosion exposure of deposited coatings in test rigs; comprehensive summary of the oxidation/corrosion behaviours of coatings and identification of possible mechanism to explain these behaviours; direction for the future development of high temperature NiCr-based corrosion resistance coatings. To achieve abovementioned objectives, following work were conducted. Short-term air thermogravimetric analysis (TGA) results of as-sprayed HVOF coatings were used to determine the air oxidation resistance of the five HVOF thermal sprayed coatings. The HVOLF and HVOGF coatings that had better air oxidation resistance were hypothesized to have better steam oxidation and fireside corrosion performance and shortlisted for the subsequent high temperature test in various atmospheres. There was no recommended process parameter sets for the laser cladding of the Ni50Cr powder. Therefore, a process window was built to help decide the most suitable parameter set. Laser cladded coating that had satisfactory metallurgical quality, acceptable thickness, maximum width and minimum dilution was selected as the candidate for subsequent high temperature exposure. The optimization process of the laser cladding parameter had been published and was not included in this thesis. The cold sprayed coating was deposited by using the Xi'an Jiaotong University's custom-made cold spray set-up. The microstructure of the deposited cold sprayed coating was acceptable after grinding several top layers of the coating (~ 500 μm), and the coating was placed into the high temperature test rigs for the further investigation of the high temperature oxidation/performance of cold sprayed coating. Longer-term air oxidation (compared with the TGA dwell time), steam oxidation and chlorine-induced fireside corrosion tests of four coatings were conducted in the simulation test rigs built in our laboratory. Following procedures were adopted to investigate the oxidation/corrosion behaviour. In the case of the air oxidation test, three samples of each coating were placed into the high temperature box furnace and was removed from the furnace after 1 h, 10 h and 100 h of exposure time, in succession. In the case of the steam oxidation test, four samples of each coating were sent into the test rig and collected after 250 h, 500 h, 750 h and 1000 h of exposure time one after another. While in the case of the chlorine-induced fireside corrosion test, two samples, with KCl and without KCl deposit on the surface of coating, were the candidates of the 250 h high temperature corrosion exposure. Adopting a relative short exposure time in the chlorine-induced corrosion test was owing to the severe attack ability of chlorine and alkali metal on material. Specimens, after the high temperature oxidation/corrosion and collected at the specific time points, can provide valuable information about the evolution of the air oxidation, steam oxidation and fireside corrosion behaviour of four coatings. Several material characterisation methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to obtain the said information. By conducting the abovementioned experiments and analysing the obtained results, the rank of the oxidation/ corrosion resistance of four coatings in various atmospheres at high temperature can be asserted. The mechanism behind the different oxidation/corrosion behaviour for various coatings can be investigated and several possible mechanisms that can explain the observed results are adopted. Finally, a promising Ni50Cr coating deposition method - HOVLF and laser cladding is recommended.

The research is aimed to evaluate thermal spray coatings to address material issues in supercritical and ultra-supercritical Rankine cycles. The primary purpose of the research is to test, evaluate, and eventually implement a coating to improve corrosion resistance and increase efficiency of coal fired power plants. The research is performed as part of a comprehensive project to evaluate the ability of titanium, titanium carbide, or titanium diboride powders to provide fireside corrosion resistance in supercritical and ultra-supercritical steam boilers, specifically, coal driven boilers in Illinois that must utilize high sulfur and high chlorine content coal. [1] The powder coatings that were tested are nano-sized titanium carbide (TiC) and titanium di-boride (TiB2) powders that were synthesized by a patented process at Southern Illinois University. The powders were then sent to Gas Technology Institute in Chicago to coat steel coupons by HVOF (High Velocity Oxy-Fuel) thermal spray technique. The powders were coated on an austenitic 304H stainless steel substrate which is commonly found in high temperature boilers, pipelines, and heat exchangers. The samples then went through various tests for various lengths of time under subcritical, supercritical, and ultra-supercritical conditions. The samples were examined using a scanning electron microscope and x-ray diffraction techniques to study microstructural changes and then determined which coating performed best.

The continuous increase of Turbine Entry Temperature (TET) in aerospace gas-turbines is the main driver for the research effort in the development of coatings for thermal and oxidation protection: i.e. Thermal Barrier Coatings (TBC). The need for TBC is particularly relevant within the combustor assembly (or simply combustion chamber) where the highest temperatures within a gas turbine, in excess of 1700 C, are registered. Due to the harsh thermal and oxidative conditions experienced within the combustion chamber, TBC are subjected to several degradation mechanisms which generally result in spallation (or delamination) of the coating. Spallation is more likely to be observed at specific locations within a combustion chamber, and acceptance limits for this quantity are specified by the Original Equipment Manufacturer (OEM). To reduce the risk that coating spallation will lead to an unplanned engine removal, in-situ coating re-deposition may be possible. However, the development of such a technology poses significant challenges. The selected deposition process must, in fact, be able to operate in a confined environment (i.e. the combustion chamber) and produce TBC of microstructure providing adequate thermal and oxidation protection properties. Moreover, a deep understanding between the process parameters and both microstructure and shape of produced coating is necessary to achieve an optimum control over the whole deposition process. Therefore, after an initial selection of Combustion Flame Spray (CFS) as TBC deposition technology, the present thesis has the following objectives: (i) analysing in-depth the physics/chemistry of coating build-up at a microscopic level (i.e. single-splat) in order to relate this to fundamental properties (e.g. adhesion and residual stress) measured at macroscopic coating level, (ii) investigating the relationship between process parameters and their effect on the material properties of the deposit, in order to determine an optimum process parameters "window" and (iii) to develop a mathematical framework that accounts for the stochastic nature of the deposition process, and has the capability to predict the deposit growth geometry with high spatial accuracy for different process parameters. For coating build-up analysis purposes, a novel set of experimental tools is developed, allowing to model fundamental flattening and solidification mechanisms with a sub-micrometre spatial resolution. For deposition parameters optimisation purposes, an extensive experimental analysis of the effect of deposition parameters including: powder morphology (size and shape), equivalence ratio, powder feed rate, carrier gas flow and torch-to-substrate standoff distance has been performed for the CFS-produced TBC to complete the lack of knowledge in literature data. Finally, the deposit growth model allows to predict, in the time domain, the three-dimensional footprint (i.e. deposit shape) and temperature of CFS and generally thermal spray deposits. For this purpose, a three-dimensional implicit finite-difference algorithm, based on two interplaying geometrical and thermal-analysis sections, has been developed. The work of this thesis thus provides a step forward in the understanding of the thermal spray deposit formation process. In fact, the determined correlation between properties at both single splat and coating level represents a powerful tool making the optimisation of process parameters-coating properties relationship more efficient as opposed to traditional trial-and-error approaches. Moreover, the developed calibration-based deposit growth model results of simple application, opening the way for spray automation in difficult-to-spray geometries and/or repair applications for several thermal spray processes.

Plasma spray processing is a technique that is used extensively in thermal barrier coatings on gas and steam turbine components, biomedical implants and automotive components. Many processing parameters are involved to achieve a coating with certain functionality. The coating could be required to function as thermal barrier, wear resistant, corrosion resistant or a high temperature oxidation resistant coating. Various parameters, such as, nozzle and electrode design, powder feeding system, spray distances, substrate temperature and roughness, plasma gas flow rates and others can greatly alter the coating quality and resulting performance. Feedstock (powder or solution precursor) composition and morphology are some of the important variables, which can affect the high end coating applications. The amount of heat a plasma plume has to offer to the particles being processed as a coating depends primarily on the dissociation of the atoms of gaseous mixtures being used to create the plasma and the residence time required for the particle to stay in the flame. The parameters that are conducive for nanostructured retention could be found out if the residence time of the particles in the flame and the available heat in the plume for various gas combinations could be predicted. If the feedstock is a liquid precursor instead of a powder feedstock, the heat that has to be offered by the plasma could be increased by suitable gas combination to achieve a good quality coating. Very little information is available with regard to the selection of process parameters and processing of nano materials feedstock to develop nanostructured coatings using plasma spray. In this study, it has been demonstrated that nano ceramics or ceramic composites either in the form of coatings or bulk free form near net components could be processed using DC plasma spray. For powder feedstock, analytical heat transfer calculations could predict the particle states for a given set of parameters by way of heat input from the plasma to the particles. The parameter selection is rendered easier by means of such calculations. Alumina nano ceramic particles are processed as a coating. During Spray drying, a process of consolidation of nano alumina particles to spherical agglomerates, parameter optimization for complete removal of moisture has been achieved. The parameters are tested for alumina nanoparticles with a plasma torch for the veracity of calculations. The amount of heat transfer from the surface of the agglomerates to the core has been quantified as a function of velocity of particles. Since preparation of nanostructured feedstock for plasma spray is expensive and cumbersome, alternative solution precursor route for direct pyrolysis of precursor to coating has been studied in case of nanocrystalline rare earth oxides. Thus, it has also been shown by this research that nanostructured coatings could be either from a powder feedstock or a solution precursor feedstock. MoSi2-Si3N4, Ni-Al2O3, W-HfC nano ceramic composite systems have been processed as a bulk free form nanocomposite with 60-70% retained nanostructures. The importance of selection of substrates, roughness and the substrate temperature for development of free form bulk components has been highlighted. The improvement in mechanical and high temperature properties associated with having such nanostructured coatings or bulk nanocomposites are revealed. These nanostructured coatings are known for their low thermal conductivity, high wear resistance and can be potentially used as steam and gas turbines coatings for improved thermal efficiency. In summary, bulk nanocomposite through plasma spray processing is a viable alternative to conventional processes such as sintering, HIP for high fracture toughness and hardness applications.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science & Engr PhD

Cerium oxide (Ceria) at nano scale has gained significant attention due to its numerous technological applications. Ceria in both doped and undoped forms are being explored as oxygen sensor, catalysis, protective coating against UV and corrosion, solid oxide fuel cell (SOFC) electrolyte and newly discovered antioxidant for biomedical applications. Therefore, there is an imminent need of a technology which can provide a cost effective, large scale manufacturing of nanoceria and its subsequent consolidation, specially using thermal spray. This dissertation aims to develop a scientific understanding towards the development of pure and doped ceria- based coating for a variety of technological applications, from SOFC applications to corrosion resistant coating. Atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) techniques for the fabrication of nano ceria coating were investigated. For feedstock powder preparation, a spray drying technique was used for the agglomeration of cerium oxide nano particles to achieve high density coating. Deposition efficiencies and coating porosity as a function of processing parameters were analyzed and optimized using a statistical design of experiment model. The coating deposition efficiency was dependent on the plasma temperature and vaporization pressure of the ceria nanoparticles. However, low standoff distance and high carrier gas flow rate were responsible for the improved density upto 86 [plus or minus] 3%.An alternative novel SPPS technique was studied for a thin film of cerium oxide deposition from various cerium salt precursors in doped and undoped conditions. The SPPS process allows controlling the chemistry of coating at a molecular level. The deposition mechanism by single scan experiments and the effect of various factors on coating microstructure evolution were studied in terms of splats formation. It was found that the precursor salt (nitrate of cerium) with lower thermal decomposition temperatures was suitable for a high density coating. The high concentration and low spray distance significantly improve the splat morphology and reduced porosity (upto 20%). The feasibility of the trivalent cations (Sm 3+ and Gd 3+) doping into cerium oxide lattice in high temperature plasma was discussed and experimentally studied. XRD analysis revealed the nano crystalline characteristic of the coating and lattice expansion due to doping. The extensive transmission electron microscopy, Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and thermo gravimetric were conducted to evaluate the precursors, and coating microstructure. Due to facial switching between Ce4+ and Ce3+ oxidation state, the cerium oxide surface becomes catalytically active. Thus, the APS ceria coatings were investigated for their applicability under extreme environmental conditions (high pressure and temperature). The air plasma sprayed coated 17-4PH steel was subjected to high pressure (10 Kpsi) and temperature (300 oF) corrosive environment. The coated steel showed continuous improvement in the corrosion resistance at 3.5 wt% NaCl at ambient temperature for three months study whereas, high pressure did not reveal a significant role in the corrosion process, and however, one needs to do further research. The ceria coated steel also revealed the improvement in corrosion protection (by 4 times) compared to the bare steel at low pH, 300 oF and 4000 Psi environment. This study projects the importance of cerium oxide coatings, their fabrication, optimization and applications.
ID: 031001377; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Adviser: Sudipta Seal.; Title from PDF title page (viewed May 21, 2013).; Thesis (Ph.D.)--University of Central Florida, 2012.; Includes bibliographical references (p. 171-182).
Ph.D.
Doctorate
Materials Science Engineering
Engineering and Computer Science
Materials Science and Engineering

This Thesis presents two different deposition techniques for the synthesis of Ti2AlC coatings. First, I have fabricated Ti2AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211TM Ti2AlC powder of size ~38 µm in an H2/O2 gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti3AlC2, TiC, and AlxTiy alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211TM Ti2AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti2AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti2AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti2AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti2AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti2AlC (101̅7) // Al2O3 (0001).
Report code: LIU-TEK-LIC-2008:15.

The purpose of this study is to characterize the plasma spray deposits used for attaching intrinsic Fabry-Perot interferometric fiber optic strain sensors. The deposits must maintain adhesion at elevated temperatures without distorting the sensors' signals. Two different material systems were tested and modeled, a nickel based alloy and yttria-stabilized zirconia. The material properties of the deposits and the thermal stresses in the system were evaluated to determine attachment lifetime of the sensors. The encapsulated sensors' signals were collected before and after plasma spraying and at elevated temperatures. The material properties of the deposits were evaluated by electron microscopy, energy dispersive x-ray spectroscopy, scratch testing, thermal fatigue testing, and nanoindentation. The thermal stresses were evaluated by Raman spectroscopy and from finite element analysis in COMSOL® Multiphysics®. Several of the sensors broke during encapsulation due to the plasma spray processing conditions and the signals experienced distortion at elevated temperatures. The sensors can be treated to remove this interference to allow for this deposit attachment. The nickel based alloy's ductility and lamellar microstructure allowed for non catastrophic relaxation mechanisms to relieve induced thermal stresses. The yttria stabilized zirconia failed catastrophically at elevated temperatures due its lack of compliance to mismatches in thermal expansion. A high melting point metallic deposit, similar to the nickel based alloy, is desirable for fiber optic sensor attachment due to its ductility, thermal expansion, and dominant relaxation mechanisms. The processing conditions may need to be optimized to allow for the sensors' protection during encapsulation.
Master of Science

Integrally heated composite tooling (IHCT) is seen as a low cost alternative to autoclave manufacturing of polymer matrix composites (PMCs). IHCTs consist of a composite tool heated by surface heaters; temperature distribution is ensured by a thermally conductive metallic layer. The main original contributions of this thesis was the development of a new method for applying copper coatings onto carbon fibre/epoxy PMCs using pulsed gas dynamic spraying, the production of larger size samples, and the characterisation of the performance of the coatings and laminates obtained. It was shown that this method has potential for producing the thermally conductive layer in an IHCT. Another contribution was the characterisation of parameters affecting temperature distribution across IHCTs through heat transfer simulations, leading to guidelines for IHCT design.

L'hydroxyapatite (HAp) et le verre bioactif (BG) 45S5 sont largement utilisés comme précurseurs des revêtements fabriqués par projection thermique pour améliorer la biocompatibilité des implants biomédicaux. Ceci, en raison de la structure similaire de l'HAp avec le tissu osseux et la haute réactivité de ce BG en milieu biologique, ce qui permet la croissance rapide du tissu osseux à sa surface. La projection combinée des deux matériaux permet d’obtenirdes revêtements hautement bioactifs et stables par rapport aux revêtements individuels. De plus, la bioactivité de l'HAp et la radio-opacité de BG45S5 peuvent être améliorées en utilisant comme matières premières des sources naturelles, telles que l'HAp issue d'os de bovins (BHAp) et en ajoutant des radiopacifiants tels que l’oxyde de bismuth (Bi2O3) à la structure du BG. Par conséquent, des revêtements combinés avec une bioactivité et une opacité accrues vis-à-vis des rayons X peuvent être obtenus pour améliorer le diagnostic non invasif par radiographie des implants biomédicaux revêtus.Cette recherche présente le développement de couches épaisses biocompatibles et opacifiées réalisées par projection thermique, flamme haute vitesse (High-Velocity Oxygen Fuel, HVOF) et plasma (Atmospheric Plasma Spray, APS). La composition chimique, la structure, la microstructure et la bioactivité des couches radio-opaques de BHAp/BG45S5+Bi2O3 obtenues sont étudiées ainsi que les propriétés des matériaux de départ. ainsi que la projection gradient du revêtement combiné. En particulier, la bioactivité de différentes architectures de revêtements (monocouches, couches à gradient de matériaux et multicouches) a été évaluée. Les résultats expérimentaux montrent que la poudre de départ de BHAp est principalement constituée d’apatite carbonée de type B avec une pureté élevée de la phase HAp. Les poudres de bioverre BG45S5 fabriquées en laboratoire présentent une composition chimique et des propriétés physiques très similaires à celles du BG45S5 commercial.Un pourcentage en poids de 1 à 20 % d’oxyde de bismuth Bi2O3 a été ajouté au verre bioactif pour modifier sa radio-opacité. Les images radiographiques des composés BG45S5 + x% Bi2O3 montrent que le bioverre avec 10% en poids d’oxyde de bismuth permet d’augmenter de 3,6 fois l’opacité du bioverre de manière homogène sans affecter de manière notable ses propriétés structurales et thermiques. En ce qui concerne les revêtements réalisés par projection thermique, la teneur en CO32- et en Mg dans la poudre de BHAp conduit à la formation de dolomite dans la phase cristalline de la surface du revêtement monocouche de BHAp réalisé par HVOF alors que celui réalisé par APS ne montre aucune phase secondaire cristalline à sa surface. Une couche d'apatite typique est mise en évidence à la surface des deux revêtements après 3 jours d'immersion dans un fluide corporel simulé (SBF), cependantle revêtement monocouche de BHAp réalisé par HVOF montre une délamination après 5 jours d'immersion. Par conséquent, la projection plasma APS a été choisie pour élaborer les revêtements monocouches de BHAp, BG45S5, BG45S5+10wt.% Bi2O3 et le revêtement à gradient de composition de BHAp/BG45S5+10wt. % Bi2O3. Le revêtement monocouche de BG45S5 + 10wt.% Bi2O3 a une microstructure et une structure amorphe similaires à celles du revêtement monocouche de BG45S5 sans Bi2O3 réalisé par projection plasma de la poudre commerciale . Cependant, une délamination et une vitesse réduite de formation de la couche d'apatite ont été observées. Le revêtement à gradient de composition de BHAp/BG45S5+10 wt. % Bi2O3 montre un taux de croissance de la couche d'apatite similaire à celui du revêtement monocouche de BG45S5+10wt. % Bi2O3. La formation d'apatite observée après 10 jours d'immersion dans un fluide corporel simulé ne présente pas de délaminage à la surface du revêtement grâce au gradient de composition du BG45S5 10 wt. % Bi2O3 combiné au BHAp
Hydroxyapatite (HAp) and bioactive glass (BG45S5) are widely employed as precursors of thermally sprayed coatings to enhance the biocompatibility of biomedical implants. This, due to the similar structure of HAp with the bone tissue and the high reactivity of BG with biological media allowing the rapid bone tissue ingrowth on its surface. The combined deposition of both materials leads to build-up highly bioactive coatings with proper stability in comparison with single coatings. Furthermore, the HAp bioactivity and the BG45S5 radiopacity can be improved by obtaining the first from natural sources, as bovine-derived HAp (BHAp), and the second through the addition of radio-opacifiers as Bi2O3 to the BG structure. Consequently, coatings with augmented both, biocompatibility and x-rays opacity that allow improving the bioactivity and facilitate the use of non-invasive diagnostic methods, can be achieved. This research presents the development of biocompatible and opacified thick coatings deposited by High-Velocity Oxygen Fuel (HVOF) and Atmospheric Plasma Spray (APS) techniques. The chemical composition, structure, and microstructure of radiopaque BHAp/BG45S5+Bi2O3 coatings were studied, as well as the properties of raw materials were also analyzed. Afterward, the bioactivity of several coatings’ architecture, i.e., monolayers, graded, and multilayers, were assessed. The experimental results show that BHAp feedstock powder is mainly carbonated B-type apatite with a high purity HAp phase. The lab-made BG45S5 powders exhibit the chemical composition and physical properties with a substantial similarity compared to commercial BG45S5. The weight percent of 1 up to 20 of Bi2O3 was added to the bioactive glass to modify its radiopacity. Radiographic images of BG45S5+Bi2O3 show that 10 wt.% of the opacified material allows increasing the opacity of the BG mixture homogeneously by 3.6 times with no considerable effects on its structural and thermal properties. Regarding the thermally sprayed coatings, CO32- and the Mg contents on BHAp lead to the formation of dolomite in the crystalline phase of the surface of single HVOF BHAp coating. Single BHAp APS coating does not exhibit any secondary phases in its surface crystalline content. A typical apatite layer is evidenced after 3 days of immersion in simulated body fluid (SBF) in the surface of both coatings and single BHAp HVOF coating show delamination after 5 days of immersion. Thus, APS was chosen to spray and analyze single BG45S5, BG45S5+10 wt.% Bi2O3 and gradual BHAp/BG45S5+10 wt.% Bi2O3 coatings. Single BG45S5+10 wt.% Bi2O3 coating shows similar microstructure and amorphous structure in comparison with the plasma-sprayed single coating using commercial BG45S5 feedstock powder without Bi2O3. However, delamination and a reduced rate of apatite layer formation are observed. Graded BHAp/BG45S5+10 wt. % Bi2O3 coating shows a similar rate of apatite layer growth compared to single BG45S5+10 wt. % Bi2O3 coating. Nevertheless, the apatite formation after 10 days of immersion in SBF and no delamination are observed on the surface of the coating due to the graded deposition of BG45S5+10 wt. % Bi2O3 combined with BHAp

Steel producers use various organic and inorganic coatings to protect cold-rolled steel (CRS) sheets from corrosion during shipment and storage. It is well known that CRS sheets can be protected from corrosion by galvanizing, phosphating, chromating, topcoating with organic, or their combinations. The chromate rinsing is particularly effective for preventing white rusting of galvanized steel. But there is an increasing interest in a replacement for the chromating process because of environmental and health concerns. The objective of the present work is to develop a chrome-free conversion coating for steel sheets. Various carboxylic acids and their salts have been studied for coating phosphated electrogalvanized (EG) steel sheets, including 10-undecenoic acid (UA), oleic acid (OA), and other fatty acids such as stearic acid (SA) and palmitic acid (PA). When they were used alone, or subsequently coated with resin, they could produce a highly hydrophobic surface and improve the corrosion resistance. Thiols such as 1-octadecanethiol (ODT) can form a self-assembled monolayer on metal substrates. This close-packed monolayer could provide an excellent corrosion resistance for EG steel sheets. It was capable of withstanding 50~60 hours of salt spray test (SST) although its thickness was only a few nanometers. The EG steel itself usually started rusting only after 2~4 hours of salt spray. In another coating system, thiols were mixed with a conventional resin to improve the corrosion resistance of EG steel. This new technique gave 100~120 hours of corrosion resistance. When the resin was applied directly on EG steel surface, its corrosion resistance was less than 72 hours. It was shown that further optimization of this technique increased the corrosion resistance to 200 hours and more in the standard SST.
Ph. D.

Aluminium alloys are frequently pre-treated by a conversion coating before application of an organic coating in order to improve the corrosion resistance and adhesive properties of the surface and the corrosion resistance provided by the system. Chromate-containing conversion coatings are commonly used for this purpose. However, legislation limits future use of hexavalent chromium compounds due to their toxic and carcinogenic nature. Therefore, alternative, so-called chromium-free conversion coatings are being developed that are more environmentally-compliant.The purpose of the present work has therefore been to contribute to a better understanding of how the aluminium substrate affects the formation and properties of conversion coatings for adhesive bonding. In particular, a chrome-free zirconium-based conversion treatment process has been investigated as a possible replacement for conventional chromate conversion treatment. The influence of the conversion time on the thickness of the formed layer on pure aluminium was investigated using complementary surface analytical techniques. The conversion time was varied between 30 and 600 seconds.In this study, the structure and composition of zirconium-based chromium-free conversion coatings on magnetron sputtered superpure aluminium and a range of aluminium-copper alloys were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of coating formation and protection. However, the presence of copper significantly influences the coating development and ultimately the performance of the conversion coatings formed on binary copper-containing aluminium alloys.The morphology and composition of the coatings have been probed using transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy, with loss of substrate through growth of the conversion coating also quantified. A comparison of the RBS spectra obtained for the superpure aluminium specimens after different immersion times revealed that zirconium (Zr) and oxygen (O) peaks were wider for longer immersion times, indicating thickening of the coating with increased immersion times. Thus, increasing the immersion time resulted in an increase in coating thickness but little change in coating composition occurred as determined by the RBS RUMP simulations. Alloying decreases the coating thickness, as well as metal consumption. Here, aspects of the corrosion behaviour of superpure aluminium and aluminium-copper alloys were also considered using electronoptical, electrochemical and surface analytical probing. The influence that short and prolonged treatment times exert on the performances of such conversion coating is discussed. The conversion coating formed after 60 s and 180 s of immersion in the zirconium-based conversion coating bath provide good corrosion resistance which can be attributed to the high stability of the compounds that constitute the surface oxide layer, and good adhesion properties.

Conventional yttria stabilized zirconia (YSZ) are widely used in the gas turbine to protect the substrate material from high temperature. But the common YSZ top coatings have limitations at higher temperature (above 1200 ℃) due to significant phase transformation and intensified sintering effect. Among the list of pyrochlores, gadolinium zirconate offer very attractive properties like low thermal conductivity, high thermal expansion coefficient and CMAS resistance. However, a lower fracture toughness than YSZ and tendency to react with alumina (thermal grown oxide) can lead to lower lifetime. Therefore, multi-layered thermal barrier coating approach was attempted and compared with single layer system. Single layer (YSZ) was processed by suspension plasma spraying (SPS). Double layer coating system comprising of YSZ as the bottom ceramic layer and gadolinium zir-conate as the top ceramic coat was processed by SPS. Also, a triple layer coating system with denser gadolinium zirconate on top of double layer system, was sprayed. Denser gado-linium zirchonate acts as the sealing layer and arrest the CMAS penetration. Isothermal oxidation performance of the sprayed coating systems including bare substrate and sub-strate with bond coat were investigated for a time period of 10hr, 50hr and 100hr at 1150℃ in air environment. Weight gain was observed in all the systems investigated. Microstruc-tural analysis was carried out using optical microscopy, SEM/EDS. Phase analysis was done using X-ray diffraction (XRD). Porosity measurement was made by water impregna-tion method. It was observed that multi-layered thermal barrier coating systems of YSZ/GZ and YSZ/GZ/GZ(dense) showed lower weight gain and TGO thickness than the single layer YSZ for all exposure time (10hr, 50hr & 100hr). The triple layer system had lower weight gain and TGO thickness compared to double layer system due to lower po-rosity content. Also, from the porosity measurement data, it could be seen that sintering effect is more dominant at 10 hr. of oxidation for all the coatings systems.

This Thesis presents two different deposition techniques for the synthesis of Ti₂AlC coatings. First, I have fabricated Ti₂AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211^TM Ti₂AlC powder of size ~38 µm in an H₂/O₂ gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti₃AlC₂, TiC, and Al_xTi_y alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211^TM Ti₂AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti₂AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti₂AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti₂AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti₂AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti₂AlC (101̅7) // Al₂O₃ (0001).

Report code: LIU-TEK-LIC-2008:15.

In a variety of engineering applications, components are subjected to corrosive environment. Protective coatings are essential to improve the functional performances and/or extend the lifetime of the components. Thermal sprayingas a cost-effective coating deposition technique offers high flexibility in coatings' chemistry/morphology/microstructure design. However, the inherent pores formed during spraying limit the use of coatings for corrosion protection. The recently developed supersonic spray method, High-Velocity-Air-Fuel (HVAF), brings significant advantages in terms of cost and coating properties. Although severely reduced, the pores are not completely eliminated even with the HVAF process. In view of the above gap to have a high quality coating, bi-layer coatings have been developed to improve the corrosion resistance of the coatings. In a bi-layer coating, an intermediate layer is deposited on the substrate before spraying the coating. The electrochemical behavior of each layer is important to ensure a good corrosion protection. The corrosion behavior of the layers strongly depends on coating composition and microstructure, which are affected by feedstock material and spraying process. Therefore, the objective of the researchis to explore the relationships between feedstock material, spraying process, microstructure and corrosion behavior of bi-layer coatings. A specific motivationis to understand the corrosion mechanisms of the intermediate layer which forms the basis for developing superior protective coatings. Cr3C2-NiCr top layer and intermediate layers (Fe-, Co- and Ni-based) were sprayed by different thermal spraying processes. Microstructure analysis, as well as various corrosion tests, e.g., electrochemical, salt spray and immersion tests were performed. The results showed a direct link between the corrosion potential (Ecorr) of the intermediate layer and the corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based coatings than Fe- and Co-based coatings was due to higher Ecorr of the coating in the galvanic couple with top layers. Inter-lamellar boundaries and interconnected pores reduced the corrosion resistance of intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.

State of the art thermal barrier coatings (TBCs) for gas turbine applications comprise (7 wt.%) yttria partially stabilized zirconia (7YSZ). 7YSZ offers a range of attractive functional properties – low thermal conductivity, high thermal expansion coefficient and high in-plane strain tolerance. However, as turbine entry temperatures are raised, the performance of 7YSZ coatings will be increasingly affected by sintering and environmental contamination, by calcia-magnesia-alumina-silica (CMAS) deposits. The effect of sintering-induced stiffening on the driving force for spallation of plasma-sprayed (PS) TBCs was investigated. Spallation lifetimes of TBC specimens sprayed onto alumina substrates were measured. A simple fracture mechanics approach was employed in order to deduce a value for the strain energy release rate. The critical strain energy release rate was found to be constant, and if this value had been known beforehand, then the rationale presented here could be used for prediction of coating lifetime. The effect of vermiculite (VM) and volcanic ash (VA) contamination on the sintering-induced spallation lifetime of PS TBCs was also investigated. The presence of both VM and VA was found to accelerate the rise in their Young’s modulus with sintering. Spallation results show that coating lifetime may be significantly reduced, even at relative low addition levels, due to the loss of strain tolerance caused by the penetration of glassy deposits. This result gives a clear insight into the role CMAS plays in destabilizing TBCs. Finally, the adhesion characteristics of ingested volcanic ash were studied using a small jet engine. The effects of engine speed and particle size were investigated. Deposition on turbine surfaces was assessed using a borescope. Deposition mainly occurred on the nozzle guide vane and blade platform. A numerical model was used to predict particle acceleration and heating in flight. It was observed that larger particles are more likely to adhere because they have greater inertia, and thus are more likely to impact surfaces. The temperature of the larger particles at the end of its flight was predicted to be below its softening point. However, since the component surface temperatures are expected to be hotter, adhesion of such particles is probable, by softening/melting straight after impact.