Дисертації з теми "THERMAL SPRAY OF COATING"

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

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.

Air plasma sprayed thermal barrier coating (APS TBC) systems are usually applied to engine components to reduce the temperature of the substrate and increase the efficiency of engines. However, failure of these coatings leads to oxidation and corrosion of the substrate. Therefore, a thorough understanding of the coating failure is necessary to predict the lifetime of coated components. This project has carried out stress analysis and prediction of subsequent failure of APS TBC systems associated with sintering of the TBC, oxidation of the bond coat (BC), substrate geometry, undulations at the coating interfaces and coating fracture toughness. Stress analysis is crucial for predicting TBC failure as stresses in the vicinity of the coating interfaces cause cracks and subsequent coating delamination. The Finite element (FE) method was used for stress analysis of TBC systems at high temperature stage and at cooling stage after operation. Initially, FE model of an axisymmetric unit cell representing the slice of a coated cylinder was used. Different radii for cylinders were used to investigate the significance of substrate curvature on coating stresses. The effect of asperities at the coating interface on residual stresses was observed using 3D models. The models were built based on the actual geometries of asperities, which were extracted from 3D SEM images of the coating interfaces. An Arrhenius approach was utilised to implement changes in mechanical and physical properties of TBC due to sintering. BC oxidation and related changes in its composition were also implemented. The accuracy of assumptions for FE models was validated by comparing the evaluated stresses against experimental results by project partners. Finally, the effects of stresses and fracture toughness of the coatings and coating interfaces on failure of the TBC system were studied, using cohesive surface modelling and extended finite element modelling (XFEM) methods.

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 aim of this thesis is to apply CFD methods to investigate the system characteristics of high speed thermal spray coating processes in order facilitate technological development. Supersonic flow phenomena, combustion, discrete droplet and particle migration with heating, phase change and disintegration, and particle impingement phenomena at the substrate are studied. Each published set of results provide an individual understanding of the underlying physics which control different aspects of thermal spray systems. A wide range of parametric studies have been carried out for HVOF, warm spray, and cold spay systems in order to build a better understanding of process design requirements. These parameters include: nozzle cross-section shape, particle size, processing gas type, nozzle throat diameter, and combustion chamber size. Detailed descriptions of the gas phase characteristics through liquid fuelled HVOF, warm spray, and cold spray systems are built and the interrelations between the gas and powder particle phases are discussed. A further study looks in detail at the disintegration of discrete phase water droplets, providing a new insight to the mechanisms which control droplet disintegration, and serves as a fundamental reference for future developments of liquid feedstock devices. In parallel with these gas-particle-droplet simulations, the impingement of molten and semi-molten powder droplets at the substrate is investigated and the models applied simulate the impingement, spreading and solidification. The results obtained shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity. The results obtained also visually describe the freezing induced break-up phenomenon at the splat periphery.

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.

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.

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.

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.

La thèse a mis en œuvre deux méthodes inverses pour l’évaluation non destructive d’un milieu multicouche anisotrope : la microscopie acoustique et les courants de Foucault. Pour la microscopie acoustique, nous avons adapté un modèle de propagation de l'onde acoustique dans un milieu multicouche, en calculant les coefficients de réflexion et de transmission d’un revêtement multicouche sur un substrat et en modélisant la réponse acoustique V(z) propre à la microscopie acoustique. Une méthode inverse utilisant l’algorithme de Levenberg-Marquardt donne accès à l’épaisseur, au module de Young et à la densité du revêtement. Pour les courants de Foucault, une méthode inverse similaire basée sur le formalisme de Tree a été mise au point, qui donne accès à l’épaisseur, à la conductivité électrique et à la perméabilité magnétique du revêtement. Après des vérifications de l’algorithme sur des matériaux massifs de référence, nous avons appliqué ces méthodes à un métal revêtu pour tuyauteries de centrale thermique : de l’acier inoxydable austénitique type 304 revêtu par pulvérisation thermique de superalliage Hastelloy C22 à base de nickel. La corrosion sèche sous l’effet des gaz chauds (air contenant du gaz SO2) amincit graduellement la paroi des tubes, ce qui peut les rendre dangereux. Il a fallu tenir compte de l’effet du grenaillage préalable à la pulvérisation, qui crée une mince couche de martensite à laquelle les courants de Foucault sont très sensibles : un modèle à trois couches substrat / martensite / revêtement a été utilisé. Nous avons caractérisé des échantillons de métal avant et après exposition à l’air avec ou sans SO2 à 650°C. Les deux méthodes d’évaluation non destructive permettent une détermination très satisfaisante de l’épaisseur de revêtement et se complètent bien. L’évaluation des caractéristiques physiques (acoustiques et électromagnétiques) met en évidence des hétérogénités du revêtement liées à son mode d’élaboration et des évolutions au cours du temps. Des pistes d’amélioration des méthodes sont proposées
In the current work, we investigate the nondestructive evaluation of a thermal sprayed coating (Hastelloy C22 Ni-based alloy) on substrate (type 304 austenitic stainless steel) using acoustic microscopy and ECT method. Two models were built for the evaluation of this kind of material: one is for acoustic V(z) measurement and the other is for swept eddy current measurement. The implementation of these two models is used for the evaluation and properties measurement of the thermal sprayed coatings, such as elastic properties, electromagnetic properties. In particular, the main achievements and results are as follows: 1. Acoustic wave propagation in an anisotropic multilayered medium was investigated. The formula for calculating the reflection and transmission coefficients of the multilayered medium on or without a substrate were derived, which is necessary for the modeling of acoustic V(z) measurement of the thermal sprayed coating on substrate. 2. A model was built for the acoustic V(z) measurement of the thermal sprayed coatings on substrate, which can deal with anisotropic multilayered media. Specifically, we used a model of multilayered coatings with graded properties on substrate to calculate the acoustic reflection coefficient of our sample. Treating the thermal sprayed coating, deposited on a 304 steel substrate, as FGMs, we evaluated the coating thickness and the Young’s modulus evolution along the depth of the coating. 3. A model was built for the swept eddy current measurement of the thermal sprayed coatings. Since before the spraying process, the surface of the substrate is usually shot-peened (SP), the coated material is considered as a three-layer medium. The coating thickness and electromagnetic properties of each of the 3 layers were determined by an effective reverse process. 4. The thermal sprayed coated material after exposure in different conditions, i.e., as-received, heat-treated in air and heat-treated in SO2 environment, and after different exposure time was evaluated by the integrity of acoustic microscopy and ECT method. The coating thickness and the electromagnetic properties of the coated material under different conditions were measured

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 purpose of this research work was to develop a nozzle capable of depositing dense CoNiCrAlY coatings via cold gas dynamic spray (CGDS) as well as compare the oxidation performance of bond coats manufactured by CGDS, high-velocity oxy-fuel (HVOF) and air plasma spray (APS) at temperatures of 1000°C and 1100°C. The work was divided in two sections, the design and manufacturing of a CGDS nozzle with an optimal profile for the deposition of CoNiCrAlY powders and the comparison of the oxidation performance of CoNiCrAlY bond coats. Throughout this work, it was shown that the quality of coatings deposited via CGDS can be increased by the use of a nozzle of optimal profile and that early formation of protective α-Al2O3 due to an oxidation temperature of 1100°C as opposed to 1000°C is beneficial to the overall oxidation performance of CoNiCrAlY coatings.

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.

Master‘s thesis deals with the forming of hypereutectic aluminium alloys using sacrificial coatings. Literature research is focused on analysing the characteristics of aluminium and its alloys. Attention is also focused to the heat treatment of aluminium alloys and the influence of alloying elements on these alloys. Analysis of characteristics and increase the useful properties of Al - Ni alloys is another object of literary research. Attention is also focused to the analysis of thermal spraying methods by focusing on the principle of individual methods and the characteristics of the coatings made by these methods. The object of experimental part is to prepare the hypereutectic aluminium alloys using sacrificial nickel based coatings. This is an unconventional manufacturing process comprising coating the surface of the aluminium substrate with using HVOF technology and subsequent thermal exposure. Furthermore, the work deals with the metallographic evaluation of prepared samples with a particular focus on the influence of temperature and time of heat treatment on the final structure of the experimental samples.

La technologie de programmation hors-ligne permet la génération de la trajectoire complexe en projection thermique. Dans le laboratoire du LERMPS, une extension logicielle appelée « Thermal Spray Toolkit » (T.S.T.) a été développée pour assister la programmation hors-ligne dans le domaine de projection thermique. Cependant, les efforts sont encore attendus pour améliorer sa fonctionnalité. Donc, l'objectif de cette thèse est d'améliorer l'application de la programmation hors-ligne en projection thermique. Selon la procédure d'application, les travaux de cette thèse se composent de trois parties.Premièrement, les efforts sont dévoués à l'amélioration du module « PathKit » dans T.S.T., afin d'optimiser la fonctionnalité de la génération de la trajectoire. L'algorithme pour la génération de la trajectoire sur le substrat courbe a été étudié pour assurer le pas de balayage constant. Une nouvelle trajectoire « spirale d'Archimède » a été développé pour réparer les défauts par la projection à froid. La réparation sur une pièce d'aluminium avec un défaut a été réalisé pour valider la trajectoire spirale d'Archimède. Deuxièmement, les modélisations ont été développées pour simuler l'épaisseur du dépôt en 2D et en 3D. Puis, Ils sont intégrés dans le logiciel RobotStudioTM comme un module individuel dit « ProfileKit ». Dans le « ProfileKit 2D », il peut évaluer les effets des paramètres opératoires sur le profil du dépôt et puis optimiser les paramètres. Dans le « ProfileKit 3D », l'épaisseur du dépôt peut être simulée selon la trajectoire du robot et la cinématique du robot.Les fonctionnalités sont validées par un dépôt de forme trapézoïdal élaboré par la projection à froid avec les pas debalayage variés.Dernièrement, l'analyse cinématique du robot a été étudiée pour optimiser la performance du robot pendant le processus de projection. Afin de mieux évaluer la performance du robot, un paramètre « overall parameter » (OP), la moyenne pondérée d'écart-type de la vitesse articulaire est introduit pour mesurer la complexité de la trajectoire du robot. Ensuite, l'optimisation du montage de la torche ainsi que l'optimisation de la disposition de la pièce sont étudiées par l'analyse cinématique du robot et le paramètre OP. Le résultat montre que l'optimisation cinématique peut efficacement améliorer la performance du robot pour maintenir la vitesse prédéfinie
The offline programming technology provides the possibility to generate complex robot trajectories in thermal spray process. In the laboratory of LERMPS, an add-in software called “Thermal SprayToolkit” (T.S.T.) has been developed to assist the offline programming in the field of thermal spray.However, efforts are still expected to improve the functionality of this software. The aim of this study is to improve the application of offline programming technology in the thermal spray process. According to the procedure of the offline programming in thermal spray, the work of this thesis consists of three parts.Firstly, efforts have been dedicated to improve the module “PathKit” in T.S.T., which aim to improve the functionality of trajectory generation. The algorithm of trajectory generation for the curved substrate surface was improved to maintain a constant scan step. A novel Archimedean spiral trajectory was developed for damage component recovery application by cold spray. The experiment of an Al5056 coating depositing on a manually manufactured workpiece with a crater defect was carried out to validate the effects of spiral trajectory with adapted nozzle speed.Secondly, numerical models were developed to simulate the coating thickness distribution in 2D and 3D, and then integrated in the RobotStudio™ as an individual module named “ProfileKit”. In the “ProfileKit 2D”, it is able to evaluate the effects of operating parameters on coating profile and optimize the parameters. In the “ProfileKit 3D”, coating thickness distribution can be simulated based on the nozzle trajectory and robot kinematics data. The functionalities were validated by the trapezoid coldsprayed coating.At last, kinematic analysis was used to provide the optimization methods for a better robot performance in thermal spraying. In order to better evaluate the robot performance, an overall parameter (OP) that is the weighted mean of standard deviation of joint speed, was introduced to measure the complexity of a robot trajectory. Afterwards, the optimal nozzle mounting method as well as the optimal workpiece placement were investigated by the kinematic analysis and the overall parameter. The result shows that the kinematic optimization can effectively improve the robot performance to maintain the predefined speed

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.

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

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.

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.

Mechanical properties of thermal spray deposited coatings are highly influenced by their microstructural characteristics. The objective of this investigation is to evaluate the mechanical properties of thermally sprayed coatings consisted of aluminum and zinc based on the coating microstructure, using an image based computational scheme. Microstructural images of coating samples were subjected to image-based finite element analysis and the results were validated by experimental tests and analytical models. Comparison of the experimental data with FEA was used to explain the microstructural basis of the mechanical characteristics of Al-Zn coatings and the differences between two methods of thermal spray techniques. It was concluded that the cold spraying technique produces higher-quality coatings with less porosity and higher hardness compared to wire arc deposition. An isotropic behavior was observed in the cold sprayed coating. Finally, the electrochemical tests showed that the coating with a higher amount of zinc had better anti-corrosion properties.

L'adhésion des revêtements est l'objectif premier de tout système afin de pouvoir apporter les propriétés de surface voulues par projection thermique. De façon conventionnelle, des traitements de sablage sont régulièrement employés afin de promouvoir des phénomènes d'ancrage mécanique entre les deux matériaux mis en contact.Néanmoins, selon la nature même des matériaux, un certain nombre de limitations peuvent être observées aussi bien d'un point de vue usage que tenue. Une fragilisation des surfaces peut en effet être remarquée dès lors qu'ils'agit du traitement de matériaux ductiles. Pour palier certaines de ces contraintes, des traitements palliatifs sont alors recherchés parmi lesquels les traitements laser apparaissent particulièrement bénéfiques dont la texturation laser. Les revêtements barrière thermique sont l'application visée de cette étude avec comme objectif une optimisation de leur durabilité à chaud (oxydation, fluage). Une sous-couche d'accroche est habituellement déposée mais les modes d'endommagement recensés semblent se concentrer autour de cette dernière. L'objectif de c etravail a donc visé à remplacer la sous-couche par une topographie de surface spécifique du substrat générée partexturation laser et permettant un ancrage mécanique suffisant aux chargement mécaniques et thermiques subis par les aubes de turbines hautes températures.Lors de l'interaction laser-matière, une élévation en température de l'extrême surface jusqu'à la température defusion et de vaporisation du matériau peut être observée et permettre la formation de motifs. Les dimensions de tels motifs sont donc liées à l¿énergie par impulsion et au nombre d¿impulsions. Pour valider de tels effets, les mécanismes de perçage ont donc été étudiés grâce à une modélisation thermo hydraulique et une validation postmortem des échantillons. Les dimensions des motifs alors contrôlées, le remplissage des surfaces texturées par des particules fondues projetées par le procédé APS a été étudié afin de minimiser le nombre de défauts proche de l'interface. Deux modes de rupture ont pu être identifiés en fonction de la morphologie de surface pour descontraintes de traction et de cisaillement. Les fissures se propagent à l'interface jusqu'à avoir des changements dedirection. L'énergie de propagation de la fissure augmente donc jusqu'à atteindre une valeur limite correspondant àla ténacité du revêtement. Dans ce cas, la tenue n'est pas fonction de la surface totale en contact mais de larépartition spatiale et l'ouverture des motifs, la seule limite de la tenue du revêtement restant la cohésion du dépôt.D'un point de vue applicatif, le but de cette étude a été de caractériser les modes d'endommagements de systèmes barrière thermique sans sous-couche pour des conditions rencontrées en service. Les mécanismes d'endommagement dus à l'oxydation et à l'allongement viscoplastique à 1100C ont donc été isolés par des essais àdes flux thermiques isothermes et cyclés, de fluage et de fatigue thermomécanique. Le traitement laser modifiant localement la microstructure des surfaces, une modification des couches d'oxydes a tout d'abord pu être identifiée.En effet, contrairement aux traitements conventionnels où la croissance d'oxyde n'est pas constante (point limitant de la durée de vie du système), l'apparition de spinelles et d'une couche dense d'alumine protectrice en surface des matériaux texturés a pu être observée. L'ancrage mécanique ainsi créé a démontré alors une durée de vie nettement améliorée face à des conditions extrêmes
Coating adhesion is requiered to rpomote specific surface properties by thermal spraying. Conventional prior-surface treatments have been developed to create anchoring zones but the adhesion strenght and their applications are limited. Laser surface texturing increases and adapts the adhesion surface. Therefore, two interface failure modes have been related to texture morphologies for tensile and shear stresses. The energy released rate at the interface increases up to coating toughness when the crack path is sharp. Mixed-mode failures have been observed with adhesive and cohesive cracks around and above pattern respectively. So, the adhesion stengyh is function of the contact aera precisely linked to pattern distribution and morphology. Thermal barrier coating system without bond coat life-span has been evaluated for thermomechanical stresses (YSZ coating on single crystal based Nickel). The bond coat has been remplaced by an adapted substrate surface topography. According ti the laser parameters (energy per pulse, pulse numbers) pattern morphology can be created. Therefore, textured surface filling by melted particles has been studies to minimize interface defaults and created mixed-mode failures for during plasma spray coatings. The drilling mechanisms have been evaluated by numerical modeling and experimental analysis. The pattern dimensions and heat affected zones has been identified. The laser treatment changes the microstructure locally.Oxydation tests have been performed to study the surface pre-tratments effects on oxide nature and mass gain rate. The damaging mechanisms ave been studied under isotherm and cyclic high temperature tests and also under creeping and thermo-mechanical fatigue tests. Grit-blasting change the natural oxides, limits life-span and bucking failure mode have been obeserved. Natural oxides have been analyzed for the textured substraes also but anchoring mechanism enables large life-span under high temperature tests. Mechanical applied stresses (constant and cyclic) validate the beneficial effects of patterned surfaces. The interface is stronger than the coating toughness and the patterns do not create early cracks under thermo-mechanical solicitations

The aim of this thesis was to modify microstructure and coating-substrate interface of CoNiCrAlY coatings deposited by HVOF and cold spray on Inconel 718 substrates. Electron beam remelting and annealing in a protective atmosphere were used to modify the coatings. Microstructure, chemical and phase composition were analyzed. The effect of beam current, transversal velocity and beam defocus on remelted depth was evaluated. As-sprayed microstructure and chemical composition of coatings were analyzed and compared with remelted samples. The effect of annealing of the as-sprayed and remelted samples was evaluated. Remelted layers exhibited dendritic structure. Chemical composition changed only after remelting of interface and part of a substrate. When only the coating was remelted, chemical composition remained the same. Phases coarsened after the annealing. Chemical composition changed after annealing due to the diffusion.

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.

Due to the current interest on intermetallic compounds (IMCs), specially for their use at high temperatures being capable to substitute superalloys and, for the lately improvement of their lack of ductility, this work evaluates the use of intermetallics as coatings by using Thermal Spray technologies, thus providing improved surface component properties.
The IMCs chosen for this study have been the following: FeAl, NiTi and NbAl3. Regarding their high Al contents, the aluminides present low densities and the possibility to form protective alumina compact layers against hostile environments. NiTi was selected as an interesting intermetallic for its shape memory properties and excellent corrosion resistance.
FeAl and NbAl3 coatings were produced by means of High Velocity Oxygen Fuel process trying to optimize the spraying conditions in order to achieve the lowest oxidation and porosity content as possible. On the other hand, Air Plasma Spraying and Vaccum Plasma Spraying were also evaluated for the production of NiTi coatings; in that case, the main purpose was the prevention of titanium oxidation and retention of autenitic NiTi phase, so that its shape memory properties could be preserved. The coatings were properly characterized by Scanning and Transmission Electron Microscopies and X-ray diffraction analysis. Afterwards, they were tested in laboratory conditions in order to study the mechanical properties such as hardness, sliding and abrasion wear and, corrosion and oxidation resistance.
FeAl coatings (obtained from a mechanically alloyed and a mechanically milled powders) were compared in terms of oxidation and wear resistance. A higher hardness was found to be correlated with a higher abrasion resistance. Iron aluminide coatings showed to have a fairly good oxidation resistance up to 900ºC. The uniform oxide scale without significantly spallation guarantees a reliable performance at such temperature.
The NbAl3 coating was about 20% harder than the hardest nitinol and iron aluminide coatings. Nb-Al based deposits displayed more wear damage than iron aluminides.
When these coatings were oxidized, they followed a logarithmic behaviour but with a much more rapid kinetics than the iron aluminide coatings. At 900ºC, whereas the steady state is reached for a weight increase below 10% in the different Fe-Al coatings, the corresponding to the Nb-Al coatings increases up to 40% due to the pesting phenomenon.
With regard to the NiTi coatings, those obtained from HVOF presented the highest retention of original austenitic phase. The hardness of such coatings is much higher than that of the iron aluminide coatings and a correlation was also found with their abrasion resistance. Finally, the as-sprayed VPS and HVOF coatings exhibitted superior corrosion resistance than APS in terms of corrosion potentials and current densities.
Els compostos intermetàl.lics han anat adquirint al llarg de les últimes dècades una gran importància en el món metaal·lúrgic, en especial per l'interès de reduir-ne la seva fragilitat i fer viable el seu ús estructural.
De tots els compostos intermetàl.lics que es coneixen, ens hem centrat com ha estudi dins de la present tesi en els aluminurs i en el nitinol (Ni-Ti). El grup dels aluminurs és bastant estudiat actualment a nivell de materials en volum degut a les expectatives que suposa poder disposar d'unes aleacions amb alta resistència, baixa densitat (degut a la presència de l'alumini) i la possibilitat d'oferir bones prestacions a alta temperatura (degut a la formació d'una capa compacta d'alúmina). Com a representació dins d'aquest grup, bàsicament cal fer menció als sistemes Fe-Al, Ni-Al i Ti-Al. Els seus elevats punts de fusió, claus per aquestes aplicacions deriven de la naturalesa del seu enllaç. Es coneixen, fins i tot, intermetàl.lics, anomenats refractaris amb punts de fusió per sobre dels 1600ºC; dos exemples en són el MoSi2 i el NbAl3. Per això, a fi de comparar dos tipus d'aluminurs, es va escollir el FeAl, com a representant del primer gruo i, el NbAl3, com a representant del segon grup.
Pel que fa al nitinol, aquest es va escollir com a compost interessant en el món dels materials ja que té el què es coneix com a efecte memòria de forma i pseudoelasticitat a través d'una transformació martensítica. També presenten una bona resistència a la corrosió.
Perquè s'han aplicat tots aquests compostos com a recobriments? Pel que fa als aluminurs degut a problemes en els mètodes de processat convencionals i, perque dóna l'opció de disposar d'un substrat amb les prestacions mecàniques desitjades protegit amb un recobriment que pugui proporcionar-li una bona resistència a l'oxidació a alta temperatura.
En el cas del nitinol, un dels principals impediments és l'econòmic ja que fabricar peces en volum d'aquest material pot resultar molt car. Algunes de les prestacions que es tenen en ment per l'ús dels recobriments obtinguts a partir d'aquesta aliatge són la resistència a la corrosió i, si es pot aconseguir, un bon comportament també a desgast.
S'ha emprat doncs la Projecció Tèrmica per l'obtenció de recobriments, principalment les tècniques: projecció per Alta Velocitat (HVOF, High-Velocity Oxy-Fuel), projecció per Plasma Atmosfèric (APS, Atmospheric Plasma Spraying) i projecció per Plasma al Buit, (VPS, Vacuum Plasma Spraying).
L'obtenció i estudi dels recobriments d'HVOF de FeAl ha permès estudiar la influència del grau d'ordre-desordre en l'estructura i el tamany de gra amb les propietats caracteritzades. Per una banda, optimitzant els paràmetres s'han aconseguit recobriments menys porosos i amb menys oxidació. La projecció de partícules de major tamany permet retenir l'estructura nanocristal.lina inicial de la pols.
S'observà en general, que els recobriments de major duresa mostraren millor resistència al desgast abrasiu, mentre que la duresa no semblà ser una variable clau en el comportament a desgast per lliscament.
La resistència a l'oxidació a alta temperatura dels recobriments de FeAl presentà resultats acceptables a 900ºC però quan es pujà a temperatures més altes, l'atac fou més accelerat.
S'han obtingut per primera vegada recobriments de Nb-Al per Projecció Tèrmica, tot i que les dificultats associades a la pròpia naturalesa de la pols i la seva irregular distribució de tamanys de partícules, han fet que els recobriments resultessin bastant oxidats, fet que suposa un empobriment d'alumini.
En respecte al NiTi, es van comparar els recobriments obtinguts per APS, VPS i HVOF. Aquests estaven formats per una varietat de zones amorfes, nanocristal.lines amb alternança de fase austenítica i martensítica.
El desgast de dits recobriments semblà ser més acusat que l'observat en el sistema FeAl. La resistència a la corrosió dels recobriments de NiTi fou major pel VPS i HVOF que no pas el d'APS ja que el mecanisme de corrosió d'aquest últim ve donat per la facilitat de penetració de la solució de clorur sòdic a través de les esquerdes del recobriment.

Yttria stabilized zirconia (YSZ) is the state of the art ceramic top coat material used for TBC applications. The desire to achieve a higher engine efficiency of agas turbine engine by increasing the turbine inlet temperature has pushed YSZ toits upper limit. Above 1200°C, issues such as poor phase stability, high sinteringrates, and susceptibility to CMAS (calcium magnesium alumino silicates) degradation have been reported for YSZ based TBCs. Among the new materials,gadolinium zirconate (GZ) is an interesting alternative since it has shown attractive properties including resistance to CMAS attack. However, GZ has a poor thermo-chemical compatibility with the thermally grown oxide leading to poor thermal cyclic performance of GZ TBCs and that is why a multi-layered coating design seems feasible.This work presents a new approach of depositing GZ/YSZ multi-layered TBCs by the suspension plasma spray (SPS) process. Single layer YSZ TBCs were also deposited by SPS and used as a reference.The primary aim of the work was to compare the thermal conductivity and thermal cyclic life of the two coating designs. Thermal diffusivity of the YSZ single layer and GZ based multi-layered TBCs was measured using laser flash analysis (LFA). Thermal cyclic life of as sprayed coatings was evaluated at 1100°C, 1200°C and 1300°C respectively. It was shown that GZ based multi-layered TBCs had a lower thermal conductivity and higher thermal cyclic life compared to the single layer YSZ at all test temperatures. The second aim was to investigate the isothermal oxidation behaviour and erosion resistance of the two coating designs. The as sprayed TBCs were subjected toisothermal oxidation test at 1150°C. The GZ based multi-layered TBCs showed a lower weight gain than the single layer YSZ TBC. However, in the erosion test,the GZ based TBCs showed lower erosion resistance compared to the YSZ singlelayer TBC. In this work, it was shown that SPS is a promising production technique and that GZ is a promising material for TBCs.

Corrosion, wear or the combined effects of these failure modes cost the industrial economies hundreds of billions of euros each year. One of the more effective ways to reduce damage due to corrosion and wear is to treat, or “engineer” the surface, so that it can perform functions that are distinct from those required by most materials. For example, steel is a very popular material, because it is inexpensive, strong and easily manufactured. Unfortunately, it is highly susceptible to corrosion in many environments and therefore needs to be coated in order to achieve a satisfactory life service. Small components can be painted, aluminized, electroplated, or clad with more corrosion resistant materials. For parts that also require wear strength, they can be thermally sprayed or coated with thin-films. Finally, when dealing with large components, the size, weight, and handling problems may limit the type of surface treatment to be considered. The aim of the research work presented in this PhD thesis was to study the interactions between the metal/ceramic and metal/metal coupling surfaces to improve the wear resistance of the materials in sliding motion. Most of my research work is included here as a collection of three papers submitted in the same format they were published or in a preprint version. Each paper is an individual piece of work with separate sections including: abstract, introduction, materials and experimental details, results and discussion, conclusions, and references. A failure analysis study was carried out on some specific mechanical components of diesel engines for industrial application. In order to increase the wear resistance of rocker arms, pushrods and valves, the deposition of ceramic coatings thermally sprayed was considered. Wear tests were performed on a plasma-sprayed ceramic coating in sliding motion against steel in order to understand the effect of a normal load applied, as well as the environmental conditions on the metal/ceramic coupling. Moreover, the influence of relative humidity on the tribological behaviour of two types of plasma-sprayed ceramic coatings and two types of HVOF-sprayed cermets in sliding motion against alumina, was evaluated through pin-on-disk testing. Different techniques for the microstructural examination were employed in this research study: Optical Microscopes (OM) and Scanning Electron Microscopes (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) microprobe were used and also X-ray Diffraction (XRD) was carried out to describe the morphology of worn surfaces and to identify the main wear mechanisms involved. In addition, surface roughness, Vickers microhardness and fracture toughness measurements were performed in order to characterise the materials subjected to wear testing.

The dissertation is split into two parts. The first part will be focused on changes in material properties found at the nanoscale, as miscibility and electronic structure can change significantly with size. The formation of classically-immiscible bimetallic nanoparticles (BNPs) becomes favorable at the nanoscale and novel catalytic properties can emerge from the bimetallic alloying. The formation of alloyed and non-alloyed BNPs is achieved through pulse laser ablation (PLA) and a significant increase in catalytic activity is observed for both. Recently discovered, the increased activity in the non-alloyed BNPs, deemed multicomponent photocatalysis, is examined and the proposed mechanism discussed. The second part of the talk will focus on thermal barrier coatings (TBCs), which are advanced, multi-layered coatings used to protect materials in high temperature environments. MCrAlY (M=Ni, Co) bond coats deposited via atmospheric plasma spray (APS) are intrinsically rough and initially the roughness provides a high surface area platform for the mechanical interlocking of the yttria stabilized zirconia (YSZ) top coat, which provides the bulk of the thermal insulation. After high temperature exposure, a protective oxide scale forms at the top coat/bond coat interface however the convex asperities of the bond coat can grow non-α-Al2O3 type oxides that can be detrimental for coating lifetime. A surface modification technique that removes the asperities while leaving intact the concavities is used to examine the role that roughness distribution has on 1100°C APS coating lifetime. Lastly, recent work validating a modelling strategy for evaluating 900°C TBC lifetimes, which can typically surpass 25 kh, is presented. Differences in coating-substrate interdiffusion behavior over 5-20 kh of 900°C exposure are discussed and reproduced with Thermo- Calc/DICTRA for three superalloys (1483, 247, X4) deposited with high velocity oxy fuel (HVOF) NiCoCrAlY coatings.

The tribological properties of cold sprayed Ni-WC metal matrix composite (MMC) coatings were investigated under dry sliding conditions from room temperature (RT) up to 400°C, and during thermal cycling to explore their temperature adaptive friction and wear behavior. Characterization of worn surfaces was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy to determine the chemical and microstructural evolution during friction testing. Data provided insights into tribo-oxide formation mechanisms controlling friction and wear. It was determined that the steady-state coefficient of friction (CoF) decreased from 0.41 at RT to 0.32 at 400˚C, while the wear rate increased from 0.5×10-4 mm3/N·m at RT to 3.7×10-4 mm3/N·m at 400˚C. The friction reduction is attributed primarily to the tribochemical formation of lubricious NiO on both the wear track and transfer film adhered to the counterface. The increase in wear is attributed to a combination of thermal softening of the coating and a change in the wear mechanism from adhesive to more abrasive. In addition, the coating exhibited low friction behavior during thermal cycling by restoring the lubricious NiO phase inside the wear track at high temperature intervals. Therefore, cold sprayed Ni-WC coatings are potential candidates for elevated temperature and thermally self-adaptive sliding wear applications.

De nombreuses pièces aéronautiques telles que les chambres de combustion sont percées d'une multitudes de trous de refroidissement. Ce perçage, généralement effectué par un laser de puissance peut induire des endommagements dans la matière percée. Sur les systèmes barrières thermiques, une fissuration pouvant conduire à l'écaillage de la barrière thermique se produit à l'interface céramique/sous-couche lors du perçage laser. Cette thèse présente des éléments de compréhension des phénomènes de formation et de propagation de la fissure interfaciale. Pour cela, un protocole spécifique a été utilisé, consistant en la réalisation de perçage interrompus. De plus, afin de comprendre l'influence de la sous-couche et de l'interface sous-couche / céramique sur cette fissuration, une large gamme d'échantillons aux propriétés morphologiques et mécaniques bien distincts ont été sélectionnés. Pour cela, la sous-couche a été modifiée par la variation des paramètres de projection, du procédé de projection ainsi que par des post traitements sur la sous-couche. Le lien entre ces interfaces et les endommagements liés au perçage laser sont présentés au travers d'observations 2D et 3D, destructives et non destructives ainsi que par l'étude des modifications des contraintes résiduelles avant et après perçage laser pour différentes barrières thermiques. De manière a réduire encore la fissure interfaciale, plusieurs stratégies de perçage sont étudiées via la variation de la puissance laser entre les différentes impulsions laser nécessaires pour percer le matériau multicouches. Les résultats obtenus offrent des perspectives intéressantes pour améliorer la résistance à la fissuration des pièces aéronautiques
Many aircraft engines parts such as combustion chambers are drilled with numerous cooling holes. These holes, generally performed by a high power laser machine can induce damages in the part materials. On thermal barrier coatings systems, cracking can lead to spalling of the coating occurs at the ceramic / bond-coat layer interface during laser drilling. This thesis presents elements of understanding of the interfacial crack formation end propagation phenomena. In this purpose, a specific protocol was used, consisting of interrupted drilling process. Moreover, in order to understand the influence of the bond coat and the ceramic / bond coat interface on cracking, a wide range of samples with specific morphological and mechanical properties were selected. With this in mind, the bond coat was modified by varying spraying parameters, spraying process and also by post treatments on the bond coat. The link between these interfaces and the associated damages due to laser drilling are presented through 2D and 3D observations, destructive and non-destructive, as well as the study of residual stress modification before and after laser drilling for several thermal barrier coatings systems. For further reducing the interfacial crack, several drilling strategies are considered through the laser peak power variation between the different laser pulses needed to drill through the multi-layer material. The results offer interesting perspectives for improving crack resistance of aeronautical engines parts

Tato práce je zaměřena na modifikaci charakteru rozhraní substrát-nástřik NiCrAlY povlaků nanesených pomocí technologie vodou stabilizované plazmy na substráty z oceli S235JRC+C. Přetavení žárové vrstvy elektronovým paprskem bylo zvoleno jako technologie pro modifikaci a dvě různé modifikace byly zkoumány. V práci byl proveden pokus o stanovení vlivu modifikací na adhezní vlastnosti nástřiku. Dále jsou v práci prezentovány analýzy mikrostruktury, fázového a chemického složení a mikrotvrdosti ve stavu před a po modifikaci. Během studie bylo zjištěno, že dochází ke změnám fázového složení jak během depozice, tak během modifikace elektronovým paprskem. Modifikace elektronovým paprskem způsobila roztavení oxidů původní mikrostruktury nástřiku, které následně rekrystalizovaly na povrchu modifikované vrstvy. Dalším získaným poznatkem bylo, že dochází ke snížení mikrotvrdosti po modifikaci, což bylo způsobeno odstraněním oxidů z mikrostruktury a promícháním materiálu substrátu a původního nástřiku. Adheze nástřiků v as sprayed stavu byla kvantifikována. V případě nástřiků modifikovaných elektronovým paprskem přesná kvantifikace nebyla možná, z důvodu předčasného porušení na rozhraní nástřik-adhezivní pojivo během adhezních testů.

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The use of coatings based in electrolytic chromium deposition causes damages to the environment, since during the process, hexavalent chromium ions are released and these are extremely toxic to nature and man. However, other technologies have been studied as alternatives to the traditional chromium electroplating process. The thermal spray high velocity oxy-fuel (HVOF) is a technique that has shown the best results considering coating quality, porosity, wear resistance, mechanical and adhesive properties. In this work, a comparative study between the hard chromium electroplated and HVOF sprayed coatings was performed, in a condition which the substrate surface has been rectified. Surface properties were checked in terms of adhesion, hardness and microstructure, while the tribological behavior was obtained by pin on disk wear tests, considering the evaluation of wear resistance, friction coefficient and wear mechanisms. The material used as substrate was the SAE 4140 steel; the coatings sprayed by HVOF were WC-Co-Cr and 316L Stainless Steel. As the conditions studied, no adhesion on substrate of 316L coating was observed. For coatings which obtained satisfactory adherence, the tungsten carbide was harder (1750 HV0,1) than chromium (1050 HV0,1). This result was attributed to the low porosity and high hardness of WC and W2C phases contained into sprayed coating. The coat WC-Co-Cr had a better performance and wear resistance, possibly, due to the formation of a great wear resistant tribofilm and the high hardness of the layer. For chromium, the delamination of a fragile tribofilm aggregated with a big quantity of cracks inside the microstructure might explain the pronounced wear.
A utilização de revestimentos a base de cromo eletrolítico traz prejuízos ao meio ambiente, uma vez que durante o processo são liberados íons de cromo extremamente tóxicos à natureza e ao homem. Outras tecnologias vêm sendo estudadas como alternativas ao processo tradicional de cromagem. A aspersão térmica por oxi- ombustível de alta velocidade (HVOF) é a técnica que tem apresentado os melhores resultados sob as óticas de qualidade de revestimento, porosidade, resistência ao desgaste, propriedades mecânicas e adesivas. Neste trabalho foi realizado um estudo comparativo entre o cromo duro eletrodepositado e revestimentos aspergidos (WC-Co-Cr e o Inox 316L) pela técnica de HVOF em uma condição em que a superfície do substrato (aço SAE 4140) foi retificada. As propriedades de superfície foram verificadas em função da adesão, dureza e microestrutura, enquanto que o comportamento tribológico foi obtido por meio de ensaios de desgaste por deslizamento pino sobre disco, com avaliação da resistência ao desgaste, coeficiente de atrito e mecanismos de desgaste. Verificou-se a não adesão ao substrato do revestimento de Inox 316L para as condições estudadas. Quanto a dureza o carbeto de tungstênio apresentou maior valor (1750 HV0,1), bastante superior ao cromo (1050 HV0,1). Resultado atribuído a baixa porosidade e a elevada dureza das fases WC e W2C contidas no revestimento aspergido. O revestimento de WC-Co-Cr teve um melhor desempenho quanto a resistência ao desgaste, possivelmente, em função da formação de um tribofilme resistente ao desgaste e a elevada dureza da camada. Para o cromo, a delaminação de um tribofilme de natureza frágil somada a uma microestrutura com elevada densidade de trincas foram fatores que puderam explicar seu desgaste mais acentuado.

Due to their ability to confer enhanced surface properties without compromising the properties of the substrate, coatings have become ubiquitous in heavy industrial applications for corrosion, wear, and thermal protection, among others. Kinetic Metallization (KM), a solid-state impact consolidation and coating process, is well-suited for depositing industrial coatings due to its versatility, low substrate heat input, and low cost. The ability of KM coatings to adhere to the substrate is determined by the quality of the interface. The purpose of this study is to develop a model to predict the interfacial quality of KM coatings using known coating and substrate properties. Of the various contributions to adhesion of KM coatings, research suggests that the thermodynamic Work of Adhesion (WAD) is the most fundamental. It is useful to define interfacial quality in terms of the critical strain energy release rate (GC) at which coating delamination occurs. Studies show that GC for a given interface is related to WAD. This study attempts to develop a theoretical model for calculating WAD and understand the relationship between GC and WAD. For a bimetallic interface between two transition metals, WAD can be theoretically calculated using known electronic and physical properties of each metal: the molar volume, V, the surface energy, γ, and the enthalpy of alloy formation, ΔHinterface; ΔHinterface is a function of the molar volume, V, the work function, φ, and the electron density at the boundary of the Wigner-Seitz cell, nWS.WAD for Ni-Cu and Ni-Ti interfaces were 3.51 J/m2 and 4.55 J/m2, respectively. A modified Four-point bend testing technique was used to experimentally measure GC for Ni-Cu and Ni-Ti specimens produced by KM. These tests yielded mean G­C values of 50.92 J/m2 and 132.68 J/m2 for Ni-Cu and Ni-Ti specimens, respectively. Plastic deformation and surface roughness are likely the main reasons for the large discrepancy between GC and WAD. At the 95% confidence level, the mean GC of the Ni-Ti interface is significantly higher than that of the Ni-Cu interface. Further testing is recommended to better understand the relationship between WAD and GC.

Aspersão térmica é um processo de produção de revestimentos com ampla aplicação industrial, utilizado na proteção de superfícies contra a corrosão e/ou o desgaste. Os setores aeronáutico e de moldes plásticos estão entre os que utilizam essa técnica para ampliação da vida útil de seus componentes. Este trabalho teve como objetivo avaliar a influência do processo de deposição de revestimentos metálicos (1248 T - Co-28,5%Mo-17,5%Cr-3,4%Si e 1256 F - Co-28%Cr-4%W-3%Ni-3%Fe), cerâmicos (CRO 192 - Cr2O3-5%SiO2-3%TiO2 e 25030 - TiO2-8%Al2O3) e composto (1342 - WC-12%Co), aspergidos pelas técnicas de combustão por chama convencional (cerâmicos) e HVOF (metálicos e composto), no substrato constituído de uma liga de alumínio AA 7475 endurecida por precipitação, visando o melhoramento de suas características tribológicas (desgaste e corrosão), sem alterar as propriedades mecânicas. A influência do processo no substrato foi avaliada por meio de ensaios de microdureza Vickers, microestruturais e de composição química. A resistência ao desgaste foi avaliada por meio de ensaios de desgaste microabrasivo com esfera, com o uso de uma solução abrasiva de alumina (Al2O3). A avaliação da resistência à corrosão de revestimentos e substrato foi realizada por meio de ensaios de polarização potenciodinâmica em meio de NaCl 3,5%. A deposição dos revestimentos por aspersão térmica não afetou significativamente a dureza, composição química e microestrutura do substrato constituído pela liga de alumínio, tornando viável a aplicação dessa técnica a essa classe de ligas. Os revestimentos foram altamente eficientes na proteção do substrato contra o desgaste microabrasivo, sendo que o revestimento constituído de WC-Co (1342) apresentou o melhor desempenho dentre eles. Quanto à corrosão, todos os revestimentos apresentaram resistênciassuperiores a do substrato, tendo o revestimento Co-Cr-W-Fe-Ni (1256 F) apresentado o melhor desempenho geral. Portanto, os revestimentos produzidos por aspersão térmica são alternativas altamente promissoras para o melhoramento tribológico das ligas de alumínio envelhecidas, sem o comprometimento de suas características mecânicas.
The thermal spray process is used in the production of coatings with a wide range of industrial application such as protection of surfaces against corrosion and/or wear. The aircraft and plastic molds industries are among the ones that use this technique to increase the useful life of components. This work aimed to evaluate the influence of the deposition process of metallic (1248 T Co-28.5%Mo-17.5%Cr-3.4%Si and 1256 F Co-28%Cr-4%W-3%Ni- 3%Fe), ceramic (CRO 192 Cr2O3-5%SiO2-3%TiO2 and 25030 TiO2-8%Al2O3) and composite (1342 WC-12%Co) coatings, deposited by flame spray (for the ceramic) and HVOF (for the metallic and composite) techniques, on the substrate of precipitation-hardened AA 7475 aluminum alloy, intended to improve the tribological characteristics (wear and corrosion), without changing its mechanical properties. The influence of the process on the substrate was measured by Vickers microhardness, microstructural tests and chemical analyses. The wear resistance was evaluated by the calowear microabrasive test, using abrasive slurry of alumina (Al2O3). The corrosion resistance evaluation of coatings and substrate was carried out using the potentiodynamic polarization test in NaCl 3.5%. The coating deposition by thermal spray did not significantly affect the hardness, chemical composition and microstructure of the aluminum alloy substrate, establishing the feasibility of using this technique on this class of alloys. Coatings were highly efficient in protecting the substrate against microabrasive wear, where of the WC-Co (1342) coating showed the best performance among them. Also, the corrosion resistance of all coatings was higher than that of the substrate of which the Co-Cr-W-Fe-Ni (1256 F) coating presented the best overall performance. Therefore, the coatings produced by thermal spray are a highly promising alternative for the improvement of the tribological properties of aged aluminum alloys, without compromising its mechanical characteristics.

The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which makes this special alloy interesting for surface engineering and particularly for thermal spray technology. In this study, the suitability of inert gas-atomised HEA powder for high-velocity-oxygen-fuel (HVOF) thermal spray is investigated. This process allows for high particle velocities and comparatively low process temperatures, resulting in dense coatings with a low oxidation. The microstructure and phase composition of the atomised powder and the HVOF coating were investigated, as well as the wear behaviour under various conditions. A multiphase microstructure was revealed for the powder and coating, whereas a chemically ordered bcc phase occurred as the main phase. The thermal spray process resulted in a slightly changed lattice parameter of the main phase and an additional phase. In comparison with a hard chrome-plated sample, an increase in wear resistance was achieved. Furthermore, no brittle behaviour occurred under abrasive load in the scratch test. The investigation of wear tracks showed only minor cracking and spallation under maximum load.