Rozprawy doktorskie na temat „Intermetallic Coatings”

This thesis is a contribution to research into the corrosion behaviour of high velocity oxy-fuel (HVOF) thermally sprayed NiAl coatings. Investigations initially concentrated on the spraying of intermetallic compounds (IMCs), in particular NiAl. Results showed that NiAl could be successfully deposited as an IMC onto a 2.25Cr-1Mo substrate to produce a high quality coating of low porosity and excellent adhesion. The feasibility of using lower cost, less corrosion resistant substrates with a high performance protective coating offers the possibility of enhanced cost effectiveness allied with increased serviceability and ongoing environmental benefits. HVOF gun spray parameters were also optimised for the coating and it was found that they have a strong influence on coating properties. The most important parameter is the oxygen-fuel ratio as it influences the temperature, stoichometry and power of the flame. Coatings were sprayed under different ratios of fuel and oxygen flow rates and evaluated in terms of porosity, oxide content, Young's modulus and hardness. It was discovered that an increase in the oxygen-fuel ratio gave a coating of lower porosity with a higher oxide content, Young's modulus and improved hardness. The coatings showed excellent resistance to isothermal and cyclic oxidation due to the formation of an adherent and continuous alumina (A1₂O₃) scale. The corrosion behaviour in air-chlorine mixtures has also been evaluated at high temperatures along with resistance to thermal cycling. Initial results showed that the coating provided improved properties compared with some conventional materials. Preliminary hot corrosion testing with one real and three synthetic ashes revealed satisfactory coating performance. It was found that the coating microstructure was influential. In addition, results in erosion testing showed that good corrosion resistance is combined with excellent protection against this form of wear.

In recent years, TiAl intermetallic alloys been widely used in aircraft and automotive industry. With the aim of improving the mechanical behavior and the oxidation resistance at high temperature of the TiAl alloys firstly designed, new intermetallic alloys of second and third generation have been successively developed. In this work, the oxidation resistance in air of four intermetallic alloys of second generation (Ti-48Al-2Cr-2Nb) and third generation ( Ti-48Al-2Nb-0.7Cr-0.3Si, Ti43.5Al-4Nb-1Mo and Ti-47Al-2Cr-8Nb) was investigated by TGA equipment under isothermal conditions in the range of 800-1000 °C. These alloys were cut from the core of bars, previously processed by Electron Beam Melting and successively heat-treated. The composition of the oxide layers was investigated by XRD, SEM-EDS and XPS. Each alloy showed different oxidation behavior at high temperatures. Layer exfoliation or spallation was observed for many samples, but at very different temperatures for the different alloys. When spallation did not happen in a significant extent the oxide layers grew according to a parabolic law. The kinetic rate constants and the activation energies were calculated. These kinetic parameters allowed to assess a rank of oxidation resistance, which can be correlated with the composition of the alloys. In order to improve the oxidation resistance of Ti-48Al-2Cr-2Nb, ceramic nitride coatings were deposited by a High Power Impulse Magnetron Sputtering (HiPIMS) method. Differently engineered TiAlN and TaAlN protective films were processed and their performances compared. Samples were submitted to thermal cycling under oxidizing atmosphere up to 850° C (40 cycles) and 950°C (100 and 200 cycles), at high heating and cooling rates. For this purpose, a burner rig able to simulate the operating conditions of the different stages of turbine engines was used. The microstructure and composition of samples before and after oxidation were investigated by several techniques: microscopy (optical and SEM-EDS), X-ray photoelectron spectrometry (XPS) and X-ray diffraction (XRD). All the TiAlN coatings differently processed provided a remarkable improvement of oxidation resistance. Good adhesion properties were observed even after repeated thermal shocks. HiPIMS pretreatments of the substrate surfaces, performed before the coating deposition, significantly affected the oxidation rate, the oxide layer composition and the coating/substrate adhesion. The oxide layers formed on the sample surface showed different thickness, depending on the presence of the protective coating and the processing path adopted for its deposition. The nitride coatings appreciably enhanced the oxidation resistance and sustained repeated thermal shocks without showing damage or spallation. Differently TaAlN coating did not improve the oxidation resistance of TiAl substrate.

Magnetron sputter deposition with single target materials was used to produce amorphous and crystalline Ti-A1-(Cr) alloy coatings on a Ti-50Al substrate. The following coatings were studied: Ti-50Al-10Cr, Ti-53Al-15Cr, Ti-50Al-20Cr and Ti-48Al. The microstructures of the coatings were studied in the as deposited condition and after devitrification and heat treatment. A random distribution of nano-precipitates was formed in amorphous as-deposited coatings. Columnar features were present when the as-received deposit had crystallised during sputter deposition. If crystallization occurred during deposition, a columnar microstructure formed with the columnar fibres being parallel to the deposition direction. For the Ti-50Al-10Cr and Ti-53Al-15Cr deposits, the crystalline domains consisted of lamellar gamma // alpha, i.e. the crystalline deposits had a gamma // alpha texture. The alpha → alpha transformation occurred during deposition. Phase competition in the alloys was studied by combining thermodynamic modelling and transformation kinetics. At 1173K, the Ti-50Al-10Cr alloy transformed to a two-phase microstructure, consisting of the gamma and C14 Ti(Al, Cr)2 phases. The Ti-53Al-15Cr alloy transformed to a three-phase microstructure, consisting of the gamma, tau and the C14 Laves phase. The gamma and tau phases were mixed finely, with gamma // tau. The Ti-50Al-20Cr alloy transformed to a two-phase microstructure, consisting of the gamma and the Cl4 Laves phase. No orientation relationship between the gamma and the C14 Laves phase was observed. Phase evolution studies at lower temperatures in the range 773K to 973K indicated that for the amorphous Ti-48Al alloy, the phase transformation path is: the amorphous phase → alpha → gamma + alpha/alpha2. A fine lamellar structure was formed, with gamma being the dominant phase. For the Ti-50Al-20Cr alloy, the phase transformation path was: the amorphous phase → gamma → gamma + Ti(Al, Cr)2. The experimental observations and the modelling results have clearly suggested a tendency of amorphous phase stabilisation via Cr addition. Thermodynamic modelling also indicated that the driving force for amorphous alloy formation is not much less than that for the precipitation of disordered solution phases. Kinetically, the amorphous phase formation during sputter deposition is related to the suppression of surface diffusion at low substrate temperatures. The temperature processing window for ordered phase formation in the Ti-Al(-Cr) alloys during magnetron sputter deposition was evaluated by the effective diffusion distance. Time dependent nucleation calculations showed that in the Ti-48Al amorphous alloy, it would be kinetically easier to precipitate the alpha phase than the gamma phase. In the case of the Ti-50Al-20Cr alloy, the gamma phase forms in preference of alpha, which is consistent with experimental observations. Diffusion phenomena at the coating/substrate interface and the oxidisation of the coatings were also studied. The experimental Cr diffusion profiles and the simulations for the Ti-Al-Cr coatings and the Ti-50Al substrate indicated that diffusion at 1173K is reasonably slow. The Ti-50Al-10Cr, Ti-53Al-15Cr and the Ti-50Al-20Cr coatings could form protective oxide scales at 1173K. When a columnar structure was present in the crystalline deposit, cracking of the coating was observed when the coating was subject to thermal cycling from elevated temperatures. It is concluded that if cracking of the coatings was to be avoided, amorphous deposits should be preferred.

Tin electrodeposits produced from aqueous electrolytes are frequently used within the electronics industry due to their high solderability and corrosion protection. One limitation to using these deposits is their spontaneous formation of long conductive filament whiskers. These whiskers grow post-electrodeposition and increase the risk of unwanted electrical shorts within electronic devices. In this thesis, tin electrodeposits produced from a proprietary bright acid Tinmac electrolyte, currently used in industry, were studied. Electrodeposits were produced using a range of current densities with and without agitation and were characterised with respect to crystallographic orientation, topography and surface finish. Moreover, the intermetallic compound (IMC) growth produced at the copper substrate-tin coating interface was assessed over a period of time as its growth is considered to be a significant driving force behind whisker formation. In addition, a technique for the electrochemical anodic oxidation of tin electrodeposits on copper substrates was developed. This technique was used throughout this project for the study of IMC growth from tin electrodeposits as it was able to effectively remove the tin whilst leaving the IMCs and substrate unaffected. Ionic liquids exhibit promising electrochemical characteristics for electrodeposition but are still not widely utilised in industry. Their ability to deposit tin coatings has been studied in the present investigation. Trials concentrated on process optimisation to produce uniform electrodeposits by varying current density, SnCl2.2H2O concentration, and electrolyte composition. These deposits were then characterised and compared to tin coatings of similar thickness produced from Tinmac with respect to topography, surface finish, crystallographic orientation, IMC growth, and whisker propensity. Electrodeposits produced from the ionic liquid electrolyte exhibited a different crystallographic texture, topography, and IMC growth compared to those produced from Tinmac. Moreover, the deposit produced from the ionic liquid featured increased whisker growth compared to those produced from Tinmac, but in a wider context, far less growth than conventional tin electrodeposits in the literature. In addition, by exploiting other electrochemical characteristics of ionic liquids, such as their large potential window, future work may be able to produce novel tin or tin alloy electrodeposits which may further reduce the whisker propensity of deposits produced in this investigation.

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.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

This work aimed to study the effect of surface oxidation of TiAl intermetallic alloys, its fracture behavior and study the formation and stability of picked coatings that could prevent these phenomena. ?-TiAl alloy (Ti46Al7Nb) was examined by three different coatings (AlCr, AlCrN and AlCrNAg). Firstly, it was carried out short-term exposure to high temperature in an inert atmosphere for all coatings. Secondly, there were experiments of cyclic exposure to high temperature in normal atmosphere and isothermal high-temperature exposure in a normal atmosphere. During high-temperature experiments in normal atmosphere mass gain was measured and it was monitored a surface texture. Finally, the most promising coatings were chosen, and those were evaluated in bending strength.

The reduction of noise and emissions is becoming increasingly important in civil aircraft jet engines as well as requirements for reduced fuel consumption and improved efficiency. This has resulted in the drive towards increasing turbine entry temperatures and the development of thermal barrier coatings (TBCs). Due to the effectiveness of the platinum-modified nickel aluminides currently used as bond coat layers for Ni-based superalloy TBCs, higher temperature ruthenium-containing bond coat layers are being examined as a possible low cost alternative to platinum. Rolls Royce have a patented process, whereby precious metal layers directly react with single crystal substrate alloys to form an aluminium containing surface coating. The aluminium is sourced from the single crystal alloy and the coating so formed has a + structure, but contains other intermetallic phases due to the reaction between the coating and the single crystal substrate. This bond coat layer acts as a diffusion barrier, which limits interdiffusion between the coating and the substrate. The aim of this research was to examine the stability of various phases within platinum and ruthenium-containing multilayer systems formed during the above reaction process and to determine the most stable intermetallics for inclusion in future coating systems. Foil samples were manufactured using multilayer sputter coating methods and the exothermic formation of these phases was examined using differential scanning calorimetry. The identification of the phases formed was carried out using X-ray diffraction. It was found that the interdiffusion between the initial multi-layers had been incomplete during the samples heat treatment, and so more intermetallic phases formed in some samples than aimed for. Hence, from the large number of samples studied it was shown that, as a result of kinetic factors, the reaction onset (or trigger) temperature was not related to the enthalpy of the intermetallic phases formed or the sample compositions within a target phase field. For the β-phase (NiAl) type intermetallic systems, the samples that produced the highest enthalpy values (i.e. the most stable intermetallic compounds) were those with the nominal compositions (in atomic %) of; ‘47Ni53Al’, ‘48Ni6Pt46Al’ and ‘51Ni7Ru42Al’. For the γ΄-phase (Ni3Al) type intermetallic systems, the highest enthalpy values were from samples with nominal compositions of ‘60Ni16Pt24Al’ and ‘74Ni5Ru24Al’

The work is aimed at studying the thermal degradation of the protective zinc coating. Its large part deals with the structure of the various stages of transition and their arrangement, the main emphasis is on analysis of brittle intermetallic phases, which arise due to increased temperature. Conclusion of the work is trying to clear itself causes degradation of this protective layer.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Blackeart malleable iron samples were galvanized using twenty different zinc bath compositions. Each bath was made with different concentration and combinations of bismuth, nickel, tin and aluminium. These additions aim not only to substitute lead, a hazardous element to the environment, but also to reduce coating thickness, usually higher than established by standards due to the great reactivity between cast iron and zinc bath. Additional studies were made to check the corrosion resistance and to check the maintenance or intensity of the coating s brightness. When using just bismuth and nickel additions to the zinc bath, it wasn t observed a considerable coating thickness reduction, but bismuth influenced bath fluidity, favoring better zinc draining and formation of more compacted and defined zinc-iron compounds. It was observed that bismuth decreased the coating s corrosion resistance, while nickel can increase the corrosion resistance when the bath has small bismuth concentrations. Tin addictions reduced the coating thickness when used together with bismuth and nickel addictions. However, tin did not only reduce the corrosion resistance, but also decreased the coating s brightness. Highest aluminium concentrations reduced the coating s thickness considerably when compared to the coating s thickness of the samples galvanized in the other baths. Aluminium also increased corrosion resistance when compared to the coatings of the samples galvanized in baths containing bismuth, nickel and tin. However excessive coating thickness reduction, caused by highest aluminium addictions to the bath, reduced corrosion resistance. Highest aluminium addictions were totally favorable to the coating s brightness. The combination of bismuth, nickel, tin and aluminium were effective on coating thickness reduction and some of these combinations also provided greater rust resistance and shinier coatings. Hence, the chemical elements added weren t detrimental to the environment and they are good alternatives to substitute lead in the hot-dip galvanizing process.
Amostras de ferro fundido maleável preto foram galvanizadas em vinte diferentes banhos de zinco, cada qual composto por concentrações e combinações variadas de bismuto, níquel, estanho e alumínio. Além de substituir o chumbo, um elemento tóxico e nocivo ao meio ambiente, objetiva-se com estas adições a redução da espessura do revestimento galvanizado, normalmente em excesso ao estabelecido em norma devido à grande reatividade dos ferros fundidos com o banho de zinco. Estudos adicionais foram realizados para a verificação da resistência à corrosão e para a verificação da manutenção ou intensificação do brilho dos revestimentos. Utilizando adições somente de bismuto e níquel ao banho de zinco não foi observada uma redução considerável da espessura do revestimento, mas o bismuto influenciou na fluidez do banho favorecendo o melhor escorrimento do zinco e a formação de fases zinco-ferro mais compactas e definidas. Observou-se que o bismuto diminuiu a resistência à corrosão dos revestimentos, mas o níquel pode aumentar a resistência à corrosão quando se têm adições menores de bismuto ao banho. Adições de estanho reduziram a espessura de camada quando utilizadas em conjunto com adições de bismuto e níquel, mas o estanho foi prejudicial à resistência à corrosão além de diminuir o brilho dos revestimentos. O alumínio em concentrações mais elevadas tornou as espessuras dos revestimentos consideravelmente menores quando comparadas às espessuras dos revestimentos galvanizados nos outros banhos estudados. O alumínio ainda aumentou a resistência à corrosão dos revestimentos em relação aos revestimentos galvanizados nos banhos contendo bismuto, níquel e estanho. Contudo a redução excessiva da espessura da camada, proporcionada por adições maiores de alumínio, fez com que a resistência à corrosão diminuísse novamente. Adições de maiores teores de alumínio foram totalmente favoráveis ao aumento do brilho dos revestimentos. A combinação de elementos químicos como o bismuto, níquel, estanho e alumínio mostrou-se efetiva na redução da espessura de camada, sendo que algumas destas combinações também proporcionaram maiores valores de resistência à corrosão e revestimentos com brilhos mais intensos. Além disso, os elementos químicos adicionados não são nocivos ao meio ambiente, sendo boas alternativas para substituir o chumbo na galvanização por imersão a quente. Palavras-chave: galvanização por imersão a quente, espessura de camada, resistência à corrosão, compostos intermetálicos zinco-ferro.

Sélectionner des traitements de surface pour l’industrie nécessite de prendre en compte :les propriétés à conférer au substrat, la nature et la géométrie de celui-ci et les caractéristiques du milieu extérieur. Certaines combinaisons de ces paramètres rendent difficile la sélection d’un traitement unique, d’où le recours à des multitraitements de surface. Dès lors, se posent les questions suivantes :

- Utiliser des multitraitements de surface peut se faire en scindant les différentes requêtes en sous-ensembles, de manière à ce que chaque traitement réponde à l’un d’eux. Dans quel ordre ces requêtes doivent-elles être introduites par rapport au substrat ?

- Comment sélectionner les traitements de surface répondant à chaque requête individuelle ?

- Comment classer des multitraitements en termes d’adéquation au problème posé ?

Dans ce travail, les première et troisième questions sont abordées, en explorant les requêtes concernant habituellement les dispositifs de moulage de l’aluminium :

- Résistance aux contraintes d’origine thermique.

- Résistance à la corrosion par les métaux fondus.

- Résistance au frottement.

L’analyse de la bibliographie relative aux traitements de surface utilisés dans ces systèmes a été analysée et des « architectures »-types ont été identifiées (chapitre 3). On prévoit, par exemple, un traitement conférant la résistance à la fatigue superficielle, ainsi qu’un revêtement étanche et résistant à l’aluminium fondu. Une barrière thermique est parfois préconisée.

Pour chacune des architectures, des traitements de surface individuels peuvent être sélectionnés. Un « facteur de performance » permettant de classer les solutions par rapport au problème de la fatigue thermique a été construit (chapitre 4) et discuté dans deux situations :

- Lorsqu’un revêtement est présent, et que les contraintes d’origine thermique (différence de dilatation thermique couche-substrat) menacent de le rompre lors de l’immersion dans un milieu corrosif à haute température. Des essais de corrosion dans de l’aluminium fondu ont été réalisés sur un acier revêtu par du nitrure de chrome dopé à l’aluminium, synthétisé par déposition physique en phase vapeur (chapitre 5 – collaboration :Inasmet).

- Lorsque des variations thermiques rapides menacent de rompre le substrat et la (les) couches. Des essais de fatigue thermique ont été réalisés sur de l’acier à outils pour travail à chaud non traité, boruré ou recouvert d’un multitraitements (zircone yttriée / NiCrAlY / boruration / acier). Le revêtement en zircone yttriée a été obtenu par projection par plasma. L’essai de fatigue thermique a été modélisé et le facteur de performance, discuté (chapitre 6).

Au chapitre 7, les architectures-types ont été introduites dans une méthodologie de sélection des multi-traitements de surface, qui a été appliquée dans deux cas :

- Celui des moules de fonderie, devant résister à la fatigue thermique et à la corrosion par l’aluminium fondu. Le facteur de performance a été extrapolé à d’autres situations qu’aux chapitres 5 et 6. Les solutions habituellement proposées pour résoudre ce problème sont retrouvées.

- Celui de deux pièces en acier frottant l’une contre l’autre en présence d’aluminium fondu.

To select surface treatments, one must account for the required functional properties, the substrate features and the solicitations the substrate must endure. Certain combinations of these parameters make it difficult to select a single surface treatment, a reason why several successive treatments are preferred. To select them, one needs to determine:

- How to divide the several requests into groups and how to stack up these groups from the substrate to the outer surface, so that each treatment deals with one specific group of requests/properties.

- How to select individual layers for each group of properties.

- How to rank the multi-treatments in terms of relevance for a given application.

In this work, one tries to answer the first and the third questions, by studying the case of aluminium foundry, in which the industrial devices frequently face the following solicitations:

- Thermal stress (thermal fatigue, thermal expansion mismatch).

- Presence of corrosive molten metal.

- Sliding wear.

In the literature, several “standard” architectures are proposed (chapter 3), like a diffusion layer reducing superficial fatigue plus a corrosion barrier layer. A thermal barrier coating is also sometimes proposed.

For each of these architectures, one can select individual treatments. To rank them, one devised a “performance index” for thermal stress (chap.4), which is discussed for two cases:

- For large differences between layer and substrate thermal expansion coefficients, when both are put into contact with a high temperature corrosive medium, the layer may be damaged. One discusses this case by examining the corrosion caused by molten aluminium for a steel substrate coated by anticorrosive chromium nitride doped with aluminium. The layer is produced by physical vapour deposition (chap. 5 – cooperation: Inasmet).

- Repeated fast surface temperature transients can also damage the substrate and/or the layer by thermal fatigue. One conducted thermal fatigue tests with samples of hot work tool steel, respectively untreated, simply borided and protected by a multilayer. In the last case, top coat is yttria stabilised zirconia, followed by a nickel superalloy and then a borided layer (undercoat). One synthesized the zirconia coating by plasma spray and one modelled the thermal fatigue (chap. 6).

In chap. 7, architectures from chap. 2 are introduced in a multi-treatment selection routine, which is applied in two cases:

- Foundry moulds for molten aluminium, withstanding both thermal fatigue and corrosion. The devised performance index is extrapolated beyond the tests of chap. 5 and 6 to treatments for this industrial application, thereby quantifying their respective merits.

- A foundry device exposed to molten metal and sliding wear.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

M. Tech. Metallurgical Engineering
Ti6Al4V alloy is the leading material for hip replacement because of its biocompatibility, elastic modulus and good yield strength, but shows poor corrosion and wear properties in simulated body fluid. Thin coatings were fabricated on Ti6Al4V alloy using Zirconium, Niobium and Titanium reinforcements with the aid of Nd:YAG Rofin Sinar laser system. Niobium reinforcement produced the best coating that displayed the best corrosion resistance and percentage increase in wear resistance in simulated body fluid as compared to the substrate. Therefore, this coating will perform best as orthopaedic implant material.

The aim of the present work is to engineer a new kind of coating material with superior quality and reliability in a cost effective way by laser cladding and ball milling process. Non equilibrium processing methods were adopted to produce different combination of phases, which are complicated, or cannot be produced by traditional fabrication processes. Advanced characterization techniques were utilized, to understand the mechanism of phase formation and the science behind our observations, thereby providing effective inputs for enhancement of outcomes. The present work deals with the alloy systems of Fe-Sn at Fe-rich end (Fe-10 wt.% Sn) and Al-Cu-Fe-B quasicrystal composition (Al61Cu25Fe12B2) for laser cladding experiments and the Ni-Al system (corresponding to NiAl and Ni3Al) for cladding by ball milling experiments. Fe-Sn intermetallics have very good hardness and magnetic properties; these properties were motivating factor to initiate the study to produce hard coatings. Among all crystalline and quasicrystalline phases of Al-Cu-Fe system, the λ and μ phases exhibit high hardness and are extremely brittle and are related to quasicrystals; the introduction of these phases into the Al matrix facilitated the production of the hard coatings of crystalline quasi-approximant phases on Al substrate. The presence of the intermetallic phases makes these coatings hard, and are good candidates for engineering applications. Since many years, Nickel aluminides were used as materials for high temperature and structural applications. These intermetallics also exhibit good thermal and oxidation properties and have good wear resistance. These properties of the intermetallics make them good candidates for coating applications. We have synthesized intermetallic particles embedded in different coating matrix. We have synthesized the coating of these composite materials which results in hard coatings with excellent strength. The thesis is subdivided into five chapters. After introducing the thesis, the second chapter contains a review of literature. It gives a brief introduction to the present problems in research. The literature review presents the current synthesis methods of thick coatings, the emerging trends in new coating methods and a brief discussion on the recent developments in coating materials. More emphasis was given to the laser prototyping, the thermal barrier coatings and the functional graded coatings. The next chapter describes the experimental techniques, the processing parameters and the characterization tools, used for understanding the microstructure evolution. This includes a brief descriptions of the alloys studied. A brief discussion of the techniques for the characterization of structure and microstructure is included in this chapter. I have also included in this chapter the challenges faced during sample preparations for different characterization techniques. The fourth chapter discusses the formation of Fe-Sn intermetallic dispersions in iron matrix by laser cladding and remelting experiments. The evolution of the secondary phase particles suggest that liquid phase separation is a major contributor. The choice of Fe-Sn alloys as clad is dictated by the fact that Fe-Sn phase diagram exhibits a miscibility gap at high temperature. Sn forms a monotectic system with iron at a composition of Fe-34 at.% Sn and a temperature of 1130 °C followed by a two phase regions of tin rich and tin lean liquid. There is a large region where liquid tin coexists with α-Fe. As the two phase alloy is cooled, the system depending on the composition undergoes a series of peritectic reactions yielding the intermetallic phases of iron and tin. We have chosen a composition of Fe 10 wt.% Sn (Fe-5 at.% Sn) for our cladding experiments. Several clad and remelt samples were prepared to understand the effect of the cooling rate on phase formations. Majority of the intermetallic phases observed in the coatings are Fe3Sn and ii FeSn2, while a large amount of Fe3Sn phase was observed in the remelt layer. Re-solidification through laser surface processing, provides an opportunity to obtain quantitative information about the solidification process and for testing the prevalent theories of phase evolution. Growth velocities of the solidification front were calculated for the clad and remelt tracks. Several clad and remelt layers are analyzed by the EBSD technique, the results of which do not suggest any significant texture during the solidification process. The composition analysis of the dispersed particles was carried out by the EDS and EPMA. Sn is primarily segregated in the cell boundaries and near the cell boundaries in the dispersed particles. With increasing laser scan rate, we observe more regions containing dispersed particles with a composition close to that of Fe3Sn. The dispersed phases are primarily Fe3Sn; however an elongated morphology at the cell boundary indicates a mixture of phases. The TEM studies on a very large number of dispersed particles confirm that most of them are Fe3Sn, and occasionally FeSn and FeSn2. In the fifth chapter we have described our attempt to synthesize a quasi-crystalline and approximant phases based on Al-Cu-Fe system. We have added a small amount of boron, to evaluate its effect on the resulting microstructure and phase formation. The quasi crystals lack ductility, which poses a major challenge in the processing. It was felt that the surface alloying techniques, in particular, laser cladding and remelting techniques can overcome this difficulty and can be adopted to utilize the properties of quasicrystals and its approximants. Most of the work that we have reported in this thesis deals with the Al rich end of the QC alloy. We have studied the effect of dilution from the Al substrate on the phase formation. Annealing experiments were carried out to study the effect of heat treatment on the phase formati experiments. The coatings were prepared by using the two step cladding process. In the first step, coatings were prepared at different laser power and clad speeds. In the second step some of the coatings were remelted at different traverse speeds to achieve different growth conditions. A few multi track samples were also processed at different processing conditions to understand the microstructural evolution. Tribological studies were carried out on some of the multi track clad and remelt samples to understand the wear behavior of coatings. Micro hardness measurements were done along the cross section of the samples, to evaluate the mechanical properties of the coatings. The sixth chapter contains two major parts. The first part deals with the synthesis and characterization of Ni-Al intermetallic particles by ball milling process. Comparative studies of the ball milled powders were carried out using high energy ball mills (P7 & SPEX and P5). Rietveld whole powder pattern fitting was carried out on powder particles obtained at different intervals of time. Typical parameters like crystallite size, lattice parameter, phase fraction and strain were calculated by this method. Further studies on milled powders were carried out by transmission electron microscope to understand the mechanism of phase formation. We show that intermetallic compound NiAl and Ni3Al can be synthesized using P7 and SPEX mills. However, the phases do not form in P5 mills under the conditions adopted by us. The second part of this chapter contains the preparation of Ni-Al intermetallic coatings using previously milled powders by P7, SPEX and P5 mills as well as fresh Ni and Al elemental powders. Comparative studies were carried out on coatings prepared by dry milling and wet milling method as well as between coatings prepared by the previously milled powders and the elemental mixture of Ni and Al powders. Coatings cross sections were examined to understand the microstructures within the coatings. As the annealing increases the diffusion of species, annealing experiments was conducted on coatings to produce better intermetallic coatings.

碩士
逢甲大學
材料科學學系
89
Thermal barrier coatings (TBCs) has been well developed to help protecting against high temperature oxidation, meanwhile applying an intermediate bond coat, typically of an MCrAlY-type metal alloy for further cyclic oxidation resistance enhancement. In previous study, demonstrating the improvement on the cyclic oxidation resistance of steel using a novel process-cathodic arc ion plated nickel aluminide. Thus, cathodic arc, electron-beam evaporation and sputter ion plated were used to deposit nickel aluminide films on nickel-based superalloy as well as stainless steel for composition purpose. The feasibility of such physical vapor deposition bond coat process can thus being facilitates. Cyclic oxidation tests were carried out at 800℃ and 1100℃, higher than previous study, to approach the real service condition. Scanning electron microscope was used to observe surface morphology of the specimens. X-ray diffractometer was used to characterize surface phases using CuKa radiation. A glow discharge optical emission spectrometer was used to analyse the composition and compositional depth profile of the specimens before and after cyclic oxidation tests. Experimental results show that nickel aluminide film can be successfully deposited by using a Ni30Al70 target material from physical vapor deposition processes. Ni2Al3 and NiAl3 were the major phases by using cathodic arc and sputter ion plating, while Aluminum is the major phase with Ni2Al3 as minor phase were obtained by using electron-beam evaporation ion plating. These physical vapor deposition types of nickel aluminide film can all provide protection against high temperature oxidation during cyclic oxidation test, although mechanisms to resist high temperature are different. When testing at 800℃, the formation of Fe3Al and NiAl phase is the primary contributor to thermal oxidation resistance due to low oxygen diffusivity in it, hence effectively protect substrate. On going to the higher test temperature 1100℃, the stable α-Al2O3 grown on the top most layer in addition to the Fe3Al or Ni3Al formation resulting form interdiffusion between coating and substrate instead prevent from further oxidation.

M. Tech. Metallurgical Engineering
Titanium alloys exhibit poor tribological characteristic which include abrasion resistance, metal to metal wear resistance and solid particle erosion and cavitation due to low surface hardness and high coefficient of friction. These poor properties have limited the application of titanium alloys as engineering tribological components, tools and parts that operate in severe wear and friction conditions. Laser processing defects such as pores, cracks and segregation pose a huge threat to the quality and the microstructure of the deposited layer. Defects caused by the parameters lead to severe wear and corrosion occurrence, hence, precise control of the parameters are crucial and it depends on the properties of the material used. It is postulated that Nickel Titanium (NiTi) is a promising candidate as reinforcement matrix material for wear resistant alloy. The wear resistance would be further enhanced if NiTi is in-situ incorporated onto titanium matrix by laser cladding to yield hard intermetallic phases. The main goal of the study is to develop corrosive-wear resistant thin surface coatings on Ti-6Al-4V alloy for automotive and aerospace applications by depositing Titanium and Nickel elemental powders to form in-situ NiTi intermetallic using laser cladding technique.