Hypersonic flight at a speed greater than Mach 5 (1715 ms-1) requires materials that can withstand temperatures up to 3000°C, high heat flux, rapid heating and disassociated reactive oxygen in the extreme environment of space and during re-entry. A number of advanced ceramic materials have melting points over 3000°C, of which the refractory metal carbides and borides are of main interest due to their excellent thermal conductivity from room temperature to over 2500°C, good chemical stability and ablation resistance at high temperatures. These materials are classified as ultra-high-temperature ceramics (UHTCs). Among the family of UHTCs, ZrB2 and HfB2 are reported as the most promising candidates to be used as thermal protection systems (TPS) for the nose tip and sharp leading edges. However, the issue of using monolithic ZrB2 and HfB2 is the phase transformation of ZrO2 and HfO2 oxide by-products at elevated temperature, leading to a volume change that results in cracking of the formed oxide scale. Hence, it is necessary to use dopants to stabilize the oxidation products of ZrB2 and HfB2 in-situ and to minimise the transformation induced cracking and thus improving the oxidation resistance. This research is focused on introducing dopants, such as Y and Ta into HfB2 and to understand its effect on the oxidation behaviour of HfB2 based UHT ceramics. The primary objectives were to: (a) Synthesize sub-micron pure and doped HfB2 powders; (b) Sinter the HfB2 based ceramics to achieve relative density > 95% (i.e. with close porosity); (c) Assess the effect of dopants on the oxidation resistance of HfB2 ceramics at high temperatures. Sub-micron pure HfB2 powder of ~200 nm was synthesized by a modified sol-gel approach combined with subsequent carbothermal reduction process using hafnium tetrachloride, boric acid, and phenolic resin as the starting materials. HfC and residual carbon were found to be the main impurity phase, owing to the lack of removal of carbon-containing species in the argon atmosphere during the heat treatment. Therefore, a precipitation approach was developed to transfer hafnium tetrachloride into hafnium hydroxide during the mixing stage to get rid of the Cl- and carbon-containing functional groups. Based on the detailed study of the formation mechanism of HfB2, it was found that the particle size of the HfB2 powders was decided by the particle size of the starting Hf source. Although the powders were slightly coarser (~400-800 nm) from the precipitation approach, importantly phase-pure HfB2 was formed at the same furnace heating conditions (1600°C/2 hrs). The precipitation method was also used to prepare doped HfB2 powders as the homogeneity of the dopants (TaB2, Y2O3) could be improved by controlling the pH values at ~8.5 to achieve the simultaneous precipitation of the dopants and HfB2 precursors. As a result, (Hf,Ta)B2 solid solution was prepared successfully at the temperature of 1600°C. Spark plasma sintering (SPS) was used to densify the pure and doped HfB2 powders. The optimized density achieved was around 97% at 2150°C without the use of any sintering aids and the addition of TaB2 slightly improved the sinterability of the HfB2 based powders due to the formation of the (Hf,Ta)B2 solid solution. The sintered density of commercial micron HfB2 powders (Treibacher) was only 94% in the same condition, and the resultant grain size (5-10 μm) is also significantly larger than that from synthesized HfB2-based ceramics (2-6 μm). The oxide impurities, such as HfO2 and B2O3, on the surface of the fine HfB2 based powders were attributed as the main reason for inhibiting further densification. The oxidation behaviours of the HfB2 based ceramics were investigated via both static oven oxidation and oxyacetylene torch testing. In low and intermediate temperature regime ( < 1600°C), it was indicated that the addition of dopants didn't significantly improve the oxidation resistance as the glassy B2O3 was the critical factor controlling the oxygen permeation rate. However, in the high-temperature regime ( > 1600°C), it was found the oxidation product was mainly tetragonal HfO2, which was stabilized by the Ta-dopants at temperatures well below the HfO2 phase transformation temperature. Therefore, the cracking and volume change due to phase transformation can be avoided and in return, oxidation resistance was improved at high temperature, which should be beneficial for the application of these materials in hypersonic aviation.

The development of molybdenum - silicon - boron (Mo-Si-B) composites having a combination of high temperature strength, creep, and oxidation residence has the potential to substantially increase the efficiency of gas turbines. The refractory nature of the αMo, Mo3Si (A15), and Mo5SiB2 (T2) phases results in good strength and creep resistance up to 1300°C. At this temperature, the formation of a borosilicate surface scale from the two intermetallic phases is able to provide oxidation resistance. However, realization of these advantages has been prevented by both a high brittle to ductile transition temperature and difficulty in forming the initial surface borosilicate to provide bulk oxidation resistance. This dissertation addresses two factors pertaining to this material system: 1) improvements to powder processing techniques, and 2) development of compositions for oxidation resistance at 1300°C. The processing of Mo-Si-B composites is strongly tied to their mechanical properties by establishing the αMo matrix, limiting impurity content, and reducing silicon supersaturation. These microstructural aspects control the brittle to ductile transition temperature which has traditionally been too high for implementation of Mo-Si-B composites. The processing here built upon the previously developed powder processing with silicon and boron nitrides which allowed for a low oxygen content and sintering of fine starting powders. Adjustments were made to the firing cycle based upon dew point measurements made during the hydrogen de-oxidation stage. Under a relatively high gas flow rate, 90% of the total water generated occurred during a ramp of 2°C /min between 450 and 800°C followed by a hold of 30 minutes. The oxidation resistance of Mo-Si-B composites was studied for a wide range of compositions. Silicon to boron atomic ratios were varied from 1 to 5 and iron, nickel, cobalt, yttria, and manganese were included as minor additions. In all these compositions, the αMo volume fraction was kept over 50% to ensure the potential toughness of the composite. For the oxidized surface glass, a silica fraction of 80 to 85% was found to be necessary for the borosilicate to have a sufficiently high viscosity and low oxygen permeability for oxidation resistance at 1300°C. For the Mo-Si-B bulk composition this corresponds to a Si/B atomic ration of 2 to 2.5. Higher viscosity compositions failed due to spallation of poorly attached, high silica scales. Lower viscosity compositions failed from continuous oxidation, either through open channels or repetitive MoO3 bubble growth and popping. Additionally, around 1% manganese was necessary for initial spreading of the borosilicate at 1300°C. In conjunction with flowing air to prevent MoO3 accumulation, oxidation weight loss rates below 0.05 mg/cm2-hr were measured. Finally, a theory is proposed here to describe the mechanisms responsible for the development of oxidation resistance. This theory involves three stages associated with: 1) generation of an initial surface borosilicate, 2) thickening of the borosilicate layer, and 3) slow parabolic oxidation controlled by the high silica surface scale.

For hypersonic flight to enable rapid global transport and allow routine space access thermal protection systems must be developed that can survive the extreme aerothermal heating and oxidation for extended periods of time. Ultra high temperature ceramics (UHTCs) are the only potential materials capable of surviving the extreme hypersonic environment however extensive research in processing science and their oxidation properties are required before engineering systems can be developed for flight vehicles. Investigating the role of oxides during processing of ultra high temperature ceramics shows they play a critical role in both synthesis of ceramic powders and during densification. During spark plasma sintering of UHTCs the oxides can result in the formation of vapor filled pores that limit densification. A low temperature heat treatment can remove the oxides responsible for forming the vapor pores and also results in a significant improvement of the densification through a particle surface physical modification. The surface modification breaks up the native continuous surface oxide increasing the surface energy of the powder and removing the oxide as a barrier to diffusion that must be overcome before densification can begin. During synthesis of UHTCs from sol-gel the B₂O₃ phase acts as the main structure of the gel limiting the transition metal oxide network. While heat treating to form diborides the transition metal oxide undergoes preferential reduction forming carbides that reduce B₂O₃ while at high temperature encourage particle growth and localized extreme coarsening. To form phase pure borides B₂O₃ is required in excessive quantities to limit residual carbides, however carbide reduction and grain growth are connected. When the UHTC systems of ZrB₂-SiC are exposed to oxidation, either as dense ceramics or coatings on Carbon-Carbon composites, at high temperatures they undergo a complex oxidation mechanism with simultaneous material transport, precipitation and evaporation of oxide species that forms a glass ceramic protective oxygen barrier on the surface. The composite effect observed between the oxides of ZrB₂-SiC enables them to survive extreme oxidizing environments where traditional SiC oxidation barrier coatings fail.

Afin d’obtenir des matériaux aux caractéristiques mécaniques, tribologiques et thermiques améliorées, nous avons élaboré des revêtements nanocomposites à base de TiN en utilisant une technique de dépôt physique en phase vapeur. Ces matériaux aux caractéristiques spécifiques peuvent être exploités pour le surfaçage d’outils de coupe de très haute dureté. En ajustant les processus d’élaboration dont dépendent la microstructure et la microchimie des revêtements, il est possible de contrôler les propriétés de ces matériaux. Cette thèse présente les résultats obtenus sur les trois systèmes de revêtement que sont Ti-Al-N, Ti-Al-Y-N et Ti-Si-N, configurés soit en réseaux superposés multicouche soit en nanocomposites. L’accent est mis sur l’étude systématique de la dureté et de la résistance à l’usure et à l'oxydation en fonction des paramètres de dépôt. En combinant la diffraction des rayons X et la microscopie électronique à transmission avec des tests physico-mécaniques sur une large gamme de configurations de revêtement, on établit une matrice processus-performance prédictive permettant de guider la fabrication de surfaces durcies. / TiN-based nanocomposite coatings were prepared using physical vapor deposition to deliver enhanced mechanical, tribological and thermal characters that can be exploited for superhard cutting tool surfacing. These properties are controlled by tailoring processing methods to tune the microstructure and microchemistry. This thesis examined three coating systems, which are Ti-Al-N, Ti-Al-Y-N and Ti-Si-N, configured variously as multilayer superlattices and nanocomposites to comprehensively correlate hardness, wear resistance and oxidation resistance with deposition parameters. Combining X-ray diffraction and transmission electron microscopy with physical-mechanical testing, over a wide range of coating configurations, enabled construction of a predictive process-performance matrix to guide the fabrication of hardened surfaces.In multilayer TiN/TixAl1-xN coatings prepared by cathodic arc deposition, the mechanical properties were controlled by the layer period that was adjusted by varying substrate rotation speed. A hardness of 39 ± 4 GPa was achieved for a superlattice period of 13 nm, where the coatings contain columnar <111> textured rock salt – type crystals connected by low-angle grain boundaries. When yttrium was introduced to the multilayers, by adding a Y - metal target powered by DC magnetron sputtering, the morphology changed from columnar to acicular grains with smaller grain size. Specifically, by fixing the period at 5.5 nm and incorporating Y from 0 to 2.4 at% the grain size decreased (from 100-200 nm to 20-30 nm) and hardness increased (from 29 ± 7 GPa to 41 ± 3 GPa). The improved performance was a consequence of solid solution hardening that arises from the misfit strain field introduced by Y (element atomic radii 2.12 Å) substitution for Ti (1.76 Å) or Al (1.18 Å), and a nanosize effect, where finer grains result in a greater volume fraction of grain boundaries that block dislocation movement. Higher Y additions also retard oxidation as high temperature (800 ºC) annealing generates Ti2O3, rather than TiO2 as in Y-free coatings, and also affects Al oxidation. Adhesion and wear resistance were not compromised by higher Y contents demonstrating that TiN/TixAl1-xN coatings can enhance mechanical properties and thermal stability. Notably, this work employed a pure Y target, instead of a Ti-Al-Y alloy target, and substrate holder rotation speed was the critical parameter, where faster substrate rotation leads to smaller periods and more uniform Y distribution. However, an Y-rich layer became progressively thicker at slower rotation with the period increasing from 5.5 nm to 24 nm. These Y-rich regions seeded crystal nucleation that reduced coherency at layer interfaces and grain boundaries to significantly degrade mechanical properties (41 ± 3 GPa to 30 ± 5 GPa). Therefore, the period and Y content work in tandem in multilayered TiN/TixAl1-xN coatings and the optimized Y content was to be 2.4 at% at a period of 5.5 nm.Nanocrystallite TiN / amorphous (a)-Si3N4 nanocomposites were fabricated by high power impulse magnetron sputtering. The introduction of silicon by controlling the Si target current can be used to modify the coating structure, tailor mechanical properties, improve wear resistance and passivate oxidation. Smaller crystal sizes promoted at higher Si content lead to TiN / amorphous (a)-Si3N4 nanocomposites, with ~10 at% Si/(Si+Ti) yielding maximum hardness (41 ± 3 GPa). Compared to TiN, Ti0.903Si0.097N showed enhanced resistance to oxidation and wear resistance, however, the TiN crystallites were not completely encapsulated by a-Si3N4 intergranular films and further optimization of the structure and property relationship can be realised.

A vehicle flying at hypersonic speeds, i.e. at speeds greater than Mach 4, needs to be able to withstand the heat arising from friction and shock waves, which can reach temperatures of up to 3000oC. The current project focuses on producing thermal protection systems based on ultra high temperature ceramic (UHTC) impregnated carbon-carbon composites. The carbon fibres offer low mass and excellent resistance to thermal shock; their vulnerability is to oxidation above 500oC. The aim of introducing HfB2, a UHTC, as a coating on the fibre tows or as particulate reinforcement into the carbon fibre preform, was to improve this property. The objectives of this project were to: i) identify a low temperature synthesis route for group IV diborides, ii) produce a powder fine enough to reduce the difficulties associated with sintering the refractory diborides, iii) develop sol-gel coating of HfB2 onto carbon fibre tows iv) improve the solid loading of the particulate reinforcement into the carbon fibre preform, which should, in turn, increase the oxidation protection. In order to achieve the above set objectives, fine HfB2 powder was synthesized through a low temperature sol gel and boro/carbothermal reduction process, using a range of different carbon sources. Study of the formation mechanism of HfB2 revealed an intermediate boron sub-oxide and/or active boron formation that yielded HfB2 formation at 1300oC. At higher temperatures the formation of HfB2 could be via intermediate HfC formation and/or B4C formation. Growth mechanism analysis showed that the nucleated particles possessed screw dislocations which indicated that the formation of HfB2 was not only through a substitution reaction, but there could have been an element of a precipitation nucleation mechanism that lead to anisotropic growth under certain conditions. The effect of carbon sources during the boro/carbothermal reduction reaction on the size of the final HfB2 powders was analysed and it was found that a direct relation existed between the size and level of agglomeration of the carbon sources and the resulting HfB2 powders. A powder phenolic resin source led to the finest powder, with particle sizes in the range 30 to 150 nm. SPS sintering of the powder revealed that 99% theoretical density could be achieved without the need for sintering aids at 2200oC. Sol-gel coatings and slurry impregnation of HfB2 on carbon fibres tows was performed using dip coating and a 'squeeze-tube' method respectively. Crack free coatings and non-porous matrix infiltration were successfully achieved. The solid loading of the fine HfB2 into the carbon fibre preform was carried out through impregnation of a HfB2 / phenolic resin/acetone slurry using vacuum impregnation. Although the sub-micron Loughborough (LU) powders were expected to improve the solid loading, compared to the commercially available micron sized powders, due to the slurry made from them having a higher viscosity because of the fine particle size, the solids loading achieved was consequently decreased. Optimisation of the rheology of the slurry with LU HfB2 still requires more work. A comparison of the oxidation and ablation resistance of the Cf-HfB2 composites prepared with both commercial micron sized HfB2 powder and Loughborough sub-micron sized HfB2 powder, each with similar level of solid loading, was carried out using oxyacetylene torch testing. It was found that the composite containing the finer, Loughborough powders suffered a larger erosion volume than the composite with the coarser commercial powders indicating that the former offered worse ablation and oxidation resistance than the latter. A full investigation of the effect of solids loading and particle size, including the option of using mixtures of fine and coarse powders, is still required.

Les composites C/C sont des matériaux très utilisés dans de nombreuses applications pour leurs propriétés exceptionnelles. Néanmoins, ils présentent l'inconvénient de s'oxyder dès les basses températures. Le travail dans cette thèse a consisté en l'élaboration de dépôts de carbures de silicium (SiC) et d’hafnium (HfC) par dépôt chimique en phase vapeur (CVD) afin de protéger en surface ces composites jusqu’à 2000°C. Cette voie d'élaboration permet l'obtention de dépôts denses et continus. Dans un premier temps, une étude thermodynamique du système chimique Hf-Cl-C-H a permis d’appréhender les conditions de dépôt d’HfC et de tracer des diagrammes de dépôt directement utilisables par l’expérimentateur. Ensuite, après avoir déterminé les conditions expérimentales de chloruration de l’hafnium, étape antérieure à la CVD, et après avoir examiné les compatibilités chimiques des deux carbures par Spark Plasma Sintering (SPS), une étude expérimentale paramétrique de la CVD d’HfC a été proposée. Cela a permis la détermination des conditions optimales de dépôt permettant l’obtention d’une protection multiséquencée HfC/SiC, les conditions de dépôt du SiC étant reprises de la littérature. En plus du procédé de CVD, un autre type de concept portant sur l'enrobage de poudres d'HfC par le SiC, puis frittées par la suite, a également été traité. Enfin, les matériaux fondés sur ces deux concepts ont été testés en oxydation à très haute température. Les résultats obtenus ont permis la validation du matériau multiséquencé à 2000°C et le matériau fritté à 1500°C. / Carbon/Carbon composites are widely used materials in many fields of application for their outstanding properties. Nevertheless, these materials have the drawback of oxidizing at very low temperatures. The aim of this work consisted in depositing by means of Chemical Vapour Deposition (CVD) coatings made of silicon carbide (SiC) and hafnium carbide (HfC) in order to protect the composite up to 2000°C in an oxidizing atmosphere. This way of manufacturing has allowed reaching dense and continuous coatings. First, a thermodynamic study of the Hf-Cl-C-H chemical system has permitted to study the influence of HfC deposition parameters and to report them into deposition diagrams. Then, after the study of experimental conditions in the metallic hafnium chlorination step and the examination of chemical compatibilities of the two carbides by Spark Plasma Sintering (SPS), a parametric study of the CVD of HfC has been carried out. This has enabled determination of optimal deposition conditions of HfC in order to manufacture an HfC/SiC multilayered protection. SiC experimental conditions were taken from the literature. Besides the materials made by CVD, another kind of material protection made of HfC powder coated with SiC and then sintered has been also studied. Finally, materials based on those two protection concepts have been oxidized at very high temperature. Results have enabled to validate the multilayered protection up to 2000°C and the HfC/SiC sintered powder up to 1500°C.

Revestimentos finos de carboneto de nióbio (NbC) puro e dopados com níquel (Ni) foram obtidos mediante a técnica de deposição reativa por magnetron sputtering, utilizando metano (CH4) como fonte de carbono (C). O filme de NbC usado como referência foi depositado aplicando uma potência de 2500 W ao alvo de Nb e, os revestimentos de NbxNiyCz foram depositados diminuindo a potência aplicada ao alvo de Nb e aumentando a potência aplicada ao alvo de Nb-Ni, dando origem à seis revestimentos com teores de Ni crescentes. As caracterizações microestrutural e estrutural dos revestimentos de NbC e NbxCyNiz foram realizadas por meio das técnicas de difração de raios-X (DRX), Espectroscopia Fotoeletrônica de Raios X (XPS), Espectroscopia Raman, Microscopia Eletrônica de Transmissão (MET) e Microscopia Eletrônica de Varredura (MEV). As propriedades mecânicas dos revestimentos foram estudas mediante a técnica de nanoindentação instrumentada, com o intuito de avaliar a dureza (H) e o módulo de elasticidade (E). A adesão dos revestimentos ao substrato foi avaliada usando ensaios Rockwell C e esclerometria linear instrumentada. A estabilidade térmica dos revestimentos foi realizada em forno com atmosfera controlada em temperaturas de 600 °C e 800 °C por 2h. Finalmente, a resistência à oxidação dos revestimentos foi estudada por meio de ensaios de Termogravimetria (TGA - \"Thermogravimetric Analysis\") de aquecimento contínuo e isotérmicos. Os resultados de adesão obtidos mostraram boa aderência (modo de falha HF1) dos filmes de NbC e NbxNiyCz ao substrato de aço AISI M2, nas condições como recém depositado e revenido a 600 °C, indicando que a deposição do gradiente de intercamadas de Cr, CrC e do gradiente CrC / NbC foi efetiva evitando falhas adesivas. A adição de Ni na estrutura dos revestimentos de NbC promoveu a formação de estruturas nanocompósitas, composta de nanocristalitos de NbC e NiCx. Adicionalmente, a introdução de níquel causou um aumento na dureza nos revestimentos como recém depositados, aumentando de 17 para 25 GPa para teores de Ni de 0 para 13 at. %, respectivamente, e, na resistência à oxidação sobre o revestimento puro de NbC, de 380 °C para 480 °C nos revestimentos com níquel. Finalmente, as análises de estabilidade térmica permitiram observar que os precipitados de NiCx se decompõem durante os tratamentos de recozimento a 600 e 800 °C, o que promoveu um aumento nos valores de dureza e módulo de Young para todos os revestimentos, atribuído ao aumento da cristalinidade dos revestimentos. / Niobium carbide (NbC) coatings doped with Nickel (Ni) were deposited by reactive DC - magnetron sputtering using methane (CH4) as carbon (C) source. Reference NbC coating was deposited with a total power of 2500 W and NbxNiyCz coatings were deposited by decreasing the power applied to the Nb target and increasing the power applied to the Nb-Ni target, giving rise to coatings with increasing Ni content. Structural and microstructural characterizations of NbC and NbxNiyCz coatings were performed using X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, Transmission Electron Microscopy (MET) and Scanning Electron Microscopy (MEV). Mechanical properties of the coatings were studied using the instrumented nanoindentation technique, in order to evaluate the Hardness (H) and Elastic modulus (E). The adhesion between coatings and substrate was evaluated using Rockwell C test and instrumented linear scratch tests. The tests for studying the thermal stability of the coatings were carried out in a controlled atmosphere chamber furnace at temperatures of 600 °C and 800 °C for 2h. Finally, the oxidation resistance of the coatings was studied by means of Thermogravimetric Analysis (TGA) tests of continuous and isothermal heating. The NbC and NbxNiyCz films in the as-deposited condition and annealed at 600 °C, showed good adhesion (failure mode HF1) to the AISI M2 steel substrate, indicating that the adhesion interlayer of the Cr, CrC and a gradient CrC/NbC layer was effective in avoiding adhesive failures. The increasing of Ni content in the structure of NbC coatings promoted the formation of nanocomposite structures, composed of a mixture of NbC and NiCx nanocrystallites. Additionally, the introduction of nickel allows increasing the hardness for the coatings in the as-deposited condition, from 17 to 25 GPa for Ni contents from 0 to 13 at. %, respectively, and, improving the oxidation resistance over the pure NbC coating, from 380 °C to 480 °C for the Ni-rich coatings. Finally, the thermal stability analyses showed that the NiCx precipitate decompose during the annealing treatments at 600 °C and 800 °C, which promoted an increase in the hardness and Young\'s modulus values for all coatings. These behaviors were attributed to the increase of crystallinity of the coatings.

Abstract des Vortrages: Titanium aluminide (TiAl) alloys are attractive lightweight materials for mediumtemperature (500°-750°C) structural applications including components such as jet engine and industrial gas turbine blades, turbocharger rotors and automotive engine valves. However, envisaged service temperatures for future advanced applications will have to be in the range of 750° to 1000°C, over which these alloys suffer from both oxidation and oxygen embrittlement. Therefore, development of surfaceengineering techniques for preventing high-temperature environmental damage is critical in exploiting the advantages of TiAl alloys to their fullest extent. Two efficient approaches to protecting candidate TiAl alloys from high-temperature (>750°C) environmental degradation have been developed at HZDR. The first technique involves a single step, namely treating TiAl alloy components directly by plasma immersion ion implantation (PIII) of fluorine using a mixture of difluoromethane and argon (CH2F2 + 25% Ar) as the precursor gas. The oxidation performance of the fluorine-implanted alloys has been evaluated by thermal gravimetric analysis (TGA) over the temperature range of 750° to 1050°C under conditions of both isothermal and thermal cyclic oxidation in air, and for times as long as 6000 h. This type of surface modification has been shown to produce a stable, adherent and highly protective alumina scale. The second technique involves the fabrication of a durable protective coating in a two-step process, namely formation of a thin aluminum-rich TiAl layer (Ti-60Al) by chemical vapor deposition (CVD) employing a mixture of inorganic precursors, followed by PIII of fluorine. Subsequent long-term oxidation exposures to air at 900°C of a GE 4822 alloy (Ti-48Al-2Cr-2Nb; alloy composition qualified for aerospace applications) have shown that the coating so developed is able to successfully prevent oxidation damage to the base material while maintaining up to 90% of its initial mechanical properties (strength and ductility).

Mo-Si-B alloys are a leading candidate for the next generation of jet turbine engine blades and have the potential to raise operating temperatures by 300-400°C. The alloys of interest are a three-phase mixture of the molybdenum solid solution (Moss) and two intermetallic phases, Mo3Si (A15) and Mo5SiB2 (T2). A novel powder metallurgical method was developed which uses the reaction of molybdenum, silicon nitride (Si3N4) and boron nitride (BN) powders to synthesize a fine dispersion of intermetallics in a Moss matrix. The covalent nitrides are stable in oxidizing environments up to 1000ºC, allowing for fine particle processing. The process developed uses standard powder processing techniques to create Mo-Si-B alloys in a less complex and expensive manner than previously demonstrated. This powder metallurgy approach yields a fine dispersion of intermetallics in the Moss matrix with average grain sizes of 2-4μm. Densities up to 95% of theoretical were attained from pressureless sintering at 1600°C and full theoretical density was achieved by hot-isostatic pressing (HIP). Sintering and HIPing at 1300°C reduced the grain sizes of all three phases by over a factor of two. Microstructure examination by electron back-scatter diffraction imaging was used to precisely define the location of the phases and to measure the volume fractions and grain size distributions. Microstructural quantification techniques including two-point correlation functions were used to quantify microstructural features and correlate the BN reactant powder size and morphology to the distribution of the intermetallic phases. High-temperature tensile tests were conducted and yield strengths of 580MPa at 1100°C and 480MPa at 1200°C were measured for the Mo-2Si-1Bwt.% alloy. The yield strength of the Mo-3Si-1Bwt.% alloy was 680MPa at 1100°C and 420MPa at 1300°C. A review of the pertinent literature reveals that these are among the highest yield strengths measured for these compositions. The oxidation resistance in air at 1000 and 1100°C was examined. The protective borosilicate surface layer formed quickly due to the close spacing of intermetallic particles and pre-oxidation treatment was developed to further limit the transient oxidation behavior. An oxidation model was developed which factors in the different stages of oxidation to predict compositions that minimize oxidation.

Abstract des Vortrages: Titanium aluminide (TiAl) alloys are attractive lightweight materials for mediumtemperature (500°-750°C) structural applications including components such as jet engine and industrial gas turbine blades, turbocharger rotors and automotive engine valves. However, envisaged service temperatures for future advanced applications will have to be in the range of 750° to 1000°C, over which these alloys suffer from both oxidation and oxygen embrittlement. Therefore, development of surfaceengineering techniques for preventing high-temperature environmental damage is critical in exploiting the advantages of TiAl alloys to their fullest extent. Two efficient approaches to protecting candidate TiAl alloys from high-temperature (>750°C) environmental degradation have been developed at HZDR. The first technique involves a single step, namely treating TiAl alloy components directly by plasma immersion ion implantation (PIII) of fluorine using a mixture of difluoromethane and argon (CH2F2 + 25% Ar) as the precursor gas. The oxidation performance of the fluorine-implanted alloys has been evaluated by thermal gravimetric analysis (TGA) over the temperature range of 750° to 1050°C under conditions of both isothermal and thermal cyclic oxidation in air, and for times as long as 6000 h. This type of surface modification has been shown to produce a stable, adherent and highly protective alumina scale. The second technique involves the fabrication of a durable protective coating in a two-step process, namely formation of a thin aluminum-rich TiAl layer (Ti-60Al) by chemical vapor deposition (CVD) employing a mixture of inorganic precursors, followed by PIII of fluorine. Subsequent long-term oxidation exposures to air at 900°C of a GE 4822 alloy (Ti-48Al-2Cr-2Nb; alloy composition qualified for aerospace applications) have shown that the coating so developed is able to successfully prevent oxidation damage to the base material while maintaining up to 90% of its initial mechanical properties (strength and ductility).