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 texturing of ultra-high temperature ceramics (UHTCs) using weak magnetic fields is studied and developed for the first time. Textured UHTCs were prepared by magnetically assisted slip casting (MASC) in weak magnetic field (B ~ 0.5 T). Analytical calculations describing the balance of torques acting on the suspended particles suggested that texture would form at such low magnetic fields. The calculations include a novel contribution of Stokes drag arising from the inhomogeneous velocity profile of the fluid during slip casting. Experimental proof-of-concept of the theoretical calculations was successfully demonstrated. Calculations of Lotgering orientation factor (LOF) based on the intensities of the (00l) family of peaks measures by XRD revealed strong c-axis crystalline texture in TiB2 (LOF = 0.88) and ZrB2 (LOF = 0.79) along the direction of the magnetic field. Less texture was achieved in HfB2 (LOF = 0.39). In all cases, the density of the textured materials was less than that of control untextured materials, indicating that texturing hindered the densification. The findings from this work confirm the potential for more cost-effective, simple, and flexible processes to develop crystalline texture in UHTCs and other advanced ceramics and give new insight into the mechanisms of magnetic alignment of UHTCs under low magnetic fields. The microstructural evolution during slip casting and pressureless sintering is investigated. The interplay between magnetic alignment and particle packing was investigated using XRD and SEM. During MASC, the suspended particles rotate into their aligned configuration. Particles that deposit at the bottom of the mold near the plaster of Paris substrate have their alignment slightly disrupted over a ~220 μm-thick region. The aligned suspended particles lock into an aligned configuration as they consolidate, leading to a uniform degree of texturing across the entire sample height of several millimeters upon full consolidation of the particle network. If the magnetic field is removed before the particles fully consolidate, the suspended particles re-randomize their orientation. Grain size measurements done using the ASTM E112 line counting method on SEM images revealed anisotropic microstructures in green and sintered textured ZrB2 materials. Smaller effective grain sizes were observed in the direction of c-axis texture than the directions perpendicular to the texture. Grain aspect ratios of 1.20 and 1.13 were observed in materials where the c-axis texture directions were parallel (PAR) and perpendicular (PERP) to the slip casting direction, respectively. Constraint of the preferred a-axis grain growth direction in the textured materials inhibited their densification compared to the untextured material. The PERP material with the preferred grain growth direction constrained along the casting direction had smaller average grain sizes than the PAR material which contained the preferred grain growth directions in the circular plane normal to the casting direction. Compression testing suggests a trend towards higher strength and stiffness in materials with higher density. Classical catastrophic brittle failure was observed in the untextured materials, but in the textured materials some samples exhibited a multiple failure mode. The PERP material tended to exhibit superior strength and stiffness to the PAR material in the classical brittle failure mode due to the orientation of the stiffer a-axis along the loading direction and smaller average grain size in the plane normal to the loading direction in the PERP condition. In the multiple failure mode, the PAR material tended to reach higher strength values after the initial failure and reach slightly higher strains before ultimate failure due to the orientation of the compliant c-axis along the loading direction and ability of the grains elongated in the plane normal to the loading direction to rearrange themselves after initial failure(s). Regardless of density or texture condition, all ZrB2 samples survived thermal shock resistance (TSR) testing. Samples were heated to 1500°C in air, held for 30 minutes, then quenched in room temperature air. After TSR testing, oxide layers formed on the surface of the materials. The specific mass gain and oxide layer thickness tended to increase with increasing porosity and were dramatically increased when open porosity was dominant as in the CTRL 1900 condition. After TSR testing, the compressive strength and strain at failure were both higher compared to the as-sintered materials. The increases in the average compressive strength were 20%, 76%, and 57% in the CTRL, PAR, and PERP conditions, respectively. The combination of the presence of the oxide layer shifting the onset of macroscale damage to higher strain values, the dissipation of load in the more porous region near the oxide layer, and the constraining effect of the oxide layer acting against the expansion of the material contributed to reinforcement of the samples after TSR testing. The CTRL material outperformed the textured materials on average in terms of strength and stiffness due to the higher density. The results suggest that reinforcement was more effective in the PAR condition than the PERP, which may be caused by the formation of a homogenous oxide layer on the PAR while the PERP formed an anisotropic layer. The work presented in this dissertation lays the foundation for affordable, energy efficient preparation of UHTCs and other ceramic materials. Equipment costs are reduced by 3 orders of magnitude, and the operating costs and energy consumption are greatly reduced. Facilitation of the preparation of textured materials opens the door to renewed investigations into their processing and performance. This work describes in detail for the first time the relationships between processing, microstructure, and properties of a textured UHTC part, providing a model for future research. Finally, the findings in this work can be used to guide process optimization, exploration of complex shapes and microstructures, and design of manufacturing schemes to create specialty textured parts for demanding structural and functional applications. / Doctor of Philosophy / Textured ultra-high temperature ceramics (UHTCs), special materials with melting temperatures above 3000°C and potential for use in thermal protection of Mach 5+ aircraft and spacecraft, were prepared by magnetically assisted slip casting (MASC) in a weak magnetic field for the first time. The magnetic field was supplied by commercially available permanent magnets which was applied to a liquid-like slurry with UHTC particles floating in it to orient the UHTC particles with their c-crystal axis along the magnetic field direction. Calculations which described the balance of rotational forces acting to align or misalign the suspended particles suggested that the UHTC particles would align in the weak magnetic field. This prediction was realized. After the liquid in the slurry was removed during MASC to leave behind an aligned particle network, the samples were densified by heating in the absence of air to 2100°C for one hour. In titanium diboride (TiB2) and zirconium diboride (ZrB2), two of the most relevant UHTC materials, strong texture was achieved; 88% and 79% of the crystals in the material were aligned along the original magnetic field direction. This is the first time that this has been reported in the scientific literature. In hafnium diboride (HfB2), only 39% of the grains were aligned. The textured materials all had lower density than the untextured materials prepared alongside them using conventional slip casting. The relationship between magnetic alignment and particle packing was investigated by observing the microstructure. During MASC, the suspended particles rotate into their aligned configuration. Particles that deposit at the bottom of the mold near the plaster of Paris substrate have their alignment slightly disrupted over a ~220 μm-thick region. The aligned suspended particles lock into an aligned configuration as they consolidate, leading to a uniform degree of texturing over across the entire sample height of several millimeters upon full consolidation of the particle network. If the magnetic field is removed before the particles fully consolidate, the suspended particles re-randomize their orientation. The findings from this work confirm the potential for more cost-effective, simple, and flexible processes to develop crystalline texture in UHTCs and other advanced ceramics and give new insight into the mechanisms of magnetic alignment of UHTCs under low magnetic fields. Because of the magnetic alignment of the particles, it is expected that the microstructure would show some difference along and across the direction that the alignment formed along the applied magnetic field. In order to determine that, the size of the grains (particles joined to each other during densification) in the materials are measured along different directions in the sample chosen for their orientational relationship to the magnetic field and casting directions. Smaller effective grain sizes were observed along the direction of magnetically aligned crystalline texture than the directions perpendicular to the texture. Because of how the crystal axes of the particles are aligned, there are differences in how the particles join each other during densification, and that results in an anisotropic microstructure where different grain sizes as a function of the magnetic field direction and the texture direction. Compression testing conducted by squeezing the samples at a fixed rate suggests a trend that indicates the samples are stronger and stiffer when the density is higher, as expected. Untextured samples abruptly failed after reaching their maximum strength value in a manner typical of brittle ceramics. Some textured samples failed in this way, but some failed at low strength values then climbed back up in strength repeatedly until they eventually gave out completely, in a crumbly mode. In the classical brittle failure mode, the PERP material with c-axis texture aligned along the sample diameter, perpendicular to the loading direction, tended to exhibit superior strength and stiffness to the PAR material with c-axis texture oriented along the height and loading directions of the sample because the stiffer crystal axis was oriented along the loading direction and the average grain size seen by the load head was smaller. In the crumbly mode, the PAR material tended to reach higher strength values after initial failure and ultimately fail later in a crumblier mode because the more compliant crystal axis was oriented along the loading direction and the grains elongated in the plane perpendicular to the loading direction could rearrange themselves better after initial failure(s) to bear more load. Regardless of density or texture condition, all ZrB2 samples survived thermal shock resistance (TSR) testing, meaning that the samples remained fully intact after experiencing a big difference in temperature in very short time. Samples were heated in a furnace to 1500°C in air, held for 30 minutes, removed from the furnace, and cooled in air. After TSR testing, the samples developed an oxide layer on the outside, in a similar manner to rust forming on a piece of metal. How much it oxidized per unit area and how thick that oxide layer was increased with increasing porosity. These quantities increased dramatically when the pores connected the interior of the sample to the outside, as in the CTRL 1900 condition. After TSR testing, the samples were stronger by 20%, 76%, and 57% in the CTRL, PAR, and PERP conditions, respectively, indicating that the oxide layer was responsible for an enhancement in strength. The results suggest that increase of strength of the oxide layer was more effective in the PAR condition than the PERP, which is believed to be caused by the formation of a homogenous oxide layer on the PAR while the PERP formed an anisotropic layer. The work presented in this dissertation reduces the start-up equipment costs associated with magnetic alignment processes by 1000 times and lays the foundation for affordable, energy efficient preparation of UHTCs and other ceramic materials. The simplicity of this technique makes it easier for future researchers to study textured materials. This work describes in detail for the first time the relationships between processing, microstructure, and properties of a textured UHTC part, providing a model for future research. Finally, the findings in this work can be used to guide process optimization, exploration of complex shapes and microstructures, and design of manufacturing schemes to create specialty textured parts for demanding applications.

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.

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.

The use of electrospun polystyrene (PS) fibers to create continuous long range ordered multi-scale porous structures in titanium diboride (TiB2) is investigated in this work. The introduction of electrospun PS fibers as a sacrificial filler into a colloidal suspension of TiB2 allows for easy control over the pore size, porosity, and long range ordering of the porous structures of the sintered ceramic. Green bodies were formed by vacuum infiltrating an electrospun-fiber-filled mold with the colloidal TiB2 suspension. The size, volume, distribution, and dispersion of the pores were optimized by carefully selecting the sacrificial polymer, the fiber diameter, the solvent, and the solid content of TiB2. The green bodies were partially sintered at 2000 C in argon to form a multiscale porous structure via the removal of the PS fibers. Aligned continuous cylindrical pores were derived from the PS fibers in a range of ~5 - 20 μm and random porosity was revealed between the ceramic particles with the size of ~0.3 - 1 μm. TiB2 near-net-shaped parts with the multi-scale porosities (~50 to 70%) were successfully cast and sintered. The multi-scale porous structure produced from electrospun fibers was characterized both thermally and mechanically, at room temperature. The conductivity ranged from 12-31 W m^(-1) K^(-1) at room temperature and the compressive strength ranged from 2-30 MPa at room temperature. Analytical thermal and mechanical models were employed to understand and verify he processing-structure-properties relationship. Finally, a method was devised for estimating the effective thermal conductivity of candidate materials for UHTC applications at relevant temperatures using a finite difference model and a controlled sample environment. This low-cost processing technique facilitates the production of thermally and mechanically anisotropic structures into near-net shape parts, for extreme environment applications, such as ultra high temperature insulation and active cooling components. / MS / Society is on the cusp of hypersonic flight which will revolutionize defense, space and transport technologies. Hypersonic flight is associated with conditions like that of atmospheric re-entry, high heat and force or specific locations of a space craft. The realization of hypersonic flight relies on innovative materials to survive the harsh conditions for repeated flight. We have created a new material with tiny holes that can help prevent heat flow from the harsh atmosphere from damaging the hypersonic craft. Thesis tiny holes are made from placing a polymer fiber in an advanced ceramic (which withstand high temperatures) and removing the fiber to leave holes. The tiny hole’s effect on strength and heat flow have been studied, to understand how the tiny holes can be made better. It is difficult to test materials in the harsh atmosphere associated with hypersonic flight, so a program has been written to estimate thermal properties of candidate materials for hypersonic flight.

Pour améliorer la durée de vie des matériaux à haute température et sous atmosphère oxydante, une solution est l’utilisation d’une protection de surface constituée de matériaux ultra réfractaires non oxydes. Un des objectifs principaux de cette thèse est la sélection et la validation expérimentale de nouvelles compositions chimiques de revêtements utilisés en condition oxydante et corrosive à ultra haute température. Les recherches s’appuient sur une démarche expérimentale physico-chimique se basant sur une approche thermodynamique et thermochimique menée au préalable pour choisir les composés. Les revêtements doivent être stables chimiquement, compatibles thermomécaniquement avec le substrat et adhérent de la température ambiante à celle d’utilisation. De plus, Ils doivent jouer le rôle de barrière environnementale et/ou de barrière thermique.Des tests d’oxydation sont réalisés au four solaire sur les systèmes de matériaux non oxydes massifs élaborés par frittage flash. En parallèle, des composites modèles constitués d’une fibre de carbone revêtue par PVD d’un revêtement métallique ultra réfractaire ont été élaborés puis chauffés par effet Joule afin de réaliser des tests d’oxydation. La compréhension des mécanismes entrant en jeu lors de l’oxydation de ces « nouveaux » revêtements est aussi un des challenges de ce manuscrit. Par ailleurs, elle aide à la classification de ces matériaux selon leur résistance à l’oxydation. / In order to improve material’s lifetime used at a temperature above 2500°C and under oxidizing atmosphere, a solution is to use a surfacing protection constituted of non oxide refractory materials. One of the main objectives of this thesis is to select and experimentally validate new chemical coating compositions which will be used under corrosive and oxidizing atmosphere at ultra high temperature (more than 2000°C). A preliminary thermodynamic and thermo-chemical study aims to select compounds. These compounds are then analyzed with physic-chemical tests. Coatings have to be chemically stable, thermo-mechanically compatible with the substrate and have to stick to the substrate from ambient temperature to more than 2000°C. Moreover, coatings have to act as an environmental barrier and/or as a thermal barrier.Two kinds of oxidation tests are made. On one hand, non oxide massive material’s systems are fabricated by spark plasma sintering in order to be tested at the solar furnace. On the other hand, composite models are fabricated by PVD. A carbon fiber is covered with ultra refractory metallic coating by PVD. Then, these composite models are heated by Joule effect in order to realize oxidation tests. Understanding mechanisms at work during the oxidation of these new coatings is another main objective of this thesis. This understanding will be also useful to classify these materials regarding their resistance to oxidation.

Les Céramiques Ultra-Haute Température (UHTC) sont des matériaux prometteurs dans le cadre d'applications en conditions extrêmes comme les parties proéminentes de véhicules à rentrée atmosphérique ou les chambres de combustion de moteurs aéronautiques. La compréhension des mécanismes d'oxydation à haute température présente donc un intérêt majeur, car les réactions en milieu oxydant limitent fortement leur durée de vie. Les matériaux ZrB2-SiC, HfB2-SiC et HfB2-SiC-Y2O3 ont été soumis pendant des durées et températures variables (jusqu'à 2400°C) à des environnements contrôlés contenant de la vapeur d'eau. Les microstructures formées ont été décrites, et les mécanismes et cinétiques d'oxydation régissant leur comportement ont été analysés. L'importance de la stabilité et de la nature de la phase vitreuse formée durant l'oxydation a été soulignée. En complément de ces analyses microstructurales, une campagne d'essais utilisant la Fluorescence Induite par Laser (LIF) a permis, via la détection in situ de la molécule BO2, de comprendre plus finement la dynamique de la phase vitreuse lors de l'oxydation. Enfin, une modélisation de la croissance de couches oxydées sur un matériau monophasé a été effectuée. / Ultra-High Temperature Ceramics (UHTC) are promising materials for applications in extreme environments such as prominent parts of atmospheric re-entry vehicles or the combustion chambers of aeronautic engines. The understanding of oxidation mechanisms at high temperature is of great interest, because reactions in oxidizing atmosphere strongly shorten their lifetime. ZrB2-SiC, HfB2-SiC and HfB2-SiC-Y2O3 materials have been subjected to controlled water vapor-containing environments for different durations and temperatures (up to 2400°C). The microstructures developed by the oxidized materials have been described, and oxidation mechanisms and kinetics governing their behavior have been analyzed. The importance of the stability and nature of the vitreous phase formed during the oxidation has been emphasized. In order to complement these microstructural analyses, tests using Light-Induced Fluorescence (LIF) have been performed, allowing us to finely understand the dynamics of the vitreous phase during oxidation thanks to the in situ detection of the BO2 molecule. Finally, a modelling of the growth of oxidized layers on a single-phased material has been performed.

Ultra-high temperature (i.e., greater than 2500°C) engineering applications present continued materials challenges. Refractory metal/ceramic composites have great potential to satisfy the demands of extreme environments (e.g., the environments found in solid rocket motors upon ignition), though general scalable processing techniques to fabricate complex shaped parts are lacking. The work embodied in this dissertation advances scientific knowledge in the development of processing techniques to form complex, near net-shape, near net-dimension, near fully-dense refractory metal/ceramic composites with controlled phase contents and microstructure. Three research thrusts are detailed in this document. First, the utilization of rapid prototyping techniques, such as computer numerical controlled machining and three dimensional printing, for the fabrication of porous tungsten carbide preforms and their application with the Displacive Compensation of Porosity process is demonstrated. Second, carbon substrates and preforms have been reactively converted to porous tungsten/tungsten carbide replicas via a novel gas-solid displacement reaction. Lastly, non-oxide ceramic solid solutions have been internally reduced to create intragranular metal/ceramic micro/nanocomposites. All three techniques combined have the potential to produce nanostructured refractory metal/ceramic composite materials with tailorable microstructure for ultra-high temperature applications.