Sharp leading edges and nose caps on hypersonic vehicles, re-entry vehicles and reusable launch vehicles are items of current research interest for enhanced aerodynamic performance and maneuverability. The unique combination of mechanical properties, physical properties, thermal / electrical conductivities and thermal shock resistance of ZrB2 make it a promising candidate material for such applications. In the recent past, a lot of work has been carried out on ZrB2-based materials towards processing as well as characterization of their mechanical, oxidation and thermal behaviour. ZrB2 based materials have been successfully processed by conventional hot pressing, pressureless sintering, reactive hot pressing and spark plasma sintering. Densification of ZrB2 gets activated when the oxide impurities (B2O3 and ZrO2) were removed from particle surfaces, which minimized coarsening. B4C is widely used as a sintering additive for ZrB2 because it reduces ZrO2 at low temperature. It is found that full densification in ZrB2 based materials by hot pressing is achieved either at 2000 C and higher temperatures with moderate pressure of 20-30 MPa or at reduced temperature (1790-1840 C) with much higher pressure (800-1500 MPa). But no study is available that identifies the dominant hot pressing mechanism at different temperatures and pressures. On the other hand, reinforcement of SiC in ZrB2 is known to increase flexural strength, fracture toughness and oxidation resistance. It has been shown that oxidation resistance of ZrB2-SiC composites is superior to that of monolithic ZrB2 and SiC. For high temperature applications in air, the residual strength (room temperature strength after exposure in air at high temperatures) of non oxide ceramics after oxidation is important. A few reports are available on residual strength of ZrB2 –SiC composite after thermal exposure at high temperatures. In contrast to the literature on composites, there are no reports available on the residual strength of monolithic ZrB2 after exposure to high temperatures. Also, previous studies on residual strength of ZrB2-SiC composites have been limited to a single temperature of exposure. But there is a need to measure the residual strength after exposure to a range of temperatures since the oxide layer structure changes with temperature. The room temperature thermal conductivity data for ZrB2 and ZrB2-SiC composite shows a wide scatter in value as well as a dependence on microstructural parameters, especially porosity and grain size. Also, there is insufficient data available for the high temperature thermal conductivity of ZrB2-SiC. Therefore, it is difficult to evaluate the effect of SiC content on thermal conductivity of ZrB2-SiC composites at high temperatures. The present thesis seeks to address some of these gaps to better understand the suitability of ZrB2 and ZrB2-SiC composites for ultra-high temperature applications. In the present work, hot pressing is used for densification of ZrB2 and ZrB2-SiC composites. Different amounts of B4C (0, 0.5, 1, 3 & 5 wt %) were used as sintering additives in ZrB2 and hot pressed at 2000 C with 25 MPa applied pressure. The hot pressed samples are characterized for their microstructural, mechanical properties and oxidation behaviour. By addition of B4C, density as well as micro-hardness increased. For lower B4C content (0.5 & 1 wt %), hot pressed ZrB2 has shown considerable improvement in flexural strength after exposure in air at 1000 C for 5 hours, while higher B4C content (3 & 5 wt %) leads to marginal or no improvement. Due to the better mechanical and oxidation behavior of composites containing SiC, the densification behavior during hot pressing was studied. The densification behaviors as well as the microstructures for hot pressing of ZrB2-20 % SiC composite were found to change in a very 0 narrow temperature range. During hot pressing at 1700 C, the densification was found to be mechanically driven particle fragmentation and rearrangement. On the other hand, thermally activated mass transport mechanisms started dominating after initial particle fragmentation and rearrangement after hot pressing at 1850 C and 2000 C. At 2000 C, the rate of grain boundary diffusion was enhanced which resulted into annihilation of dislocation. The effect of SiC contents (10, 20 & 30 vol %) on mechanical and oxidation behavior of ZrB2-SiC composite were also studied. The average micro-hardness and fracture toughness of ZrB2-SiC composites increased with SiC content. But the flexural strength of ZrB2-20 vol % SiC composites was found to be the highest. Oxidation and residual strength of hot pressed ZrB2 -SiC composites were evaluated as a function of SiC contents after exposure over a wide temperature range (1000-1700 C). Multilayer oxide scale structures were found after oxidation. The composition and thickness of these multilayered oxide scale structures were found to depend on exposure temperature and SiC content. After exposure to 1000 C for 5 hours, the residual strength of ZrB2 -SiC composites improved by nearly 60 % compared to the as-hot pressed composites with 20 & 30 vol % SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 C for 5 hours. A drastic degradation in residual strength was observed in composites with 20 & 30 vol % SiC whereas strength was retained for ZrB2-10 % SiC composite after exposure to 1700 C for 5 hours in ZrB2 –SiC. Therefore, residual strength of ZrB2-10 % SiC composite was measured at different exposure times (up to 10 hours) at 1500 0C. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of exposure. Since the ZrB2-20 vol % SiC composite showed the maximum strength, the dependence of strength on various microstructural as well processing parameters was also studied. It was found that porosity, grain size as well as surface residual stress due to grinding influenced the strength of ZrB2-20 vol % SiC composites. Finally, thermal diffusivity and conductivity of hot pressed ZrB2 with different amounts of B4C and ZrB2-SiC composites were investigated experimentally over a wide temperature range (25 – 1500 C). Both thermal diffusivity as well as thermal conductivity was found to decrease with increase in temperature for all hot pressed ZrB2 and ZrB2-SiC composites. At around 200 C, thermal conductivity of ZrB2-SiC composites was found to be composition independent. Thermal conductivity of ZrB2-SiC composites was also correlated with theoretical predictions of the Maxwell-Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB2 and ZrB2-SiC composites at room temperature were determined by Wiedemann-Franz analysis using measured room temperature electrical conductivity of these materials. It was found that the electronic thermal conductivity dominated for all monolithic ZrB2 whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB2-SiC composites. The heat conduction mechanism at high temperature was also studied by measuring the high temperature electrical conductivity of ZrB2 and ZrB2-SiC composites. The effect of porosity on thermal diffusivity and conductivity was also studied for ZrB2-20 vol % SiC composites.

Ce travail se place dans le cadre des matériaux thermo-structuraux utilisés dans les applications aéronautiques et aérospatiales. Les composites HfB2-SiC et ZrB2-SiC se sont avérés très prometteurs pour ces domaines, dans la mesure où ceux-ci présentent de bonnes tenues à l’oxydation à très haute température. L’enjeu principal de ce travail réside dans l’amélioration de leurs performances (durée de vie, résistance aux attaques chimiques et aux chocs thermiques) ainsi que dans la compréhension des mécanismes d’oxydation impliqués dans deux environnements distincts (sous oxygène moléculaire, à pression atmosphérique, de 1450 K à 2000 K et sous oxygène dissocié, à faible pression partielle d’oxygène, de 1800 K à 2200 K). Pour cela, une première étape a consisté à élaborer, par Spark Plasma Sintering (SPS), plusieurs nuances de matériaux homogènes en termes de microstructure et de densité relative. Leur comportement dans ces deux environnements a ensuite été étudié grâce à l’utilisation de dispositifs d’oxydation adaptés (four solaire et analyseur thermogravimétrique) et de techniques de caractérisation complémentaires (microscopie électronique à balayage, diffraction des rayons X, spectroscopies Raman et de photo-électrons). / This work is part of thermo-structural materials used in the aeronautic and aerospace fields. HfB2-SiC and ZrB2-SiC composites turned out to be very promising for these areas insofar as they present good oxidation resistance at very high temperature. The main issue of this work consists in improving their performances (lifetime, chemical attacks and thermal shocks resistance) and in the understanding of the oxidation mechanisms involved in two different environments (under molecular oxygen, at atmospheric pressure, from 1450 K to 2000 K and under dissociated oxygen, at low oxygen partial pressure, from 1800 K to 2200 K). To this end, a first step consisted in synthesizing, by Spark Plasma Sintering (SPS), several compositions of homogeneous materials in terms of microstructure and relative density. Then, their behavior in both environments has been studied thanks to the use of adapted oxidation facilities (solar furnace and thermogravimetric analyzer) and complementary characterization techniques (scanning electron microscopy, X-ray diffraction, Raman and photo-electron spectroscopies).

Zirconium- and titanium- based compounds (borides, carbides and nitrides) are of importance because of their attractive properties including: high melting temperature, high-temperature strength, high hardness, high elastic modulus and good wear-erosion-corrosion resistance. The ultra high temperature ceramics (UHTCs) - zirconium diboride (ZrB2) and zirconium carbide (ZrC) in combination with SiC are potential candidates for ultra-high temperature applications such as nose cones for re-entry vehicles and thermal protection systems, where temperature exceeds 2000°C. Titanium nitride (TiN) and titanium diboride (TiB2) composites have been considered for cutting tools, wear resistant parts etc. There are problems in the processing of these materials, as very high temperatures are required to produce dense composites. This problem can be overcome by the development of composites through reactive hot processing (RHP). In RHP, the composites are simultaneously synthesized and densified by application of pressure and temperatures that are relatively low compared to the melting points of individual components. There have been earlier studies on the fabrication of dense ZrB2-ZrC, ZrB2-SiC and TiN-TiB2 composites by the following methods: Pressureless sintering of preformed powders at high temperatures (1800-2300°C) with MoSi2, Ni, Cr, Fe additions Hot pressing of preformed powders at high temperatures (1700-2000°C) with additives like Ni, Si3N4, TiSi2, TaSi2, TaC Melt infiltration of Zr/Ti into B4C preform at 1800-1900°C to produce ZrB2-ZrC-Zr and TiB2-TiC composites RHP of Zr-B4C, Zr-Si-B4C and Ti-BN powder mixtures to produce ZrB2-ZrC, ZrB2-SiC and TiN-TiB2 powder mixtures at 1650-1900°C Spark plasma sintering of powder mixtures at 1800-2100°C There has been a lack of attention paid to the conditions under which ceramic composites can be produced by simple hot pressing (~50 MPa) with minimum amount of additives, which will not affect the mechanical properties of the composites. There has been no systematic study of microstructural evolution to be able to highlight the change in relative density (RD) with temperature during RHP by formation of sub-stoichiometric compounds, and liquid phase when a small amount of additive is used. The present study has been undertaken to establish the experimental conditions and densification mechanisms during RHP of Zr-B4C, Zr-B4C-Si and Ti-BN powder mixtures to yield (a) ZrB2-ZrC, (b) ZrB2-SiC, (c) ZrB2-ZrC-SiC and (d) TiN-TiB2 composites. The following reactions were used to produce the composites: (1) 3 Zr + B4C → 2 ZrB2 + ZrC (2) 3.5 Zr + B4C → 2 ZrB2 + 1.52rCx- 0.67 (3) (1+y) Zr + C → (1+y) ZrCx- 1/ (1+y) (y=0 to 1) (4) 2 Zr + B4C + Si → 2 ZrB2 + SiC (5) 2.5 Zr + B4C + 0.65 Si → 2 ZrB2 + 0.5 ZrCx + 0.65 SiC (6) 3.5 Zr + B4C + SiC → 2 ZrB2 + 1.5 ZrCx + SiC (5 to 15 vol%) (7) (3+y) Ti + 2 BN → (2+y) TiN1/(1+y) + TiB2 (y=0 to 0.5) (a) ZrB2-ZrC Composites: The effect of different particle sizes of B4C (60-240 μm, <74 μm and 10-20 μm) with Zr on the reaction and densification of composites has been studied. The role of Ni addition on reaction and densification of the composites has been attempted. The effect of excess Zr addition on the reaction and densification has also been studied. The RHP experiments were conducted under vacuum in the temperature range 1000-1600°C for 30 min without and with 1 wt% Ni at 40 MPa pressure. The RHP composites have been characterized by density measurements, x-ray diffraction for phase analysis and lattice parameter measurements, microstructural observation using optical and scanning electron microscopy. Selected samples have been analyzed by transmission electron microscopy. The hardness of the composites has also been measured. The results of the study on the effect of different particle sizes B4C and Ni addition on reaction and densification in the stoichiometric reaction mixture as follows. With the coarse B4C (60-240 μm and <74 μm) particles the temperature required are higher for completion of the reaction (1600°C and above). The microstructural observation showed that the material is densified even in the presence of unreacted B4C particles. The composite made with 10-20 μm B4C and 1 wt% Ni showed completion of the reaction at 1200°C, whereas composite made without Ni showed unreacted B4C (∼3 vol%) and the final densities of both the composites are similar (5.44 g/cm3). Increase in the temperature to 1400°C resulted in the completion of the reaction (without Ni) accompanied with a relative density (RD) of 95%. The composites produced with and without Ni at 1600°C had similar densities of 6.13 g/cm3 and 6.11 g/cm3 respectively (~97.3% RD). The Zr-Ni phase diagram suggests that the addition of Ni helps in formation of Zr-Ni liquid at ~960°C and leads to an increase in the reaction rate up to 1200°C. Once the reaction is completed, not enough Zr is available to maintain the liquid phase and further densification occurs through solid state sintering. The grain sizes of ZrB2 and ZrC phases after 1200°C are 0.4 μm and 0.3 μm, which are much lower than those reported in literature (2-10 μm), and may be the reason for reducing the densification temperature to 1600°C for stoichiometric ZrB2-ZrC composites. The effect of excess Zr (0.5 mol), over and above the stoichiometric Zr-B4C powder mixture, on reaction and densification of the composites is as follows. The formation of ZrB2 and ZrC phases with unreacted starting Zr and B4C is observed at 1000°C and with increase in temperature to 1200°C the reaction is completed. Since microstructural characterization reveals no indication of free Zr, it is concluded that the excess Zr is incorporated by the formation of non-stoichiometric ZrC (ZrCx-0.67). This observation is supported by lattice parameter measurements of ZrC in the stoichiometric and non-stoichiometric composites which are lower than those reported in the literature. X-ray microanalysis of ZrC grains in the stoichiometric and non-stoichiometric composites using transmission electron microscopy confirmed the presence of carbon deficiency. The composite produced at 1200°C showed the density of 6.1 g/cm3 (~97% RD), whereas addition of Ni produced 6.2 g/cm3 (~99% RD). The reduction in densification temperature for the non-stoichiometric composites is due to the presence of ZrCx even in the absence of Ni. The mechanism of densification of the composites at 1200°C is attributed to the lowering of critical resolved shear stress with increasing non-stoichimetry in the ZrC, which leads to plastic deformation during RHP. An additional mechanism may be enhanced diffusion through the structural point defects created in ZrC. The hardness of the composites are 20-22 GPa, which is higher than those of reported in literature due to the presence of a dense and fine grain microstructure in the present work. In order to verify the role of non-stoichiometric ZrC the study was extended to produce monolithic ZrC using various C/Zr ratios (0.5-1). Here again, stoichiometric ZrC does not densify even at 1600°C, whereas non-stoichiometric ZrC can be densified at 1200°C. (b) ZrB2-SiC Composites: Since ZrB2 and ZrC do not have good oxidation resistance unless they are reinforced with SiC, the present study has been extended to produce ZrB2-SiC (25 vol%) composites using Zr-Si-B4C powder mixtures. The samples produced at 1000°C showed the formation of ZrB2, ZrC and Zr-Si compounds with unreacted Zr and B4C and as the temperature is increased to 1200°C only ZrB2 and SiC remained. A fine grain (~0.5 μm) microstructure has been observed at 1200°C. During RHP, it was observed that the formations of ZrC, Si-rich phases and fine grain size at low temperatures was responsible for attaining the high relative density at a temperature of ~1600°C. The relative densities of the composites produced with 1 wt% Ni at 40 MPa, 1600°C for 30 min is 97% RD, where as composites without Ni showed a small amount of partially reacted B4C; extending the holding time to 60 min eliminated the B4C and produced 98% RD. The hardness of the composites is 18-20 GPa. (c) ZrB2-ZrC-SiC Composites: Since ZrC plays a crucial role in densification of ZrB2-ZrC and ZrB2-SiC composites, the study has been extended to reduce the processing temperature for ZrB2-ZrCx-SiC composites by two methods. In one of the methods, Si is added to the non-stoichiometric 2.5Zr-B4C powder mixture which is resulted in ZrB2-ZrCx-SiC (15 vol%) composites with ~98% RD at 1600°C. In another method, SiC particulates are added to the non-stoichiometric 3.5Zr-B4C powder mixture to yield ZrB2-ZrCx-SiCp (5-15 vol%) composites at 1400°C. The density of the 5 vol% SiC composite is 99.9%, whereas addition of 15 vol% SiC reduced the density to 95.5% RD. The mechanisms of densification of the composites are similar to those observed in ZrB2-ZrC composites. The hardness of the composites is 18-20GPa (d) TiN-TiB2 Composites: ZrB2, ZrC, TiB2, and TiN are members of the same class of transition metal borides, carbides and nitrides; however, their densification mechanisms appear to be different. In earlier work, the RHP of stoichiometric 3Ti-2BN powder mixtures yielded dense composite at 1400-1600°C with 1 wt% Ni addition, whereas composites without Ni required at least 1850°C. The major contributor to better densification at 1600°C (with Ni) appeared to be the formation of local Ni-Ti liquid phase at ~942°C (Ti-Ni phase diagram). The present work explores the additional role of non-stoichiometry in this system. It is shown that Ti excess can lead to a further lowering of the RHP temperature, but with a different mechanism compared to the Zr-B4C system. Excess Ti allows the transient alloy phase to remain above the liquidus for a longer time, thereby permitting the attainment of a higher relative density. However, eventually, the excess Ti is converted into a non-stoichiometric nitride. Thus, the volume fraction of a potentially low melting phase is not increased in the final composite by this addition. The contrast between these two systems suggests the existence of two classes of refractory materials for which densification may be greatly accelerated in the presence of non-stoichiometry, either through the ability to absorb a liquid-phase producing metal into a refractory and hard ceramic structure or greater deformability. Conclusions: The study on RHP of ZrB2-ZrC, ZrB2-SiC, ZrB2-ZrC-SiC and TiN-TiB2 composites led to the following conclusions: • It has been possible to densify the ZrB2-ZrC composites to ~97 % RD by RHP of stoichiometric Zr-B4C powder mixture with or without Ni addition. The role of B4C particle size is important to complete both reaction as well as densification. • Excess Zr (0.5 mol) to stoichiometric 3Zr-B4C powder mixtures produces dense ZrB2-ZrCx composite with 99% RD at 1200°C. The densification mechanisms in these non-stoichiometric composites are enhanced diffusion due to fine microstructural scale / stoichiometric vacancies and plastic deformation. • In the case of ZrB2-SiC composites, the formation of a fine microstructure, and intermediate ZrC and Zr-Si compounds at the early stages plays a major role in densification. • Starting with non-stoichiometric Zr-B4C powder mixture, the dense ZrB2-ZrCx-SiC composites can be produced with SiC particulates addition at 1400°C. • Non-stoichiometry in TiN and ZrC is route to the increased densification of composites through enhanced liquid phase sintering in TiN based composites that contain Ni and through plasticity of a carbon-deficient carbide in ZrC based composites.