SiHf(B)CN-based ultra-high temperature ceramic nanocomposites: Single-source precursor synthesis and behavior in hostile environments

Hf-containing ultra-high-temperature ceramics (UHTCs) are being pursued for Thermal Protection Systems (TPSs) for high-temperature applications (i.e., future hypersonic vehicles) in harsh environments. Most of these ceramic composites have been prepared using traditional powder techniques; however, the grain sizes of the resulting composites are limited to the micrometer range. Furthermore, nano-sized Hf-containing materials have proven to exhibit tremendously improved structural/functional properties, even at elevated temperatures, compared with microcomposite ceramics. Single-source precursors (SSPs) have yielded promising results in the processing of ceramic nanocomposites; moreover, these composites exhibit unique properties, e.g., high-temperature stability and high-temperature oxidation and corrosion. The objective of this work was to synthesize new Hf-containing ultra-high-temperature ceramic nanocomposites (UHTC-NCs) using SSP-based methods and to investigate their behavior in harsh environments. In the research presented in this PhD thesis, focus was first placed on the synthesis of novel Hf-containing SiHfCN and SiHfBCN amorphous UHTC-NCs derived from polysilazane. Amorphous SiHfCN and SiHfBCN ceramics were prepared from commercial polysilazane (HTT1800, AZ-EM), which was modified through reactions with Hf(NEt2)4 and BH3·SMe2 and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase compositions using spectroscopic techniques (FTIR, Raman, and MAS NMR spectroscopy) and via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments on SiHfCN and SiHfBCN samples in inert gas atmospheres (Ar and N2) at temperatures ranging from 1300 to 1700 °C revealed the conversion of the amorphous materials into nano-structured UHTC-NCs, whose high-temperature decomposition and crystallization were also investigated. It was found that β-SiC/HfCxN1-x nanocomposites were obtained from SiHfCN upon annealing at 1500 °C. Depending on the annealing atmosphere, HfCxN1-x/HfB2/SiC (annealing in argon) and HfNxC1-x/Si3N4/SiBCN/C (annealing in nitrogen) nanocomposites were obtained from SiHfBCN annealed at 1700 °C. The results demonstrate that the conversion of single-phase SiHf(B)CN into UHTC-NCs is thermodynamically controlled and thus offer insight toward the development of nano-structured ultra-high-temperature stable materials with tunable compositions. The second focus of the present study was the development of dense Hf-containing ceramic monoliths via pressureless sintering (PLS) or spark plasma sintering (SPS) and the development of ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) methods. Dense amorphous ceramic monoliths were prepared upon annealing pyrolytic ceramics in nitrogen at 1300 °C. Dense SiHfCN- and SiHfBCN-based UHTC-NCs were successfully prepared via SPS at 1850-1950 °C using high heating rates (~450 °C/min.) and high pressures (≥ 100 MPa). The obtained UHTC-NCs were investigated via spectroscopic analyses (XRD and Raman spectroscopy) and electron microscopy (SEM and TEM) with regard to their phase evolution and microstructure. Despite the very high sintering temperatures, the microstructures of the prepared dense UHTC-NCs remained rather fine, with grain sizes varying from 165 nm down to a few tens of nm. The hardness and elastic modulus of the dense SiHfCN were found to be 26.8 and 367 GPa, respectively, whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa (measured by nanoindentation). Additionally, Cf/SiCN and Cf/SiHfBCN CMCs were fabricated via a simple and low-cost PIP route. Cf/SiC-SiCN and Cf/SiC-SiHfBCN materials with pyrolytic carbon coatings were synthesized using hybrid techniques (CVI and PIP). The bending strength of the prepared CMCs resulted in the observation of brittle fracture surfaces only in the Cf/SiHfBCN material, indicating strong interfacial bonding between the fibers and the matrix; the much higher values of bending strength observed for Cf/SiC-SiCN and Cf/SiC-SiHfBCN resulted from the fact that weak interfaces (pyrolytic carbon) lead to transfer loading. This finding of the present work suggests that a single-source precursor route is suitable for the preparation of a variety of (ultra)-high-temperature ceramics, such as amorphous ceramics, UHTC-NC monoliths, and CMCs. Moreover, we explored the behavior of the prepared materials in harsh environments, e.g., their high-temperature stability with respect to decomposition and crystallization and their oxidation, corrosion and ablation behavior. High-temperature annealing experiments revealed that the SiHfCN and SiHfBCN materials exhibited improved high-temperature stability with respect to decomposition compared with non-modified SiCN. The oxidation behavior of the SiCN, SiHfCN and SiHfBCN ceramic powders was studied via thermogravimetric analysis (TGA) in air at 1200-1400 °C, revealing that the modified SiHfCN and SiHfBCN ceramics exhibited poorer oxidation resistance than that of SiCN. However, parabolic oxidation kinetics of SiHfCN and SiHfBCN were observed, wherein the parabolic rate (Kp) that was obtained from the equation K_p=〖 (∆m/(S_BET×m))〗^2×t^(-1) indicated that the amorphous SiHfBCN ceramic powder exhibited enhanced oxidation resistance compared with that of the SiHfCN. Furthermore, the oxidation behavior of SiHfBCN ceramic monoliths was investigated in a tube furnace (stagnant air, up to 100-200 h). The microstructure and phase composition of the monoliths’ oxide scale was investigated via XRD and microscopy (SEM, BSE and EPMA). The results revealed that the oxidation of the SiHfBCN ceramic monoliths followed typical parabolic kinetics, indicating that the oxidation diffusion was controlled by a passive oxide layer. However, the microstructure and composition of the oxide scale were strongly dependent on temperature. A continuous oxide layer, consisting of cristobalite and hafnia (m- and t- HfO2), was observed at 1200 °C; however, at 1400 °C, it became a discontinuous oxide layer and its composition changed to cristobalite, HfO2 and HfSiO4. Thus, the wide range of Ea values (174 and 140 KJ mol-1, depending on the Hf content) obtained from the apparent or corrected oxidation kinetics indicate the complex nature of their oxidation process, which might be the result of a wide variety of oxygen-controlling mechanisms in both the inward oxygen transport into the oxide scale (borosilicate or silica, hafnia, or hafnium silicate) and the outward transport of gas produced by oxidation reactions. Additionally, an investigation of the oxidation of the prepared dense UHTC-NCs at high temperature revealed that both samples exhibited parabolic behavior. Interestingly, the parabolic oxidation rates of the SiHfCN were comparable to those of other UHTCs (e.g., HfC-20 vol% SiC), whereas the parabolic oxidation rates of the SiHfBCN were 3 to 4 orders of magnitude lower. The results obtained in this study indicate that amorphous Hf-containing Si(Hf)BCN ceramics nanocomposites and nanoscale Hf-containing UHTC-NCs are promising candidates for high-temperature applications in harsh environments. The behavior of Cf/SiCN and Cf/SiHfBCN under subcritical hydrothermal conditions was also investigated at temperatures of 150-250 °C for exposure times of 48, 96 and 240 h. The effect of the ratio between the surface area of the sample and the volume of water used (S/V ratio) on the corrosion behavior of the prepared CMCs was analyzed. For S/V ratios greater than 0.18, the exposure of the CMCs to hydrothermal conditions led to a gain in mass, whereas at lower S/V ratios, a mass loss of the samples was recorded. Because the behavior of the studied samples was representative and reliable at small S/V ratios, both investigated CMC samples were concluded to exhibit active corrosion behavior in a subcritical hydrothermal corrosive environment. Based on the corrosion experiments performed at an S/V ratio of 0.075, the data for the mass loss as a function of the corrosion time and temperature were used to rationalize the corrosion kinetics of the Cf/SiCN and Cf/SiHfBCN samples. Both materials were shown to exhibit excellent stability under subcritical hydrothermal conditions. The corrosion rate of Cf/SiHfBCN was found to be lower than that of Cf/SiCN; furthermore, an SEM investigation indicated that spallation occurred in the Cf/SiCN samples, whereas the ceramic matrix remained attached to the individual carbon fibers in Cf/SiHfBCN. The results of the present study indicate that the incorporation of Hf and B into the SiCN matrix leads to significant improvement in its hydrothermal corrosion performance. Finally, the ablation mechanism of the Cf/SiHfBCN ceramic composites after treatment in a laser ablation environment was investigated. The microstructure and ablation behavior of this composite were studied using SEM combined with EDS. The formation of porous HfO2, molten HfO2 and SixOyHfz yielded fibers with good protection from oxidation and the laser beam. Three regions with different ablation behaviors are proposed based on the temperature distribution. The ablation center exhibited bubble-like structures, corresponding to the melting of HfO2 and SiHfxOy layers that covered the ends of the carbon fibers, and moreover, eroded carbon fibers that retained their original shape were also observed. In the transition region, carbon sheets and oxidation-product particles (HfCxOy and SiO2) peeled off from the eroded fibers and the matrix because of the high vapor pressure. Additionally, the growth of SiC grains and glass with bubble structure, corresponding to SiO2 with inclusions of B2O3 and SiO gas, was observed.