Synthesis and properties of carbon fiber reinforced UHTC composites

In Korea, a research project named “Development of ultra-abrasion resistant Hf- and Ta- based ceramic composites” was performed at 2013-2016. 8 research groups including national institutes, universities and a company in Korea and Germany joined for the project. In order to fabricate ablation resistant ultra-high temperature ceramic matrix composites (UHTC-CMC), the improvement of powder synthesis, crushing, dispersion, shaping, densification by several methods and coating process of UHTC were sequentially performed. Nano-UHTC powders having the average particle size of about 100-200 nm were synthesized at 1500-1600oC through the optimization of processing conditions and the application of spark plasma sintering (SPS) apparatus. The metal basis purity (except Zr, Hf), oxygen content and maximum synthesis capacity of the nano-UHTC powders were above 99.9%, 0.5wt% and 170g/day, respectively. Highly concentrated UHTC slurries up to 55vol% were successfully fabricated by the modification of the UHTC powders and the application of new dispersion technologies. Dense (98%) and nano-structured UHTC-SiC composites were fabricated through low temperature sintering process. For that purpose, reactive spark plasma sintering (R-SPS) was applied at 1450 - 1850oC under 40-120MPa pressure using silicides-(B4C)-C sintering additive systems. Ultra-fine (200 - 300 nm) and homogeneously distributed UHTC and SiC grains were observed in the dense nano-UHTC composites due to the high energy milling of raw powders, the molecular-level homogeneity of the source materials in the silicides, and low sintering temperature by R-SPS. Silicides, which may strongly decrease the high temperature properties of the composites, were nearly completely consumed during the densification process. The Vickers hardness, Young’s modulus and fracture toughness of the nano-UHTC composites were about 20 GPa, 300 GPa and 3 MPa m1/2, respectively. The ablation behavior of pure HfC and the nano HfC-SiC composites was compared through oxy-acetylene torch test. After testing at 2700-2800oC for 30 minutes in air, both the materials retained their shapes. The thickness of the oxide scales were 150-300 micrometer after the test. The carbon fiber fabrics, with and without the infiltration of HfC nano-particles, were supplied to the research groups for the matrix densification by precursor impregnation and pyrolysis (PIP), liquid alloy infiltration (LAI), and pressure assisted sintering such as hot pressing (HP) and spark plasma sintering (SPS) methods. The maximum bending strength of the Cf/HfC-SiC-based UHTC-CMC made by PIP, LAI and SPS were 70, 160 and 122MPa, respectively. The low bending strength was caused by the difference of the coefficient of thermal expansion between the fiber and the matrix. The CMC made by HP and SPS showed well-developed fiber pull-out behavior. The thermal shock resistance of the UHTC was clearly improved by the fabrication of the CMC. Highly dense HfB2, HfC and TaC coating with the thickness of up to 150 micrometer were deposited on the UHTC and UHTC-CMC substrates by using vacuum plasma spray (VPS) coating process. HfC coating effectively reduced the formation of HfO2 on the HfC-SiC substrates after the ablation test at 2800oC for 30min, but the spalling of the coating was observed after the testing. In the Cf/HfC-SiC CMC, relatively large portion of the carbon fibers survived after the ablation test at 2700oC for 20minutes in air. In conclusion, ablation resistant ultra-high temperature ceramic matrix composites (UHTC-CMC) were successfully fabricated by optimization of the processing for UHTC