Effect of reinforcement additions on mechanical, thermal, oxidation and ablation properties of ZrB2 based ultra-high temperature ceramic composites

Reference Presenter Authors
14-078 Rahul Mitra Kashyap, S.(Indian Institute of Technology Kharagpur); Mallik, M.(National Institute of Technology Durgapur); Mitra, R.(Indian Institute of Technology Kharagpur);

A comparative study has been carried out on creep behavior, thermal properties as well as and ablation resistance of ZrB2-based composites with varying amounts of SiC, Si3N4, LaB6 or ZrC as reinforcement and processed by pressureless sintering to understand the microstructure-property relationships. The ZrB2-based composites having SiC volume fraction in the range of 10-40 vol.% have been processed with relative densities >98% by pressureless sintering at 2000 oC using B4C and C as additives. The thermal conductivity and coefficients of thermal expansion of the aforementioned composites have been measured, and correlated with their thermal shock resistance. Based on the studies related to ablation and thermal shock, 20 vol.% SiC has been found to be the most optimum reinforcement content. Therefore, ZrB2-20 vol.% SiC (ZS) composites along with additions of ZrC, Si3N4 or LaB6 have been processed by hot pressing or spark-plasma sintering at 2000 oC for 30 minutes using uniaxial pressure of 30 MPa, so as  to possess >99% of the theoretical density. Compressive creep tests performed at constant load in air at different combinations of stress (93-140 MPa) and temperature (1300-1425  oC) have shown addition of Si3N4 as detrimental. Analysis of creep stress exponents and activation energies suggest grain boundary diffusion and viscoplastic flow of glassy interfacial phase as rate controlling below and above 1350 oC, respectively. The resistance against degradation of these composites during isothermal/cyclic exposure at 1200 or 1300 oC or ablation by heating through neutral oxy-acetylene flame at >2000 oC is contributed by formation of an adherent film of SiO2, as well as thermal protection provided by ZrO2 layer. The mechanisms of creep, oxidation and ablation of the investigated composites will be discussed.