Experimental and computational analysis of thermo-oxidative-structural stability of ZrB2-SiC-Ti during arc-jet testing

The development of new ultra-high temperature ceramics for thermal protection system (TPS) of hypersonic cruise and re-entry vehicles requires performance-qualification testing under simulated flight conditions. The present work, encompassing experiments and computational analysis, critically analyzes the thermo-oxidative-structural stability of flat surface disks of spark plasma sintered ZrB2-18SiC-xTi composites (x=0, 10, 20; composition in wt%) under arc jet flow with heat flux of 2.5 MW/m(2) for 30 seconds. Such testing conditions effectively simulate the aero-thermal environment in ground facility, as experienced by hypersonic vehicles. Based on the extensive XRD, SEM-EDS and electron probe microanalyzer based analysis of the surface/sub-surface of arc jet exposed ceramics, the oxidation mechanisms are qualitatively discussed. Importantly, thick oxide layers (similar to 400-950 mu m) were found to be adherent, thereby providing good structural stability of such ceramics for reusable TPS. The careful finite element (FE) analysis with high quality structural elements, being generated using Hyper Mesh, was conducted to understand the underlying reasons for observed oxidation. Such analysis allows us to determine the temporal evolution of through-thickness temperature distribution. FE-based calculations were subsequently validated using experimentally measured backwall temperatures. The thermodynamic feasibility of competing oxidation reactions at the analytically computed front wall temperatures was thereafter realistically assessed to support the oxidation mechanisms. Taken together, the present work provides guidelines for better understanding of the thermo-oxidative-structural stability of ceramics under arc jet testing and also establishes the good stability of ZrB2-18SiC-20Ti composites for potential application in TPS of hypersonic space vehicles.