Evaluation of High Temperature Composites Thermal Properties under Different Heat Flux Conditions

The thermal protection of structures in vehicles، at instantaneous high thermal shocks، would be more effective and economically feasible among other thermal protection methods using the passive heat shields especially charring the ablative composites. The most important limitations reported are lack of compiled knowledge on designing heat shield with optimal thickness under real conditions and high surface erosion rate and low mechanical strength of char layer of a composite created by ablation process. In this paper SiAlON ceramic composites، reinforced with short carbon fiber، are identified as high performance heat shields for challenging these limitations. Ablation rate and effective thermal diffusivity at different external heat fluxes are determined and calculated using oxyacetylene flame test and modeling of temperature distributions in ablation process for evaluation of thermal protection performance and effective thermal diffusivity of this composite، as a thermal protection system. The results of this work have indicated that the carbon fiber reinforced SiAlON ceramic composite can be considered as a high ablation heat shield. Under the same condition of ablation test، SiAlON ceramic composites reinforced by carbon fiber show higher ablation performance relative to other commercial carbon fiber reinforced composite heat shields. At 8500 and 5000 kWm-2 external heat flux the ablation rates of this composite are 0. 075 and 0. 026 mms-1، respectively. Also، at 2500 kWm-2 external heat flux and test duration time of less than 25 s، this composite displays an adequate thermal shock protection with maximum flexural strength loss of about 23. 4 %.