High-temperature thermal stability of Hi-Nicalon™ SiC fiber/SiC matrix composites under long term cyclic heating

The high-temperature thermal stability of C-coated standard grade Nicalon™ SiC fiber and of C or BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composites under long-term cyclic heat exposure in air has been studied. Woven fabric composites fabricated by the PIP process were cyclic heat-exposed at 1273 and 1673 K for up to 1000 h with one heat exposure cycle of 200 h. The degradation of the composites was evaluated by a three-point flexure test at room temperature. The oxidation of the composite occurred from the matrix phase due to the existence of macro- and micro-pores that exist in the matrix. Degradation of the strength was observed for the C-coated SiC fiber/SiC composites after heat exposure for 200 h at 1273 K. On the contrary, the BN-coated Hi-Nicalon™ SiC fiber/SiC composite showed only a slight degradation even after a 1000-h exposure at 1273 K. However, at 1673 K, this compositeʹs strength degraded after an 800-h exposure.