Thermal-shock behavior of a Nicalon-fiber-reinforced hybrid glass-ceramic composite

A Nicalon-fiber-reinforced hybrid composite with a matrix of barium magnesium aluminosilicate (BMAS) glass with silicon carbide whiskers was subjected to thermal shock from elevated to ambient temperatures. The combination of SiC whisker and BMAS glass resulted in a hybrid matrix with a lower thermal expansion than that of the fibers, inducing tensile stresses in the fiber upon thermal shock. This stress state resulted in microstructural damage in the form of fiber cracking and cracking along the fiber/ matrix interface, as opposed to the conventional matrix cracking which is typically observed in ceramic-matrix composites. Significant damage in the composite was only observed after three thermal shock cycles. Flexural resonance measurements, used to evaluate thermal shock-induced changes in Young's modulus, showed a reduction in modulus that correlated well with the onset of microstructural damage. Finally, fiber push-out tests, performed to evaluate changes in fiber/matrix interface strength after thermal cycling, indicated a slight decrease in interfacial strength, which was attributed to recession of the carbon-rich fiber surface during thermal shock.