FABRICATION OF SILICON-NITRIDE BASED COMPOSITES BY IMPREGNATION WITH PERHYDROPOLYSILAZANE

A low viscosity perhydropolysilazane which is converted to Si-N ceramics with a 70-90 wt% yield by pyrolyzing at temperatures above 800-degrees-C, was used as a matrix precursor. The preform of SiC(w)/Si-N composites was prepared by slip casting, and impregnated with the polysilazane. The impregnated specimen was cured at 100-degrees-300-degrees-C in a N2 gas under the pressure of 0.05-0.1 MPa, and fired to 1200-degrees-C. To obtain dense composites, impregnation, curing and firing processes were repeated. The performs of C(f)/Si-N and C(f)/SiC(w)/Si-N with unidirectional aligned fiber were prepared by filament winding. The prepregs were cured and fired, and the densifying processes were repeated. The densities of the C(f)/Si-N and C(f)/SiC(w)/Si-N were increased to 90-96% of the theoretical values. Such extensive densification resulted from the excellent infiltrativity and the high ceramic yield of the polysilazane. Owing to the high density of the matrix and the adequate shear strength of the fiber/matrix interface, the three-point flexural strength of the C(f)/SiC(w)/Si-N was substantially high, 695 MPa.