STRENGTHENING OF SILICON-CARBIDE CERAMICS BY SURFACE NITRIDATION DURING HOT ISOSTATIC PRESSING

Surface strengthening of SiC by in situ surface nitridation during post-hot isostatic pressing (post-HIPing) in N2 was investigated. The formation of a thin (5-15-mu-m) layer of submicrometer beta-Si3N4 on the surface of SiC was obtained at 1850-degrees-C and 200 MPa. While SiC HIPed in Ar attained a mean bending strength of 660 MPa, a significant increase in strength (with a maximum fracture stress above 1000 MPa) was observed for the SiC/Si3N4-layer composite material. Generation of residual compressive stresses on the surface layer caused by the differences in thermal expansion may account for the observed strengthening. Thus, in situ surface nitridation by post-HIPing in N2 may offer an attractive way to improve surface-sensitive mechanical properties of complex-shaped SiC components.