Pressureless sintering of silicon nitride/boron nitride laminate composites

Composites made from these materials are densified by hot-pressing because of the reluctance of BN to sinter; For example, h-BN similar to that used in this study was hot-pressed for 1.5 hrs at 1740DGC under 25 MPa with no apparent sintering observed between BN particles. It is the strong covalent bonds on the basal planes that make for an unreactive, non-wetting compound which severely hampers the densification of BN. Previous research on Si3N4/BN composites has shown that during the hot-pressing step, the liquid phase formed in the Si3N4 cells is forced into the BN cell boundaries. This glassy phase, which has a composition that is a function of the sintering aids used and the silica on the surface of the silicon nitride, acts as a "glue" to hold the BN platelets together. It is proposed that if the sintering aids are added in the appropriate proportion directly to the BN powder, as well as to the Si3N4 laminate composites may be densified by pressureless sintering.