VERY HIGH-PRESSURE SINTERING OF CUBIC BORON-NITRIDE WITH PSZ ADDITIVE

High pressure sintering under the condition corresponding to the thermodynamically stable region for cubic boron nitride in a phase diagram of boron nitride was studied to demonstrate the availability of PSZ as a sintering additive of cubic boron nitride. In a specimen without additives, because of rigidity of cubic boron nitride, the sufficiently high pressure was not applied on the surface of cubic boron nitride particles effectively except for the portion of surface where the cubic boron nitride particles contacted with each other, where inversion into hexagonal boron nitride occured. In PSZ-added specimens, however, PSZ behaved as a pressure transmitting medium to the surface of cubic boron nitride particles. Consequently, phase transition into hexagonal boron nitride was prevented. On the other hand, part of partially stabilized high temperature phase t-ZrO2 of PSZ underwent conversion into stable phase m-ZrO2 for the specimens doped with PSZ less than 12 vol%. The cubic boron nitride sintered body with PSZ of 12 vol% had the highest microhardness of above Hv 2300. It was concluded that PSZ was effective as a sintering additive to obtain cubic boron nitride sintered bodies with a compact texture.