Texture and anisotropy of hot-pressed h-BN matrix composite ceramics with in situ formed YAG

Abstract

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot-pressing using different contents of 3Y2O3−5Al2O3 (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y3Al5O12 (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y2O3 and Al2O3, and a coherent relationship between h-BN and YAG was observed with [ 010 ] h − BN// [ 111 ] YAG and (002)h-BN//(321)YAG. In the YAG liquid phase environment formed during hot-pressing, plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction, which give these composite ceramics anisotropy in their mechanical and thermal properties. The highest texture degree was found in the specimen with 30 wt% YAG, which also possesses the highest anisotropy degree in thermal conductivity. The aggregation of YAG phase was observed in the specimen with 40 wt% YAG, which resulted in the buckling of h-BN plates and significantly reduced the texture degree.[Figure not available: see fulltext.]