Texture and Anisotropy of Hot-Pressed h-BN Matrix Composite Ceramics With in Situ Formed YAG

Textured h-BN matrix composite ceramics were prepared by hot-pressing using different contents of 3Y2O3-5Al2O3 (mole ratio of 3:5) as the liquid phase sintering additive. During the hot-pressing process, the liquid Y3Al5O12 (YAG) phase with 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 // [1̅11]YAG and (002)h-BN // (321)YAG. Plate-like h-BN grains in the YAG liquid phase environment were rotated under the action of the uniaxial sintering pressure, making their basal surfaces preferentially arranged perpendicular to the sintering pressure, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure, 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 content, which also possesses the most anisotropic thermal conductivity. The aggregation of YAG phase resulting in the buckling of h-BN plates was observed in the specimen with 40 wt.% YAG content, which significantly reduced its texture degree.