Fabrication and mechanical properties of h-BN based composites containing dual glass phases

In this work, h-BN based composites containing amorphous silica and ytterbium silicate glass phases were successfully fabricated by in situ hot pressing process. The powder mixtures of h-BN, Yb2O3, SiO2 and sintering additive (Al2O3) were hot pressed at 1880 °C for 1 h under 30 MPa in Ar atmosphere. In the composites, β-Yb2Si2O7 phase produced from the reaction between Yb2O3 and SiO2 disappears completely and transforms to Yb-Si-Al-O glass when the amount of Al2O3 is ≥1.5 wt.%. The Vickers hardness, flexural strength, and compressive strength reached the maximum values of 2.38 ± 0.08 GPa, 337 ± 22 MPa and 950 ± 34 MPa, respectively, as 1.5 wt.% Al2O3 was added. The strengthening effects were attributed to the fine spherical microstructure of Yb-Si-Al-O glass particles, strong [AlO4] coordination state, and residual compressive stress in glass phases.