In Situ Reaction of B ₂ O ₃ with AlN and/or Si ₃ N ₄ to Form BN‐Toughened Composites

Abstract

BN‐toughened oxide matrix composites were formed by the in situ reaction of B 2 O 3 with Si 3 N 4 and/or AlN. A lowtemperature transient liquid phase aids densification at <1000°C, and the process tends to produce an intrinsically homogeneous microstructure. Mechanical properties and microstructure of the composites formed in situ were compared to those of composites prepared by conventional means from oxide and BN powders. Fracture toughness and flexural strength of the nearly isotropic in situ formed composites ranged from 2.82 to 3.66 MPa · m 1/2 and 130 to 320 MPa, respectively, with Young's moduli of 100 to 110 GPa. Densities achieved ranged from 90% to 97% of estimated theoretical densities. The strength and toughness values are intermediate to the extreme values for the anisotropic composites formed by hot‐pressing mixed powders.