Thermal stability and mechanical performance of multiply heat-treated α-sialon ceramics densified with rare earth oxides

Abstract

Typical α-sialon starting compositions, of formula Ln_0.33Si_9.3Al_2.7O_1.7N_14.3, were densified by hot-pressing using Ln₂O₃ as sintering additives, where Ln = Nd, Dy, and Yb. The as-sintered materials were heat-treated at 1450°C for 96 hours and then re-sintered at 1800°C for 1 hour to observe the overlapping effects of both Ln₂O₃ and multiple heat-treatment on thermal stability of the Ln-α-sialon phase and also the change in microstructure. The kinds of grain boundary phases which occurred also affected the results. The hardness, fracture toughness and flexural strength of the materials were evaluated using indentation and three-point bending tests, respectively. Mechanical tests and detailed microstructural analysis have led to the conclusion that a multiple-mechanism is involved, with debonding, crack deflection, crack bridging, and elongated grain pull-out all making a significant contribution towards improving the fracture toughness. Nd-containing specimens were tough with a highest indentation fracture toughness K_1C of 7.0 MPa m^1/2. In contrast, Dy- and Yb-containing specimens were hard and brittle with a highest Vickers hardness H_V10 of 18.0 GPa. All re-sintered specimens underwent β→α transformation to some degree, leading to a degradation of mechanical properties as a consequence.