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

Typical alpha-sialon starting compositions, of formula Ln(0.33)Si(9.3)Al(2. 7)O(1.7)N(14.3), were densified by hot-pressing using Ln(2)O(3) as sinterin g additives, where Ln = Nd, Dy, and Yb. The as-sintered materials were heat -treated at 1450 degrees C for 96 hours and then re-sintered at 1800 degree s C for 1 hour to observe the overlapping effects of both Ln(2)O(3) and mul tiple heat-treatment on thermal stability of the Ln-alpha-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 f lexural strength of the materials were evaluated using indentation and thre e-point bending tests, respectively. Mechanical tests and detailed microstr uctural analysis have led to the conclusion that a multiple-mechanism is in volved, with debonding, crack deflection, crack bridging, and elongated gra in pull-out all making a significant contribution towards improving the fra cture toughness. Nd-containing specimens were tough with a highest indentat ion fracture toughness K-1C of 7.0 MPa m(1/2). In contrast, Dy- and Yb-cont aining specimens were hard and brittle with a highest Vickers hardness H-V1 0 of 18.0 GPa. All re-sintered specimens underwent beta -->alpha transforma tion to some degree, leading to a degradation of mechanical properties as a consequence. (C) 2000 Kluwer Academic Publishers.