ALBERT

Notes: Abstract Sialon ceramics were discovered simultaneously (but independently) in late 1971 at Newcastle University and also at the Toyota Research Laboratories in Japan. During the 30 years since their original discovery, the Newcastle laboratory has made a significant contribution to current understanding of the science and technology of these materials. Sialons are of interest as engineering materials for high temperature (〉1000°C) applications because they can be pressureless-sintered to high density and be designed to retain good mechanical properties even up to ≈1350°C, whereas competing metallic materials are weaker and prone to corrosion. A characteristic disadvantage of all nitrogen ceramics is that an oxide additive is always included in the starting mix to promote densification, and this remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since the glass melts at ≈1000°C, the high temperature properties of the final ceramic are in fact determined by the properties of the grain-boundary glass. The most common method of improving high-temperature performance is to heat-treat the material at temperatures of 1100–1350°C in order to devitrify the glass into a mixture of crystalline phases. More specifically it is desirable to convert the glass into a sialon phase plus only one other crystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max ≈1400°C) in contact with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compounds which satisfy these requirements. The present paper gives an overall review of this subject area and then summarises recent work at Newcastle aimed at total removal of residual grain boundary glass. This has been achieved by: (1) a post-preparative vacuum heat treatment process to remove the grain boundary glass from silicon nitride based ceramics in gaseous form, (2) above-eutectic heat-treatment (AET) of sialon-based ceramics to crystallize grain-boundary liquid into five-component crystalline sialon phases.

Notes: Abstract Typical α-sialon starting compositions, of formula Ln0.33Si9.3Al2.7O1.7N14.3, 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 K1C of 7.0 MPa m1/2. In contrast, Dy- and Yb-containing specimens were hard and brittle with a highest Vickers hardness HV10 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.