Papers by Keyword: SiAlON Glass

Abstract: This paper provides an overview of the crystallisation of an oxynitride glass likely to remain in a silicon nitride ceramic following firing. The crystallisation process was studied using both differential thermal analysis (DTA) and separate isothermal heat treatments in a tube furnace under nitrogen. The activation energy for the crystallisation process was determined by DTA. The nucleation temperature, Tg + 40°C, which corresponded to the maximum volume fraction of crystalline phases, agreed closely with the optimum nucleation temperature of Tg + 35°C, found from DTA. The optimum crystal growth temperature was observed to be 1210°C and yielded the - and -polymorphs of yttrium disilicate. Heat treatments at other temperatures indicated the development of phase assemblages which contained different polymorphs of yttrium disilicate as well as silicon oxynitride. Not all of the polymorphic transformations of yttrium disilicate were observed by DTA unless some crystallisation exotherms were deconvoluted, indicating that DTA analysis of the crystallisation of complex systems requires careful interpretation. It is, however, possible to simplify the system by substituting some yttrium by lanthanum. This stabilises the -polymorph of yttrium disilicate. The activation energy for crystallisation was observed to be similar to that for viscous flow of Y-Si-Al-O-N glasses.

Abstract: Oxynitride glasses are found at grain boundaries, i.e. triple point junctions and intergranular films, in silicon nitride based materials as a result of cooling of liquid phases formed by reaction of sintering additives with silicon nitride and silica present on the nitride surface during the densification of the ceramics. The glass chemistry, particularly the content of modifying cation, usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic affect the properties of silicon nitride such as fracture toughness and creep at high temperature. As nitrogen substitutes for oxygen in silicate and alumino-silicate glasses, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or as the size of the rare earth cation decreases, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have a strong impact on the mechanical properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of previous work on oxynitride glasses and outlines the effect of glass composition on their properties and discusses the implications for high temperature behaviour of Si3N4 ceramics.

Abstract: Oxynitirde glasses are found at triple point junctions and as intergranular films in silicon nitride based ceramics. The glass chemistry, particularly the content of modifyer,usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic control the properties of silicon nitride, in particular, creep at high temperature. It is known that, as nitrogen substitutes for oxygen in silicate and aluminosilicate glass networks, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or different rare earth cation are substituted, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have implications for the high temperature properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity and discusses the effects on high temperature behaviour of silicon nitride ceramics.

Abstract: M-Si-Al-O-N glasses (where M = Y or rare earth cation) are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of these oxynitride glass phases determine the properties of the material, in particular, high temperature mechanical behaviour. Investigations on oxynitride glass formation and properties have shown that nitrogen increases the glass transition and softening temperatures, viscosity, elastic modulus and hardness. By changing the cation ratios or the type of rare earth cation incorporated, properties such as viscosity can be increased further. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity. These effects have important implications for silicon nitride based ceramics where amorphous intergranular films control high temperature properties such as creep resistance.