Papers by Author: Michael J. Pomeroy

Abstract: Silicon nitride is recognised as a high performance material for both wear resistant and high temperature structural applications. Oxide sintering additives such as yttrium oxide and alumina are used to provide conditions for liquid phase sintering, during which the additives react with surface silica present on the Si3N4 particles and some of the nitride to form an oxynitride liquid which allows densification and transformation of - to -Si3N4 and on cooling remains as an intergranular oxynitride glass. This paper provides an overview of liquid phase sintering of silicon nitride ceramics, grain boundary oxynitride glasses and the effects of chemistry and structure on properties. As nitrogen substitutes for oxygen in oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics.

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: This paper provides an overview of the preparation of M-Si-Al-O-N glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in sialon glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. If changes are made to the cation ratios or different rare earth elements are substituted, properties can be modified. The effects of these changes on mechanical properties of silicon nitride based ceramics and sialons are discussed. New research on M-Si-Al-O-N-F glasses is outlined.

Abstract: The preparation of bulk glasses in Ca-Si-Al-O-N-F system with the composition in equivalent % of 28e/oCa:56e/oSi:16e/oAl:100-X-Ye/oO:Xe/oF:Ye/oN are reported. The glass formation behaviour and properties of this new range of glasses are examined in detail. Fluorine decreases the glass transition temperature, the density and the mechanical properties of the glasses while nitrogen increases them. Therefore, it appears that fluorine acts as a network modifier while, on the contrary, nitrogen acts as a network former even in presence of fluorine.

Abstract: Ca-Sialon glasses have been known for some time [1] and they are effectively calciumalumino- silicate glasses containing nitrogen which improves their mechanical properties. Calciumalumino- silicate glasses containing fluorine are known to have useful characteristics as potential bioactive materials [2]. Therefore, the combination of both nitrogen and fluorine additions to these glasses may give useful bioglasses with enhanced mechanical stability.Addition of fluorine to oxynitride glasses was not reported previously and this paper gives the first report of the glass forming regions (and evaluation of some properties) in the Ca-Si-Al-O-N-F system. Within the previously defined [1] glass forming region in the Ca-Si-Al-O-N system, homogeneous, dense glasses are formed. Addition of fluorine extends the glass forming region but also increases the reactivity of the glass melts. One major problem is fluorine loss as SiF4, but also loss of nitrogen, which affects the final composition and results in porous samples. To suppress the fluorine loss and CaF2 precipitation, consideration of the ratio of cations to fluorine and the coordination number of Al atoms is important. Discussion of the role of cations in these oxyfluoronitride glasses is presented.

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: Oxynitride glasses are effectively alumino-silicate glasses in which nitrogen substitutes for oxygen in the glass network, resulting in increases in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. Calcium alumino-silicate glasses containing fluorine are known to have useful characteristics as potential bioactive materials. Therefore, the combination of both nitrogen and fluorine additions to these glasses may give useful bioglasses with enhanced mechanical stability. This paper gives a review of oxynitride glasses and reports glass formation and evaluation of glass properties in the Ca-Si-Al-O-N-F system. Within the previously defined glass forming region in the Ca-Si-Al-O-N system, homogeneous, dense glasses are formed. However, addition of fluorine affects glass formation and reactivity of the glass melts and can lead to fluorine loss as SiF4, but also nitrogen loss, and cause bubble formation. At high fluorine and high Ca contents under conditions when Ca- F bonding is favoured, CaF2 crystals precipitate in the glass. It was found that fluorine expands the glass forming region of Ca-Sialon system and facilitates the solution of nitrogen into the melt.

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: Y-SiAlON glasses of composition 36.5 Y: 42.3 Si: 21.2 Al with different amounts of N (0, 5, 8, 15 and 22 in e/o) were produced by melting appropriate mixtures of powders under flowing nitrogen at 1715°C. This composition is known to give B-phase (Y2SiAlO5N) on crystallisation at temperatures below 1050°C. In this work, the effect of nitrogen in the starting glass composition on the crystalline phases formed is discussed. High temperature in-situ XRD analysis was performed on powdered glass samples up to 1150°C by using a Philips X’pert PRO MPD (Multi Purpose Diffractometer) with a HTK1200 Oven Camera (Anton Paar, Austria). As expected, the results show that different nitrogen contents affect the crystalline phases formed. In all glasses, yttrium apatite silicate forms first, followed by crystallisation of B-phase. The phase transformation from B-phase to Iw-phase (Y3Si2Al[O,N10] i.e. 10 e/o N) takes place at relatively low temperatures (1050°C) for the lower nitrogen containing samples (5 and 8 e/o), whereas, the transformation does not take place for the glasses with higher nitrogen contents even at the maximum temperature studied (1150°C). This work also confirms that there is a correlation between the temperature where the first crystals appear and the amount of nitrogen in the starting glass.

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.