Papers by Keyword: Oxynitride Glass

Abstract: Si_5.4Al_0.6O_0.6N_7.4 (β-Sialon)/ Y_1.75Si_2.625Al_1.0O_7.5N_1.25 (oxynitride glass) composite solders with different ratio were designed using SiO₂-Al₂O₃-AlN-Y₂O₃-Si₃N₄ mixture powders to join Si₃N₄ ceramic to itself. It was found that both solder composition and bonding temperature have strong influence on the microstructure and strength of the joints. As far as using a pure oxynitride glass solder was concerned, the thickness of the seam turned narrow with the increase of temperature, and the strength had a peak value at 1550 °C. When increasing the designed content of β-Sialon, serious composition separation occurred in the seam at low bonding temperatures, and the strength was also lower than as using pure oxynitride glass. Increasing bonding temperature, the strength of the joints was improved and reached the maximum value of near 80 MPa at 1600 °C for the designed 20%β-Sialon. In this instance, large amounts of fine elongated β-Sialon grains were uniformly distributed in the seam. However, for the 60%β-Sialon containing solder, some pores appeared in the seam and its strength was very low at bonding temperatures.

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: Silicon nitride is one of the major structural ceramics that has been developed following many years of intensive research. It possesses high flexural strength, high fracture resistance, good creep resistance, high hardness and excellent wear resistance. These properties arise from the processing of the ceramic by liquid phase sintering and the development of microstructures in which high aspect ratio grains and intergranular glass phase lead to excellent fracture toughness and high strength. The glass phase softens at high temperature and controls the creep rate of the ceramic. The purpose of this review is to examine the development of silicon nitride and the related sialons and their processing into a range of high-grade structural ceramic materials. The development of knowledge of microstructure–property relationships in silicon nitride materials is outlined, particularly recent advances in understanding the effects of grain boundary chemistry and structure on mechanical properties. This review should be of interest to scientists and engineers concerned with the processing and use of ceramics for structural engineering applications.

Abstract: Rapid cooling rates and quenching have traditionally been associated with glass formation. Hampshire et al. [1] investigated oxynitride glasses cooled in a tungsten resistance furnace at approximately 200oC/min and found that fast cooling rates were only important near the limits of the glass-forming region. In the current work on various M-Si-Al-O-N (M=Y, La, Yb, Nd) systems, it was found that even at a relatively slow cooling rate glass formation was still possible for a wide range of compositions. Different cooling rates were investigated to determine the minimum cooling rate at which a glass will form. Quantitative X-ray analysis of melted compositions indicated the relative amounts of amorphous phase and crystalline phase.

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: Thermal shock resistance of the RE-Si-Mg-O-N glasses (RE = La, Nd, Yb, Lu) with 0 and 20 eq.% of nitrogen was investigated by the indentation-quench method based on propagation of Vickers cracks. Crack growth was measured on the same sample for a test series of different quenching temperatures. Thermal shock resistance of the studied materials was determined as a temperature difference resulting in 10 % growth of the initial cracks (∆T10) and by the thermal shock parameter R calculated from the material properties. Although the comparison of ∆T10 and R values as a function of glass composition revealed some differences between these two approaches, also a common trend was observed. Thermal shock resistance increased with the fractional glass compactness resulting from RE type and N content increase.