Crystallization of glasses in the system (Zr)-Y-Si-Al-O-(N)

Abstract: Glass is a group of materials that has been used for structural and functional purposes for many centuries. Nevertheless, interest in the science and technology of glass has expanded significantly in recent years as a result of the need to develop new materials for advanced applications. Examples of new materials in which glasses of various kinds are an essential constituent include glass ceramics which are prepared by crystallizing glass, glass matrix composities, glass fibre reinforced composities and optical-fibre glass. Sintered covalent ceramics, used for example for wear resistance and high temperature applications often also contain a small proportion of glass phase which is added intentionally to aid densification during sintering. In the development of such materials it has proved necessary to widen knowledge of the physical, chemical and mechanical properties of glass and how these are influenced by composition and microstructural morphology. The aim of the work presented in this thesis is to develop a better understanding of the crystallization behaviour of glasses in the system YSi-Al-0-(N) with small additions of ZrO2 as a nucleating agent. Interest in the Y-Si-Al-0-(N) system stems not only from the fact that it forms the basis of glass phases added as sintering aids found as microstructural constituents in sintered Si3N4-based ceramics but also because glasses in this system have potential to form a relatively new group of refractory glass-ceramics. The study of compositions within this system in this work has involved investigation of: the glass formation in the Al2O3-Y2O3-Si02 system under conditions of laser melting; the crystallization behaviour and properties of selected oxide and oxynitride glasses with / without addition of zirconia; the role of zirconia addition on the crystallization behaviour of Y-Si-Al-0-(N) glasses and on the melting of Y-Si-Al-0-(N) glass-ceramics; the YAG crystallization from an oxynitride glass relevant to the improved heat treatment of YAG-based glass-ceramics and YAG/beta-SiAlON materials. Techniques used in the investigations involved laser glass melting, conventional furnace glass melting, glass-ceramic furnace heat treatment, scanning electron microscopy, room temperature X-ray diffractometry, high temperature X-ray diffractometry, differential thermal analysis, differential scanning calorimetry, dilatometry, infrared spectroscopy, Vickers hardness and density measurements. The glass-forming region in the system Y2O3-Al2O3-SiO2 under conditions of laser melting was found to be more extensive than in earlier studies. No glass formed in compositions with less than 20 wt % SiO2. 6 wt % of ZrO2 can be dissolved in the investigated composition of the yttria-alumina-silica liquid at 1700°C. When larger amounts are added a cubic crystalline phase of "yttria-stabilized zirconia" with a composition of Y0.15Zr0.8501.9 forms. The hardness and density of the zirconia-containing glasses are slightly higher than for the zirconiafree glasses. The glass transition temperature and softening temperature are lowered by the addition of zirconia. The Zr4+ cation acts as a glass-network modifier causing lowering of the viscosity of the glass with a consequent decrease of the nucleation density. The optimum nucleation temperature of the ZrO2-containing glasses is marginally lower than that of the ZrO2-free glass. ZrO2 acts as a crystal growth modifier rather than a nucleating agent during the crystallization of the ZrO2-containing glasses. Addition of ZrO2 promotes the formation of the y-Y2Si2O7 phase. Due to the lower viscosity of the retained glass in the ZrO2-containing glass-ceramics the rates of the phase transformations involved in the melting process are increased. The YAG crystallization from a ZrO2-free oxynitride glass was studied by high temperature X-ray diffractometry. It was found that the crystallization occurred preferably at free surfaces.