Preparations Of Y-Mg-Si-Al-O-N Oxynitride Glass And Glass-ceramics And Investigations On The Correlations Between Their Compositions, Structures And Properties

Aluminosilicate oxynitride glasses have many excellent properties such as much higher glass transition temperature, visocosity, microhardness, strength, elastic modulus, fracture toughness and much lower thermal expansion coefficient in comparision with their corresponding oxide glasses. Oxynitride glass-ceramic with some crystalline phases and small amounts of residual glassy phase, a kind of promising non-oxide ceramic materials, can be prepared after appropriate heat treatments performed on oxyntride glass. However, besides long duration time, the melting temperatures for preparing aluminosilicate oxynitride glasses are usually higher than 1700℃, which results in decomposition or oxidation of the silicon nitride powder as raw materials. Therefore, it is still very difficult to prepare homogeneous and transparent aluminosilicate oxynitride glasses by far. High heat treat high temperature from 1100℃to 1400℃and long duration time over ten hours are required to prepare aluminosilicate glass-ceramics from their corresponding oxynitride glasses. Furthermore, crystalline phases in the glass-ceramics are very complex and difficult to control, which leads to the unstable properties of aluminosilicate glass-ceramic.The Y-Mg-Si-Al-O-N system was chosen to prepare the oxynitride glasses and the glass-ceramics in the present thesis. Glass composition design, glass melting experiments, DSC, IR, X-ray diffraction, SEM in combination with EDS methods were used to investigate a new method for preparing Y-Mg-Si-Al-O-N oxynitride glasses, the influences of cation composition and their ratios on the structures and properties of Y-Mg-Si-Al-O-N glasses and glass-ceramics, and the relations between heat treatment schedules, including one-stage heat treatment and two-stage heat treatment, and phase transformation, crystallinity, microstructures and mechanical properties of Y-Mg-Si-Al-O-N glass-ceramics. The results are listed as follows:1. A kind of new process for the preparation of aluminosilicate oxynitride glasses has been provided in this thesis. This process involves preparing oxynitride glasses at relative low melting temperature under locally carbon monoxide reducing atmosphere, which lowered the melting temperature form above 1700℃to about 1560℃. Decomposition and oxidation rates of silicon nitride powder in the raw materials decrease significantly during this glass preparation process. Furthermore, the glass melt can be quenched immediately and the devitrification for the glasses during the melt cooling process can be avoided. Therefore, oxynitride glasses prepared by this process are more homogenous and transparent than the glasses prepared by conventional melting quench method applied for the preparation of oxynitride glasses. Moreover, a lower glass melting temperatures and locally reducing atmosphere get rid of the limitation of the conventional oxynitride glass preparation route in which a high temperature gas-pressure furnace was required.2. The dependences of structures and properties of the Y-Mg-Si-Al-O-N oxynitride glasses on the cation composition, including Mg/Y and Al/Si ratios, were investigated in detail in this thesis. With increasing Mg/Y ratio, some MgO4 tetrahedra are present in the network structure of Y-Mg-Si-Al-O-N glasses, which induces some Al3+ ions forming five or six-fold coordination groups. Therefore, the number of AlO6 or AlO5 groups and the Al-O-Si bridging increase. The number of non-bridging oxygen (NBO) and Al-O-Si bridges in the Y-Mg-Si-Al-O-N glasses increase as the Al/Si ratio is increased. The variation of glass structure leads to changes of the glass properties. For Y-Mg-Si-Al-O-N glasses, glass transition temperature decreases non-linearly, chemical durability decreases linearly and the mechanical properties increase first and then decrease as the Mg/Y ratio is increased. With increasing Al/Si ratio in the Y-Mg-Si-Al-O-N glasses, glass transition temperature decreases, whereas chemical durability and mechanical properties increases.3.14Y14Mg48Si24A183O17N glass with excellent mechanical properties in the Y-Mg-Si-Al-O-N system was chosen as parent glass. Multiphase glass-ceramics were obtained after different heat treatments performed on the parent glass samples. YMgSi2O5N phase, a major crystalline phase and the only nitrogen-containing phase, appears in all 14Y14Mg48Si24A183O17N glass-ceramics samples. Apparent crystallization activation energy of 14Y14Mg48Si24A183O17N glass is 378kJ/mol and 397kJ/mol respectively based on the modified Kissinger and the Ozawa methods. It is obvious that the activation energy is far lower than those of glasses in the Y-Si-Al-O-N system. For 14Y14Mg48Si24A183O17N glass, bulk nucleation is the predominant nucleation mechanism.4. For Y-Mg-Si-Al-O-N glasses, the activation energy and the initial crystallization temperature decrease and the heat treatment time gets shorten as the Mg/Y ratio is increased. Therefore, it gets easier to prepare Y-Mg-Si-Al-O-N glass-ceamic with high volume fraction of crystalline phases. When 7eq.% yttrium in 28Y48Si24A183O17N glass was substituted by 7eq.% magnesium, the type of the crystalline phases precipitated are totally changed, whereas further replacement of 7eq.% yttrium by 7eq.% magnesium in 21Y7Mg48Si24A183O17N leads to small change in crystalline phases.5. The influences of heat treatment on crystalline product, volume fraction of crystalline phase, microstructure and mechanical property of 14Y14Mg48Si24A183O17N and 28Y48Si24A183O17N glass-ceramics have been investigated systematically. Heat treatment has small effects on the transformation of crystalline products in the 14Y14Mg48Si24A183O17N and the 28Y48Si24A183O17N glass-ceramics. All 14Y14Mg48Si24A183O17N glass-ceramics contain YMgSi2O5N, MgSiO3 and Mg3Al2(SiO4)3 three crystalline phases. The order is always YMgSi2O5N>MgSiO3>Mg3Al2(Si04)3 sorted by their contents in the glass-ceramics.28Y48Si24A183O17N glass-ceramics with Y3A15O12 as the dominant crystalline phase were prepared in the temperature range of 1100-1300℃. Furthermore,28Y48Si24A183O17N glass-ceramics with Y3Al5O12 as single crystalline phase have been prepared by treating 28Y48Si24A183O17N parent glass samples at or under 1200℃. Small amounts of O'-sialon phase starts to precipitate from 28Y48Si24A183O17N glass samples at temperatures above 1200℃. Heat treatment has great effects on volume fractions of crystalline phases in the 14Y14Mg48Si24A183O17N and the 28Y48Si24A183O17N glass-ceramics. Volume fraction of crystalline phases in glass-ceramic increases with increasing nucleation temperature or nucleation hold duration. The increase of crystallization temperature leads to sharp increase in volume fraction of crystalline phases in 28Y48Si24A183O17N glass-ceramic, whereas small increase in volume fraction of crystalline phases in 14Y14Mg48Si24A183O17N glass-ceramics. When crystallization temperature is increased to 1160℃, the rod-shape YMgSi2O5N crystal grains grow fully and distribute uniformly in the 14Y14Mg48Si24A183O17N glass-ceramics. However, when the crystallization temperature is higher than 1160℃, some rod-shape crystals become bent. Branches of dendritic Y3Al5O12 crystals in 28Y48Si24A183O17N glass-ceramics get thicken with increasing crystallization temperature. Crystallization temperature has small effect on three-point bending strength of 14Y14Mg48Si24A183O17N glass-ceramics. The strength increases first and then decreases with increasing crystallization temperature. The glass-ceramics obtained at 1160℃shows the maximum strength. Three-point bending strength of 28Y48Si24A183O17N glass-ceramics increases as the crystallization temperature increases.