Development of Si3N4 based ceramic materials containing MgF2

Ceramic materials have hardness, inertness, high compressive strength, weak sliding and shears strengths and relatively good wear and chemical resistance. Silicon nitride and oxynitride ceramics have attracted interest for high-temperature engineering applications because of their excellent properties, high strength, wear resistance, high decomposition temperature, oxidation resistance, thermal shock resistance, low coefficient of friction, resistance to corrosive environments. One of the most import advantages of ß-Si3N4 and ß-SiAlON ceramics are that they have ß grains with high aspect ratio like fiber which lead to excellent fracture toughness. Due to very low diffusivity of ß grains they can not be densified by using conventional solid-state sintering methods. Instead, densification has been achieved by means of liquid-phase sintering.The main objective of the present study was to improve Si3N4 based ceramic materials containing MgF2.Effect of conventional oxide additives and fluoride addition to glass system on densification behaviour in the production of Si3N4 based ceramics via liquid phase sintering was compared. In comparison of conventional oxide and fluoride additive system with liquid phase sintering, about 50-150?C less eutectic temperature was achieved for fluoride system. In addition to this, glass forming region was enhanced and so, high relative density values was obtained at lower temperatures. Sintering studies were systematically carried out in the range of 1450-1750°C for 0.5-4 hour by pressureless sintering techniques.Characterization of resulting products were carried out by using Archimedes principle to determine densities of sintered samples, X-Ray Diffraction Analysis for phase definition (??ß transformation), Scanning Electron Microscope (SEM) for microstructure analysis and Energy Dispersive X-ray spectroscopy (EDS or EDX) for elemental analysis and elemental composition maps. Mechanical properties of resulting ceramics were determined by using micro hardness testing and indentation technique.The effect of fluoride and oxide additives on density, hardness, fracture toughness and ??ß transformation of resulting ß- Si3N4 and ß- SiAlON ceramics were investigated. Fluoride addition to liquid phase gave lower eutectic temperature, faster densification and higher ß:? ratio.