Thermal analysis on the milled Al+B₂O₃+Si+WO₃ system to synthesize Al₂O₃-WxSiy-WxBy powders

Composites including silicide of tungsten and boride of tungsten intermetallic compound as reinforced agents in alumina matrix have a good mechanical properties in low and high temperature (high friction resistance, strength, resin stance to creep and relatively high thermal shock resistance), moreover, they have chemical neutral with a high corrosion resistance in a high corrosive and high temperature environment. The purpose of this study is to investigate thermal analysis, phase and microstructural evaluation during synthesis of above mentioned composite by combustion aluminothermic processing in three systems 1-Al+Si+WO₃, 2-Al+B₂O₃+WO₃ and 3-Al+B₂O₃+Si+WO₃. A Ball-milled starting material according to stoichiometric ratio was thermal analyzed (DTA-TGA) for each of above mentioned system. The XRD results never show any new phase during ball-milling even up to 10 hours. The forming of tungsten silicide (Wx Siy ) is in lower temperature in comparison with tungsten boride (Wx By ) during the thermal analysis experiments. The presence of Si in the Al+B₂O₃+WO₃ system facilitates the formation of tungsten borides. The microstructural observations show a uniform and dense distribution of silicide and boride of tungsten in the alumina matrix. Silicide phases are small grain with spherical morphology whereas; the boride phases are coarser and relatively elongated with irregular morphology. (author)