OXIDATION OF SILICON-NITRIDE SINTERED WITH YTTRIA AND MAGNESIA CONTAINING NITROGEN RICH LIQUID

Oxidation of pressureless sintered Silicon Nitride densified with a Nitrogen rich liquid phase in Y2O3-AlN-SiO2 and MgO-AlN-SiO2 systems have been studied. Thermogravimetric studies indicate a parabollic kinetics. Activation energies are 188 and 430 kJ/mole for the oxidation of materials densified with 11.89 and 23.15%. additives containing Y2O3 and 390 kJ/mole for those densified with 21% additives containing MgO.XRD studies show that the oxidation products are SiO2(Cristobalite), Si2ON2,YAlO3 (above 1250C) and Y2Si2O7 in case of Yttria containing additives and SiO2(Cristoballite),Si2ON2, MgSiO3(Protoenstatite),Mg2SiO4(Forsterite) and MgAl2O4(Spinel) in case of Magnesia containing aditives. XPS study shows the presence of a silicate phase in the outermost laver of the oxidised surface. Oxidation of MgO containing material is higher than that of Y2O3 containing material.