Effect of Y sub 2 O sub 3 on the Si sub 3 N sub 4 - carbon interaction at high temperatures

The most important problems in improving the properties of silicon nitride based ceramics are connected with studying the influence of additives and raising the purity of the raw material, in particular, reducing the oxygen content, because it affects the heat conduction and mechanical and other properties of the material. Yttria is among activators that best promote compaction of silicon nitride in sintering or hot pressing. The aim of the present work is to study the action of yttria additive on the high temperature interaction between silicon nitride and carbon. The method of pulsed reduction extraction in a flow of carrier gas is used to study the effect of Y sub 2 O sub 3 on the interaction between Si sub 3 N sub 4 and carbon in the range of 1500-3500 deg C. High-temperature dependences of the degree of extraction of oxygen (in the form of CO) and nitrogen from Si sub 3 N sub 4 , Y sub 2 O sub 3 , and a proportioned mixture of them (from 2 to 5% Y sub 2 O sub 3 ) are obtained. It is established that small additives of Y sub 2 O sub 3 diminish substantially the thermal stability of Si sub 3 N sub 4 . It is assumed that Y sub 2 O sub 3 has a pseudocatalytic effect on the Si sub 3 N sub 4 -carbon interaction at high temperatures. The Temkin-Shvartsman method and published values of the Gibbs free energy are used for thermodynamic calculation of the change in the Gibbs free energy for evaluation of the possibility of some hypothetical reactions.