Influence of additives content on the high temperature oxidation of silicon nitride based composites

A life prediction tool for mechanical and electrical applications of electroconductive structural ceramics is essential in order to know the limit for engineering uses. The aim of this work was to study the influence of additives content on the oxidation behaviour, in pure oxygen between 900 and 1400 degreesC, of two fully dense Si3N4-35 vol.% TiN composites. For this purpose, a hot pressed material (HP), containing 3.7 wt.% of Y2O3 and Al2O3 as sintering aids, was compared to an hipped material (HIP), containing 0.4 wt.% of the same additives. Up to a temperature T < similar to 1200 degrees C, where the oxidation of the composite is mainly governed by the preferential oxidation of TiN, the two materials exhibit paralinear kinetics with very close oxidation resistance. Contrarily, the hipped material shows a better oxidation resistance at T > similar to 1200 degreesC, when the oxidation of the Si3N4 matrix takes place. The formation of a compact silica sub-scale acts as an efficient diffusion barrier leading to asymptotic kinetics, with final weight gains exhibiting a negative temperature dependence. In the case of the HP material, i.e. in presence of a higher content of additives, a deterioration of the protective nature of the scale is provoked by the increased mobility of the impurity cations (Y3+, Al3+) linked to a decrease of the viscosity of the secondary glassy phase. The kinetics have a paralinear shape up to 1400 degreesC, with final weight gains increasing as a function of temperature. Therefore, this study confirms the deleterious influence on oxidation resistance of additives used for a better sintering of powders and the beneficial effect of hot isostatic pressing for which lower amounts of aids are necessary in comparison with hot pressing. (C) 2003 Elsevier Science Ltd. All rights reserved.