Oxidation resistance and electrical properties of silicon carbide added with Al2O3, AlN, Y2O3 and NiO

Because of its excellent thermal, mechanical and electrical properties sili con carbide is widely used for heating elements. Nevertheless these element s are affected by electrical ageing (increase of electrical resistivity dur ing use). This phenomenon is generally attributed to oxidation but no satis factory answer has been presently found to reduce its effects. The aim of t his study is to obtain a better understanding of the degradation of the ele ctrical properties through the oxidation behavior of hot pressed samples co ntaining various amount of additives. Eight dense SiC ceramic samples with Al2O3, AlN, Y2O3 and NiO additives were prepared by hot pressing. The influ ence of these additives on sintering, oxidation behavior and electrical pro perties was evaluated. Formation of an yttrium garnet phase leads to liquid phase sintering but decreases the oxidation resistance. The dependence of electrical resistivity with temperature can be explained by the presence or not of a metallic phase formed between Ni and Si. This secondary phase per mits a low (< 5 Omega . cm) and almost constant value of the electrical res istivity from ambient temperature up to 950 degreesC to be obtained. (C) 20 01 Kluwer Academic Publishers.