SYNTHESIS OF MOLYBDENUM DISILICIDE ON MOLYBDENUM SUBSTRATES

Molybdenum electrodes are widely used in the glass-making industry for electrical resistance heating of glass melts. The electrodes are expo sed to an oxidizing environment at temperatures around 1600 degrees C, conditions which result in accelerated oxidation and eventual failure of the electrodes. One approach to protect molybdenum at elevated tem peratures is to synthesize a coating system based on molybdenum disili cide (MoSi2), which has excellent oxidation resistance at high tempera tures due to the formation of a ''self-healing'' layer of silica. A po tential problem is the mismatch in the coefficient of thermal expansio n between molybdenum and MoSi2, which would result in spalling of the film, on cooling from elevated temperatures. The tendency to spall can be minimized by producing multilayer films. This paper will focus on the synthesis of molybdenum disilicide on molybdenum substrates. Thin films of molybdenum and silicon were deposited (using d.c./r.f. magnet ron sputtering) either as single layers or in a multilayer structure, under different processing conditions. The molybdenum layer thickness was varied between 50 nm and 1000 nm and the Si/Mo atomic ratio was ma intained at 3 (i.e. a stoichiometric excess of Si). The as-deposited f ilms were subjected to a diffusion anneal treatment at 1000-1100 degre es C (in vacuum or an Ar ambient) for different times. Details of the phase transformations, characterized by X-ray diffraction analysis, an d microstructural development (through scanning electron microscopy) w ill be presented. The results indicate that dense molybdenum disilicid e can be synthesized on molybdenum substrates under carefully controll ed processing conditions.