Reactions between tungsten and molybdenum thin films and polycrystalline diamond substrates

Reactions between thin sputtered tungsten and molybdenum films and polycrystalline diamond substrates were studied in the temperature range 675-1120 degrees C. The mass transport effects and carbides that formed were characterized by Rutherford backscattering (RBS), X-ray diffraction and transmission electron microscopy (TEM) methods. It was found that for tungsten films, W2C formed at low annealing temperatures (800 degrees C), whereas at higher temperatures only WC was observed. For molybdenum films only Mo2C formed. The formation of W2C at the vacuum surface was favored relative to the metal-diamond interface. The reverse was true for Mo2C where, in addition, a surface layer of graphite formed at temperatures of 1020 degrees C. No similar graphite formation occurred in WC films below 1100 degrees C. From both the RBS spectral profiles and TEM observations a model for interstitial and/or grain boundary diffusion assisted carbide formation is suggested. These results for tungsten are consistent with the Bene' rule for the sequence of compound phase formation in metal-metal couples.