DIAMOND DEPOSITION ON STEEL WITH CVD TUNGSTEN INTERMEDIATE LAYER

Diamond deposition on steel with enhanced adhesion has been performed by using a CVD tungsten intermediate layer between substrate and diamond as a diffusion barrier for iron and carbon. The tungsten buffer layers of 15-45 mu m thickness with highly faceted surface relief have been grown on #R18 steel (79% Fe, 17% W, 4% Cr) by the CVD process from tungsten hexafluoride in a ''hot-wall'' reactor. This technique is suitable for large-scale production with deposition rate up to a few millimeters per hour over a large area. The diamond films were grown in a d.c. are discharge in 5%CH4/H-2 mixture. The samples characterized by X-ray diffraction, Auger electron spectroscopy and cross-section examination revealed a tri-layer coating structure: diamond/tungsten carbide/tungsten. The carbide layer, consisting mainly of WC phase with a small fraction of W2C phase, provides good adhesion, as confirmed by Rockwell indentation tests. As determined from Raman spectra, the films were under high compressive stress, sigma approximate to 7 GPa, originating from the large mismatch in thermal expansion coefficients of steel and diamond. Yet the adhesion accommodates the stress without film delamination.