Mechanism of different adhesive strength of chemical vapor deposition diamond coatings on WC-Co and Si3N4: from surface morphologies during growth

Abstract Substrate has an essential influence on the growth process of diamond grains and the quality of the diamond coating. In this paper, the diamond coatings on Co-cemented tungsten carbide (WC-Co) and silicon nitride (Si3N4) substrates were deposited by hot filament chemical vapor deposition (HFCVD) technique to study the effect of growth process on adhesive strength of coating. The WC-Co and Si3N4 substrates were treated by etching and polishing process, respectively. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the diamond coatings. Furthermore, the adhesive strength and tribological properties of the as-deposited diamond coatings were evaluated by indentation tests and dry sliding tests. The results showed that the diamond coating deposited on WC-Co had a considerable stronger adhesive strength and a higher friction coefficient than that on Si3N4, which may partly result from the flat surface morphologies of Si3N4 substrate that was due to no etching pretreatment. Thereafter, a novel etching method for Si3N4 surfaces by hydrofluoric acid (HF) was proposed to roughen the surface, and a new diamond coating with enhanced adhesive strength was fabricated on Si3N4. Hence, for the hard substrate, etching the surface, rather than polishing the surface, played an importance role on improving the adhesion strength of diamond coating.