Improved nanostructured diamond adhesion on cemented tungsten carbide with boride interlayers

Abstract

The application of diamond coatings for strengthening cemented tungsten carbide has been previously attempted, but suffers from delamination. Plasma enhanced chemical vapor deposition boriding improves the strength of cemented carbides by forming WCoB, W2CoB2, and/or CoB phases using controllable diborane stoichiometry; this research explores these borides as interlayers for nanostructured diamond coatings. Diamond deposition occurred between 600°C and 1100°C at 100°C increments for 30min to 4h. Raman spectroscopy showed enhanced diamond growth compared to untreated WC-Co, however predominantly WCoB and CoB phase interlayers suffered from diamond film delamination. Examination by scanning electron microscopy and energy dispersive X-ray spectroscopy showed interfacial cobalt clusters on these interlayers. Predominantly W2CoB2 phase interlayers showed improvement with a reduction in reactive cobalt, and improved adhesion of nanostructured diamond coatings. Diamond on W2CoB2 was well adhered with deposition temperature dependent nanoindentation hardness ranging from 10 to 60GPa and elastic modulus of 400 to 750GPa. Scratch testing revealed cohesive and adhesive failure of the diamond coatings at 5N±2N and 8N±2N respectively and epoxy pull testing resulted in a surface adhesion tensile strength of 8.2MPa±0.1MPa. The W2CoB2 phase is shown to be desirable as an interlayer for improved nanostructured diamond adhesion on cemented tungsten carbide. Prime novelty statementThe scope of diamond deposition parameters used and the comparison of interlayer effect for CVD diamond coatings on ternary borides (WCoB, W2CoB2) with a cemented tungsten carbide substrate has not been previously examined to this level.