Strengthening of cemented tungsten carbide by boriding is used to improve the wear resistance and lifetime of carbide tools; however, many conventional boriding techniques render the bulk carbide too brittle for extreme conditions, such as hard rock drilling. This research explored the variation in metal-boride phase formation during the microwave plasma enhanced chemical vapor deposition process at surface temperatures from 700 to 1100 °C. We showed several welladhered metal-boride surface layers consisting of WCoB, CoB and/or W 2 CoB 2 with average hardness from 23-27 GPa and average elastic modulus of 600-730 GPa. The metal-boride interlayer was shown to be an effective diffusion barrier against elemental cobalt; migration of elemental cobalt to the surface of the interlayer was significantly reduced. A combination of glancing angle x-ray diffraction, electron dispersive spectroscopy, nanoindentation and scratch testing was used to evaluate the surface composition and material properties. An evaluation of the material properties shows that plasma enhanced chemical vapor deposited borides formed at substrate temperatures of 800 °C, 850 °C, 900 °C and 1000 °C strengthen the material by increasing the hardness and elastic modulus of cemented tungsten carbide. Additionally, these boride surface layers may offer potential for adhesion of ultra-hard carbon coatings.
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, W 2 CoB 2 , and/or CoB phases using controllable diborane stoichiometry;this research exploresthese borides as interlayers for nanostructured diamond coatings. Diamond deposition occurred between 600 °C and 1100 °C at 100 °C increments for 30 min to 4 hrs. Raman spectroscopy showed enhanced diamond growth compared to untreated WC-Co,however predominantlyWCoB and CoBphase interlayerssuffered from diamond film delamination. Examination by scanning electron microscopy and energy dispersive X-ray spectroscopy showed interfacial cobalt clusters on these interlayers. Predominantly W 2 CoB 2 phase interlayers showed improvement with a reduction in reactive cobalt, and improved adhesion of nanostructured diamond coatings. Diamond on W 2 CoB 2 was well adhered with deposition temperature dependentnanoindentationhardness ranging from 10 to 60 GPaand elastic modulus of 400 to 750 GPa. Scratch testing revealed cohesiveand 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 8.2 MPa ± .1MPa. The W 2 CoB 2 phaseis shown to be desirable as an interlayer for improved nanostructured diamondadhesion on cemented tungsten carbide.
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