Characterisation of smooth fine-grained diamond coatings on titanium alloy by TEM/EELS, Raman spectroscopy and X-ray diffraction

Smooth fine-grained diamond (SFGD) coatings have been deposited by a two-step microwave PACVD process on titanium alloys at 600 °C. The first step consists in depositing, from a methane-rich CH4–H2 mixture, a sacrificial sp2-carbon containing layer that can diffuse partly or completely during the diamond growth which is carried out in a second step from a CO2–CH4 inlet mixture. The coating microstructure and carbon hybridisation state were studied by X-ray diffraction, visible and UV Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The results are compared with those of classical rough polycrystalline diamond coatings deposited from a 8% CO–H2 mixture. The SFGD surface roughness is low, in the 15–35 nm range. Both the surface roughness and the size of the coherent diffracting domains of the SFGD coatings decrease with increasing inlet CH4 content. While the polycrystalline coatings exhibit a competitive growth of grains, the SFGD coatings present an overall columnar microstructure, especially at the lowest CH4 content. At higher inlet CH4, the amount of smaller grains increases because of some twinning and/or renucleation and a stronger 〈011〉 texture is observed. The EELS measurements show a clear diamond structure with very little evidence of sp2-carbon incorporation. The visible and UV Raman spectra show a more intense sp2-bonding contribution for all coatings, especially those deposited from CH4-rich mixtures. Finally, it is shown that smooth diamond coatings can be deposited at moderate temperature with a sufficient diamond quality for mechanical applications.