Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H 2 /CH 4 /N 2 gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5-6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C 2 species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H 2 /CH 4 gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H 2 /CH 4 /N 2 plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer H α line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/H α ratios with the coating roughness and grain size were also confirmed with different set of gas flows/plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H 2 /CH 4 /N 2 gas mixture. The RMS roughness of 4 nm and grain size of 5-6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/H α species in the plasma. Wear tests were performed on the OrthoPOD ® , a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.

The effect of changing the N2/CH4 feedgas ratio on the structure and mechanical properties of microwave plasma chemical vapor deposited diamond films grown on Ti–6Al–4V alloy substrates was investigated. The relative concentration of CH4 and N2 (in a balance of H2) was shown to strongly influence film structure, hardness, and adhesion. For high CH4 concentration (15% by volume), nanostructured diamond films with roughness magnitudes of 15–30 nm, good adhesion and a high hardness value of 90 GPa was obtained. A distinct correlation was found between the nanoindentation hardness of the deposited film and the N2/CH4 ratio in the plasma, as well as a correlation of hardness to the ratio of the Raman peak intensities (1332 and 1555 cm−1). Scratch adhesion testing of nanostructured diamond films showed delamination at a critical force of 33 N using acoustic emission techniques. These results demonstrate that nanostructured diamond films can be tailored on metallic surfaces with hardness ranging from 10 GPa (medium-hard) to 100 GPa (superhard) and may be considered for wear resistant applications such as in the design of articulating medical implant devices.