The effect of changing the N 2/CH 4 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 CH 4 and N 2 (in a balance of H 2) was shown to strongly influence film structure, hardness, and adhesion. For high CH 4 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 N 2/CH 4 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. © 2002 American Institute of Physics.