Initial programme target of 15 m thick coatings significantly exceeded. Potential of diamond as a wear-resistant coating for abrasive valve service clearly demonstrated. Films with low residual stress appear to exhibit the longest lifetimes. When the coatings fail, failure tends to occur catastrophically along the coating-substrate interface. The grain boundaries do not appear to represent an easy route for crack propagation, with the inter-granular adhesion strength appearing to be as high as that of the grains themselves.
Further research is necessary to better characterise and understand the failure mechanisms.
In general, excellent quality diamond growth has been achieved on both the Co and Ni binder substrates investigated in this work. However, good adhesion was found to be critically dependent on substrate finish, pre-deposition treatment and the growth procedure. Substrate size also played an important role in our reactor, most likely through an increased non-uniformity of coating thickness and stress distribution for the largest substrates. No benefits were seen from the limited work carried out on the use of interlayers of a number of other materials. Many factors influence adhesion and more work is required to better understand these and hence optimise this critical property.