Microstructure and properties of polycrystalline diamond coatings synthesized by HFCVD at high methane concentrations

In this paper we present the results of experimental studies of the dependence of the structure and mechanical properties of diamond films grown by HFCVD method in an H$_2$/CH$_4$ atmosphere on variations in the methane concentration (from 5.6 to 19.3 vol.%). Films synthesized at 5.6 vol.% CH4 have a columnar microcrystalline structure. The microstructure of films grown at high methane contents (12.2-19.3 vol.%) contains both individual diamond crystals and dendritic clusters, the calculated values of the crystal sizes of such films are $\sim$5 nm. Depending on the deposition parameters, the hardness and elastic modulus of diamond films varied from 50.4 and 520 GPa to 95.15 and 974.5 GPa, respectively. With an increase in the CH$_4$ concentration during deposition, an increase in residual tensile stresses occurs. In addition, the dependence of residual stresses on the film thickness is presented. Ultrananocrystalline coatings have, on average, order of magnitude lower values of surface roughness than microcrystalline ones; in all films, an increase in roughness is observed with an increase in their thickness.