Mechanical performance upgrading of CVD diamond using the multilayer strategy

The present work is a comparative study of different combinations of microcrystalline (MCD) and nanocrystalline diamond (NCD) varieties (monolayers, bi-, tri- and fourfold layers) grown by hot filament CVD technique on silicon nitride ceramic substrates. The Hertzian stress states under static indentation are estimated and compared on the basis of the von Mises stress parameter. The diamond multilayer morphology and the structural quality were evaluated step-by-step by SEM, AFM and UV-Raman spectroscopy. By using the multilayer strategy, starting with a MCD adherent layer on Si3N4 ceramic substrates and finishing with a NCD smooth layer at the top, it was possible to keep the surface roughness at a lower RMS level, of around 90 nm. The fourfold multilayer configuration allowed obtaining 10 μm thick diamond coatings that endure an indentation load of 800 N with a Brale cone, before film spalling-off. This behaviour is determined by the high bonding strength of the initial MCD layer grown on the Si3N4 ceramic substrate, where the von Mises stress (J21/2) is estimated as 27 GPa.