Diamond coating of steel at high temperatures in hot filament chemical vapour deposition (HFCVD) employing chromium interlayers

Since diamond coating via hot filament chemical vapour deposition (HFCVD) processes is possible, the coating of steel substrates is in the focus of research programs. As the direct deposition onto steel is not possible yet, the role of interlayers is of extraordinary importance. In this work, we deposited diamond onto steel samples with chromium carbide interlayers at temperatures above 800 °C without film delamination after deposition. Further advantages of this process are an improved chemical activation of the surface at high temperatures, a higher growth rate and a higher quality (sp3-ratio) of the diamond film. The mismatch in thermal expansion of diamond, interlayer and steel leads to high residual compressive stresses in the diamond layer, but about 30% less than theoretically expected. We ascribe this stress reduction to the specific thermal expansion characteristics of a certain class of steels, which show a α-γ phase transformation during cooling down from 850 °C. This transformation from closer packed γ-phase (fcc-structure, austenite) to α-phase (bcc-structure, ferrite) comes along with a volume expansion of approx. 1% and enables the reduction of the high residual compressive stresses in the diamond film, analysed via Raman spectroscopy.