Protective double-layer coatings prepared by plasma enhanced chemical vapor deposition on tool steel

Hydrogenated diamond-like carbon films were prepared on high-speed steel substrates using low pressure radio-frequency capacitively coupled discharge (13.56 MHz) using methane mixed either with hydrogen or argon. A dc self-bias was induced by the rf discharge and accelerated the ions towards the substrates during the whole deposition process. Prior to the carbon film deposition and to improve the adhesion, the substrates were subject to plasma nitriding and coated with a silicon oxide layer using the same reactor. The deposited films were optically characterized (UV-IR) and by using a combination of Rutherford backscattering spectroscopy and elastic recoil detection the atomic composition was determined. The carbon films high hardness (~ 18 GPa) was assessed from indentation tests. Adhesion tests revealed critical loads up to 13.6 N for the carbon films deposited on steel substrates using silicon oxide interlayer. Friction coefficient varied from 0.02 against diamond and 0.23 against steel counterpart. The results suggest that hard carbon films can be deposited on steel substrate using a silicon oxide intermediate layer deposited by the same plasma process with commercial potential.