Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel

Thick and soft a-C:H:Si coatings containing more than 45% hydrogen (thickness: 25-27 μm, hardness: 6 GPa, Young's Modulus 38 GPa and low ratio of sp³bonds) were deposited by PACVD with a DC pulsed discharge on nitrided (duplex sample) and non-nitrided austenitic stainless steel (coated sample). After deposition, the chemical, microstructural and tribological properties were studied. Finally, the adhesion and the atmospheric corrosion resistance of a-C:H:Si coatings were also investigated. In pin-on-disk tests, the friction coefficient using an alumina pin of 6 mm in diameter as counterpart, under 0.59 GPa Hertzian pressure was 0.05 for the coated samples and 0.076 for the duplex samples. These values were more than one order of magnitude smaller than the friction coefficient of the nitrided sample without coating, which was around 0.65. In the coated samples, the wear loss could not be measured. In ball-on-disk tests under dry sliding conditions, the coatings were tested under different Hertzian pressures (1.29, 1.44 and 1.57 GPa) using a steel ball with a diameter of 1.5 mm as counterpart. Using a normal load of 9 N, the a-C:H:Si coating of the coated samples was broken and detached thus leading to a coefficient of friction of around 0.429. However, in contrast to that, the friction coefficient of the duplex samples remained stable and reached as maximum a value of 0.208. In abrasive tests, mass loss was undetectable in both duplex and coated samples. Furthermore it could be seen that the a-C:H:Si film showed only some smaller grooves and no severe damage or deformation. On the contrary, severe damage was observed in the only nitrided sample. With respect to adhesion, the critical load to break the coating was higher in the duplex sample (27 N) than in the only coated sample (16.3 N). By chemical analysis using the salt spray fog test, the duplex sample remained clean, but the coated sample failed and presented film delamination as well as general corrosion.