Bias-graded deposition of diamond-like carbon for tribological applications

Hydrogen-free diamond-like carbon coatings of thickness 1.5 μm were deposited on 440C stainless steel substrates and silicon wafers via DC magnetron sputtering of graphite target at power density of 10.5 W/cm². RF bias voltage was applied in the range of -20 to -150 V in two configurations: constant bias and bias-graded. The structure, surface topography, hardness and tribological behavior of these coatings were investigated. The surface roughness (R_a) of the coating decreased from 5.8 to 3.4 nm as bias voltage increased. Results from nanoindentation showed that the hardness of the coatings deposited at high bias voltage reached at 30 GPa and toughness (plasticity during indentation deformation) from approximately 50 to 60%. The hardness of the coating deposited with bias-graded configuration was comparable to the coatings deposited at high bias voltage but the toughness and adhesion strength were significantly improved. Ball-on-disc tribotests were carried out under dry, water- and oil-lubrication conditions. The result showed that the bias-graded coating had a very low coefficient of friction (down to 0.07 in oil). Furthermore, after 2.5 km sliding against alumina and steel (6 mm in diameter) at a high load of 10 N with and without lubrication, no sign of damage was observed on the wear track. Bias-graded sputtering deposition is an appropriate technique to combine the hardness, toughness and adhesion strength into one coating for demanding tribological applications.