Tribocorrosion Behaviour of Duplex S/Cr(N) and S/Cr(C) Coatings on CoCrMo Alloy in 0.89% NaCl Solution

In the present study, magnetron sputtering was used to deposit two types of duplex coatings, S/Cr(N) and S/Cr(C), onto ASTM F75 CoCrMo cast alloy, aiming at improving the tribocorrosion behaviour of the medical alloy. The duplex coatings were fabricated and structured such that the inner layer comprised carbon S phase with a CoCrMo matrix by sputtering a CoCrMo alloy target in the presence of carbon containing atmosphere. The duplex coatings were found to be elastically compatible with the CoCrMo alloy, and possess higher hardness and a larger hardness-to-modulus ratio than the uncoated alloy. Tribocorrosion tests were conducted in 0.89% NaCl solution at 37oC under elastic contact and unidirectional sliding conditions. The results showed that the duplex S/Cr(N) coating was very effective in improving the tribocorrosion behaviour of the alloy at all applied potentials, in terms of reduced friction and much improved resistance to total material loss by up to more than one order of magnitude. On the other hand, the duplex S/Cr(C) coating was effective only at open circuit and anodic potentials, but not at cathodic potentials. The uncoated alloy exhibited low friction and low total material loss at a large cathodic potential and experienced peculiar frictional behaviour at the tested anodic potential. The results are discussed in terms of material transfer, hydrogen charging, third body effects and synergism between wear and corrosion.