Ni–Mo–Co ternary alloy as a replacement for hard chrome

Abstract

Hard chrome is the most extensively used electroplated coating in the aerospace and automotive industries due to its attractive properties such as high hardness and excellent wear resistance. However, due to the health risks associated with the use of hexavalent chromium baths during electroplating, there is a need to identify an alternative to this coating. In this study a nickel–molybdenum alloy with cobalt as the alloying element has been developed. The coating was characterized for its micro hardness, wear resistance, coefficient of friction and corrosion resistance. The coating was also subjected to heat treatment at temperatures in the range of 200°–600°C. It was observed that the micro hardness of Ni–Mo–Co (730KHN) alloy coating under optimized conditions is apparently quiet similar to that of the most probable substitute Co–P (745VHN) and hard chrome (800VHN) coatings. The tribological properties like the wear rate and coefficient of friction of the 400°C heat treated Ni–Mo–Co coating were noticed to be better compared to hard chrome coating. The electrochemical impedance and polarization studies showed that the corrosion resistance of heat treated Ni–Mo–Co alloy was better than as-deposited Ni–Mo–Co and Ni–Mo coating.