Effects of Crack Density on Wettability and Mechanical Properties of Hard Chrome Coatings

Abstract

Hard chrome is an important coating used widely in the industry, yet the understanding of its plating process in relation to surface crack development and corresponding properties has not been fully established. This research has investigated the development of surface crack of hard chrome through the variations of chromic acid concentration, catalyst content and plating temperature, and subsequently examined how crack density contributes to wettability and mechanical properties in dry and lubricated environments. The study has revealed that an increase in crack density in the low-to-medium crack range (150–400 crack/cm) is generated due to the decrease in the chromic acid-to-catalyst volume ratio and the increase in temperature. These process parameter adjustments has led to reduction of cathodic current efficiency and hydrogen gas development which can ultimately generate stress in the deposits. An increase in crack density has contributed to the marked improvement of wettability with a decrement of the contact angle from 8.5° to 4.2°. Hardness has also been found to increase from 720 to 830 HV. Furthermore, crack density increment has also resulted in the reduction in wear rate of the coatings in a non-lubricated condition. The hardness of the coating and the presence of cracks appear to largely influence the improvement of the wear resistance.