The effect of thermal fatigue on the hardness of hard chromium electroplatings

Due to the low coefficient of friction and also high hardness, electrodeposited chromium layers possess a high resistance to erosion. However, at high temperatures and in particular when objects are subjected to thermal stress, the hardness and, as a result, resistance to erosion decreases. In this research, the effect of thermal fatigue on the hardness of hard chromium electroplatings subjected to different thermal cycles was studied. The coatings were applied electrochemically on a 32NiCrMo145 steel, and possessed different degrees of contraction and crack density, namely low contraction, cracked and high contraction. It was found that after 50 cycles, the hardness of all types of coating examined in this study decreased from above 12.7±0.05 GN m−2 to below 7.3±0.05 GN m−2, whereas the hardness of the heat-affected zone (HAZ) of the substrate increased from 5.6±0.05 to about 8.2±0.05 GN m−2. These changes are discussed in the light of phase changes that occur and oxide formation during heating.