Failure mechanism analysis of electroplated chromium coating in repeated scratch tests under subcritical loads

Abstract

Unlike traditional progressive scratch tests, which gradually increase the load to the critical failure level, repeated scratch tests assess the failure behavior of a coating under the repeated application of subcritical loads. It typically provides a more accurate representation of the actual application conditions of a well-designed coating system, where overcritical conditions are not commonly encountered. In this study, the failure behavior of electroplated chrome coating under subcritical loads was evaluated through repeated scratch tests. The subcritical loads correspond to 20 %, 40 %, and 60 % of the critical load at which the coating experiences significant spalling. The results show that the coating exhibits different failure modes in repeated scratch tests compared to progressive scratch tests, in the following order: tensile crack, coating internal delamination, small spalling pit, and large spalling pit. The reason for this difference arises from the repeated loading in the repeated scratch test, and coating failure is influenced by the repeated frictional action of the indenter, which can be considered a cyclic wear test at low loads. The cutting and plasticity ratio shows that there are two deformation mechanisms, micro-plowing and micro-cutting, for scribing coating by the indenter and that the micro-cutting mechanism is dominant in the scribing process. As the load and the number of cycles increase, the proportion of micro-cutting continues to rise. Repeated scratch tests at subcritical loads provide a more comprehensive assessment of failure at different loads than progressive scratch tests and give clearer insights into the coating failure formation process.