Contact fracture mechanism of electroplated Ni–P coating upon stainless steel substrate

The contact fracture property and mechanism of electroplated Ni–P coating on stainless steel substrate were investigated using ball indentation testing, through a comprehensive experimental and numerical approach. First, the elastoplastic properties of both coating and substrate were evaluated using micro-indentation tests. Next, ball indentation test with large contact force was performed, such that the brittle coating on ductile substrate suffers from cracks, including ring crack (propagates circumferentially) and radial cracks (propagates radially), owing to the coating bending effect. The fracture nucleation process was investigated using the acoustic emission technique (AET). In addition, finite element method (FEM) with cohesive zone model (CZM) was carried out to compute stress field and simulate crack initiation around the impression during the test. By using the comprehensive experimental/computational framework, the nucleation process (mechanism) of such a complicate crack system was clarified. The present technique and fracture mechanism may be applicable to the analysis of structural integrity of other brittle coatings.