Evaluation of adhesion durability of Ni-P coating using repeated Laser Shock Adhesion Test

This study aims to evaluate the adhesion strength and durability of Ni-P coatings in a non-contact manner using the Laser Shock Adhesion Test (LaSAT). A hard coating of electroless Ni-P deposited on carbon steel substrate (SKD11) was prepared, and the effect of post heat treatment on adhesion was comprehensively investigated in this study. With the LaSAT, strong elastic waves induced by pulsed YAG laser irradiation applies tensile stress to the coating/substrate interface resulted in interfacial delamination. Computation of the elastic-wave propagation using finite element method (FEM) was carried out to estimate the interfacial tensile stress. In addition, the adhesion durability was investigated by repetitive pulsed-laser irradiation, and the relationship between adhesion strength and the number of laser irradiation cycles until delamination was obtained. Finally, the effects of heat treatment on the adhesion strength and durability of the Ni-P coating were discussed. Owing to the postheat treatment, it is found that the hardness of coating, adhesion strength and durability were improved. Indentation test reveals that the coating hardness increases from 792 HV to 1163 HV. LaSAT quantitatively evaluated the adhesion strength with increasing from 84.4 MPa to 143.9 MPa. In addition, repetitive LaSAT reveals that adhesion durability improves twice or more. This result was verified using the tensile adhesion test and microscopic observations of the interface. Therefore, the present LaSAT is a quick measurement for the adhesion durability of hard coating, and it may shed some light on quality control of surface coatings.