Abstract: In order to reveal the failure mechanism of coating, both scratch test on Ni-P-Al2O3 complex coating surface and the simulation of scratch process with finite element method are carried out. Scratch test shows that transverse surface cracks in an interval between two neighboring cracks appear on the scratch groove surface of Ni-P-Al2O3 complex coating. Finite element simulation of scratch process indicates distributing rules of stress in the coating surface and interface at different stages of scratch process, and reveals coating surface cracks generating mode and forming mechanism. Scratch process is divided into three stages that are stylus tip indenting coating surface, slipping on coating surface and rising from coating surface. In former two stages the scratch groove is formed on the coating surface and the surface cracks are induced by stylus tip acting on coating surface. The stress analysis shows that there are two ways of forming coating surface cracks. One is that coating interface cracks are firstly induced, thus they are propagated to the coating surface, and finally the surface cracks are formed. Other is that cracks are directly formed in the coating surface. Because they are induced by maximum tensile stress, surface cracks are modeⅠ. After stylus tip rising from coating surface, biggish residual stresses remain in the coating. Maximum residual tensile stress is first residual principal stress at the coating interface below the contacting central. Those will supply the basis of coating design and application.

Key words: Complex coating, Finite element method, Scratch test, Stress analysis, Surface crack