Study on Thick Coating Prepared by Composite Process of Brush Plating and Electric Contact Strengthening

1.6 mm nickel coating was prepared on the surface of nodular cast iron by composite process of brush plating and electric contact strengthening. The microstructure of raw coating and strengthened coating was analyzed by ultra depth of field microscope and hardness tester, and the coatings were tested by file according to QB/T3821-1999. The results show that the defects such as cracks and holes increase with growing of coating. The overall microstructure characteristics of the raw coating can be roughly divided into three areas, such as the "crack area" near the substrate, the "pore area" in the middle and the "loose area" in the surface. In the process of grain deposition of brush plating, the formation of defects is related to the collision and mergence between grain clusters. After strengthening, the regional characteristics basically disappear, the gaps are pressed, and few defects exist at the outer edge of coating, with coating’s hardness increasing from 473.50 HV to 540.32HV. This is mainly due to the coupling effect of heat and press of electric contact strengthening. The resistance between defect and interface is highest with heating concentrated, and the drag forces at the interface or gaps are weakest, so the defects there are filled more easily. After file test, it is found that the coating is peeled off before strengthening, but not after strengthening,indicating that the electrical contact strengthening could improve the adhesion between the coating and substrate. The further analysis of the interface shows that there are local mutual fusion, bridging and inlay between the coating and the matrix.