Defect analysis of Ag coatings on aluminum castings

Cast aluminum alloys are one of the most widely used structural materials in electrical engineering due to their good mechanical properties, high thermal and electrical conductivity. Passive Al₂O₃ layer formation on the aluminum surface reduces its electrical conductivity. Thus, the Ag coating electrodeposition on the aluminum substrate material allows increasing the electrical conductivity. The coating quality depends not only on the composition and on the quality of the electrolytic bath and technological parameters of the deposition, but a very important factor is the surface material pre-treatment prior to coating. In this study, the causes of the defect occurrence in the form of blisters on the Ag coating surface were analyzed. Blistering is a characteristic sign of local low adhesion strength of the coating to the substrate. Microscopic analysis showed the presence of AlSi fragments between the coating and the substrate, which were formed during machining of the Al casting surface. The difference in electrical conductivity between the AlSi particles and the aluminum matrix created an inhomogeneous field during electroplating, which led to the formation of defects in the coating. The presence of pores on the Al casting surface, resulting from low-pressure casting conditions, may also have contributed to defects formation in the coating.