STRENGTH EVALUATION AND EFFECT OF GRAPHITE ON STRENGTH OF ELECTROLESSNICKEL-PLATING ON CAST-IRON

The effect of graphite on the strength of electroless nickel plating o n cast iron was studied. Specimens of cast irons with four types of gr aphite and of 0.4% carbon steel were prepared and machined into plates with dimensions of 10 mm x 3 mm x 80 mm. Electroless nickel plating, about 40 mum thick, was deposited on the test pieces. The plated test pieces were tested by three-point bending tests using both acoustic em ission (AE) and microscopic observation to evaluate the strength of th e plating film. It was found that the first AE signal was generated wh en the cracks initiated and the final AE signal was generated when the film was fractured by crack penetration into the film. In addition it was found that film cracks on cast iron were initiated by the graphit e existing at the interface between the plating film and the substrate , and propagated to the surface of the film, unlike carbon steel. The strength of the plating film on cast iron measured by this method, dec reased more sharply with increasing amount of graphite than with graph ite shape. Observations of cast iron surfaces at early stages of plati ng showed that the nickel was deposited only on the matrix and not on the graphite. It is believed that the non-deposited areas of the cast iron acted as types of defects. It is concluded that the strength of e lectroless nickel plating film on cast iron is strongly influenced by graphite on the surface.