Electroplated Overlays for Crankshaft Bearings

Overlays of either lead-indium or lead-tin-copper are electroplated onto both lead-bronze and aluminum alloy crankshaft bearings to improve seizure resistance and conformability during the initial running-in period. In addition, both the corrosion resistance, particularly of lead-bronze, and the effective fatigue strength of the composite bearing are improved by this layer. The life of the overlay is largely dependent upon the diffusion rate of the low melting point species to the substrate. Thus, migration of either the indium or the tin will determine both the corrosion and wear rates of the overlay. Owing to the processing requirements, aluminum bearings require a nickel or copper interlayer prior to final overlaying with either of the lead alloys. For diffusion control reasons, when depositing lead-tin-copper onto lead-bronze it is usual to have a thin nickel dam to retard the formation of copper-tin intermetallics, which under given conditions may reduce the overall strength and adhesion; lead-indium does not require such a dam on lead-bronze. The principal differences between the two overlays lie in their respective fatigue and wear properties. Thus, lead-indium has a higher fatigue strength but lower wear resistance than lead-tin-copper. This paper compares these two major overlays and considers the selection criteria for the overlay employed.