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Kalubowila, Prematilake Wimaladarma (1984) Diffusion problems in lead-tin overlay bearings. Doctoral thesis, City of London Polytechnic.
The corrosion of lead-tin overlay bearings in hot lubricating oils, through the depletion of tin in the overlay, has been recognised as a problem of some severity in the automotive bearing industry. The inter-diffusion of tin in the overlay and the copper in the underlying substrate results in the formation of copper-tin intermetallic compounds, consequently depleting tin in the overlay. This renders the bearing subject to corrosion in degraded oil. Attempts have been made to slow down this diffusion process by placing electro- deposited barriers between the bonding material and the overlay. The most common barrier is a thin electro-deposit of nickel. Claims have been made that this nickel barrier retards the growth of intermetallic compounds. However, no evidence to this effect has been found in the present work. The nickel barrier, as well as other barriers, forms intermetallic compounds with tin at almost the same rate as when a barrier is not present.
This work was carried out to investigate the problem of diffusion in detail and to search for possible solutions. The kinetics of the diffusion processes involved was studied. After experimenting on a number of barriers, two successful barriers were discovered. These are electrodeposited alloys of Cu-P and Cu-B. A mechanism of prevention of intermetallic compound growth and tin depletion by the two barriers is discussed.
Another important outcome of the investigation is that it has been established that the diffusion of copper and nickel are the rate controlling elements in the process of formation of the respective intermetallic compounds with tin. This disproves the frequently held view that the diffusion of tin, commonly associated with its low melting point, is the rate controlling step.
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