The effect of microstructure and lead content on the tribological properties of bearing alloys

Owing to the presence of lead in the alloy, leaded tin bronzes have excellent seizure resistance in bearing applications where lubrication may become absent or impaired. However, ecological concerns over the toxicity of lead have caused a growing demand for a feasible substitute. This requires a thorough understanding of the role of microstructure and lead content on the tribological properties of leaded tin bronze bearings. In this study, the effect of these qualities was tested with a specialised device simulating thrust bearings in boundary lubrication conditions, as well as with conventional unlubricated pin-on-disc testing. Ball-on-disc testing was also tried to see the effect of reversing the pin-on-disc material pair on tribological behaviour in dry sliding. The presence of lead in the tested bronze alloys was found to increase the capability of the alloy to sustain a lubricant film. The effect of the actual amount of lead was small beyond 4 wt%. In pin-on-disc testing, the microstructure of the alloy was found to have a significantly higher effect on tribological behaviour than lead content. Fine microstructure and small lead globule size were found to lead to a tendency to microcracking at the lead/copper matrix interfacial boundaries, resulting in rapid wear rates and low friction coefficients in dry sliding. The capability of lead to smear between sliding surfaces was verified in ball-on-disc testing.