Friction and wear characteristics of different Pb-free bearing materials in mixed and boundary lubrication regimes

• Tribological studies were carried out on Pb-free engine bearing materials.
• Test materials behave differently when rotational speed is varied.
• Al-Sn based material shows higher friction in the mixed and boundary regime.
• In boundary lubrication, PAI based overlay has better friction and wear properties.
• Worn surfaces exhibited different damages and differ from material to material.

Traditional bearing materials contain different amounts of lead (Pb) because of its friction reducing properties. However, in view of the negative health and environmental impact of Pb, there is growing emphasis on restricting the usage of Pb in engine bearings. Owing to this, new bearing materials that provide at least comparable tribological performance to that of Pb containing alloys are being developed and some new Pb-free materials are being already used in engine bearing applications. It is, however, still unclear how these new engine bearing materials would perform in mixed and boundary lubricated conditions. In this study, a block-on-ring test setup was employed to investigate the tribological performance of several bi-metal and multi-layer Pb-free bearing materials with different compositions of bearing lining and overlay plating. Pb-containing bearing material was also studied as a reference material. Friction and wear properties of these bearing materials were investigated and their wear mechanisms under lubricated conditions have been analysed. Bearing material with Polyamide-Imide based overlay containing graphite and MoS2 exhibited better friction and wear properties than Pb-based and Al–Sn based materials. Pb-containing bearing material shows higher wear of material and Al–Sn based material has shown higher friction compared with the other test materials.