Tribological Behaviour of Pb-free Engine Bearing Materials
Engine bearings are amongst the most critical components of an internal combustion engine that support and allow smooth rotation of the crankshaft. They are designed to operate under hydrodynamic lubrication condition where the bearing and shaft surface are separated by a thick lubricant film. However, they also occasionally operate under mixed and boundary lubrication conditions particularly during starting, stopping and load, speed and temperature variations. Under these conditions, tribological performance of bearing materials is crucial for the satisfactory performance of the engine.
Traditionally, the most extensively used engine bearing materials have been Copper-Lead (Cu-Pb) based linings and Pb based overlays because of the friction reducing properties of Pb. However, due to the adverse health and environmental impact of Pb, there is growing emphasis on restricting the usage of Pb in engine bearings. Owing to this, new Pb-free bearing materials that provide at least comparable or superior tribological performance to that of Pb containing materials are being developed. Some of these materials have already been introduced in engine bearing applications.
There are, however, only few research results in the open literature as to how these new engine bearing materials would perform in mixed and boundary lubricated conditions. The objective of this work is to evaluate and understand the tribological performance of selected Pb-free engine bearing materials and compare their performance with that of the traditional Pb-containing material. To understand the damage mechanisms in the traditional Pb-containing bearings, a full set of main and connecting rod bearings from field test run in Euro V truck engines in long haulage application with European diesel fuel and with slightly longer oil drainage interval were investigated.
Furthermore, laboratory tests on Pb-free engine bearings with different compositions of lining and overlay materials were carried out with a block-on-ring test set up in order to evaluate their tribological performance. For this study, aluminum-tin (Al-Sn) based lining with no overlay; Cu-based linings with overlay of polyamide-imide (PAI) containing MoS, 2 and graphite, Al-Sn based overlay and Sn based overlay were studied. Cu-Pb lining with Pb-based overlay was also studied as a reference.
Investigations on a full set of main bearings and connecting rod bearings from field test revealed that the major damage mechanisms were 3-body abrasive wear leading to exposure of lining material, flaking of overlay material due to surface fatigue, formation of compound layer composed of Sn, Cu and Ni and cavitation damage. Laboratory tests on Pb-free bearing materials have shown that Al-Sn based lining with no overlay shows higher friction than the other materials at lower rotational speed. For Al-Sn based lining and Pb-based overlay materials, the decrease in friction is relatively sharp as rotational speed increases compared with the PAI based overlay. Test samples with overlay of PAI containing graphite and MoS 2 exhibited better friction and wear properties than Al-Sn based and Pb-based materials. Under steady-state conditions, Pb-containing bearing material shows higher wear and Al-Sn based material has shown higher friction. In addition, Sn-based and Pb-based overlays have shown similar friction behaviour when rotational speed is varied. For relatively longer test durations, samples with Sn overlay exhibited comparable friction and wear with that of Pb-based overlay material.