Improved Embeddability for Polymeric Bearing Overlays

Recent engine developments toward higher loads (down-sizing) and thinner oil films have increased the severity of plain bearing operating conditions [1]. These factors, combined with lower viscosity oils, have resulted in a greater sensitivity of bearings to damage by foreign debris particles. Traditional highly embeddable materials, such as lead, are being progressively phased out. This lead-free trend observed in the passenger car market is likely to spread to the truck market in the future. As a result, it is becoming increasingly challenging to balance the conflicting hard and soft requirements of bearing materials. Although new generations of bearing materials, particularly polymeric overlays, have shown excellent fatigue and wear capabilities [2], they would benefit from enhanced embeddability properties. This demand has led MAHLE to take a new approach with the development of a polymeric overlay material that has both hard and soft characteristics. This newly developed soft-phase copolymer resin has been synthesized from monomers selected to give the desired properties. Conventional polyamide-imide (PAI) monomers have been combined with polydimethylsiloxane (PDMS) macromonomers. PDMS was selected to improve embeddability as it is softer and offers more flexibility than PAI. Via a polymerization reaction, chains of hard, fatigue resistant PAI are alternately combined with short chains of PDMS. This produces a polymer matrix which has a very fine distribution of soft phase due to the microphase segregation created as the soft and hard segments of neighboring polymer chains preferentially align with each other [3,4]. The relative lengths of the hard and soft sections can be “tuned” to produce domains of differing size and therefore adjust the balance of properties. Experiments have been carried out varying the overall percentage of PDMS and also with the molecular weight of the PDMS segments. Initial embeddability testing has shown an improvement in embedment over current polymer products and further work is ongoing to optimize this new resin system.