Tribological behaviour of polymers in lubricated contacts

Abstract: An important issue in hydropower and other industries is the increasing demand for introduction of environment friendly solutions. Mineral oil based lubricants have long been used in various sliding bearings in hydropower stations. Their use in aqueous environments however raises concerns about the environmental impact if they leak into downstream water. This has necessitated research in replacing mineral oils with more bio-degradable lubricants and the ultimate goal of ‘oil-free’ hydropower generating machines. Replacing oil with water however poses many challenges. Due to considerably lower viscosity of water compared to that of turbine oils, the water lubricated bearings are likely to operate in boundary/mixed lubrication regime for relatively longer period. Therefore choice of the materials and their tribological performance are very important for the bearings operating in the boundary/mixed lubrication regimes. Application of compliant polymers in water lubricated bearings introduces many advantages which cannot be achieved with coatings (DLC, etc.) or ceramics. Most previous tribological studies on polymers have been carried out in dry conditions and only a few studies in presence of water have been reported. This work is thus aimed at investigating the tribological behavior of some selected polymer materials in water lubricated conditions. The results of these studies provide an insight into polymers’ tribological performance in boundary/mixed and hydrodynamic lubrication regime and associated wear mechanisms in presence of water. Polymers also enhance performance of oil lubricated bearings. Application of polymers in the oil lubricated bearings provides a smooth transition from oil lubricated Babbitt bearings to water lubricated polymer bearings. Therefore a part of this thesis is also aimed at investigating the tribological characteristics of several polytetrafluoroethylene based materials at the onset of sliding (break-away friction) at different pressures and temperatures. The results of this studyshow significantly lower breakaway friction of PTFE materials compared to Babbitt at all pressures and temperatures. SEM investigations revealed wear modes of the PTFE materials and the abrasive nature of hard fillers. Bronze-filled, carbon-filled and pure PTFE were found to provide lower and more stable break-away friction and generally superior properties compared to the other materials.