Tribochemistry of Lubricant Additives on non-ferrous Coatings for reduced Friction, Improved Durability and Wear in Internal Combustion engines

• Main Author: Haque, Tabassumul
• Published: University of Leeds 2007
• Subjects: 621.89
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487511

Increasing demand for improved fuel economy 'and environmental protection has meant that there has been increased focus on the implementation of non-ferrous materials and coatings which have excellent tribological properties for automotive applications. Since all . . commercially. available lubricants are typically optimised for ferrous materials, it is therefore expected that lubricants will not perform in an optimised way on alternative surfaces: It is therefore crucial to understand how the existing lubricants interact with non- . . ferrous coatings to be able to sensibly approach the design of new additives for non-ferrous surfaces. In this study, CrN and DLC coatings which have excellent generic tribological properties, chosen for a full evaluation of their tribochemistry using both model and fully formulated oils. Tribological tests were performed using three test rigs, namely pin-on-plate, min~ traction machine (MTM) and cam/follower rig. The pin-on-plate tests were performed to evaluate friction, wear and fturability of CrN and DLC coatings under boundary lubrication .. conditions. Based on the results obtained from pin-on-plate tests, a' few coatings were chosen for cam/follower tests. In addition, MTM tests were performed to investigate the effect of slidelroll ratio and lubrication regime on the tribologicalltribochemical performance of a particular DLC coating. Based on the MTM results, the link between the results obtained from pin-on-plate and cam/follower rigs were assessed. Using the state-of-the-art surface analysis techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS), both physical and chemical characterisation of tribofilms formed by lubricant additives on CrN and DLC coatings have been characterised. Finally, they are correlated with the tribological results and also compared with those observed in conventional ferrous systems. This study has shown that the tribological performance of CrN and DLC coatings are oil dependent. The physical and chemical analyses of tribofilms suggest that the CrN coating responds in a similar manner to steel surfaces to the conventional additives. Optimisation 'of lubricant additives has been found to be immensely important, especially for the effective lubrication of110n/low-hydrogenated DLC coatings.