Investigation of wear and scuffing behaviour of ferrous thermal spray coatings for aluminum engines

The development of lightweight internal combustion engines using materials such as cast aluminum alloys represents one of the most significant technological developments in the automotive industry. These engines reduce weight, which in turn reduce fuel consumption and emission. However, poor wear resistance and low seizure load of unprotected Al-Si alloys are a major drawback for applications involving sliding contact in automotive engine blocks. The wear resistance of cast aluminum parts can be improved by depositing coatings on the sliding surfaces. In this respect, iron based coatings deposited through a thermal spray process may play an important role in improving wear resistances of aluminium parts used in the automotive industry. These coatings can be produced economically and be easily deposited on the curved surfaces in ambient air atmosphere. In this research, two promising thermal spray deposition processes were considered: These were (i) plasma transfer wire arc thermal spraying (PTWA) process, and (ii) high velocity oxy-fuel (HVOF) process. The research work presented in this dissertation primarily focussed on the wear behaviour of low carbon steel thermal spray coatings which were applied using PTWA and HVOF processes deposited on engine grade cast aluminum alloy substrates. The main objective of the work was to characterize the micromechanisms of wear that control the wear rates of the coatings. Several new wear mechanisms that were previously unknown in thermal spray coatings were identified. In addition, the effect of the environment on the wear performance of coatings was investigated. The importance of controlling the atmospheric conditions during the sliding contact of coated aluminum components was established. Detailed analyses of compositions and microstructures of iron based coatings that were produced using PTWA and HVOF thermal deposition processes showed that the wear resistances of the coatings were sensitive to the production method. A model to calculate the friction induced contact temperature increase was developed and used to explain the differences in the wear rates of the coatings.; Wear maps for thermal sprayed coatings have been constructed for the first time. The wear maps constructed showed the wear rates as a function of the loading conditions (load and velocity). The potential industrial application of wear maps includes prediction of scuffing behaviour of lightweight engines coated by thermal spray coatings. A laboratory experimental method has been developed based on information provided on the wear maps to simulate the wear mechanisms seen in the scuffed engines.