Wang, G., Nie, X., and Tjong, J., "Investigation into Mixed and Hydrodynamic Frictions of PEO Coatings and Cast Iron," SAE Int. J. Fuels Lubr. 9(1):23-31, 2016, doi:10.4271/2016-01-0491.
A linerless aluminum (Al) engine block has potential to reduce the weight of an automotive engine and improve the fuel economy. However, the Al cylinder surface of an aluminum engine block is not usually strong enough to withstand the sliding wear against piston rings. A few surface processing technologies are used to protect the surface of cylinders. Among them, a thermal spraying coating, such as plasma transferred wire arc (PTWA) is already popular. Plasma electrolytic oxidation (PEO) coating is also proposed for increasing the wear resistance of aluminum-silicon (Al-Si) alloys and reducing the friction between the cylinder and piston. In this work, two different PEO coatings with a thickness of around 23 μm were prepared on an Al-Si alloy A356, and a high speed pin-on-disc tribometer was used to study the tribological behavior of the coatings at oil lubricant conditions. A cast iron sample was also used to do similar tribological tests for comparison. The coefficient of friction (COF) vs surface roughness (Ra: 0.2 - 0.8 μm) and sliding speeds (up to 6.07 m/s) were particularly studied. The results show that the COF significantly decreased with the increase of sliding speeds, and a smoother coating surface generally exhibited a lower COF and a steeper descent rate of the COF. While such observations seem true for both PEO coatings and the cast iron sample, the polished PEO coatings can have a lower COF than cast iron. The study indicates that the Al-Si alloy with PEO coatings could be further explored as a feasible solution to reduce the weight and improve the fuel efficiency of an Al engine.