Comparative Study of Tribological Characteristics of Magnetron Sputtered CrN and Carbon Doped CrCN Coatingson Automotive Piston Rings Under Boundary Lubrication

Friction and wear are major challenges in automotive engines, and they can lead to significant operating issues such as increased fuel consumption, component damage, heat generation, loss of performance, etc. Friction and wear between piston rings and cylinder liner are significant issues in automotive engines, and researchers continually focus on reducing them to improve engine performance and efficiency. Numerous studies have been carried out to minimise friction and wear losses. However, coating is one of the promising techniques used in the automotive industry. PVD-deposited, CrN-coated piston rings demonstrated excellent tribological properties. However, CrN coatings become less effective under conditions of lubrication starvation, exhibiting a high coefficient of friction. In the present study, PVD-based magnetron sputtered CrN and carbon-doped CrCN coatings were applied to the surface of the piston rings. The structure, elemental composition, surface morphology, thickness, and hardness of the coatings were characterised systematically. The coated samples underwent tribological testing under loads of 50, 80, and 110 N to assess the coefficient of friction at the piston ring-cylinder liner interface under the boundary lubrication condition and evaluate the wear behaviour of the coated ring samples. The carbon-doped CrCN coating exhibits a low coefficient of friction at each load and exhibits a 16.59% reduction in the coefficient of friction compared to the CrN coating. This could be ascribed to the graphitisation effect of the CrCN coating. However, both coatings showed almost equal and minimal wear loss. Finally, the possible worn morphology of coated samples was proposed based on their microstructure, film deposition, and Raman's spectroscopy results.