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To improve the thermal efficiency of next-generation engines, the cylinder pressure will be increased further, and thermal load is expected to increase. In addition, to reduce friction loss, the use of low viscosity oils is also being considered. These changes will make the sliding condition of piston rings much more severe, especially for Heavy Duty Diesel (HDD) engines. Therefore, piston rings require surface treatment on their peripheral surface, not only to prevent wear and scuffing, but also to reduce the friction force. Consequently, Diamond-like Carbon (DLC) coatings have been adopted recently as surface treatment for piston rings. However, heat resistance of DLC coatings is known to be low, and suggestions have been made that these coatings are not suitable for use under high temperature environment.