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. As a result, there is possibility that the current DLC coatings may not allow piston rings to function properly in the next-generation HDD engines, which will have higher thermal load and even more severe sliding condition.
For that reason, development of new surface treatment for piston rings, to be used in next-generation HDD engines, was conducted. In this paper, the development of advanced CrN coating as a replacement to DLC coating will be reported. This advanced CrN coating is a nano-composite CrN coating in which substitutional solid solution element X is added by ion plating method. In this paper, this coating will be referred to as the Advanced Nano-composite CrN-X (Nano-α) coating.
Initially, basic sliding properties of the Nano-α coating required by the piston ring were confirmed. Test rigs as well as floating liner device were used to confirm the wear resistance, scuffing resistance, and friction performance. Next, heating test was performed to confirm the heat resistance of the Nano-α coating, the most critical property of the next-generation HDD engines. And finally, to determine the suitability of Nano-α coating for piston rings, durability test was conducted with the current production HDD engine.
Results of the tests confirmed that the sliding properties of the Nano-α coating were similar to or better than the conventional CrN coating and DLC coating that are currently used in the market. In addition, friction performance of the Nano-α coating improved further when MoDTC was added to the engine oil. Also, the heat resistance of Nano-α coating was experimentally confirmed to be exceptional compared to the DLC coating. The effectiveness of this newly developed Nano-α coating as a surface treatment for the piston rings, to be used in the next-generation HDD engines with much higher thermal load, will be reported in this paper.