Nitriding has been utilized for surface treatment of automotive parts as a typical cheap and reliable processing. Recent requirement to reduce the environmental burden, denies usage of cyanides or cyanates and excessive exhaustion of ammonia gases, which are widely used in practice. Among several candidates, the plasma nitriding might be well recommended as an alternative, environmentally benign surface treatment. Typical features among the above three approaches are first introduced in the present paper to demonstrate the usefulness of plasma nitriding. Both the commercial high alloy steel with type of SR34 and the Fe-14Cr binary alloy were used to understand the feasibility of plasma nitriding. Among various surface-treated steel automotive parts, a piston ring was employed to consider the possibility of plasma nitriding for alternative surface treatment method with comparison to the gas nitriding, the chromium wet-plating and the CrN PVD coating. Both the hardness testing and the scuffing load measurement were utilized for mechanical evaluation of the surface-treated samples. In the case of the normal plasma nitriding, the trade-off-balancing is held for the surface hardness vs. scuffing load relationship. Since the gas-nitriding data were plotted on the obtained master curve, the normal plasma nitriding can be a candidate surface treatment to be working instead of the gas nitriding processes in industry. Of much concern is the advanced plasma nitriding with formation of multi-striped microstructure. Although further validation is necessary to demonstrate its effectiveness as a reliable surface treatment for industry, the above normal trade-off-balancing curve can be extended in the direction to enhance the scuffing load or the wear toughness even at the same level of surface hardness. Both the chromium wet plating and the CrN ion-plating data might well be classified with this new trade-off-balancing curve for mechanical design of piston ring. This high qualification in the mechanical performance must be indispensable for the environmentally benign surface treatment.
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