Surface Fatigue Cracking Behavior of a CrN-Coated Tool Steel Influenced by Sliding Cycles and Sliding Energy Density

Light-weighting of vehicles is one of the challenges for transportation industry due to the increasing pressure of demands in better fuel economy and environment protection. Advanced high strength steels (AHSS) are considered as prominent material of choice to realize lightweight auto body and structures at least in near term. Stamping of AHSS with conventional die materials and surface coatings, however, results in frequent die failures and undesired panel surface finish. A chromium nitride (CrN) coating with plasma nitriding case hardened layer on a die material (duplex treatment) is found to offer good wear and galling resistances. The coating failure initiates from fatigue cracking on the coating surface due to cyclic sliding frictions. In this work, cyclic inclined sliding wear test was used to imitate a stamping process for study on development of coating fatigue cracking, including crack length and spacing vs. sliding-cycles and sliding energy densities. The purpose of this study was to examine the effect of testing cycles and sliding energy work on evolution of coating fatigue cracking. In addition, the proportion of total energy transferring to surface energy as a path of energy dissipation through generation of new surfaces of cracks was analyzed. The result indicated that such an energy conversion was insignificant; suggesting that the computing simulation output should also include information of stresses on the die surface and more attention should be paid on response of the coated die material to the stresses.