The wear mechanism of hybrid layer “PN+CrN” during the hot forging process

Purpose: One of the most perspective development directions of surface engineering is related to hybrid technologies, which best fulfil the expectations of the industry concerning the obtainment of adequate properties of the surface of tools and machine components. The best known and widely employed hybrid technology of surface treatment using the diffusion phenomenon is the combination of gas nitriding or plasma nitriding (PN+CrN) followed by the deposition of hard, wear resistant coatings by the PVD methods. In this paper the wear mechanisms of forging dies, covered with the PN+CrN hybrid layer were analysed. Design/methodology/approach: The hybrid layers PN+CrN selected for testing were obtained by means of a multi-stage technology of surface treatment encompassing plasma nitriding (PN) and arc-evaporation process. Maintenance tests were conducted at Institut für Umformtechnik und Umformmaschinen in Hannover, according to different forging time. Findings: It was demonstrated that the dominating mechanisms of the wear process of forging dies covered with the PN+CrN hybrid layer are: thermo-mechanical fatigue of the CrN coating and thermo-mechanical fatigue and plastic deformation of steel substrate. Research limitations/implications: The CrN coating is of great importance in the wear process of forging dies covered with the PN+CrN hybrid layer. Very important in the die wear process is the resistance of the CrN coating to brittle cracking. CrN coating reduces, and as a result significantly limits, stresses initiated in the substrate in the forging process by the operation of external impacts. According to the results of simulation tests, 3 μm-thick CrN coating limits stresses initiated in material and hence reduces the possibility of plastic deformation occurrence in substrate. Practical implications: The obtained results of the tests have been practically applied in the surface engineering laboratory to develop modern surface engineering solutions aimed at increasing the effectiveness of different hybrid layers PN+CrN. Originality/value: In order to ensure the required level of effectiveness of the use of layered composites of the PN+PVD type to increase the durability of forging dies, it is necessary to properly select the composites on the basis of the analysis of the intensity of forging dies wear mechanisms.