Synthesis, characterization, and tribological evaluation of HPPMS (Cr1 − xAlx)N + MoSy coatings

Lubricants are applied to reduce friction between workpieces and tools in many processes, such as cold forging. Due to significant advances in solid lubricant coatings deposited by physical vapor deposition (PVD) in the recent years, coatings took over the tasks of lubricants in numerous applications. However, most attempts to realize dry metal cold forging of steel failed so far. The combination of (Cr1 − xAlx)N and MoSy within a PVD coating by means of high power pulsed magnetron sputtering (HPPMS) to gain outstanding tribological properties such as low coefficient of friction through solid lubrication combined with increased abrasive wear resistant provided by the excellent mechanical properties of (Cr1 − xAlx)N is a promising candidate in order to replace lubricants on highly loaded tools in dry cold forging. HPPMS technology offers outstanding advantages in terms of high hardness and dense morphology. In the present work (Cr1 − xAlx)N + MoSy coatings were deposited on AISI D2 tool steel by means of HPPMS using an industrial scale PVD unit. A custom target consisting of a CrAl half and a MoS2 half in form of triangles was used, which leads to a variation of the chemical composition of the deposited coatings dependent on the substrate position within the coating chamber. After the characterization of the coatings, a pin-on-disc tribometer (PoD) was used to simulate the tribological behavior during dry contacts. Contact region was analyzed by Raman spectroscopy after the tribological tests. Increasing Cr and Al contents by decreasing Mo and S contents leads to considerable influences on the coating properties such as the increase in mechanical properties from HU = 8.89 to 18.44 GPa. A reduction of the coefficient of friction (CoF) against 16MnCr5 with the addition of Mo and S was observed. (Cr0.74,Al0.26)N + MoS0.76 coating exhibited the best results regarding the tribological behavior owning a CoF = 0.3. (Cr1 − xAlx)N + MoSy coatings with a nanolayer architecture deposited using HPPMS technology have a high potential for lubricant-free cold forging of steel.