Mechanical and tribological characterization of self-lubricating (Cr1-xAlx)N coatings for deposition on complex-shaped forging tools

Abstract

Cold forging processes in production engineering enable a high material utilization with a considerable energy and resource efficiency. In such processes, liquid and solid lubricants are commonly used to reduce the friction between forming tools and workpieces. However, using of lubricants in tribological cold forging processes is unfavorable to economic, ecological and legislative perspectives. To overcome disadvantages of lubricants and conduct lubricant-free cold forging, the physical vapor deposition (PVD) technology for coating of metal forming tools can be applied. Such coatings contain soft and hard constituents. Self-lubricating systems such as MoS2 will be applied as soft constituents. Furthermore, incorporation of MoS2 into the hard matrixes such as (Cr1-xAlx)N is required, in order to increase the life time of coated tools under high tribological loads in dry cold forging. In the presented work, self-lubricating coatings in system (Cr1-xAlx)N MoSy with two different nitrogen contents for the application in dry cold forging of steel were investigated. The coatings were deposited on tool steel X155CrMoV12 (DIN 1.2379, AISI D2) using a hybrid technology, consisting of direct current and high power pulse magnetron sputtering dcMS/HPPMS in an industrial scale PVD unit. In order to investigate the distribution of coating thickness s along the length of complex-shaped forging tools Ltool, an experimental tool was designed and the aspect ratio s/Ltool was determined. The coating and substrate/coating compound properties were investigated. Tribological model tests were conducted using a pin-on-disc (PoD) tribometer, which simulates dry contacts in cold forging. The analyses were performed using pins made of 16MnCr5 (DIN 1.7131, AISI 5115) and included the determination of friction coefficients. Furthermore, contact region was studied by Raman spectroscopy after the tribological tests. Based on the results, the coating system (Cr1-xAlx)N MoSy exhibits a high potential for application in dry cold forging of steel. Moreover, an increase of nitrogen content in the coatings leads to an enhancement of adhesion strength between substrate and coating and the tribological behavior of the coated tools against 1.7131.