Tribological Behavior of (Cr1-xAlx)N/WSy PVD Tool Coatings for the Application in Dry Cold Forging of Steel

Abstract

In cold forging processes, there is a strong demand to avoid lubricants due to economic, ecological and legislative aspects. PVD coatings on metal forming tools may enormously reduce tool and workpiece wear in cold forging or deliver special functions even in the absence of lubricants. However, the abdication of lubricants goes along with the requirement that the dry tribological system has to withstand the increased tribological loads. The extreme tribological conditions in cold forging process which can lead to contact pressures of more than pCP = 2,500 MPa are critical for the success of a tool coating. (Cr1 xAlx)N/WSy coatings promise a solution to replace lubricants on highly forced tools in dry cold forging. Wear resistant (Cr1 xAlx)N together with the self-lubricating WSy deposited using PVD technology, offers an extension of tool life without lubricants. The aim of the current contribution is the evaluation of the tribological behavior of the (Cr1-xAlx)N/WSy PVD coatings under dry sliding conditions. At the Surface Engineering Institute (IOT) (Cr1-xAlx)N/WSy coatings with different chemical content ratios of WS2 were deposited in an industrial high power pulsed magnetron sputtering (HPPMS) coating process. A special target which consists of a Cr0.50Al0.50 half and a WS2 half in form of triangles was used, which led to a variation of the chemical composition of the deposited coatings according to the position of the sample on the holder. The coated samples of X155CrMoV12 (DIN 1.2379, AISI D2) tool material were investigated. A Pin-on-Disc (PoD) tribometer was used for tribological investigations of the coated samples under dry sliding conditions. Under the given tribological conditions depending on the WS2 content, a significant wear reduction was observed for the (Cr1-xAlx)N/WSy coatings. A reduction of the friction coefficient from μ = 0.70 for uncoated sample to μ = 0.25 with the WSy content was detected. The investigated (Cr1-xAlx)N/WSy coatings are a promising alternative to lubricants for the application in dry cold forging process of steel.