Impact testing of duplex and non-duplex (Ti,Al)N and Cr–N PVD coatings

Abstract

An impact test has been used to evaluate the impact resistance of duplex (i.e. pre-nitrided) and non-duplex PVD (Ti,Al)N and Cr–N coatings. The impact tests were carried out using a cyclic loading system developed at the University of Hull. Two different balls (tungsten carbide and hardened SAE 52100 steel) were used as impact bodies. After the test, samples were observed on the scanning electron microscope in order to investigate coating failure. Experimental results revealed that both duplex (Ti,Al)N and Cr–N coatings displayed better impact resistance than their non-duplex counterparts. None of the duplex coatings exhibited adhesive failures up to 5×104 impacts when both tungsten carbide and hardened steel balls were used. Conversely, all non-duplex coatings displayed adhesive failures at lower numbers of impact cycles. The nitrided case minimised substrate deformation under the hard PVD coatings so that fewer cohesive failures occurred in the duplex coatings. Moreover, the improvements of the duplex treatment could be more markedly noted when tungsten carbide balls were used, since higher stresses are expected to occur for this impact body. A mechanism involving material transfer from the ball to the coated surface was observed to occur only for the duplex (Ti,Al)N coating. A larger amount of transferred material took place for tests carried out with hardened steel balls, probably owing to their relatively lower hardness in comparison to that of the tungsten carbide balls. Such a mechanism was not observed to occur for the duplex Cr–N coating, which seemed to be the most suitable coating among all those tested, to be used in dynamic loading wear applications.