Development of advanced duplex surface systems by combining CrAlN multilayer coatings with plasma nitrided steel substrates

Abstract

Both modelling analyses and experimental work have recently confirmed that the failure of thin coatings on steel, especially under high loads for some demanding applications, is related to the lack of mechanical support from the substrate, which means that the exceptional hardness, toughness and wear resistance of state of the art PVD coatings cannot be fully exploited. To this end, two types of new duplex surface systems have been developed from this research: i) PVD (specifically Closed Field UnBalanced Magnetron Sputter Ion Plating, CFUBMSIP) CrAlN multilayer coating deposited on ex-situ, 20-h active-screen plasma nitrided (ASPN) steel samples, and ii) in-situ 3-h, high power medium frequency pulsed plasma nitriding (HPMFPPN) treatment, followed by a similar CrAlN coating, but performed in a serial, single batch process in an otherwise conventional CFUBMSIP equipment. The microstructures of these two duplex surface engineered systems were characterised using XRD and SEM, while their hardness, adhesion, load bearing capacity and tribological properties were compared by micro-hardness tests, scratch adhesion tests and pin-on-disc sliding wear measurements. Post-test examinations were conducted and the results confirmed that a hard coating on a nitrided substrate can fully exploit the advantages of both the nitrided sub-surfaces for increased load bearing capacity and of the PVD coating for increased hardness, high temperature stability and extreme wear resistance. Finally, the advantages and disadvantages of these two new duplex surface systems were compared and their potential applications were discussed.