Investigation of interfacial microstructure of CrN coatings on HSS substrates pretreated by HIPIMS for adhesion enhancement
In this study, six dc Magnetron Sputtered (dcMS) CrN hard coatings were deposited on pretreated High Speed Steel (HSS) to achieve different interface architectures. The aim was to correlate the interfacial microstructure to the adhesion of the coatings. The substrates were pretreatment using the Ionized Physical Vapor Deposition (IPVD) method High Power Impulse Magnetron Sputtering (HIPIMS) using a Cr target in an inert atmosphere varying the substrate bias ($U_b$) between 0 V and 1100 V at ambient temperature as well as at a substrate temperature of 400$^\circ$C. The deposition parameters were chosen to show how kinetically induced diffusion, etching and implantation changes the interface chemistry and structure and to investigate their effect on the adhesion on the film. At elevated temperatures, the diffusion will be thermally driven. Annealing of the deposited samples were, therefore, performed at 900 K in an Ar atmosphere. The films were characterized employing XRD, HR-TEM, A-STEM and by scratch test measurements to see how the the interface microstructure can be correlated to the adhesion of the coating. The study shows that a sputter cleaned substrate surface with well preserved crystal structure of the substrate enhances the adhesion of the coating by promotion of local epitaxial growth. However, annealing was also shown to have a large effect on the adhesion enhancement by allowing for interdiffusion in the interface region and due to promotion of interface strain relaxation. Implantation of target material on the other hand had limited influence on the adhesion compared to the clean oxide free surfaces. The low adhesion improvement when gradually changing the chemical composition at the interface is assumed to stem from that the radiation induced defects and strain diminished the positive effect of this gradient.
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