Pretreatment of cutting tools by plasma electrolytic polishing (PEP) for enhanced adhesion of hard coatings

Machining tools such as milling, turning, or drilling tools are necessary and important in a wide range of industrial production sectors. In order to increase the tool service life, their surfaces are finished with a high-quality hard coating. Prior to coating, the surface has to be thoroughly cleaned from contaminations created during the tool manufacturing process to ensure sufficient adhesion between coating and tool surface. This contribution demonstrates the versatility of plasma electrolytic polishing (PEP) applications ranging from cleaning to polishing of precious metal surfaces. In particular, the process optimization of PEP as a suitable pretreatment step for the surface of cemented carbide cutting tools, based on cobalt containing tungsten carbide (WC-Co) is reported. The tool surface micro-structure was characterized by various analytical techniques such as SEM, AFM and XRD. Additionally, XPS analysis was performed to investigate the compositional change of the tool surface. The results of surface analyses showed that WC is dominantly dissolved while Co is remained on the tool surface in the form of oxide/hydroxide after PEP treatment at about 110 V. In order to investigate the cleaning effect of PEP, the process was performed on the tool with a defined grease contamination. ATR-FTIR and XPS analyses revealed that PEP treated tool surfaces obtain less contamination remained on the surface compared to contaminated and as-received tools. The cutting edge radius of the tool is measured using White-Light-Interferometry. The treatment does not induce a major deformation of the cutting edge. Instead, the radius can be enlarged evenly in a controlled way. After pretreatment, a TiN hard coating layer was deposited by reactive HiPIMS and tribological tests were performed. The PEP treated tools show a significant increase in tool life and layer adhesion. •The WC-Co based tool surface was pretreated by plasma electrolytic polishing (PEP).•Optimal process parameters for PEP of WC-Co based inserts were identified.•The PEP process enables the cleaning of tool surfaces while minimizing oxidation.•Cutting edge sharpness was ensured by decreasing the process voltage.•Adhesion and tool life time of TiN coatings on PEP treated tools were improved.