An excellent adhesion of hard coatings to steel substrates is paramount in practically all application areas. Conventional methods utilise Ar glow etching or arc discharge pretreatments that have the disadvantage of producing weak interfaces or adding droplets respectively.
A new tool for interface engineering is high power impulse magnetron sputtering (HIPIMS). HIPIMS is based on conventional sputtering with extremely high peak power densities reaching 3 kWcm-2 at current densities of >2 Acm-2. HIPIMS of Cr, Nb and V was used in an industrial size batch tool coater to prepare interfaces on 304 stainless steel, M2 high speed steel (HSS) and gamma- titanium aluminide alloys. During the pretreatment, the substrates were bombarded with highly energetic metal ions produced in a HIPIMS plasma. The substrates were to -600 V and -1000 V. The bombarding flux density reached peak values of >100 mAcm-2 and consisted of Cr1+, V1+ and Nb1+ but also doubly charged metal ions Cr2+ , V2+ and Nb2+ as observed by mass and optical emission spectroscopy. The gas ion to metal ion ratio was found to be 1:1 for Cr and Nb. The adhesion was evaluated for a coating deposited by conventional magnetron sputtering consisting of a 0.5 µm thick CrN base layer and a 2.5 µm thick nanoscale multilayer stack of CrN/NbN with bilayer thickness of 3.4 nm with typical hardness of HK0.025 = 3100 and residual stress of -1.8 GPa. HIPIMS V pretreatment was used for TiAlCN/VCN multilayer stack with hardness of HK0.025 = 3100. In HIPIMS pretreatment by Cr, V and Nb ions, the adhesion values on M2 HSS reached scratch test critical load values of LC > 50 N and were equal or better than pretreatments by cathodic vacuum arc. HIPIMS pretreatment introduced metal ions in the substrate and promoted epitaxial growth over large lateral areas on polycrystalline steel and γ-TiAl. Due to the droplet-free pretreatment by HIPIMS, large scale defects in the coating are minimised and the performance of the coating in tribological and corrosion applications is enhanced.
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