Influence of deposition temperature and time during PVD coating of CrN on corrosive wear in liquid aluminium

Erosive and corrosive wear are two major life-limiting factors in die casting dies. To resist the corrosive and erosive attack from molten metal flow the tool surface needs to be hard and chemically inert. It has been indicated that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid aluminium. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. In this work, physically vapour deposited (PVD) CrN coatings were applied on hot work tool steel specimens and treated in an aluminium melt. Substrate temperature and deposition time were varied to give coatings with individual properties. Type and density of defects as well as thickness of the coatings were characterised for all coatings. The influence of substrate temperature and deposition time on coating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage. It is clearly demonstrated that liquid aluminium corrosion of CrN-coated tool steel is initiated at defects which penetrate through the coating, and localised corrosion pits are formed. Subsequently, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects through the coating, which, for PVD CrN, was achieved by increasing the coating thickness.