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 casti ng dies. To resist the corrosive and erosive attack from molten metal flow the toot surface needs to be hard and chemically inert. It has been indicat ed that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid alumini um. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. In this work. physically vapou r deposited (PVD) CrN coatings were applied on hot work tool steel specimen s and treated in an aluminium melt. Substrate temperature and deposition ti me were varied to give coatings with individual properties. Type and densit y of defects as well as thickness of the coatings were characterised for al l coatings. The influence of substrate temperature and deposition time on c oating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage. It is clearly demonstrated that liquid alumi nium corrosion of CrN-coated tool steel is initiated at defects which penet rate through the coating, and localised corrosion pits are formed. Subseque ntly, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects thr ough the coating, which, for PVD CrN, was achieved by increasing the coatin g thickness. (C) 2001 Elsevier Science B.V. All rights reserved.