Substrate surface finish effects in duplex coatings of PAPVD TiN and CrN with electroless nickel-phosphorus interlayers

The ability to deposit hard coating materials on to a wide range of substrate surface finishes and still maintain adequate structural and functional properties is a very desirable objective in the quest to optimise product economics. A better surface finish requires longer and more elaborate finishing processes which will result in substantial additional costs. In this investigation two different groups of substrate surface roughnesses were prepared for a number of appropriate testing procedures. The tests involved in this study were microhardness, surface profilometry, scratch adhesion and the wet sand rubber wheel abrasive test. Electrochemical corrosion testing was also carried out. The results reveal that the substrate roughness can be optimised to achieve the best compromise in terms of cost and performance. The addition of an electroless nickel-phosphorus (ENiP) interlayer, in particular, is beneficial in isolating the substrate from near-surface effects and in permitting the use of rougher surface finishes.