Influence of plasma-nitriding and surface roughness on the wear and corrosion resistance of thin films (PVD/PECVD)

The application of PVD coatings on low-alloy steels for the protection against wear and corrosion, may allow the substitution of high-alloy steel materials, which are used for applications under mechanical-corrosive loads. This could prolong the component lifetime and save costs. A pretreatment method, which may improve the corrosion resistance of PVD coated low-alloy steel, is plasma-nitriding of the substrate. So far there were no investigations to quantify the influence of plasma-nitriding on the corrosion behaviour of PVD coated low-alloy steel. Hence one objective of this study was to quantify this influence by means of polarisation tests with determination of corrosion potentials and corrosion current densities. Furthermore the influence of plasma-nitriding of the substrate on the wear properties of PVD coated low-alloy steel was investigated. Moreover the influence of surface roughness on the corrosion properties of PVD coated low-alloy steel was an aim of this study. The investigated PVD coatings were new graded zirconium carbide layers (ZrCg) and graded chromium carbide layers (CrCg). The tests revealed, that higher roughness leads to worse corrosion protection by the investigated PVD coatings. This can be explained by the number of defects in the layer, which rises with increasing roughness and which allows the penetration of corrosive media to the substrate and thus accelerates corrosion underneath the film. The tests in artificial sea water in accordance with DIN 50905 did not show corrosive attacks on the tested films themselves. Plasma-nitriding of the substrate resulted in significantly higher corrosion potentials and significantly lower corrosion rates for the PVD coated specimens. The positive influence of plasma-nitriding of the substrate on the wear behaviour has been less pronounced. Only when higher normal forces were applied, which allowed the counter body to have direct frictional contact with the substrate, the wear was reduced due to plasma-nitriding. In summary, plasma-nitriding of the substrate is beneficial for achieving good corrosion and wear properties for PVD coatings on low-alloy steel. However a special focus has to be put on a proper pretreatment of the surface. which has to be as smooth as possible. (C) 2008 Elsevier B.V. All rights reserved.