The multilayer effect in abrasion - optimising the combination of hard and tough phases

In the present investigation it is shown that the wear resistance of multilayer PVD coatings, combining relatively soft but tough chromium (Cr) and harder but more brittle chromium nitride (CrN): exceeds that of both its phases. This striking exception to the simple rules of mixtures - the multilayer effect in abrasion - is investigated with respect to physical mechanisms, layer thickness dependence and implications for multilayer design for abrasive applications. Multilayers of Cr and CrN were deposited on high speed steel using reactive magnetron sputtering. The coatings were characterised with respect to microhardness and abrasive wear resistance against a slurry of 2.5 mu m diamond abrasives. The abraded surfaces were investigated by scanning electron microscopy and optical profilometry to reveal the microscopic wear mechanisms. A physical model for the layer thickness dependence for wear resistance of multilayer coatings was developed. Despite its simplifications. the model agrees very well with the experimental results. The advantage of Cr layers is their tendency to deform plastically rather than becoming removed by wear; the drawback is lack of penetration resistance. The penetration resistance, however, is the main advantage of the CrN layers. It was concluded, both theoretically and experimentally, that the multilayer effect is explained by the reduction of abrasive penetration depth in the Cr layers due to the underlying harder CrN layer. (C) 1999 Elsevier Science S.A. All rights reserved.