ETCHING BY IONIC BOMBARDMENT BEFORE DEPOS ITION - EFFECT OF GAS QUALITY ON THE ADHESION OF AN ALUMINUM FILM TO AN XC-70 STEEL SUBSTRATE

The adhesion strength of radiofrequency sputtered aluminium thin films to XC 70 steel substrates was measured by scratch testing the samples . Before deposition, the substrate surface was etched by ion bombardme nt either in an argon or in a nitrogen atmosphere and the influence of the etching time was studied. The scratch test critical load which wa s very weak on a mechanically polished substrate increased significant ly with increasing ion etching time, the best results being obtained i n the case of nitrogen ion etching. The effects of the etching pretrea tment on the scratch tracks, coating microstructure and substrate surf ace were observed by scanning electron microscopy. Furthermore, the in terfacial domain between the substrate and the coating was studied by XPS. From these studies, it appeared that the increase in the critical load values resulted from the elimination of the layer produced by me chanical polishing of the substrate and a deeper penetration of the co ating into the substrate, unrelative to the nature of the ion etching gas. The additional increase observed in the case of nitrogen etching pretreatment was attributed to the formation of a nitrided domain on t he substrate surface, even if contributions arising from other mechani sms cannot completly be disregarded.