Adhesion and failure mechanisms of tungsten-carbon containing multilayered and graded coatings subjected to scratch tests

This paper aims at studying the fracture resistance and adhesion of tungsten-carbon based coatings, deposited by d.c. reactive magnetron sputtering on stainless steel substrates, with respect to their microstructure and thickness. The coatings are multilayer structures consisting of periodic layers of ductile tungsten W and hard tungsten-carbon W(C) layers, containing 14 at.% of carbon, and gradient structures based on a gradual incorporation of carbon in the tungsten deposit to eliminate the W(C)/W interface effect. This investigation is motivated by the need to understand the mechanical behavior of promising composites as protective coatings in aerospace industry in terms of morphology, hardness measurements and scratch test experiments. The main trends are the following: a columnar W layer at the coating/substrate interface may contribute to prevent the delamination of the coating from the substrate surface, and a dense hard W(C) layer at the surface of the coating delays the first cracking of the structure. The mechanical behavior of the multilayered coatings is characterized by a partial detachment of the coating at layer interfaces, whereas the existence of one well defined coating/substrate interface for the gradient coating leads to the complete removal of this coating. Good adhesion between the hardest and thickest multilayer coating and the steel substrate is observed. Such coatings with many more layers have already exhibited an excellent resistance to erosion.