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

This paper aims at studying the fracture resistance and adhesion of tungste n-carbon based coatings, deposited by d.c. reactive magnetron sputtering on stainless steel substrates, with respect to their microstructure and thick ness. The coatings are multilayer structures consisting of periodic layers of ductile tungsten W and hard tungsten-carbon W(C) layers, containing 14 a t.% of carbon, and gradient structures based on a gradual incorporation of carbon in the tungsten deposit to eliminate the W(C)/W interface effect. Th is investigation is motivated by the need to understand the mechanical beha vior of promising composites as protective coatings in aerospace industry i n terms of morphology, hardness measurements and scratch test experiments. The main trends are the following: a columnar W layer at the coating/substr ate interface may contribute to prevent the delamination of the coating fro m 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 t he coating at layer interfaces, whereas the existence of one well defined c oating/substrate interface for the gradient coating leads to the complete r emoval of this coating. Good adhesion between the hardest and thickest mult ilayer coating and the steel substrate is observed. Such coatings with many more layers have already exhibited an excellent resistance to erosion. (C) 1999 Elsevier Science S.A. All rights reserved.