Mechanical properties of W and W(C) thin films: Young's modulus, fracture toughness and adhesion

This contribution presents an investigation and the determination of the mechanical properties of tungsten (W) and tungsten-carbon (W(C)) single layer coatings deposited on steel substrates, in order to optimize a W/W(C) bilayer coating for mechanical applications. The coatings are first tested by three-point bending, to determine Young's moduli. An acoustic emission detector, adapted to the bending device, allows the detection of the first stages of cracking in the coating and furthermore, by an analytical treatment, the determination of the fracture resistance. The adhesion of the coating on this substrate is analyzed from microtensile experiments, performed with a device adapted to a scanning electron microscope. A theoretical formalism is applied to analyze the stress redistribution in the cracked coatings. The elastic moduli of W and W(C) thin films are found to be very close to the bulk values of W. The determined fracture properties (toughness) ranged from 1 to 2.5 MPa m(1/2) and from 0.2 to MPa m(1/2) for the W and the W(C) coatings, respectively. Their differences might be related to the film morphology. The adhesion of these coatings on stainless steel substrates appears good; experimentally no debonding was observed in either system. (C) 1998 Elsevier Science S.A. All rights reserved.