Identification of the wear mechanism on WC/C nanostructured coatings

Abstract

A series of WC/C nanostructured films with carbon contents ranging from 30 to 70at.% was deposited on M2 steel substrates by magnetron sputtering of WC and graphite targets in argon. Depending on the amorphous carbon (a-C) incorporated in the coatings, nanocrystalline coating (formed mainly by WC1−x and W2C phases) or nanocomposite (WC1−x/a-C) were obtained with tunable mechanical and tribological properties. Ultrahardness values of 36–40GPa were measured for the nanocrystalline samples whilst values between 16 and 23GPa were obtained in the nanocomposite ones depending on the a-C content. The tribological properties were studied using a pin-on-disk tester versus steel (100Cr6) balls and 5N of applied load in dry sliding conditions and the failure modes by scratch adhesion tests. Three different zones were identified according to the observed tribological behavior: I (μ>0.8; adhesive wear), II (μ: 0.3–0.6; abrasive wear) and III (μ~0.2; self-lubricated). The wear tracks and the ball scars were observed by scanning electron microscopy (SEM) and Raman spectroscopy in order to elucidate the tribochemical reactions appearing at the contact and to determine the wear mechanism present in each type. A correlation among structure, crystalline phases, a-C content and tribomechanical properties could be established for the series of WC/C coatings and extended to understand the trends observed in the literature for similar coatings.