Mechanical properties of WC coatings evaluated using instrumented indentation technique J. G. La Barbera-Sosa* 1 , Y. Y. Santana 1 , J. Caro 2 , D. Chicot 3 , J. Lesage 3 , M. H. Staia 1,4 and E. S. Puchi-Cabrera 1,3,4 Instrumented indentation tests have been conducted on the surface and cross-section of the two WC based coatings obtained by means of high velocity oxygen fuel thermal spray techniques in order to investigate their mechanical properties. The yield strength of the coatings has been determined by means of the Zeng and Chiu model. In addition, the absolute hardness of the coatings has been determined by means of the modified proportional specimen resistance model. Additionally, conventional spherical indentation tests were carried out on the surface of the coatings, aimed at determining the yield strength by means of the equations employed in classical elastic contact mechanics. The analysis indicates that the yield strength determined from the Zeng and Chiu model is of 3?5 and 3?1 GPa for the WC–10Co–4Cr and WC–12Co coatings respectively. The absolute hardness of the WC–10Co–4Cr coating was found to be of 11?1 GPa, whereas that of the WC–12Co was of 9?3 GPa. Keywords: WC–Co coatings, Instrumented indentation tests, Spherical indentation, Elastic contact mechanics equations, Yield strength Introduction High velocity oxygen fuel (HVOF) thermal spray coat- ings are widely employed in many different applications in order to increase wear, friction and corrosion resistance. 1 In addition, the remarkable characteristics of these coatings allow their use in many important engineering fields, such as aerospace, for the manufacture of sensible motor components and in machinery used in mining and mineral processing industry. 2 WC–Co coat- ings have been widely employed to increase wear resistance of different engineering components, exhibiting their best results as electrolytic hard chromium plating substitutes, particularly in the manufacture and main- tenance operations of both military and civil aircraft components, which include, among others, landing gear parts, propeller hubs and hydraulic actuators, for which fatigue properties are of utmost importance. 3 Particularly, HVOF WC–Co–Cr coatings have been shown to have remarkable properties, and therefore, these are widely employed for the protection of landing gear parts. These coatings exhibit superior wear and corrosion resistance in comparison with electrolytic hard chromium plating, without compromising the fatigue resistance of the substrate to a large extent. 4 On the contrary, in some cases, it has been shown that such coatings give rise to an increase in the fatigue strength of the substrate–coating system. 5,6 It is widely acknowledged that the microstructural characteristics of HVOF thermal spray coatings depend significantly on the spraying conditions employed during the deposition of the powders. Among the most important deposition conditions are the spray distance, powder feeding rate, particle temperature and speed at which the particles achieve the substrate surface. In turn, the coatings’ microstructure allows a rational explana- tion of the mechanical and tribological performance of the coating in service. The correlation between the microstructural characteristics of WC–Co and WC–Co– Cr coatings with hardness, elastic modulus, fracture toughness, residual stresses, as well as wear and fatigue resistance have been widely investigated in the past few years. 7–15 In a recent investigation, Houdkova´ et al. 13 studied different thermal spray coatings including WC–12Co and WC–17Co. In this work, more than 50 instrumented indentation tests were conducted at random on the surface of the coatings. It was found that the different hardness and elastic modulus curves could, in general, be classified in three different groups, depending on the behaviour described: hard particles, soft matrix and mixture of both regions. This work allowed the conclusion that the properties pertaining to hard 1 School of Metallurgical Engineering and Materials Science, Faculty of Engineering, Universidad Central de Venezuela, Los Chaguaramos, Caracas 1041, Venezuela 2 CTM Centre Tecnolo` gic, Av. Bases de Manresa, 1. 08242 Manresa, Spain 3 Universite´ Lille Nord de France, USTL, LML, CNRS, UMR 8107, F-59650, Villeneuve d’Ascq, France 4 Venezuelan National Academy for Engineering and Habitat, Palacio de las Academias, Postal Address 1723, Caracas 1010, Venezuela *Corresponding author, jose.labarbera@ucv.ve ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 20 December 2013; accepted 6 February 2014 DOI 10.1179/1743294414Y.0000000261 Surface Engineering 2014 VOL 30 NO 7 498