Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings Pornthep Chivavibul ⁎ , Makoto Watanabe, Seiji Kuroda, Kentaro Shinoda Composites and Coatings Center, National Institute for Materials Science, Ibaraki, Japan Received 5 February 2007; accepted in revised form 12 June 2007 Available online 27 June 2007 Abstract Twelve commercially available WC–Co powders with different average WC grain sizes (0.2, 2, and 6 μm) and cobalt contents (8, 12, 17 and 25 wt.%) were sprayed on carbon steel substrates using High Velocity Oxy-Fuel (HVOF) spraying process. Hardness, Young's modulus, and fracture toughness of the coatings were measured. While the hardness and Young's modulus decreased with increasing cobalt content from 1600 to 1100 Hv and from 400 to 300 GPa respectively, the fracture toughness remained in the range from 4 to 6 MPam 1/2 . The coatings with 2 μm carbide showed lower hardness than those deposited from 0.2 and 6 μm carbide. These measured mechanical properties were discussed with the help of microstructures of the coatings investigated by scanning electron microscopy, X-ray diffraction and chemical analysis. Finally, the hardness of the binder phase in these coatings was estimated to range from 1000 to 1300 Hv by applying the mixture rule for composites to the experimental data, demonstrating that such hardening of the binder phase is a key factor affecting the mechanical properties of the coatings. © 2007 Elsevier B.V. All rights reserved. Keywords: WC–Co; Carbide size; Co content; Mechanical properties; Binder phase 1. Introduction Sintered WC–Co materials have a good combination of high modulus, high wear resistance, and adequate fracture toughness, making them the choice material for a wide range of industrial applications such as metal-cutting tools and wear resistant components [1]. Recently, WC–Co cermet surface coatings have been used to enhance the wear resistance of various engineering components. Many thermal spraying techniques such as air plasma spraying (APS) and high velocity oxy fuel (HVOF) spraying can be applied to deposit the WC–Co coatings, how- ever, the properties of coatings strongly depend on spraying technique. Compared to other spraying techniques, HVOF spraying is one of the best methods for depositing conventional WC–Co cermet powder because the higher velocities and lower temperatures experienced by the powder result in less decom- position of the WC during spraying process [2]. Consequently, coatings with higher amount of retained WC and less porosity are obtained. However, compared to sintered WC–Co, for which the sintering atmosphere, temperature and time have been carefully controlled, HVOF-sprayed WC–Co coatings still suffer from decomposition and decarburization during spraying process leading to formation of detrimental phases such as W 2 C, W, and amorphous or nanocrystalline Co–W–C phase [3,4]. Many efforts to reduce the degree of decomposition and decarburiza- tion of WC–Co powder have been made such as one using a gas shroud to introduce an inert gas in HVOF spraying [5]. In-flight phenomena in HVOF spray of WC–Co and coating formation have been studied by Li et al. [6,7]. Their works focused on the modeling and feedback control of the HVOF process. The simulation results demonstrated that the particle- melting degree and the particle velocity strongly influence coating microstructure formation and they can be adjusted by the chamber pressure and fuel/oxygen ratio. When the particle- melting ratio is not high, many large particles are bounced off as they hit the substrate and the deposited coatings tend to have a high porosity. Therefore, a higher melting degree is considered to be desirable to obtain dense coatings. Furthermore, it is generally accepted that the higher particle velocity results in denser coating. Besides these process parameters, the particle- melting degree and particle velocity depend on the morphology Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2007) 509 – 521 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. E-mail address: chivavibul.pornthep@nims.go.jp (P. Chivavibul). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.06.026