Sliding wear properties of HVOF sprayed WC–20%Cr 3 C 2 –7%Ni cermet coatings Yasunari Ishikawa a,1 , Seiji Kuroda a, ⁎ , Jin Kawakita a , Yukihiro Sakamoto b , Matsufumi Takaya b a National Institute for Materials Science, 1-2-1, Sengen, Tsukuba-shi, Ibaraki-ken, 305-0047, Japan b Chiba Institute of Technology, 1-17-1, Tsudanuma, Narashino-shi, 270-2213, Japan Received 26 April 2006; accepted in revised form 5 October 2006 Available online 20 November 2006 Abstract WC–20 mass%Cr 3 C 2 –7 mass%Ni powder was sprayed onto low-carbon steel substrates by a commercial high velocity oxygen-fuel (HVOF) spray process as well as by an improved HVOF process equipped with a gas shroud attachment. The latter process utilizes a nitrogen gas flow to shield the region between the spray gun and the substrate in order to suppress the material's degradation caused by reaction with air such as oxidation and decarburization. Some coatings were further heat-treated in air at 773 K for 30 h to form a thin oxide film on the surface. The sliding wear properties of these coatings against an iron pin were evaluated by using a pin-on-disk wear tester. The specific wear rate of the as-sprayed cermet coatings prepared under the conventional spray condition was about three times higher than that of the chrome plating but by using the gas shroud, the wear rate was reduced to the same level with the chrome plating. The specific wear rate could be further decreased by the oxidation heat-treatment. It was found that a proper amount of oxides existing on the surface or within the coatings have a great beneficial effects on the wear properties such as to promote the transition from severe wear to mild wear and thus to reduce the wear rate remarkably. XPS analysis of the transfer particles collected from the wear track revealed a shift in the oxidation state of iron depending on the wear condition. © 2006 Elsevier B.V. All rights reserved. Keywords: High velocity oxy-fuel spray; Gas-shroud; Tungsten carbide cermet; Wear property 1. Introduction The hard chrome plating process is known to have detrimental effects to the environment and the operator's health. Therefore, research for alternatives of chrome plating has been widely carried out in the world [1,2]. High velocity oxy-fuel (HVOF) sprayed cermet coatings are adopted increasingly in industry as an alternative for hard chromium plating as they can provide a superb corrosion and wear resistance if adequate powder is properly processed. For example, large efforts have been made to find hard chrome alternatives for aircraft landing gears and it has been shown that the sliding wear rates of HVOF sprayed WC/17Co or WC/10Co4Cr against many different types of materials is much less for both the coatings and mating materials than for hard chrome [1]. HVOF process owes both its heating power and acceleration force to a supersonic jet flame made from combustion of a mixture of oxygen and fuel. With this technique sprayed particles with speeds over 500 m/s and with temperatures up to around 2300 K can be deposited onto a wide range of substrate materials. Such particles impinge onto a target substrate often in the semi-molten state and pile up to form coatings. However, for some cermet powders such as WC–Cr 3 C 2 –Ni used in this study, it was more difficult to make a fully dense coating. When one tried to decrease the coating porosity by commercial HVOF equipment, operating parameters needed to be set to a high-energy side and such spray conditions tended to Surface & Coatings Technology 201 (2007) 4718 – 4727 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. National Institute for Materials Science (NIMS), Composites and Coatings Center, 1-2-1, Sengen, Tsukuba-shi, Ibaraki-ken, 305- 0047, Japan. Tel.: +81 298 59 2444; fax: +81 298 59 2401. E-mail address: KURODA.Seiji@nims.go.jp (S. Kuroda). 1 Present address: Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa-ken, 221-8755, Japan. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.007