Effect of cathodic arc plasma treatment on the properties of WC–Co based hard metals S. Sinan Akkaya a , Erdem Sireli b , Berk Alkan a , M. Kursat Kazmanli a , Mustafa Ürgen a, ⁎ a Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469 Maslak, Istanbul, Turkey b Böhler Sert Maden ve Takım Inc., Kartal, Istanbul, Turkey abstract article info Available online 1 October 2011 Keywords: Cathodic arc plasma treatment Cemented carbides Eta phase Hard metals Corrosion resistance In this study, ion bombardment in a cathodic arc physical vapor deposition system was applied on WC–Co hard metal surfaces aiming to benefit from the diffusion acceleration effect, and to investigate the role of this effect on the surface composition, morphology and corrosion resistance of the materials. Chromium ions obtained via cathodic arc evaporation were accelerated under low (-150 V) and high (-1000 V) bias voltages in order to apply coating–bombardment cycles to sample surfaces. Substrate temperatures were measured by an optical pyrometer during the processes. The treated samples were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Temperature measurements showed that the sample temperature could be controlled precisely by adjusting the bias voltage. Temperatures in the range of 750–1200 °C were measured during the treatment depending on the duration of the high bias voltage cy- cles. XRD analysis showed η phase formation in the near surface regions of all treated samples. The amount of the formed η phase was shown to be dependent on the heating–cooling regime that varied with the applied mode of bias. The corrosion behavior of the samples was investigated by immersing treated and untreated samples in a solution of 5% H 3 PO 4 containing 1 g/l Zn +2 for 24 h at 50 °C. The samples were investigated via SEM observations after immersion. Cathodic arc plasma treated samples showed a better resistance to corrosion in this environment. © 2011 Elsevier B.V. All rights reserved. 1. Introduction WC–Co hard metals (cemented carbides) are widely used as cutting tools, seals and bearings due to their excellent mechanical properties, especially high hardness and wear resistance [1–3]. They have been the subject of numerous studies aiming to explain and improve their properties. One of the major concerns in WC–Co hard metals is the for- mation of eta “η” phase. If there is a deficiency of carbon, the formation of the “η” phase is possible during sintering. However, in the case where excessive carbon is present in the structure, graphite formation may occur. The presence of graphite or eta phase in hard metals is not desir- able since they influence the mechanical properties negatively [1, 4]. The eta phase is a carbide phase in the form of M 3 W 3 C (M 6 C) or M 6 W 6 C (M 12 C) (M=Fe, Co…) with very high bulk modulus. The bulk modulus of M 12 C is 462 GPa, which is larger than that of diamond (436.8 GPa) [5–7]. The eta phase coexists with WC at around 1280–1450 °C, at optimal carbon content, though upon cooling it separates into WC and Co [1]. If no carbon deficiency is present, the eta phase is not found at room tem- perature unless cooled very rapidly. Although the η phase is not wanted in the structure of WC–Co based hard metals that are mainly used for cutting tools, it may be beneficial for the applications where high wear resistance is needed, such as drawing dies, as claimed by Perez and Pauty [8]. Mikus [9] used eta phase formation as promoter for the formation of a graded structure composed of larger grains at the hard metal sur- face. In his work, the eta phase is deliberately formed under decarbur- izing atmosphere and then separated into a WC–Co structure by means of specialized heat treatment. Through the dissociation of the eta phase into WC and Co, resulting grain growth is obtained. This method is used to achieve grain size tailored structures at near sur- face regions of hard metals. Another area of concern about WC–Co hard metals tools is corrosion resistance [10, 11]. Since WC–Co has a non-homogeneous composite structure, it suffers from galvanic corrosion [11]. Cobalt binder gets dis- solved in an aggressive environment while WC remains mostly unaf- fected, hence a WC skeleton is left behind which cannot withstand working conditions [3, 12, 13]. It is possible to increase the corrosion re- sistance by controlling the binder amount, WC grain size, addition of Cr 3 C 2 or substitution of binder metal with Ni and its alloys [1, 12–15]. However, all of these processes used for increasing the corrosion resis- tance bring along the risk of reduced toughness. As demonstrated in the earlier work by Şireli [16] and Çorlu and Ürgen [17, 18], low energy ion bombardment can be implemented Surface & Coatings Technology 206 (2011) 1759–1764 ⁎ Corresponding author. Tel.: + 90 212 2856999; fax: + 90 212 2853427. E-mail address: urgen@itu.edu.tr (M. Ürgen). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.09.046 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat