Int. Journal of Refractory Metals & Hard Materials 27 (2009) 130–139
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Sparking plasma sintering of nanometric tungsten carbide Jinfeng Zhao, Troy Holland, Cosan Unuvar 1, Zuhair A. Munir * Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis CA 95616, USA
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Article history: Received 13 May 2008 Accepted 3 June 2008
Keywords: Tungsten carbide Spark plasma sintering W2C phase Abnormal grain growth
a b s t r a c t The consolidation of nanometric powders of WC by the spark plasma sintering (SPS) method was investigated over the temperature range 1425–1800 °C under a uniaxial pressure of 126 MPa. Nominally stoichiometric WC powders with a grain size in the range 40–70 nm could be consolidated to near theoretical densities (99.1%) with a grain size of 305 nm when heated at a high rate to 1750 °C with no hold time. The sintered material, however, contained W2C and the effect of addition of carbon on the presence of this phase was investigated with these powders and with powders having a grain size of 12 nm. The effect of carbon on abnormal grain growth (AGG) was investigated as a function of temperature and carbon addition. The effect of heating rate up to the sintering temperature and the hold time at temperature was also investigated. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten carbide, WC, is an attractive material for many applications due to a combination of physical and mechanical properties (high melting point, high hardness, low coefficient of friction, and good electrical conductivity) and chemical stability (high corrosion and oxidation resistance) [1]. It is widely used in wear-resistant coatings, cutting and drilling tools, high temperature electrical contacts, and conductive protective layers in sensor applications. Because of its low fracture toughness, metallic phases (e.g., Co, Ni, and Fe) are usually added to tungsten carbide, forming cemented carbides. Metallic phases are also added to aid in the consolidation of WC through liquid-phase sintering. However, such additions lower the hardness and decrease the corrosion resistance of the carbide. As a consequence recent efforts have been directed toward forming pure (binder-free) WC, with a focus on the use of submicrometer and nano-powders. The sintering of such powders is expected to lead to a fine-grained product with anticipation of increased hardness, toughness, and abrasion resistance [2,3]. However, the use of fine particles has been reported to lead to significant grain growth, which has been attributed to physical and chemical characteristics of the powder [4] and the anisotropy of surface energy of the WC [5]. Abnormal grain growth (AGG) is often observed in the synthesis of cemented carbides, especially when the starting WC powder is fine-grained [6]. While grain growth is enhanced by the presence of a liquid phase in the case of cemented carbides, AGG is also observed under conditions where
* Corresponding author. E-mail address:
[email protected] (Z.A. Munir). 1 Present address: Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden CO 80401, USA. 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.06.004
no liquid phase exists [6]. The presence of AGG in WC has been reported to increase fracture toughness but decrease hardness [7,8]. Abnormal grain growth has also been reported in the sintering of WC containing excess carbon [6,8,9]. Excess carbon is added to eliminate the formation of the W2C phase. The occurrence of this phase, which is thermodynamically stable above 1255 °C, is attributed to the presence of a surface layer of tungsten oxide, which when reduced during sintering in a graphite die, provides the excess tungsten. Expectedly, this effect is increased as the particles size is decreased [6,8]. Although it improves sinterability of WC, W2C contributes to a decrease in hardness, Young’s modulus, and fracture toughness [10]. In contrast to normal grain growth, abnormal grain growth is not predicted by the Lifshitz–Slyozov–Wagner (LSW) theory [11], and various explanations for its presence have been put forth [12]. These include the effect of carbon [6,13], the existence of large grains in the initial powder [8], and other factors [12]. Theoretical analysis of AGG in WC, where both solid/liquid and solid/solid interfaces exist has focused on the existence of singular surfaces arising from the anisotropy of surface energy [14]. The two-dimensional nucleation on the singular surfaces leads to the preferential exaggerated growth and the formation of high aspect ratio grains. The observed effect of carbon on AGG has been explained in terms of a reduction in the two-dimensional nucleation energy barrier in WC [8]. The use of spark plasma sintering (SPS) to successfully consolidate WC powders (without a binder) has been demonstrated in several investigations. An example of these is a recent work in which 0.4 lm WC powders were consolidated to a relative density of up to 97.6% under 60 MPa pressure and a current of 2800 A with a 2-min hold time [15]. The dense carbide had fracture toughness and hardness values of 6.6 MPa m1/2 and 2480 kg mm2, respectively.
J. Zhao et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 130–139
In this paper, we present and discuss results of an investigation on the sintering of nanometric powders of WC using the spark plasma sintering (SPS) method. 2. Experimental materials and methods Two commercial WC powders with different average particle sizes (40–70 and 12 nm) were used in this research. Fig. 1a and b show, respectively, the scanning electron microscope (SEM) image and the X-ray diffraction (XRD) pattern of the powder with a reported grain size in the range 40–70 nm. This powder was obtained from Inframat Advanced Materials (Farmington, CT). The XRD pattern, Fig. 1b, shows the presence of the WC phase only, with no discernable peaks for carbon, tungsten, tungsten oxide, and other carbide phases. The 12-nm powder, with characteristics shown in Fig. 2, was supplied by Pittsburgh Mineral & Environmental Technology (PMET) (New Brighton, PA). The powder was 98.0 at% WC with 2.0 at% excess C. All sample preparations ware performed in a glove box with an O2 and H2O concentrations of
