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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 11759–11765 www.elsevier.com/locate/ceramint
VC and Cr3C2 doped WC-based nano-cermets prepared by MA and SPS N. Al-Aqeelin, K. Mohammad, T. Laoui, N. Saheb Mechanical Engineering Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia Received 21 March 2014; received in revised form 31 March 2014; accepted 1 April 2014 Available online 13 April 2014
Abstract WC powders with average crystallite size of 10 nm were obtained through planetary ball milling of micron sized WC powder. Nanosized-WC powders were then mechanically milled with both 9 and 12 wt% Co, and the grain growth inhibitors VC and Cr3C2 in the range 0.2–0.8 wt%. Powder mixtures were then consolidated using spark plasma sintering (SPS) at two different temperatures, i.e. 1200 and 1300 1C. The microstructure, densification and mechanical properties of the resulting sintered nano-cermets were analyzed as a function of the type and amount of grain growth inhibitors, Co content, and sintering temperature. In general, the addition of VC and Cr3C2 was found to reduce densification. Nevertheless, the effect was found to be lower in Cr3C2 containing compositions with higher Co contents. The grain size variation was found by both a conventional line intersection method on FESEM micrographs, and by using grain analyzer software. VC was found to be comparatively more effective in restricting undesirable grain growth during the sintering processes. Moreover, the micro-hardness (HV30) and fracture toughness were measured using micro-indentation and the results were compared for each composition to comparatively assess the individual effect of the inhibitors. Increasing the concentration of inhibitors was found to restrict grain growth even more and higher hardness values were obtained but only up to a critical inhibitor concentration, beyond which the hardness degrades. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: WC-based; Nanocrystalline alloys; Mechanical alloying; Spark plasma sintering
1. Introduction Cemented carbides are known for their high hardness and toughness, and are widely used in cutting tool industry [1–3]. WC is a common choice for these carbides, as it exhibits high melting temperature (2785 1C) and high hardness values (16–22 GPa) [4]. WC, being very hard, is cemented with soft ductile metals such as Co, acting as a binding material and enhancing the mechanical properties and performance in cutting applications. Cobalt in particular is used to lower the consolidation temperature during sintering in the WC–Co system, in addition to enhancing the degree of densification of the composite [5]. Cobalt, as a second phase binding material, can be mixed with the WC, the hard matrix, through mechanical alloying, which results in a composite exhibiting a n
Corresponding author. E-mail addresses:
[email protected] (N. Al-Aqeeli),
[email protected] (K. Mohammad),
[email protected] (T. Laoui),
[email protected] (N. Saheb). http://dx.doi.org/10.1016/j.ceramint.2014.04.004 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
combination of improved hardness and toughness properties [6–9]. Mechanical alloying was preferred as it is known to be simple, versatile, economically viable and a scalable technique for production of large quantities of processed powders [10]. Higher amounts of binding materials in the mixture will lead to higher toughness values at the expense of hardness [4,5,11]. Therefore, controlling their amount in the mixed composition is very crucial in attaining adequate combination of properties in the resulted cermets. Multiple studies, in this regard, have been devoted to the analysis of the effect of binder addition on the cermets' properties [6,7]. One of the major challenges in developing usable materials via powder metallurgy route is consolidating the resulting powder mixtures, which is usually performed using a variety of techniques. These challenges are primarily related to the occurrence of undesirable grain growth during sintering, which is linked to following mechanisms. Firstly, the dissolution of the smaller particles which are having a larger ratio of surface atoms compared to larger particles and are more unstable due to larger surface energy; thus, tend to coalesce and promote grain growth.
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Secondly, nanocrystalline WC–Co system has comparatively higher free surface energy and hence is more susceptible to grain growth. This will result in the loss of the nanostructure, degrading mechanical properties of the resulted cermets. Therefore, ultrafine-grained and nanostructured WC–Co composites have attracted great interest in the field of high-performance hard materials [12]. One of the preferred routes to overcome these issues is a nonconventional advanced sintering process, such as spark plasmasintering (SPS), which is shown to be beneficial in a variety of studies [13,14]. SPS technique is chosen over conventional sintering processes because it exhibits higher heating rates and densification kinetics, in addition to lower sintering temperatures and shorter holding times under controlled high-pressure [11,15]. Further relevant reduction in co-efficient of friction and wear rate by 20 times in materials, when consolidated by the SPS technique was found by Espinosa-Fernández et al. [16]. Nevertheless, faster grain growth kinetics under SPS can also occur and add to the possibility of undesirable grain growth [17,18]. Therefore, limiting grain growth during the SPS process is a challenge for researchers when consolidating WC–Co cermets. To overcome this, the use of grain growth inhibitors during the sintering process helps reducing the exposed surface area of WC, thus limiting material transfer and hence grain growth rate [19]. Material transfer is also hindered as inhibitors reduce the WC solubility in cobalt. The cobalt binder possibly creates radicals over the WC surfaces and thus retard the solid transfer of material between grains [6]. Inhibitors also prevent transfer of phases and hence reduce the possibility of grain growth by creating an interface between WC and Co particles [18]. In addition, the amount and type of inhibitors added to the powder mixture, is another parameter that needs to be controlled, which if ignored may not inhibit grain growth leading to adverse effects [6], especially for the WC–Co system [20]. Different grain growth inhibitors have been used by researchers, and it has been found that VC and Cr3C2 are the most effective in restricting grain growth, due to their considerable solubility and mobility in liquid cobalt at lower sintering temperatures [18]. Nevertheless, systematic studies that highlight the specific role of these inhibitors need to be carried out. The effect of adding different kind of inhibitors to the WC-based cermets was presented in our previous work, however for fixed amounts aiming to assess their individual effect [21]. Therefore, the aim of the present study is to investigate the concentration effect of two grain growth inhibitors, i.e. VC and Cr3C2, in the range of 0.2–0.8 wt%, added to WC–9Co and WC–12Co nano-cermets. The powder mixtures were sintered at 1200 and 1300 1C via SPS to study the effect of different consolidation temperatures. The effect of changing the WC particle size from micron- to nano-scale was also investigated. Finally, an assessment of densification and mechanical properties was carried out. 2. Experimental procedure The starting powder materials used in this study are listed in Table 1 and supplied by William-Rowland Co. UK. Nano-WC
Table 1 Characteristics of starting powder. Powder
Purity
FSSS (lm)
WC Co VC Cr3C2
99.99% 99.9% 99.9% 99.9%
3.5 1.3 2.02 1.82
Table 2 Sample composition and sintering temperature. No.
Sample composition
Sintering temp. (1C)
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
WC(10 nm)–9Co–0.2VC WC(10 nm)–9Co–0.4VC WC(10 nm)–9Co–0.6VC WC(10 nm)–9Co–0.8VC WC(10 nm)–9Co–0.2Cr3C2 WC(10 nm)–9Co–0.4Cr3C2 WC(10 nm)–9Co–0.6Cr3C2 WC(10 nm)–9Co–0.8Cr3C2 WC(10 nm)–9Co–0.2VC WC(10 nm)–9Co–0.4VC WC(10 nm)–9Co–0.6VC WC(10 nm)–9Co–0.8VC WC(10 nm)–9Co–0.2Cr3C2 WC(10 nm)–9Co–0.4Cr3C2 WC(10 nm)–9Co–0.6Cr3C2 WC(10 nm)–9Co–0.8Cr3C2 WC(10 nm)–12Co–0.2VC WC(10 nm)–12Co–0.4VC WC(10 nm)–12Co–0.6VC WC(10 nm)–12Co–0.8VC WC(10 nm)–12Co–0.2Cr3C2 WC(10 nm)–12Co–0.4Cr3C2 WC(10 nm)–12Co–0.6Cr3C2 WC(10 nm)–12Co–0.8Cr3C2 WC(10 nm)–12Co–0.2VC WC(10 nm)–12Co–0.4VC WC(10 nm)–12Co–0.6VC WC(10 nm)–12Co–0.8VC WC(10 nm)–12Co–0.2Cr3C2 WC(10 nm)-12Co-0.4Cr3C2 WC(10 nm)–12Co–0.6Cr3C2 WC(10 nm)–12Co–0.8Cr3C2
1200 1200 1200 1200 1200 1200 1200 1200 1300 1300 1300 1300 1300 1300 1300 1300 1200 1200 1200 1200 1200 1200 1200 1200 1300 1300 1300 1300 1300 1300 1300 1300
powders were prepared through ball milling of the micronsized WC powders originally received from the manufacturer. Different amounts of both VC and Cr3C2, specifically 0.2, 0.4, 0.6, and 0.8 wt% were mechanically alloyed with WC–9Co and WC–12Co, as shown in Table 2. The milling experiments were carried out in a planetary ball mill (Fritsch Pulverisette 5) with an adjusted ball to powder ratio of 10:1. In addition, milling was performed in an inert Ar environment to reduce contamination of the powders. Ethyl-alcohol was added as a process control agent (PCA) in order to avoid excessive cold welding of the powders. Enhanced homogeneity powders mixture along with effective dispersion of WC within the Co matrix was achieved by using high energy probe sonication. Each milled powder composition was placed in a cylindrical 20 50 50 mm3 graphite die, setup with graphite thin layer
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sheet inserts and SPS sintered (Type HP D-50, FCT Systeme, Rauenstein, Germany) at 1200 1C and 1300 1C; Fig. 1 represents a schematic view of the setup. The SPS was carried out at a heating rate of 100 1C/min, for 10 min at 50 MPa pressure and under vacuum. The sintering pulsed electric current and pulse duration cycle are shown in Fig. 2. Density measurements of the sintered nano-cermets were taken using the Archimedes principle using an electronic densimeter MD-300S, Alfa Mirage, SG. Moreover, hardness (HV30) was recorded, for each sample using a universal hardness testing machine (Zwick-Roell, ZHU250, Germany). The fracture toughness was evaluated using crack length generated, produced by using micro-indentations on the cemented carbides. Crack measurements were taken using optical microscopy and then employing the mathematical relationship between fracture toughness and crack length: pffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffiffiffiffi K 1c ¼ A HV P=∑l ð1Þ
Fig. 1. Schematics of spark plasma sintering [21].
Fig. 2. Time–temperature and pressure–time variation during SPS [21].
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where K1c is the fracture toughness MNm 3/2; HV is the hardness in MPa; P is the indentation load in N; Σl is the sum of crack lengths in mm and A is the constant factor 0.0028. Hardness was measured by Vickers indentation with 30 kg load. Grain size measurement was carried out by a linear intercept method while FE-SEM and optical microscopy analysis (MEIJI-Techno microscope, Japan) were also used for morphological and microstructural characterization. 3. Results and discussion Fig. 3 shows the densification data following the aforementioned procedure for different compositions of grain growth inhibitors. It was found that almost all the samples showed a densification reduction despite containing different types and concentration of inhibitors. This reduction in densification can be attributed to limited diffusion due to different reasons. First, addition of inhibitors leads to the formation of thin layers of VC and or Cr3C2 on the surface of the WC particles, which limits the atomic diffusion and migration of Co [2,22], thus, reducing the final densification. Also, the formation of voids or microporosity lowering densification, as a result of the addition of inhibitors, has also been reported by Bonache et al. [23,24]. Moreover, all the range of added inhibitors containing lower Co-concentrations, i.e. 9 wt% Co, showed lower densification both at lower and at higher sintering temperatures. However, this density difference is minimized at a higher sintering temperature in combination with higher Co-contents, i.e. 12 wt% Co, as shown in Fig. 3. This is because higher binding material contents can coat more WC surface area, which causes a large number of particles to re-arrange and enhance diffusion and hence densification, even at lower sintering temperatures. Further, increasing sintering temperatures in combination with higher cobalt concentration, have shown to increase the diffusion kinetics at higher temperatures and leading to higher densification. Changing the amount of either inhibitor in the powder compositions containing higher amount of binding material and consolidated at higher temperatures, did not show any appreciable change in attained densification, however 0.6 wt% was found to be the saturation point. Higher amount of inhibitors resulted in further densification reduction. The influence of the type of inhibitor i.e. Cr3C2 and VC, on sample densification was analyzed and it was noticed that samples containing Cr3C2 show comparatively higher densification. Lower densification in samples with addition of VC can be linked to the presence of higher amounts of micro-porosity, as shown in Fig. 6. A higher amount of porosity in VC-rich samples has also been reported by Zhan et al. [25]. Addition of 0.6 wt% of either inhibitor to 9 wt% Co at 1300 1C can be assumed as the saturation point, and further addition can lead to the formation of precipitates at the WC/Co grain boundary, further restricting material diffusion and hence densification, as shown in Fig. 3. This saturation effect was not noticed at 12 wt% cobalt, probably because the solubility limit was not reached, which led to an increased densification in this particular case beyond 0.6% of VC, when amount of cobalt was increased from 9 to 12 wt%. All the consolidated powder
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Fig. 3. Densification dependence on amount and type of grain growth inhibitor, and amount of cobalt and sintering temperature of the resulted nanocomposite; (a) 1200 1C and (b) 1300 1C.
Fig. 4. Influence of amount and type of grain growth inhibitors, and amount of cobalt and sintering temperature on crystallite size of the resulted nanocomposite; (a) 1200 1C and (b) 1300 1C.
compositions were also analyzed by X-ray diffraction, to follow the crystallite size development with the different experimental conditions. It was observed that the crystallite size remained almost unchanged for all the compositions, when the sintering temperature was increased from 1200 1C to 1300 1C. It was also observed, as shown in Fig. 4, that the addition of inhibitors was effective in restricting grain growth and maintaining a nanostructure for the whole composition range below 1 wt%. However, it was observed both at lower and higher sintering temperatures that VC is more effective than Cr3C2 in restricting crystallite sizes growth particularly at higher addition of binding material.
Powder compositions containing ultrafine WC with either grain growth inhibitor, could not densify well, even with the addition of high amounts of binding material, when consolidated at 1200 1C. This resulted in lower HV30 values, as shown in Fig. 5. However, values between 1800 and 1900 HV30 were achieved at higher sintering temperatures, i.e. 1300 1C. Samples with 9 wt% Co showed comparatively higher hardness than samples with a higher cobalt concentration, as the amount of softer binding material in the latter samples is higher. Furthermore, comparing critically both inhibitors, chromium carbide seems to lead to comparatively higher hardness values for all set of compositions. The most plausible reason could be linked to the findings published recently by Zhan et al. [25],
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Fig. 5. Influence of amount and type of grain growth inhibitors, and amount of cobalt and sintering temperature on Vickers Hardness of the resulted nano-cermets; (a) 1200 1C and (b) 1300 1C.
Fig. 6. FESEM micrographs showing comparatively higher amount of micro-porosity in all samples containing VC.
where it was found that addition of Cr3C2 produces larger lattice distortion in the binder phase, which strengthens the binder phase and hence improves the average HV30 values of the composite. The same researcher also found out that VC increases the sample microporosity volume percentage, hence lowering HV30 values and densification. Similar results have been observed in our work. The microporosity in samples with
different VC and Cr3C2 contents was also analyzed by FESEM, as shown in the micrographs presented in Fig. 6. The amount of inhibitor added to each composition is small enough i.e. less than 1 wt% to be fully distributed within the WC–Co matrix. Complete and homogeneous distribution reveals the validity of our results. Elemental distribution mapping was also carried out by FESEM. The results show
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Fig. 7. Mapping of WC–Co/Cr3C2, showing complete and homogeneous phase distribution.
Fig. 8. Mapping of WC–Co/VC, showing complete and homogeneous phase distribution.
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a homogeneous phase distribution of WC–Co–Cr3C2 and WC– Co–VC in the samples, as shown in Figs. 7 and 8. 4. Conclusions Planetary ball milling was successfully used to reduce micron size WC particles into the nano-scale range. It was found that higher surface area of WC nanoparticles resulted in finer grain sizes, although requiring higher amounts of binding material to achieve full densification. Further, higher kinetics of diffusion in these finer WC particles furthering addition to the spark plasma sintering process itself, gave rise to grain growth, which may degrade the mechanical properties of the nano-cermets. The addition of grain growth inhibitors has played a definite role in restricting the grain growth and hence developing a finer microstructure which enhances the mechanical properties. Both VC and Cr3C2 have shown to affect the final densification due to limited diffusion. Adding 0.6 wt% of inhibitor was found to be the saturation point in 9 wt% Co. However, this saturation point was not observed when the amount of cobalt was increased to 12 wt% and consolidated at 1300 1C. VC was found to be more effective in restricting grain growth; nevertheless, samples containing higher amounts of VC showed more microporosity. Cr3C2 was found to be more effective in providing comparatively higher densification and higher hardness values. Acknowledgment The authors wish to acknowledge the financial support from King Fahd University of Petroleum & Minerals (KFUPM) to complete this work. References [1] T. Kagnaya, C. Boher, L. Lambert, M. Lazard, Mechanisms of WC–Co cutting tools from high-speed tribological tests, Wear 267 (2009) 890–897. [2] Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide – a review, Int. J. Refract. Met. Hard Mater. 27 (2009) 288–299. [3] Y.M. Egashira, S. Hosono, S. Takemoto, Fabrication and cutting performance of cemented tungsten carbide micro-cutting tools, Precis. Eng. 35 (2011) 547–553. [4] K.-M. Tsai, C.-Y. Hsieh, H.-H. Lu, Sintering of binderless tungsten carbide, Ceram. Int. 36 (2010) 689–692. [5] H. Kim, I. Shon, Consolidation of ultra fine WC and WC–Co hard materials by pulsed current activated sintering and its mechanical properties, Int. J. Refract. Met. Hard Mater. 25 (2007) 46–52. [6] L. Sun, T. Yang, C. Jia, J. Xiong, VC, Cr3C2 doped ultrafine WC–Co cemented carbides prepared by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 29 (2011) 147–152. [7] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383–502.
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