Different effects of Cr3C2 and VC on the sintering behavior of WC–Co materials

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Highlights

  • Combination of DSC, TGA, EGA and dilatometer was used to study sintering behavior of VC and Cr3C2 doped WC–Co.

  • VC doped WC–Co has a CO2 outgassing reaction between 900 and 1100 °C.

  • Cr3C2 doped WC–Co has a strong CO2 outgassing reaction in the 1200–1400 °C range.

  • VC doping has a lower melting temperature than Cr3C2 doping.

Abstract

Cr3C2 and VC are two typical grain growth inhibitors used in producing fine grain microstructure in WC–Co materials. In this investigation, the sintering processes of Cr3C2 and VC doped submicron WC–Co powders were studied by using dilatometry, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and evolved gas analysis (EGA) by Fourier Transform Infrared Spectrophotometry (FT-IR). The combination of these methods can provide a comprehensive understanding of sintering processes of Cr3C2 and VC doped WC–Co powders during heating. Results show that, below 900 °C, similar sintering behaviors were observed for both Cr3C2 and VC doped WC–Co powders, showing a wax vaporization process in the range of 200 °C to 400 °C, and tungsten oxide reduction by carbon occurring around 800 °C. Onset of densification begins around 900 °C. Differences in sintering reaction and densification were observed above 900 °C. First, VC doping shows an outgassing reaction and a clear shrinkage inflection, increase in shrinkage rate, in the dilatometer curve between 900 and 1100 °C, while Cr3C2 doping shows only slight outgassing and no shrinkage inflection in the dilatometer response within the same temperature range. Second, Cr3C2 doping shows a strong outgassing reaction in the 1200–1400 °C range, which was not evident in the VC doped material. Third, Cr3C2 doping also shows a higher melting temperature than VC doping. Possible mechanisms for these different sintering reactions and resulting sintering behaviors have been discussed in this paper.

Introduction

Cobalt cemented tungsten carbide (WC–Co), consisting of large volume fractions of tungsten carbide (WC) particles in a cobalt (Co) metal matrix, is widely used in tool materials for numerous manufacturing industries including automotive and aerospace manufacturing, oil and gas drilling, geothermal energy exploration, mining, construction and other applications where extreme wear resistance is required [1], [2], [3], [4], [5], [6], [7], [8]. Mechanical properties of cemented tungsten carbides can be tailored by controlling WC grain size and Co binder content. Generally, fine WC grain size can increase hardness and wear resistance of WC–Co materials. However, for ultrafine and submicron WC particle sizes, grain growth during sintering can often result in grain sizes much larger than the original submicron scale after sintering. To produce these ultrafine/submicron WC–Co grades, a grain growth inhibitor is necessary for reducing grain growth during sintering. Two commonly used grain growth inhibitors are VC and Cr3C2. Mechanisms of VC and Cr3C2 on grain growth inhibition have been previously studied and documented in existing literature [9], [10], [11], [12], [13], [14]. It is generally believed that VC can form a thin layer of (W,V)Cx on the WC surface, while Cr3C2 can dissolve in the Co binder phase, both mechanisms resulting in the suppression of dissolution and re-precipitation of WC during liquid phase sintering, thus retaining the desired fine WC grain size [9], [10], [11], [12], [13], [14].

Most of the studies in the literature are focused on the effects of VC and Cr3C2 on grain growth during liquid phase sintering or after liquid phase sintering [9], [10], [11], [12], [13], [14]. Inhibition of grain growth during liquid phase sintering is thus well understood. As described above, VC forms (W,V)Cx to inhibit grain growth and Cr3C2 dissolves into the liquid cobalt phase to inhibit grain growth. However, there are limited investigations regarding the effects of VC and Cr3C2 on either densification or grain growth behavior during the heating process before liquid phase formation. Taniuchi [15] provided one study on solid state sintering of VC doped WC–Co at 1200 °C, and found VC dissolved in the Co phase after a 1 h hold. Wang [16] determined that VC can inhibit grain growth of nanosized WC during solid state sintering at temperatures much below eutectic temperature. Elfwing [17] reported a solid state sintering experiment using Cr3C2 doped WC–Co, and showed undissolved Cr particles after hot pressing 1 h at 1200 °C. These studies have clearly indicated that there are interactions between grain growth inhibitors VC, Cr3C2 and WC–Co during solid state sintering. For ultrafine/submicron WC–Co materials, solid state sintering is critical since most densification and grain growth are completed before liquid phase formation [18]. Therefore, it is important to understand the effects of VC and Cr3C2 on the sintering behavior of WC–Co during the heating process, so the sintering profile can be optimized to improve microstructure and densification of ultrafine or submicron WC–Co systems.

In this study, different thermal analysis methods are employed to understand the effects of VC and Cr3C2 on sintering behavior of WC–Co during the heating process. These methods include thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), evolved gas analysis (EGA) by Fourier Transform infra-red spectrophotometry (FT-IR), and dilatometry. TGA, DSC and EGA signals can be monitored simultaneously during a single heating cycle. The combination of this information can provide a comprehensive understanding of sintering reactions during the heating process. Further, dilatometry data can provide additional insight into correlations between sintering reactions and densification. The purpose of this study is to investigate sintering reactions of VC and Cr3C2 doped submicron WC–Co materials during heating and understand differences in sintering behavior between VC and Cr3C2 doped submicron WC–Co materials during solid state sintering. Findings of this study can be used as reference to design sintering recipes for producing ultrafine or submicron WC–Co grades.

Section snippets

Materials and methods

Powder samples were prepared with 10 wt.% cobalt and submicron WC with 0.17 wt.% Cr3C2 or VC. The starting raw materials WC, VC and Cr3C2 were fine submicron sized particles with < 1 μm FSSS. Powders were attritor milled for 5 h in heptane and vacuum dried after milling. Paraffin wax was used as the organic binder. Green bar samples (16 g each) were pressed under 140 MPa using a hydraulic press. The pressed green bars were used for dilatometer experiments. Powder samples were used for DSC–TGA–EGA

Results

Fig. 1 shows TGA, DSC, and EGA results of VC doped WC–Co and Cr3C2 doped WC–Co after heating to 1425 °C. For VC doped WC–Co, in Fig. 1(a), several sintering reactions are seen at different stages of heating. Between 200 °C and 400 °C TGA shows significant weight loss, DSC shows an endothermic peak, and EGA shows several outgassing peaks. After this reaction, the next sintering reaction occurs in the 700 °C to 900 °C temperature range, showing weight loss in the TGA, an endothermic peak in DSC, and

Discussion

The results from combined thermal analysis by TGA, DSC, EGA and dilatometry reveal three major differences between VC doped and Cr3C2 doped submicron WC–Co during the heating cycle of a sintering process. These differences include the following: (1) VC doping indicates an obvious outgassing reaction and a clear shrinkage inflection in the dilatometer curve between 900 °C and 1100 °C, while Cr3C2 shows only slight outgassing and no shrinkage inflection within the same temperature range; (2) Cr3C2

Conclusions

In this investigation, the sintering behavior of Cr3C2 and VC doped submicron WC–Co powder were studied by using dilatometry, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and evolved gas analysis (EGA). The combination of these methods provides a comprehensive understanding of sintering processes of Cr3C2 and VC doped WC–Co powders during heating. Several sintering reactions were observed during heating of Cr3C2 and VC doped submicron WC–Co powders. Sintering

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