The Effect of Cr₃C₂ and TaC Additives on Microstructure, Hardness and Fracture Toughness of WC-6Co Tool Material Fabricated by Spark Plasma Sintering

Abstract

Spark plasma sintering has been used to successfully produce WC-6Co-xCr₃C₂, WC-6Co-xTaC (x = 0.2, 0.6, 1.0) and WC-6Co-xCr₃C₂-xTaC (x = 0.1, 0.3, 0.5) cemented carbides. The spark plasma sintered compacts were investigated by scanning electron microscopy, hardness tests and fracture toughness tests. The results were compared to an additives-free WC-6Co cemented carbide consolidated under the same process parameters and commercial ISO K10 inserts. By using Cr₃C₂ and TaC additives, it is possible to improve the hardness and fracture toughness of WC-Co cemented carbides. The best combination of hardness (1936 ± 15 HV₃₀) and fracture toughness (10.38 ± 0.46 MPa∙m^1/2) was obtained by the WC-6Co-1Cr₃C₂.

Keywords:
Spark plasma sintering; Cemented carbide; Chromium carbide; Tantalum carbide; Mechanical properties

1. Introduction

Several authors’ studies^1-3 have concentrated on the relationship of the microstructure and properties of the cutting zone in industrial applications. It is well known that WC-Co cemented carbides are usually used as tool materials for machining, mining, cutting and drilling tools as well as wear resistance parts. It is worth mentioning that the most likely dominant wear mechanisms of WC-Co tools at low speeds/temperatures during the machining process are abrasion, followed by adhesion at moderate speeds/temperatures and then diffusion at high speeds/temperatures. Of course, all of them cannot occur simultaneously, and the dominant wear mechanism depends on the machining conditions and work materials^4-5. Morphologically, WC-Co cemented carbides consist of hard WC embedded in ductile Co^6-9. Increasing the WC fraction can increase hardness and wear resistance, while fracture toughness can increase by increasing the Co content^9-10. That is why WC-Co cemented carbides with 5-10 wt% Co are the most promising materials offering a very good balance of hardness and fracture toughness¹⁰. Moreover, to increase the hardness and fracture toughness, some authors modified the structure of these materials with additives such as Cr₃C₂¹¹, TaC^12-13, TiC¹³, VC¹² and NbC¹⁴. Siwak and Garbiec in a previous study¹⁵ proved that an addition of 2 wt% Cr₃C₂ and 2 wt% TaC to WC-5Co cemented carbides increased the hardness from 1512 to 2105 and 1725 HV₃₀ respectively. The same relationship was confirmed by other authors. Su et al.¹⁰ showed that an addition of 0.4 wt% TaC to WC-9Co increased the hardness from 927 to 1124 HV₃₀. The same effect was observed by Espinosa-Fernández et al.¹¹, where an addition of 1 wt% Cr₃C₂ to WC-12Co cemented carbides increased the hardness from 1503 to 1668 HV₃₀ for conventional sintering and from 1847 to 1872 HV₃₀ for spark plasma sintering. In recent years spark plasma sintering has become increasingly more attractive for the fabrication of WC-Co tool materials in view of the rapid heating and cooling, short holding time and controllable high compaction pressure over conventional powder metallurgy methods^16-17. That is why in this paper the results on the spark plasma sintering of WC-6Co tool materials with additions of Cr₃C₂ and TaC additives are reported. The goal of the work was to study the effect of Cr₃C₂ and TaC additions to the basic WC-6Co cemented carbide on the microstructure and main mechanical properties such as hardness and fracture toughness.

2. Experimental procedure

Nanocrystalline WC-6Co (99.9%, grain size 40-80 nm), microcrystalline Cr₃C₂ (99.9%, particle size ~6 µm) and microcrystalline TaC (99.9%, particle size ~3 µm) powders supplied by Inframat Advanced Materials, USA were used as the initial powders. The morphology of the powders was examined by scanning electron microscopy using a Vega 5135 (Tescan, USA) microscope with an energy dispersive X-Ray spectrometer. The WC-6Co and Cr₃C₂, WC-6Co and TaC, WC-6Co and Cr₃C₂ and TaC powders were mechanically mixed for 5 h using a mixer, similar to a Turbula shaker-mixer. The obtained powder mixtures were densified by spark plasma sintering using an HP D 25-3 furnace (FCT, Germany). The sintering temperature of 1450°C was reached at the heating rate of 600°C/min. The pressure level on the specimens was kept constant at 60 MPa throughout the sintering process. The vacuum level of the sintering chamber was set at 5 Pa. After a 5 min holding time, the spark plasma sintered compacts were rapidly cooled to room temperature. Cylindrical specimens with dimensions of 20 mm in diameter and 5 mm high were fabricated.

Vickers hardness measurements using a hardness tester FV-700 (Future-Tech, Germany) were carried out by applying a load of 294.2 N for 7 s. The fracture toughness (K_1c) was calculated based on the crack length measured from the corner of the indenter made by Vicker’s indentation using the following equation (1):¹⁸

(1) ${\mathit{K}}_{1\mathit{c}}=0,15·\sqrt{\frac{\mathit{HV}30}{\Sigma \mathit{l}}}$

where HV 30 is the Vickers hardness measured under a load of 294.2 N and Σl is the total length of cracks initiated from the corners of the indenter. The microstructure of the sintered materials was observed on polished cross-sectioned surfaces by scanning electron microscopy using an Inspect S (FEI, Netherlands) microscope. The scheme of the experimental procedure of fabricating the cemented carbide specimens and their examination is presented in Figure 1. A photograph of one of the obtained cemented carbide specimens is presented in Figure 2.

Figure 1
The scheme of the experimental procedure of the fabricating the WC-Co cemented carbide specimens and their examination.

Figure 2
The photograph of one of the SPSed WC-Co cemented carbide specimen.

3. Results and Discussion

Figure 3 shows the second electron SEM micrographs of the initial powders morphology. The main component of premixed WC-6Co powder is WC, whose particles are multi-angular in shape in contrast to the Co powder particles which are spherical. The Cr₃C₂ powder particles are generally rounded and slightly agglomerated in contrast to the TaC powder particles which are spherical in shape but significantly agglomerated. The EDS analysis showed that only the main peaks corresponding to the basic elements are clearly visible in the spectra. It means that no contaminations were detected above the EDS detection limit.

Figure 3
Second electron SEM micrographs and EDS analysis results of initial powders: (a) WC-6Co, (b) Cr₃C₂ and (c) TaC, various magnification.

Figure 4 shows the backscattered electron SEM micrographs of the obtained sintered compacts. The microstructure observations allowed the authors to highlight the ceramic and the binder distribution. The bright and grey contrast phases are the WC ceramic and Co binder respectively. The black areas correspond to pores. Rumman et al.¹⁹ explained that in WC-Co cemented carbides pores tend to decrease in size with an increasing compaction pressure. They applied a compaction pressure ranging between 30 and 80 MPa and obtained a reduction in porosity from 3.12 to 2.12%. Moreover, the pores found on the low compaction pressure specimens were bigger and less homogenously distributed as opposed to the high compaction pressure specimens. In this work, the microstructures are very fine, with a uniform grain-size distribution, only in the basic specimen and the specimen with the addition of 0.2 wt% Cr₃C₂. In other specimens with 0.2 wt% additives, porosity with pores up to a few µm is apparent within the microstructure. The cemented carbides with 0.6 wt% additives have bigger pores non-uniformly distributed in the microstructure, whereas microstructural defects in the form of carbide agglomerates are noted in the WC-6Co-0.6TaC specimen. The same carbide agglomerates are also noted in the WC-6Co-0.5Cr₃C₂-0.5TaC specimen, but the size of these agglomerates is much higher. Probably one of the reason for carbide agglomerates formation is the mixing process in dry conditions and the resulting segregation. On the other hand, the same mixing process was used for preparing the WC-5Co-2Cr₃C₂ and WC-5Co-2TaC powder mixtures¹⁵ and no carbide agglomerates were present in the microstructures. Anyway, one of keys to preventing the agglomeration phenomenon might be to use ultrasound in the process of mixing the suspension consisting of the initial powders and liquid as in work¹⁹, where aluminum and alumina powders were ultrasound mixed using anhydrous methanol. The specimens with 1.0 wt% Cr₃C₂ and TaC show an obviously finer microstructure than those in the specimens with the 0.6 wt% of these additives, even in the WC-6Co-1TaC specimen microstructural defects are also present. It means that increasing the additive contents in the powder mixtures can prevent agglomerations of additive particles and lead to obtaining a fine microstructure without microstructural defects after the spark plasma sintering process, as shown in¹⁵.

Figure 4
Backscattered SEM micrographs of spark plasma sintered WC-6Co-xCr₃C₂ (a) x = 0.2, (b) x = 0.6, (c) x = 1.0 and WC-6Co-xTaC (d) x = 0.2, (e) x = 0.6, (f) x = 1.0 and WC-6Co-xCr₃C₃-xTaC (g) x = 0.1, (h) x = 0.3, (i) x = 0.5 and (j) WC-6Co cemented carbides.

The calculated hardness and fracture toughness value of the additive-free WC-6Co and WC-6Co cemented carbides with additives up to 1 wt% are shown in Table 1. Figure 5 shows the hardness as a function of additive contents and Figure 6 shows the fracture toughness as the same function. As expected, the Vickers hardness increased with increasing the additive contents. The basic WC-6Co cemented carbide has the hardness of 1728 HV₃₀. The hardness obtained in this work is higher than the value reported in the open literature for similar materials^8,20. Furthermore, modification of the structure by the addition of 0.2, 0.6 and 1.0 wt% Cr₃C₂ has an influence not only on an increase in hardness but also on an increase in fracture toughness. The specimen with the highest content of Cr₃C₂ (1 wt%) is characterized by the highest hardness of 1936 HV₃₀ and fracture toughness of 10.38 MPa·m^1/2 of the obtained materials. In turn, the obtained cemented carbides with the TaC additive, independent of the content of this additive, are characterized by a slightly lower fracture toughness than the basic cemented carbide and ranged between 9.29 MPa·m^1/2 for the highest content of TaC and 9.76 MPa·m^1/2 for the medium content. The hardness of these materials is slightly higher than that of the basic material and ranged between 1784 and 1799 HV₃₀. Higher hardness is noted for the cemented carbide with both the Cr₃C₂ and TaC additives and a value up to 1870 HV₃₀ by WC-6Co-0.5Cr₃C₂-0.5TaC was achieved. The fracture toughness of this material is 9.48 MPa·m^1/2. In turn, the lowest hardness of 1732 HV₃₀ is noted for WC-6Co-0.3Cr₃C₂-0.3TaC and results directly from the porosity which is present in the microstructure (Figure 4h), because the porosity is a critical parameter that affects the hardness of cemented carbides¹⁹. On the other hand, this material is characterized by the highest fracture toughness of 11.02 MPa·m^1/2 of the obtained specimens. For comparison, the hardness values in this work were compared against commercial inserts supplied by Baildonit, Poland from the ISO K10 group such as H10 (WC-6Co) and H10S (WC-4.5Co-4.5TaC-NbC), whose hardness was found to be 1600 and 1650 HV₃₀ respectively²¹. The presented results of the commercial inserts clearly show the positive effect of additives on hardness. Obviously the WC-Co cemented carbides obtained by the authors are characterized by a much higher hardness. For another comparison, Mahmoodan et al.²² investigated conventionally sintered WC-10Co-xCr₃C₂ (x = 0.3, 0.6, 1.0) cemented carbides and obtained a hardness not exceeding 1800 HV₃₀ but the fracture toughness exceed 23 MPa·m^1/2. Similar results were obtained by Sun et al.²³ where WC-11Co-xCr₃C₂ (x = 0.2, 0.4, 0.6, 0.8) ultrafine cemented carbides were fabricated by spark plasma sintering. The highest hardness of 1922 HV₃₀ and fracture toughness of 14.14 MPa·m^1/2 were noted for an ultrafine WC-11Co-0.6Cr₃C₂ cemented carbides. Of course, it should be kept in mind that direct comparison of other authors’ results with the obtained mechanical properties is hampered due to, e.g. the differences in WC ceramic grain size, various contents of Co binder and used additives as well as the fabrication method and process parameters in each paper.

Table 1
Hardness and fracture toughness of spark plasma sintered WC-6Co cemented carbides with varying weight content of Cr3C2 and TaC.

Material	Hardness, HV30	Fracture toughness, MPa∙m1/2
WC-6Co	1728 ± 16	9.71 ± 1.14
WC-6Co-0.2Cr3C2	1749 ± 29	10.32 ± 0.43
WC-6Co-0.6Cr3C2	1831 ± 26	10.02 ± 0.39
WC-6Co-1Cr3C2	1936 ± 15	10.38 ± 0.46
WC-6Co-0.2TaC	1784 ± 21	9.52 ± 0.67
WC-6Co-0.6TaC	1799 ± 20	9.76 ± 0.20
WC-6Co-1TaC	1790 ± 11	9.29 ± 0.18
WC-6Co-0.1Cr3C2-0.1TaC	1815 ± 19	9.32 ± 0.13
WC-6Co-0.3Cr3C2-0.3TaC	1732 ± 8	11.02 ± 0.47
WC-6Co-0.5Cr3C2-0.5TaC	1870 ± 21	9.48 ± 1.33

Figure 5
Hardness as function of additive contents of spark plasma sintered WC-6Co cemented carbides. Corresponding assay data are presented in Table 1.

Figure 6
Fracture toughness as function of additive contents of spark plasma sintered WC-6 Co cemented carbides. Corresponding assay data are presented in Table 1.

4. Conclusions

Hardness is one of the most important factors that determine how well WC-Co tools will withstand extreme cutting conditions. In this work the effect of various weight contents of Cr₃C₂ and TaC additives on the microstructure, hardness and fracture toughness are analyzed. The obtained results clearly show that increasing the additive contents can prevent the agglomeration of carbides and lead to obtaining a fine microstructure with a higher hardness exceeding 1900 HV₃₀ and a fracture toughness exceeding 9 MPa·m^1/2. Moreover the cemented carbides with the Cr₃C₂ additive are characterized by better hardness and fracture toughness than with the TaC additive. The best combination of hardness of 1936 ± 15 HV₃₀ and fracture toughness of 10.38 ± 0.46 MPa·m^1/2 was obtained by the WC-6Co-1Cr₃C₂.

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Publication Dates

• Publication in this collection
  11 May 2017
• Date of issue
  May-Jun 2017

History

• Received
  29 Nov 2016
• Reviewed
  08 Mar 2017
• Accepted
  05 Apr 2017