Effects of initial particle size distribution and sintering parameters on microstructure and mechanical properties of functionally graded WC-TiC-VC-Cr3C2-Co hard alloys

doi:10.1016/j.ceramint.2016.11.086

Ceramics International

Volume 43, Issue 2, 1 February 2017, Pages 2686-2696

https://doi.org/10.1016/j.ceramint.2016.11.086 Get rights and content

Abstract

Functionally graded WC-TiC-Co cemented carbides with a Co content gradient and not comprising the η-phase were prepared employing hot-pressing sintering technique by introducing pre-designed both gradients of Co contents and WC grain sizes. The effects of initial hard phase particle size distribution and sintering parameters on the microstructure and mechanical properties of functionally graded cemented carbides were investigated by fabricating coarse-particle, fine-particle and hybrid-particle (combining coarse-particle and fine-particle) functionally graded cemented carbides employing hot-pressing sintering technique. Various hot sintering temperatures (1325 ℃, 1350 ℃, 1375 ℃ and 1400 ℃) and soaking times (15 min, 30 min, 45 min, 60 min) were tested in order to determine the optimal parameters of hybrid-particle functionally graded WC-TiC-Co cemented carbides with the assistance of VC and Cr₃C₂ as grain growth inhibitors and PVP as dispersant. The experimental results showed that excellent mechanical properties are achieved for sintering at 1375 ℃ soaked for 45 min with a hardness of 1986 kgf/mm², a TRS of 1752 MPa and a fracture toughness of 13.05 MPa m^1/2. Hybrid-particle functionally graded WC-TiC-Co cemented carbides generated a good liquid-phase sintering at 1375 ℃ soaked for 45 min. Specimens sintering for 1325 ℃ and 1350 ℃ maintained the pre-designed Co gradient, whereas the specimens sintering for 1375 ℃ maintained a smaller Co gradient and the specimen sintering for 1400 ℃ had no Co gradient. The excessive grain growth of fine-grained WC resulted the WC size difference disappear of coarse-grain and fine-grain, which weaken the Co phase migration pressure from surface layer to core layer. Furthermore, Co liquid phase distribution behavior is normally related with temperature, soaking time and pressure. The five-layer structure developed in this paper provides the surface with exceptional combination of high wear resistance and high toughness, which is favorable for the application as metal cutting tools.

Introduction

Cemented carbides, teeth in industry, have been universally used in metal cutting, wood machining and rock drilling due to their excellent toughness and inherent wear resistance [1], [2], [3]. Generally, cemented carbides are of a large volume percentage of the “hard” phase, which renders the alloys preeminent wear resistance and hardness embedded within a tough and soft metal binder phase, which endows the alloys considerable toughness and strength and facilitates the sintering to realize a fully dense bulk [4]. As a consequence, increasing the content of hard phase results in superior wear resistance but decreased toughness, whereas raising the content of metal phase brings about higher toughness yet reduced wear resistance. This is because of the main drawback associated with the contradictory of increasing hardness and toughness simultaneously, conventional cemented carbides are less satisfactory in certain applications demanding both desired hardness and toughness.

In recent years, due to its industrial significance, massive efforts have been performed to produce WC cemented carbide featuring perfect combination of high toughness and high hardness. There are some basic ways of obtaining cemented carbide with superior integrative mechanical properties. For one way, recent developments in the field of nanostructured materials [5], [6], [7], [8], [9], [10] suggest that the formation of nanocrystalline (the mean WC grain size smaller than 100 nm) grain structure has the makings of dramatically improving the mechanical properties of cemented carbides. However, few approaches that fabricate nanocrystalline WC–Co cemented carbides are commercialized and nanocrystalline WC–Co powders tend to rapid grow resulting in the nano-grain lose their nanoscale characteristics during conventional sintering [11]. There is no doubt that grain refinement contributes much to enhancing hardness, however, it is unclear whether nanosized particles offer any benefit as far as fracture toughness is concerned. For another, surface coating may be an effective approach to enhance the mechanical properties of the materials [12], [13], [14], coating on the cemented carbide surfaces with hard material (TiNC, TiAIN, etc.) or soft material (WS₂, CaF₂ etc.) may improve the mechanical properties. There are large amounts of literatures reporting on coating materials, whereas literatures on hard film with good toughness or soft film with high hardness are rather limited. Besides this, coating process, just as chemical vapor deposition, often requires high temperature, consequently, cracks would inescapably form in the coatings and lead to insert failure because of the thermal expansion coefficient difference between substrate and coating. Functionally graded cemented carbides (FGCCs), an ideal material for use in industrial applications, are considered as the most feasible approach to the balance between the fracture toughness and the wear resistance. The effect of the graded structure on the mechanical properties of FGCCs has been reported by many literatures [15], [16] previously.

The preparation of functionally graded cemented carbide (FGCC) has attracted immense interest, quite a few processes have been developed to produce FGCC with various types of graded structures, such as solid-phase sintering, infiltration, carburized sintering and so on [17], [18], [19], [20], [21]. Unfortunately, these processes are rather complex and hard to be commercialized. Subsequent hot isostatic pressing is necessary to eliminate the residual porosity of alloys sintered by solid-phase sintering; green porous compact with a certain sized pores must be prepared before infiltration, while cemented carbide compact deviating from normal carbon content is indispensable for carburized sintering; As far as pressureless sintering is concerned, an integral addition is forming agent just as wax, during dewaxing, free C produced from the decomposition of organic forming agent would penetrate inside the material and result in the carburizing of WC and Co. Moreover, the evaporation of Co is unavoidable for vacuum pressureless sintering.

The hot-pressing sintering (HPS), as a suitable and economical method to facilitate the sintering of cemented carbide, which yielded such benefits as external high pressure and rapid sintering at relatively lower temperature comparing with conventional sintering approaches, has drawn considerable attention and has been demonstrated at the industrial level. The external pressure improves the consolidation of the cemented carbides [22], therefore hot-pressing requires a shorter sintering time in comparison with vacuum sintering [23], [24]. Furthermore, the hot-press sintering can effectively inhibit the grain growth of the cemented carbides [25], [26].

Featuring high crystallinity, less defects, slight micro-strain and coarse substructure, coarse WC grains which lower the crack growth rate by generating twin-slip coordination deformation, provide the alloy with good toughness [27], [28], [29], [30], [31], [32], while fine WC grains contribute to the increase in alloy hardness and wear resistance. Therefore, an alloy surface with both coarse grains and fine grains has the potential to implement superior combination of high toughness and high hardness.

In the present study, by employing hot-press sintering, coarse-particle functionally graded WC-TiC-Co cemented carbides (CFGCC), fine-particle functionally graded WC-TiC-Co cemented carbides (FFGCC) and hybrid-particle functionally graded WC-TiC-Co cemented carbides (HFGCC) were fabricated with the auxiliary of dispersant (PVP) and composite grain growth inhibitors (Cr₃C₂ and VC). The effect of hybrid-particle combining coarse-particle and fine-particle on the microstructure and mechanical properties of the developed functionally graded WC-TiC-VC-Cr₃C₂-Co based cemented carbides was investigated. Moreover, various hot sintering temperatures (1325 ℃, 1350 ℃, 1375 ℃ and 1400 ℃) and soaking times (15 min, 30 min, 45 min, 60 min) were tested in order to determine the optimal parameters of the fabricated hybrid-particle functionally graded WC-TiC-VC-Cr₃C₂-Co cemented carbides.

Section snippets

Preparation

Powders of tungsten carbide (3 µm, 99.9% purity, Jinan Institute of metallurgical science Co. Ltd., China); Powders of tungsten carbide (0.95 µm, 99.9% purity, Xiamen Golden egret special alloy Co. Ltd., China); Powders of titanium carbide (400 nm, 99.8% purity, Shanghai Chaowei nanotechnology Co. Ltd., China); Powders of cobalt (1.26 µm, 99.6% purity, Jinan Institute of metallurgical science Co. Ltd., China); Powders of vanadium carbide (200 nm, 99.9% purity, Shanghai Xiangtian nano materials Co.

Relative density

Fig. 2 demonstrates the effects of sintering temperature and soaking time on the relative density of HFGCC. It is evident that the density increased with the increasing sintering temperature and soaking time in the whole experimental range, whereas the effectiveness is not significant after sintering temperature at 1350 ℃ or soaking time for 45 min. The relative density of HFGCC reaches 98.73% when sintering at 1375 ℃ for 45 min, which indicates that the samples developed in our study are quite

Conclusion

Hybrid-particle functionally graded WC-TiC-VC-Cr₃C₂-Co cemented carbides are successfully fabricated by employing hot-press sintering. The effect of hybrid-particle combining coarse-particle and fine-particle on the microstructure and mechanical properties of the functionally graded cemented carbides were investigated. .

(1)
The hybrid of coarse grains and fine grains brought about preeminent improvement on the mechanical properties of functionally graded cemented carbides. The beneficial influence

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51475273).

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Citation Excerpt :
It is believed that only a small amount of TiC and Cr3C2 (nearly 2 wt%) in WC-Co inhibits the grain growth, decreasing the impact toughness [26,27]. However, WC-TiC-VC-Cr3C2–Co cemented carbides exhibit excellent mechanical properties, such as hardness of 1986 kgf·mm−2, TRS of 1752 MPa and fracture toughness of 13.05 MPa m1/2 KIC [28]. The previous work about TiC-reinforced WC-based cemented carbides was based on powder sintering, however, the sintering process is quite complex and requires multi-stage temperature control [29–31].
WC-based cemented carbides with different amounts of TiC (0.9, 1.2, 1.5, 1.8 and 2.1 wt%) were synthesized by pressureless melt infiltration at 1200 °C for 90 min in vacuum furnace using Cu–10Ni–5Mn–3Sn binder alloy and cast WC, Ni and TiC powders. The microstructural evolution, new phase formation, and related mechanical properties of melt infiltrated alloys were investigated. Results revealed that partially-cast WC and TiC decomposed and formed Ni₂W₄C, W, (Ti,W)C_1-x and annular-structured solid solution when the content of TiC was in the range of 1.2–2.1 wt%, whereas Ni₂W₄C and W were not observed in the alloy with 0.9 wt% TiC. Annular-structured solid solution was characterized by high carbon with relatively small amounts of W and/or Ti. An increase in TiC content promoted the formation and growth of Ni₂W₄C and (Ti,W)C_1-x phases. However, the annular structure with a continuous network developed into discontinuous net-like distribution and gradually disappeared. Addition of an appropriate amount of TiC improved the Rockwell hardness (HRA) and transverse rupture strength (TRS); however, the impact toughness always decreased with the addition of more amount of TiC. The alloys with 1.8 wt% TiC, 1.5 wt% TiC, and 0.9 wt% TiC exhibited the best mechanical properties with Rockwell hardness of 93.6 HRA, TRS of 1823.4 MPa, and impact toughness of 6.33 J cm⁻², respectively.

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Effects of initial particle size distribution and sintering parameters on microstructure and mechanical properties of functionally graded WC-TiC-VC-Cr3C2-Co hard alloys

Abstract

Introduction

Section snippets

Preparation

Relative density

Conclusion

Acknowledgements

J. Alloy. Comp.

J. Alloy. Comp.

Appl. Surf. Sci.

Int. J. Refract. Metals Hard Mater.

Mater. Sci. Eng. A

Mater. Let.

Int. J. Refract. Metals Hard Mater.

Ceram. Int.

Scr. Mater.

J. Alloy. Comp.

Int. J. Refract. Metals Hard Mater.

Tribol. Int.

J. Alloy. Comp.

Int. J. Refract. Metals Hard Mater.

J. Alloy. Comp.

Int. J. Refract. Metals Hard Mater.

Int. J. Refract. Metals Hard Mater.

Int. J. Refract. Metals Hard Mater.

Effects of initial particle size distribution and sintering parameters on microstructure and mechanical properties of functionally graded WC-TiC-VC-Cr₃C₂-Co hard alloys