Microstructural evolution and development of mechanical properties of spark plasma sintered WC–Co cemented carbides for machine parts and engineering tools

Abstract

WC–Co, WC–Co–Cr₃C₂ and WC–Co–TaC–NbC cemented carbides were spark plasma sintered and the microstructure and main mechanical properties of the obtained specimens were investigated. A series of WC–6Co cemented carbides was heated to the sintering temperature of 1400 °C at 200 and 400 °C/min at compacting pressures of 50 and 60 MPa. It was shown that the specimens spark plasma sintered at 400 °C/min and at 60 MPa possess the best mechanical properties. These parameters were applied for sintering WC–6Co cemented carbides with addition of grain growth inhibitors such as Cr₃C₂ and TaC–NbC. The influence of the grain growth inhibitors content was studied. The X-ray diffraction test results show that decarburization of the WC phase occurred and carbon deficient W₂C and η (Co₃W₃C, Co₆W₆C) phases were formed during spark plasma sintering, wherein an increase in compacting pressure from 50 to 60 MPa results in a diminution in the carbon diffusion processes. The mechanical properties of the cemented carbides were defined. The best ratio of hardness and fracture toughness was obtained for WC–6Co–1Cr₃C₂: hardness was 1808 ± 19 HV₃₀ and fracture toughness was 10.17 ± 0.27 MPa m^1/2.

Introduction

Despite the fact that tungsten carbide–cobalt (WC–Co) hard materials cause allergies and contain probable human carcinogenic agents [1], [2], they are still the most dominant tool materials in terms of industrial applications for machining, mining, cutting and drilling tools as well as wear resistance parts. WC–Co cemented carbides display a unique combination of mechanical properties such as high hardness, excellent wear resistance, good toughness and strength than that of other hard materials [3], [4], [5], [6]. Morphologically, they consist of the hard hexagonal WC ceramic phase embedded within a soft and tough Co binder phase. Increasing of the WC phase content in the sintered compacts leads to raising of the hardness and wear resistance and decrease of the fracture toughness [6]. Moreover, the presence of secondary phases synthesized in the sintered compacts as a result of decarburization of the WC phase, and carbon diffusion during sintering may reduce its mechanical properties [7].

It is well known at present time that industrial interests are focused on synthesis of WC–Co cemented carbides with high hardness and fracture toughness. These materials with a reduced grain size to a submicron meter or nanometer range can meet the growing demands of industry. Thus WC-based and other nanocrystalline materials have received a great deal of attention as advanced engineering materials with improved physical and mechanical properties [3], [4], [5], [6], [8]. As shown by Liu et al. [4], the hardness, strength and toughness of WC–Co cemented carbides improve greatly when the WC grain size is reduced to below 500 nm. However, the rapid growth of ultra-fine WC grains occurs during heating even though the liquid phase is not formed. A variety of new sintering technologies are employed to diminish the effect of grain growth and to achieve an efficient WC–Co material densification [8], [9]. The densification of WC–Co powder mixture has been accomplished by conventional sintering [10], hot pressing (HP) [11], hot isostatic pressing (HIP) [12], microwave sintering [9], [13], high-frequency induction sintering [14], [15], pulse plasma compaction (PPC) [16] and spark plasma sintering (SPS) [17], [18]. The SPS technique has a well-known advantages owing to its such as rapid heating and cooling rates, short holding time and small compacting pressure changes. For this reason, this technology has attracted increasingly more attention in the investigations of nanocrystalline and ultrafine WC–Co cemented carbides. As shown by [4], [8], [18], grain growth during SPS can be effectively inhibited. Modifying the microstructure of WC–Co cemented carbides with grain growth inhibitors such as Cr₃C₂ [19], TaC [20], [21], TiC [21], VC [20] and NbC [22] can efficiently inhibit WC grain growth and allow one to increase both the hardness and fracture toughness.

The purpose of this paper, which is a continuation of the paper [23], is to study the effect of adding some grain growth inhibitors such as Cr₃C₂ and TaC–NbC on the microstructure evolution and the main mechanical properties of SPSed WC–6Co cemented carbides.