Development of new Cr-based hardmetals by liquid phase sintering and spark plasma sintering

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Traditional hardmetals based on a cobalt-based tungsten carbide (WC) system are currently the most commonly used hard materials in a wide range of applications, in particular rock drilling tools and cutting tools for metals, plastics, wood and composite materials, due to their outstanding properties in terms of hardness, fracture toughness and wear resistance. The interest in finding alternative binders has been driven by the need for the protection of human health and the environment, since binder alloys including cobalt (Co) and nickel (Ni) are reported as being carcinogenic to human health and toxic chemicals for the environment. In addition, Co binder has another important drawback: the fluctuation in its high price due to its poor availability. Thus, this investigation aims to develop novel hardmetals with outstanding properties avoiding the use of Co and Ni. Within this framework, a chromium-based (Cr-based) alloy is proposed as an alternative binder, as a consequence of its easy availability, reduced price, lower toxicity compared to Co and/or Ni alloy, and the possibility of providing an improvement in the oxidation and wear resistance of the hardmetals. In the present investigation, the development of Cr-based WC hardmetals is based on a combination of thermodynamic modelling (Thermo-Calc with database TCFE7) and experimental studies. Firstly, Thermo-Calc is used to optimise the compositional design by studying the effect of adding extra iron (Fe) or extra iron and carbon (Fe/C) contents on the phase formation. Then, the hardmetals are processed by a powder metallurgy (PM) route including mechanical milling of the powders and two different sintering techniques: liquid phase sintering (LPS) and spark plasma sintering (SPS). Commercial powders are used as raw materials to prepare Cr-based WC hardmetal powders with 70 vol.% of reinforcement (hard WC) by mechanical milling at 350 rpm during 20 h of milling time. The average size of the WC particles embedded in the Cr-based WC hardmetal powders is close to 80 nm, which demonstrates that high-energy milling is an efficient way to produce nanoscale WC particles in these Cr-based hardmetal powders. Then, Cr-based WC hardmetals with the designed compositions (a 3 wt.% of extra iron content and extra carbon contents varying from 0 to 2 wt.%) are processed by liquid phase sintering at 1450 °C for 30 min. The LPSed Cr-based WC hardmetal with extra 3 wt.% Fe and 1 wt.% C contents has achieved the best hardness and facture toughness values (1647 HV30 and 6.0 MPam1/2). However, the fracture toughness achieved is still not sufficiently high due to the existence of large brittle carbides within the microstructure, together with a porosity greater than 3%. Thus, the viability of using an alternative processing route by field assisted solid-state sintering is also studied, in order to limit or prevent the formation of undesirable brittle carbides and further improve the hardness/fracture toughness relationship in these materials. Consequently, the parameters for the consolidation by spark plasma sintering are optimised based on their effect on shrinkage, phase formation, microstructure and mechanical properties. Thus, a sintering temperature of 1350 ºC, a heaing rate of 400 °C, and an applied pressure of 80 MPa during a holding time equal to 10 min, are beneficial experimental conditions to improve the relative densification of the bulk sample while maintaining a nanosized WC grain. The SPSed Cr-based WC hardmetal with an extra 3 wt.% Fe content and extra 0.5 wt.% C content reaches the best combination of hardness and toughness fracture values (2219 HV30 and 8.2 MPam1/2). This extremely high hardness comes from the uniform distribution of nanosized prismatic WC grains (around 100 nm), whereas the good toughness is due to the achievement of a near full densification (> 99%), the existence of a thin (W,Cr)2C interphase between WC and Cr2O3 acting as a bonding phase, and to the inhibition of the formation or growth of undesirable brittle carbides. In addition, this hardmetal achieves the highest compressive strength in a temperature range between 25-600ºC, which is also related to its high hardness and toughness values. All the SPSed Cr-based WC hardmetals evaluated in this study have reached a higher oxidation resistance than Fe-based and Co-based WC hardmetals under the same oxidation conditions, mainly due to their high values of activation energy for oxidation. Another important property required for hardmetals, as is the case of wear resistance, is found to be outstanding for these SPSed Cr-based WC hardmetals, even under aggressive wear conditions, in comparison to other commercial Co-based WC hardmetals. In conclusion, the newly developed SPSed Cr-based WC hardmetals presented in this work exhibit an excellent combination of hardness, toughness, oxidation resistance and wear resistance. Furthermore, these Cr-based WC hardmetals have a lower price and are less toxic than Co-based or Ni-based WC hardmetals. Therefore, Cr-based hardmetals seem to be promising materials for their introduction in industrial applications, in particular for those in which high oxidation and wear resistances are demanded.
Cr-based hardmetals, Liquid phase sintering, Spark plasma sintering
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