International Journal of Refractory Metals and Hard Materials
Control of eta carbide formation in tungsten carbide powders sputter-coated with (Fe/Ni/Cr)
Introduction
The constant increase in the use of WC-based cemented carbides in new applications, where cobalt is not welcome, and the threat from the depleting resources of cobalt led to a great deal of research into the development of alternative cemented carbides compatible with the WC–Co based ones.
Early attempts have been made [1], [2], [3], [4], [5], [6], [7], [8], [9] to find a satisfactory alternative binder phase to cobalt in WC hard metals with similar mechanical properties. The efforts made in the replacement of cobalt by other transition metals of VIII group, as iron or nickel [7], [8], [9], [10], [11], [12], revealed that it is possible to promote the densification of WC–(Fe/Ni) at temperatures near those used for WC–Co cemented carbides. However, it might be more difficult to obtain competitive mechanical properties and to find the optimum choice of carbon content for such alloys. The most suitable compositions, from a carbon control point of view, are those where neither η-phase (M, W)6C nor graphite are formed during the thermal cycle needed to sinter the material [13], [14]. A vertical section of the phase diagram W–C–Fe, calculated to 10 wt.% Fe (Fig. 1a), shows that only very close compositions, between the points a and b, fulfill these conditions [15]. Moreover, in the present system, this range of compositions corresponds to carbon contents higher than the stoichiometric. The addition of Ni to the W–C–Fe system can enlarge the suitable composition range and move this region to more favourable carbon contents, closer to the stoichiometric, depending on the Ni:Fe ratio, as illustrated in Fig. 1b [12], [16], [17].
When the composition of Fe-rich binders is adjusted there are no occurrences of free carbon or of brittle η-phase in the WC composites and the mechanical properties, such as hardness, toughness and transverse rupture strength (TRS), indicate similar or even superior values to the WC–Co system [7], [8], [9], [10], [11], [12], [13]. The role of chromium in the binder has also been investigated, because it acts as a strong grain growth inhibitor and improves oxidation and corrosion resistance [13]. However, the presence of this element, which has more affinity to the carbon than the iron, obliges to an increase of the carbon content to avoid the formation of η-phase [12].
In the authors’ own studies [18], [19], [20], [21], tungsten carbide powders were sputter-deposited with stainless steel 304 AISI (71%Fe, 8%Ni, 18%Cr, 2%Mn and 0.07%C) by an innovative technique to produce composite powders. The aim of the selection of this binder is to induce two roles: (i) to improve the quality of the powder surfaces and (ii) to establish the optimum composition for the different elements studied. The structure, morphology and chemical distribution of the coated powders have already been investigated [18], [19] and revealed that all the WC particles were uniformly coated and show “rough” surfaces, coming from the columnar growth of the thin film deposited [18], [19]. The coated powder presents a WC major crystalline phase, traces of W2C coming from the bulk, and a ferrite b.c.c. structure (Fe-α) for the sputtered layer.
The processing of these coated powders does not need a pressing binder, commonly used in the WC based cemented carbide powders. Sintering performed in a vacuum atmosphere provides high relative densities, ∼96%, at a relatively low temperature of 1325 °C for compacts with an initial content of 10 wt.% of stainless steel [20], [21]. This is a consequence of the low liquidus temperature, T ≃ 1150 °C, and good wettability of the liquid phase to the powders to be sintered, together with the nanometer character and the highly uniform distribution of the coating induced by the sputtering process [18], [19], [20], [21]. However, these qualities can enhance the formation of η-phase during the deposition process and sintering.
In this work, an attempt has been made to study η-phase formation with respect to the heating temperature, holding time, binder composition, namely the Ni/(Fe + Cr) ratio, and the carbon content. The optimization of the best ratio between elements with affinity and no-affinity to carbon was envisaged. The conclusions might be applied to coated and uncoated powders.
Section snippets
Experimental
WC powder particles were coated with stainless steel by d.c. magnetron sputtering. The deposition chamber was tailored to powder coating by a rotation and vibration of the substrate holder [18].
The powder is a fully carburized WC (9–10 μm) (H.C. Starck, HCST-Germany), which contains, besides the WC phase, traces of W2C phase. The coatings were sputter-deposited on the WC particles from a target, consisting of a commercial type AISI 304 stainless steel disc (71%Fe, 8%Ni, 18%Cr, 2%Mn and 0.07%C).
Structural evolution during heating
The sputtered powders have different amounts of coating, variable between ∼1 and 15 wt.% (C–WC1 to C–WC5, in Table 1). The coated powder C–WC6 has a different binder composition with a higher Ni/(Fe + Cr) ratio, when compared with that of the other coated powders.
As-deposited coated powders do not reveal by X-ray diffraction the presence of η-phase. In order to evaluate the formation of phases during heating, the C–WC3 powder with 5.7 wt.% of binder (Table 1) was heated to different temperatures
Conclusions
The formation of η-phase in composite powders of WC sputter-coated with stainless steel AISI 304 starts at about 750 °C, increases up to 1100 °C, and continues until the maximum studied temperature, 1400 °C. The η-phase, in the temperature range 1200–1325 °C, can be represented approximately as (Fe2.3Ni0.3)(Cr0.6W2.8)C ((M, W)6C). The main binder elements (Fe, Ni, Cr) are easily accommodated in the (M, W)6C structure.
The amount of η-phase is higher in coated than in the admixed powders with similar
Acknowledgements
The authors wish to thank Dr. Maxim Avdeev and Dr. Dimitry Khalyavin for the help in the analysis of XRD data. The author C.M. Fernandes gratefully acknowledges the financial support of the POCTI programme of the Portuguese Foundation for Science and Technology (FCT) and European Social Fund (FSE).
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