Control of eta carbide formation in tungsten carbide powders sputter-coated with (Fe/Ni/Cr)

Abstract

Composite powders of tungsten carbide with variable coating contents of Fe/Ni/Cr based alloys (1–15 wt.%) were prepared by sputtering. The coated powders have revealed a good performance to shape-forming and sintering. However, the presence of nanocrystalline coatings could enhance the formation of a brittle phase during sintering, named η-phase, (M, W)₆C. To investigate this effect, the powders were heat-treated at variable temperatures, up to 1400 °C. For comparison, conventionally prepared powders were studied, too, and the content of η-phase was found to be higher for the sputter-coated ones. This result is attributed to the content of binder elements with affinity to carbon, like chromium, enhanced by the difference on binder characteristics, such as nanocrystallinity and morphology. It was concluded that an effective control of the reaction can be achieved by increasing in the binder composition the ratio between the elements without affinity and those with affinity to carbon and/or enlarging the carbon content.

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.