Hot isostatic pressing of WB4 and WB4-TaB2 based ultrahard materials
Introduction
High hardness is a material property related to the presence of symmetrical three-dimensional atom networks where bonds are short and strong (i.e. sp3 bonds in diamond) [1]. Transition metal borides, especially those based on Pt group metals (i.e. ReB2, OsB2, RuB2), belong to this type of ultrahard materials [[2], [3], [4], [5]]. However, their industrial application is far from reality due to their extremely high cost [5,6]. Tungsten based borides are much cheaper and, therefore, are being thoroughly investigated for their potential use as wear resistant materials [7]. Presently, research is focused on tungsten tetraboride (WB4), the one with the highest B/W molar ratio, for which theoretical calculations predict a hardness above 45 GPa [1]. The experimental confirmation of this fact is difficult because, like other borides, WB4 powders are very difficult to sinter [[8], [9], [10]]. In addition, it has been found that the WB4 hexagonal phase is metastable, decomposing to WB2 above 1200 °C [11].
So far, WB4 powders are obtained from B and W powder mixtures either by “in-situ” reaction sintering [12], arc-melting [13] or mechanical alloying followed by heating up to 1500 °C [14,15]. According to these authors, high B/W ratios and the addition of elements like Ta, Mn, Cr or Mo tend to stabilize the WB4 phase [11,13,16]. Apart from using pressure-assisted methods, other researchers have proposed the addition of metallic nickel to boron and tungsten powder mixtures in order to enhance densification via liquid phase sintering [12]. However, some residual porosity is still present even after hot uniaxial pressing at 1650 °C and 30 MPa for 1 h. Although the microhardness values reported for these W-B-Ni materials reach 49 GPa (with 0.49 N of applied load), those obtained with 9.8 N are below 28 GPa. In addition, flexural strength and fracture toughness values are very low (≈ 300 MPa and ≈4.5 MPa.m1/2 respectively). All these data suggest that the best starting powders and processing methods for obtaining functional WB4- based hard materials are still to be found. This work is focused on analyzing the materials obtained by applying hot isostatic pressing to WB4-B and WB4-B-Ta powder mixtures, where densification is obtained at temperatures as low as 1350 °C.
Section snippets
Experimental procedure
As-received tungsten boride powders were supplied by Supermetalix, Inc. USA. These materials were obtained by arc-melting of mixtures of W, Ta and B elemental powders [13]. Their chemical composition was analyzed by means of ICP-EOS and IR spectroscopy (Table 1). These data show that the B/W atomic ratio is 6.52 (63% higher than that corresponding to the WB4 phase). These high B/W ratios are reported to be necessary for stabilizing the WB4 hexagonal phase [[11], [12], [13], [14], [15], [16]].
Densification and microstructures of WB4-B and WB4-B-Ta HIPed specimens
The density of Alloy 1 increases from 5.4 to 6.4 g/cm3 when the HIP temperature raises from 1100 °C to 1350 °C (Table 3). According to XRD analyses and SEM images (Fig. 5, Fig. 6), these samples only consist of WB4 and amorphous boron. Optical images, used to distinguish free boron regions (pointed by red arrows in Fig. 7) from pores (pointed by blue arrows), confirm this fact. Porosity measurements are in agreement with those calculated assuming a theoretical density of 6.58 g/cm3 for Alloy 1 (
Conclusions
WB4 based ultrahard materials have been obtained by applying glass encapsulated HIP to WB4-B and WB4-B-Ta powders mixtures. Processing conditions (i.e. temperature, pressure and time) have been tailored in order to retain the hard WB4 hexagonal phase after sintering. Hardness values over 24 GPa (HV5) have been obtained by HIP at 1350 °C-150 MPa for 1 h. Contrary to that reported by other authors, it has been confirmed that metallic Ta additions enhance the decomposition of tungsten tetraboride
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships, which may be considered as potential competing interests.
Acknowledgments
HILTI Corporation is gratefully acknowledged by the financial support of this work. Thanks are also due to Supermetalix Inc. for providing the starting WB4 based powders.
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