Ultrahigh-pressure densification of nanocrystalline WB ceramics
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Phase-pure nanostructured WB ceramics are hot pressed at ultrahigh pressures of 1.0 to 3.0 GPa and high temperatures of 700 to 1000 C (UHPHT) for 60 min. The UHPHT samples are nanograin size from 15 to 40 nm. Our experimental observation shows that ultrahigh pressure could improve densification, and the density of WB samples could reach 99.4% of theoretical. The comparative experiments carried out at ambient pressure and temperatures of 550 to 1100 C for 60 min indicate that the external pressure was favorable for phase-pure and highly dense WB formation. In addition, the UHPHT samples give a high hardness value of 28.9 0.8 GPa. Transition metal borides (TMBs) recently have drawn great attention as promising heat-resistant, wear-resistant alloys and coatings1 because of their high melting points, high hardness values, outstanding wear resistance, and high-temperature electrical resistance.2–4 Many processes are available for the production of TMBs at ambient pressure (group IVa TMBs have been reported),5–7 including self-propagating combustion synthesis,8 chemical vapor deposition (CVD),9 and spark plasma sintering.10,11 Tungsten borides based on W2B5 as a component of cermets and tool materials,12,13 and boron-rich WB2 have been much studied.14,15 It is well known that WC materials are widely used in industry. WB, as a hard alloy similar to WC, might have some applications in industry. Unfortunately, there is little literature about well-sintered bulk WB. Ultrahigh pressure (UHP) is an effective way for synthesizing phase-pure, and nearly fully densified samples (NiAl and ReB2).16,17 In this communication, phase-pure nanocrystalline WB compacts were prepared under UHP. The phase composition, microstructure, and density of WB samples were investigated. In addition, the effect of pressure on reaction temperature and reaction rate was discussed. The starting materials were powders of tungsten (99% purity, 44 mm) and amorphous boron (96.56% purity, 0.62 mm). The proportion of W to amorphous B was fixed to a molar ratio of 1:1. The powder mixture was pressed into a cylinder shape (8.0 mm in diameter and 4.0 mm in height), placed in a capsule made of hexagonal boron nitride (hBN) with a sandwichlike configuration, and then contained in a pyrophyllite UHP cell. The UHPHT experiments were carried out in a DS6 8 MN a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0082 J. Mater. Res., Vol. 25, No. 4, Apr 2010
cubic press.18 The experimental details were described elsewhere.17 The samples were first compressed to 1.0 to 3.0 GPa, heated to 500 to 1000 C, maintained at the UHPHT conditions for 60 min, and then quenched to room temperature. We also carried out experiments at ambient pressure and temperatures of 550 to 1100 C for 60 min in argon. The prepared specimens were characterized by x-ray diffraction (XRD, model DX-2500, Dandong, China), scanning election microscopy (SEM, model JSM-6490, JEOL, Akishima, Japan), and density measurements. Figure 1(a) shows the
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