Abstract
A geometrical model for sintering nickel-doped tungsten is proposed which consists of a pendular ring at the tungsten neck surface plus a nickel-enriched tungsten grain boundary. As compared to Kingery's model, the force equation contains two additional terms to account for the effect of surface tension at the nickel-vapor interface, and the effect of the interfacial tension, γ WNi, at the tungsten surface, including a factor for the dihedral angle. Nickel does not wet tungsten completely. The contact angle at 1200 °C, as determined by scanning electronmicroscopy, is 35 °. Therefore, wetting is restricted to places of suitable geometry, especially grain boundaries. The activating effect of nickel on the sintering of 0.5 μm tungsten powder at 1000 °C is due to the enhancement of grain boundary self-diffusion in tungsten. At 1100 °C an additional contribution from rearrangement has to be considered. The behaviour of nickel at elevated temperatures on tungsten surfaces is qualitatively similar for flat tungsten surfaces and tungsten particles. In order to obtain a homogeneous nickel distribution in the tungsten compact, nickel has to be added by evaporation or in the form of chemical solution. Complete homogenization is not possible with tungsten-nickel powder mixes.
Published Version
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