The carbide reduces the ductility of tungsten and leads to brittle intergranular fracture. These cracks are inevitably formed in tungsten during spark plasma sintering through carbon diffusion due to the action of graphite mould. Here, we describe a protective foil composed of two different carbides forming elements (molybdenum and tantalum) to minimize the formation of carbides. The cross section diagram shows that tantalum foil inhibits the diffusion of carbon into tungsten, while molybdenum foil is not an effective diffusion barrier. Thermodynamic kinetics simulations show that the inhibited carbon diffusion in tantalum is due to high solubility and low diffusivity. Furthermore, thermodynamically stable tantalum carbide prevents further diffusion of carbon at the tantalum tungsten interface. For the opposite reason, carbon diffuses more rapidly not only in molybdenum but also at the molybdenum tungsten interface. This research provides strategies to minimize carbon diffusion during spark plasma sintering, as well as an intuition for the development of structural materials for extreme carbon-containing environments.
Tungsten compact was prepared by the combined process of spark plasma sintering and hot isostatic pressing. Samples manufactured from commercial powders by spark plasma sintering (1500-1600°C) showed a relative density of more than 90.4%. After 2 h of hot isostatic pressing at 1650 °C, the improved relative density in the range of 96.4% to 97.2% was obtained. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis show that the sintered samples mainly consist of tungsten, and trace amount of tungsten is combined with oxygen due to surface oxidation. Strength measurements are made by biaxial bending tests using pistons against the three balls. The maximum bending strength of 761 MPa was obtained by the combined process of spark plasma sintering and hot isostatic pressing. The increase in strength is explained by an increase in relative density rather than the effect of particle size.
Graphene nanosheets (GNF) were consolidated by spark plasma sintering (SPS) at 1100 °C and 30 MPa. The sintered materials were characterized by TEM, BET and Raman spectra. In addition to unmodified GNF, it also contains graphene sheets and onion-like carbon (OLC) particles. Phase separation and functionalization of the particle surface are performed by oxidation with nitric acid vapor. According to THE XPS data, the oxygen content in the oxidized sintered material increases from 17.8AT to 28.8AT. % increased from 3 hours to 6 hours with the oxidation time. At the same time the graphene sheet and the OLC disappeared. Gas phase oxidation prevents the destruction of sintered pellets. A possible mechanism of OLC formation during SPS was proposed. The potential application of SPS in carbon phase transition is discussed.
