Fabrication of sintered tungsten by spark plasma sintering and investigation of thermal stability

Tungsten has been considered as the most promising candidate for plasma-facing materials (PFMs) in a next generation fusion reactor. It is well known that commercialized ITER (International Thermonuclear Experimental Reactor) grade tungsten is manufactured by the mechanical processing at high temperature after sintering to ensure a high density with an improved structural stability. In this study, in order to obtain the high density sintered tungsten with more enhanced structural stability, spark plasma sintering (SPS) method was employed, where a pulsed direct electric current was applied during heat treatment of powders with a pressure in the specimen. It is well known that by utilizing SPS, high-density sintered materials at a relatively lower temperature for a shorter time could be achieved compared to the other conventional sintering methods. In particular, in this study, reduction in H-2 atmosphere and two-step sintering were introduced to remove the residual oxygen and achieve the full densification with suppressed grain growth at relatively low operating temperature. In an optimized condition, a fully densified sintered tungsten with a relative density of 99.9% and an average grain size of 4.4 mu m was fabricated. The thermal stability of tungsten specimens was evaluated by high heat flux (HHF) test, where the surface temperature was set up to 2300 degrees C by nitrogen plasma. Then, the microstructural changes of the specimen surface have been examined after the HHF test. As a result, it was confirmed that the high-density sintered tungsten samples fabricated by SPS show an excellent microstructural stability for PFMs.