A self-passivating tungsten bulk composite: Effects of silicon on its oxidation resistance

Abstract

A series of W-xSi bulk composites was prepared by SPS at the same condition. It was found that the phase compositions and their distribution state have great influence on the sample's oxidation resistance performance. For the low Si content composite, the nano-SiO2 particles generated from dissolved Si densely packed on the tungsten oxide grain boundary, which can effectively prevent the growth of WO3 grains and cracks expanding in the oxide layer below 1000 °C. However, as the temperature increases to 1000 °C and the oxidation time is prolonged, more nanoparticles will precipitate out from matrix and grow to form a cross-linked network structure. This growth will lead nano-SiO2 particles to unevenly distribute on the surface of WO3 grains, which causes WO3 grains to expose in the air and volatilize. On the other hand, for the high Si content composite, a continuous double oxide layer originating from W5Si3 is formed on the surface of the sample, the outer layer is a nano-porous SiOy layer, and the inner layer is a dense composite oxide layer composed of polycrystalline WO3 nanoparticles and amorphous SiOy. This double oxide layer is much more stable than the nano-SiO2 particles and allows the sample to resist oxidation for longer time at 1000 °C. Thus, a continuous distribution state of oxidized W5Si3 is the key factor for getting crack-free oxide scale.