Carbon contamination of elemental beta-boron promoted a stable boron carbide by spark plasma sintering

Abstract

The effect of carbon contamination on the elemental beta boron (β-B) from the graphite components of spark plasma sintering (SPS) was investigated. The X-ray diffraction, analytical electron microscopy (AEM) and Raman spectroscopy witnessed boron carbide (B13+xC2-x where x=0.05–0.35), when SPS temperature was above 1750 °C. The degree of carbon contamination became severe with increasing SPS temperature to 1800 °C and 1850 °C and formed a single-phase rhombohedral boron carbide with about 12 wt% of carbon. Our observations indicate that the carbon gets kinetically trapped into β-B to form boron carbide with a primitive unit cell consisting mainly of icosahedral B11C and three-atom chain of CBB configurations. After 6 months of natural aging, it has been revealed that boron carbide formed at and below sintering temperatures of 1800 °C is not a thermodynamically stable phase, and transformed reversibly to β-B structure. X-ray photoelectron spectroscopy ascertained that there are no longer B-C bonds and the existence of more CC bonds in a 6 months aged specimen, suggesting carbon is no longer strongly bonded to boron, but precipitates as graphitic structure. Further examination using atom probe tomography and AEM confirmed the existence of carbon nanoclusters within grains and high concentration of carbon segregating onto grain boundaries. The phase transformation of boron carbide to mostly boron resulted in a significant loss of hardness and modulus in a bulk SPS specimen. This study underscores the importance of solid solution and local bonding environment in icosahedral boron-rich solids for phase stability and sustained properties.