Densification of carbon-rich boron carbide nanopowder compacts
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Noel Vanier and Cheng-Hung Hung PPG Industries, Inc., Allison Park, Pennsylvania 15101 (Received 12 May 2006; accepted 1 February 2007)
The densification behavior of 20–40-nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction, and electron microscopy. The sintering onset temperature was higher than expected from a nanoscale powder (∼1500 °C); remnant B2O3 kept particles separated until B2O3 volatilization, and the graphite coatings imposed particle-to-particle contact of a substance more refractory than B4C. Solid-state sintering (1500–1850 °C) was followed by a substantial slowing of contraction rate attributed to the formation of eutectic liquid droplets more than 10× the size of the original nano-particles. These droplets were induced to form well below the B4C-graphite eutectic temperature by the high surface energy of nanoparticles. They were interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded detection of continued solid-state sintering. Starting at 2200 °C, a permanent and interconnected liquid phase formed, which facilitated rapid contraction by liquid phase sintering and/or compact slumping. I. INTRODUCTION
Boron carbide is the third hardest material (Knoop: 2800, 100 g load1) following diamond and cubic boron nitride. Because of its hardness and low weight (theoretical density: 2.52 g/cm3), it is the premier material for personal armor. It is used as a nozzle material for slurry pumping and grit blasting because of its excellent abrasion resistance and for nuclear shielding applications based on boron’s high neutron absorption cross section.2 Its crystal structure is complex because of the highly covalent nature of its interatomic cohesion; twelve-atom boron-rich icosahedra reside at the corners of a rhombohedron, each icosahedron is bonded to six others via direct bonds, and three-atom intericosahedral chains reside between the icosahedra.3 Limited substitution of boron for carbon in the iscosahedra and chains allows boron carbide to exist as a solid solution from stoichiometric B4C at 20 mol% carbon to 8.8 mol% carbon. Mechanical as well as ballistic performance improves with decreasing porosity, i.e., with increasing fired relative density; however, until recently, pressureless sintering of pure B4C to high relative density has proved to be difficult. Achieving near-theoretical density had required hot pressing, which precludes the formation of complex
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0155 1354 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 22 Jan 2015
shapes. Additives such as SiC, Al2O3, TiB2, AlF3, and W2B5 have been used as sintering aids to increase pressureless sintered density,4–7 but the second phases formed often have deleterious effects on mechanical behavior.2 The best known additive for B4C is carbon, most successfully added in the form of phenolic resin, which distributes carbon around
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