Microwave-assisted pressureless sintering of silicon-reinforced boron carbide composites

We report on a rapid sintering method that relies on gyrotron-produced millimeter wave irradiation to densify boron carbide monoliths. This microwave-assisted process requires a lower sintering temperature (<1400 degrees C) and no applied pressure. The rapid, 20-min long treatment yielded composites with densities that exceeded 95% compaction. Microwave sintering did not coarsen grains and retained a nanocrystalline structure of the boron carbide. We incorporated different ratios of silicon metal into the ceramic green-bodies via high-energy ball milling and determined the effects of these inclusions on the resulting properties of refractory carbide composites. We determined that silicon reacted with carbon in the 3-atom chain of boron carbide and formed cubic silicon carbide interstitial phases in a non-equilibrium process. Its optimal 11 wt % concentration maximized the relative hardness of the resulting monoliths. This rapid sintering mechanism and customizable ceramic matrix composite fabrication route are promising avenues for subsequent fabrication of refractory carbides with nanosized grains that will enable higher fracture toughness.