B4C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B4C–B4C contact between particles permitted via volatilization of B2O3 particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B4C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B4C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B2O3 to form B4C and CO gas, thereby extracting the B2O3 coatings, permitting sintering to start at ∼1350°C.

This is a companion work to our previous study on the pressureless sintering of boron carbide (B 4 C). The Vickers hardness and indentation fracture toughness of B 4 C compacts were measured after various sintering heat treatments. Increases in hardness and decreases in indentation fracture toughness as the grain size decreased in sintered B 4 C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries.

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Zirconium diboride and a zirconium diboride/tantalum diboride mixture were synthesized by solution‐based processing. Zirconium n‐propoxide was refluxed with 2,4‐pentanedione to form zirconium diketonate. This compound hydrolyzed in a controllable fashion to form a zirconia precursor. Boria and carbon precursors were formed via solution additions of phenol–formaldehyde and boric acid, respectively. Tantalum oxide precursors were formed similarly as zirconia precursors, in which tantalum ethoxide was used. Solutions were concentrated, dried, pyrolyzed (800°–1100°C, 2 h, flowing argon), and exposed to carbothermal reduction heat treatments (1150°–1800°C, 2 h, flowing argon). Spherical particles of 200–600 nm for pure ZrB2 and ZrB2–TaB2 mixtures were formed.

were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 11501-15501C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB 2 -B 4 C-SiC in the form of TaB 2 and/or TaSi 2 increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi 2 proved to be a more effective additive than TaB 2 . Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions.