Reactive Synthesis of B.sub.4C-CrB.sub.2, B.sub.4C-TiB.sub.2, AND B.sub.4C-TiCrB.sub.2 Heterophase Ceramics by Spark Plasma Sintering

The reactive synthesis of heterophase refractory ultrahard B.sub.4C-based composites by spark plasma sintering (SPS) was examined. To produce heterophase B.sub.4C + TiB.sub.2 + CrB.sub.2 ceramics, the chemical reaction between boron carbide and chromium oxide and between boron carbide and titanium carbide resulting in boron carbide-chromium diboride and boron carbide-titanium diboride composites was previously studied. The reactive sintering of B.sub.4C + Cr.sub.2O.sub.3 + C and B.sub.4C + TiC mixtures using boron carbide powders obtained from the Zaporizhzhya Abrasive Plant and Donetsk Chemical Reagent Plant (Ukraine) was compared. The boron carbide powders differed in the ratio of B.sub.13C.sub.2 and B.sub.4C phases and particle sizes. The reactively synthesized TiB.sub.2, CrB.sub.2, and CrTiB.sub.2 boride phases positively influenced the SPS consolidation and properties of the boron carbide composites. The B.sub.4C-CrB.sub.2 and B.sub.4C-TiB.sub.2 ceramics subjected to Vickers hardness testing under a load of 98 N showed HV levels of 23-29 GPa and 26-28 GPa. The ceramics demonstrated brittle fracture according to the Half-penny model, with a fracture toughness of 3 MPa*m.sup.1/2 for B.sub.4C-CrB.sub.2 and 4.4 MPa*m.sup.1/2 for B.sub.4C-TiB.sub.2. The 90 vol.% B.sub.4C-5.5 vol.% TiCrB.sub.2-4.5 vol.% C ceramics with ~33 GPa hardness and ~ 4 MPa*m.sup.1/2 fracture toughness were produced by reactive SPS from a mixture of B.sub.4C (Zaporizhzhya Abrasive Plant), 6.6 wt.% TiC, and 11 wt.% Cr.sub.2O.sub.3. The high strength of TiCrB.sub.2 ceramics was attributed to the stress-strain state, where the matrix phase of boron carbide was subjected to compressive stresses. The high hardness and fracture toughness allow the B.sub.4C-TiCrB.sub.2 composite to be classified as an ultrahard ceramic material.