REACTIVE SYNTHESIS OF B₄C–CrB₂, B₄C–TiB₂, and B₄C–TiCrB₂ HETEROPHASE CERAMICS BY SPARK PLASMA SINTERING

Abstract

The reaction synthesis of heterophase refractory ultrahard B₄C-based composites by spark plasma sintering (SPS) was examined. To produce heterophase B₄C + TiB₂ + CrB₂ ceramics, the formation of boron carbide–chromium diboride and boron carbide–titanium diboride composites through chemical reaction between boron carbide and chromium oxide and between boron carbide with titanium carbide was previously studied. A comparative analysis between the reactive sintering of B₄C + Cr₂O₃ + 2C and B₄C + TiC mixtures using the boron carbide powders produced by the Zaporizhzhya Abrasive Plant (KB-ZAK) and the Donetsk Plant of Chemical Reagents (KB-DZHR), Ukraine, was performed. These boron carbide powders differ in the ratio of B₁₃C₂ and B₄C phases and particle size. The positive influence of reactively synthesized TiB₂, CrB₂, and CrTiB₂ boride phases on SPS and properties of the boron carbide composites was shown. The B₄C–CrB₂ and B₄C–TiB₂ ceramics subjected to Vickers hardness testing under a load of 98N showed HV levels of 23–29 and 26–28 GPa, respectively. The ceramics demonstrated brittle fracture according to the Half-penny model and fracture toughness of 3 MPa × m^1/2 for B₄C–CrB₂ and 4.4 MPa × m^1/2 for B₄C–TiB₂. The ceramics of composition 90 vol.% В₄С–5.5 vol.% TiCrB₂–4.5 vol.% C with ~33 GPa hardness and ~ 4 MPa × m^1/2 fracture toughness were produced by reactive SPS from a mixture of boron carbide (KB-ZAK), 6.6 wt.% TiC, and 11 wt.% Cr₂O₃. The high strength of TiCrB₂ ceramics was attributed to the stress-strain state, where the matrix phase of boron carbide was actually subjected to compressive stresses. The high values of hardness and fracture toughness allow the B₄C–TiCrB₂ composite to be classified as an ultrahard ceramic material.