Mechanical and corrosion properties of composite  ZrB₂–SiC ceramics with oxide additions

Abstract

Hot pressing was used to produce compact ceramics samples with the following composition (wt.%): 60 ZrB₂ + 20 SiC + 20 (Al₂O₃ + 32 t-ZrO₂). The tetragonal modification of zirconium oxide in the eutectic was stabilized by yttrium oxide. The porosity of the samples was 3–5%. The mechanical properties of the ceramics (hardness HV, fracture toughness K_Ic, tensile strength d_f, compressive strength Y, grain-boundary strength S, and bending strength s) were studied. Study of the microstructure and elemental composition of the phases revealed that a defect-free structure formed in the ZrB₂-based composite through strong Van Der Waals’ adhesive phase interaction at the SiC–Al₂O₃ interface, which increased the fracture toughness to 9.4 MPa × m^1/2. In turn, this increased the grain strength from 0.64 GPa for the base composite to 3.46 GPa for the ZrB₂–SiC composite with an oxide addition. An addition of Al₂O₃ + 32 wt.% t-ZrO₂ was introduced in sufficient quantities not only to reduce the fracture stress, but also to promote plastic deformation of the material for high-temperature bending strength. Study of the oxidation process showed that the weight increment of the 60 wt.% ZrB₂ + 20 wt.% SiC + 20 wt.% (Al₂O₃ + 32 wt.% t-ZrO₂) sample at 1600 °C for a holding time of 1 h was stabilized by the formation of dense oxide scale on the material, while the weight increment of the ZrB₂ + 20 wt.% SiC sample and, consequently, the scale thickness increased monotonically. The scale formed on the ZrB₂ + 20 wt.% SiC samples with an addition of Al₂O₃ + 32 wt.% t-ZrO₂ consisted of an upper Al₂SiO₅-based layer 50 μm thick with ZrO₂ inclusions and a lower ZrO₂-based layer up to 80 μm thick with Al₂SiO₅ inclusions. The eutectic Al₂O₃ + 32 wt.% t-ZrO₂ oxide addition to the basic ZrB₂–SiC system had higher oxidation resistance and thus prevented the diffusion of oxygen into the material.