Anomalous oxidation behaviour of pressureless liquid-phase-sintered SiC

The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) SiC, an important non-oxide engineering ceramic, was investigated, and was found to be universally anomalous. Thermogravimetry oxidation tests performed in oxygen in the temperature range 1000-1225 °C on three PLPS SiC ceramics fabricated with different combinations of Al₂O₃-RE₂O₃ (RE = Gd, Sc, or Sm) as sintering aids indicated that the oxidation is in all cases passive and protective, but unexpectedly anomalous in the sense that the oxidation resistance does not scale inversely with temperature. In particular, in all cases it was observed that there is less oxidation above 1100 °C than below, in clear contradiction to the expectation for a diffusional process. Exhaustive characterization of the oxide scales by scanning electron microscopy, X-ray energy dispersive spectrometry, and X-ray diffractometry, together with detailed modeling of the oxidation curves, showed that the origin of this universal anomalous oxidation behaviour lies in the marked crystallization within the oxide scale of rare-earth silicates that act as effective barriers against the inward diffusion of oxygen thus improving notably the oxidation resistance. A strategy is proposed to provide PLPS SiC, and probably other SiO₂-scale-forming ceramics that are sintered using rare-earth oxides, with the superior oxidation resistance at moderate temperatures (i.e., <1100 °C) that they do not currently have.