Influence of grain size and microstructure on oxidation rate and mechanism in sintered titanium carbide under high temperature and low oxygen partial pressure

Abstract

Titanium carbide (TiC) pellets were prepared by Spark Plasma Sintering (SPS). Three different microstructures were synthesized with average grain sizes of 340nm, 1.34μm and 25μm, respectively. Prior to oxidation treatment, TiC samples were implanted with xenon ions. This noble gas is used as a marker of the initial surface. Oxidation behavior was studied by Rutherford Backscattering Spectroscopy (RBS) and electronic microscopy through thermal treatment at 1000°C for 10h and under different low oxygen partial pressure (OPP). No detectable oxidation was observed after thermal treatment at OPP≤2.10−7mbar whereas an oxide layer is formed at OPP=2.10−6mbar. Thickness, composition and morphology of the oxide layer depend on the initial microstructure of the material. Numerous grain boundaries and the porosity (≈10%) of the nano-grained material were responsible for the enhanced penetration of oxygen which resulted in the formation of a thick oxide layer. This layer is composed of external oxide and interfacial band sublayers. Ti4O7 was identified by XRD (X-Ray Diffraction) as being the main phase in the oxide layer concerning the nano-grained material whereas a relatively thick layer of TiO2 (rutile) is formed for larger grain microstructures. We proved the presence of carbon in the interfacial band as well as possible evacuation paths for CO/CO2 gas. There is less release of xenon from nano-grained material than larger grained materials during oxidation at OPP=2.10−6mbar for 20h attesting to the superior gas-tightness of the oxide layer in this case. A schematic model is then proposed to explain growth mechanisms and properties of the oxide layer.