Zirconia growth on zirconium carbide single crystals by oxidation

Zirconium carbide is an important high-temperature structural material. Results of oxidation studies reported in the literature show a complex picture. In this investigation, three ZrC single crystals have been oxidized at high temperature and in Ar/O2 mixtures at O2 composition, respectively, (105.5±0.3) ppm for ZrC-A, (119±2) ppm for ZrC-B and (130.7±0.5) ppm for ZrC-C. The analysis of the kinetic data acquired through an apparatus based on two oxygen sensors, appropriately connected to a Quadrupole Mass Spectrometer (QMS), has been matched with the characterization of the oxidized samples. This has been performed by Auger Electron Spectroscopy (AES), Electron Probe Micro-Analysis (EPMA), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Raman techniques. Through an appropriate processing of the experimental data, it has been possible to identify the main steps of the high-temperature ZrC oxidation process: (i) ZrC substrate interacts with O2 producing C that is retained into the sample. According to AES analysis, carbon has been detected in the amorphous state; (ii) during the cooling treatment, due to crack formation, a relatively small fraction of C is burnt as revealed by CO and CO2 formation. The characterization of the oxidized samples by XRD and Raman techniques has identified the presence of both the ZrO2-c and ZrO2-m phases.