XPS studies of surface oxidation of metal carbides

Metal carbides MC (HfC, NbC, ZrC, TaC and TiC) that were exposed to air under the ambient temperature condition for a period of time were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and Raman. Fresh MC samples exposed to the air for different time (fresh, one day, one week and one quarter) illustrated unexpected oxidation degree of the surfaces. XRD tests provided very limited information, since the oxidation process occurred from the surface at the nanometer scale. However, the XPS tests provided more comprehensive information for the MOx (HfO2, ZrO2, Nb2O5, Ta2O5 and TiO2) film and MC (HfC, NbC, ZrC, TaC and TiC) interlayer. Meanwhile, the HRTEM and Raman tests could give some additional or supporting information for XPS tests, when the samples were exposed to the air for longer time (one week or one quarter). Meanwhile, this research is the first to show that the oxidation progress of the metal surfaces of HfC, NbC, ZrC, TaC and TiC can take place in air under the ambient temperature condition and the oxidation film is about 2-5 nm. Meanwhile, a new oxidation mechanism of metal carbides MC (HfC, NbC, ZrC, TaC and TiC) exposed to air under the ambient temperature condition is also proposed, considering the obtained Raman, TEM and XPS results that could clearly confirm the formation of HfO2 and free carbon.