Multi-spectroscopic analysis of high temperature oxides formed on Cobalt-Chrome-Molybdenum alloys

Thanks to their thermal stability, resistance to oxidation and mechanical strength, cobalt-chrome molybdenum alloys are considered an ideal alloy for high temperature applications. The surface oxide layer evolves as a function of time and temperature, changing its chemical structure and increasing its thickness from a few nanometers to various microns. Making use of various diffractographic and spectroscopic techniques, namely X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and glow-discharge optical emission spectroscopy, coupled with complementary analysis, this work gives new insights on the chemical bonding, crystallographic structure, thickness and elemental composition of the oxide layers as a function of both time and temperature of oxidation. Results show that the initial nanometric passive layer of Co3O4 evolves into a metastable, sub-micrometric CoCrO4 structure and finally stabilizes into a micrometric Cr2O3 at the highest temperatures. This paper fills a fundamental gap in the understanding of the chemistry and stability of Cobalt-based alloys used for high temperature applications, such as in poppet or exhaust valves, aerospace components or hot gas turbines. Once calibrated, this innovative, complete surface characterization approach can be ideally extended to other metallic alloys.