In situ Raman monitoring of chromium oxide scale growth for stress determination

Raman spectra of a growing chromia (Cr2O3) layer generated by oxidation of pure chromium at 750 °C under 150 mbar of oxygen were recorded in situ every 5 min. The wavenumber evolution of the main Raman band versus oxidation time was interpreted in terms of mechanical stress development. Comparison with Raman spectra of a fully relaxed spalled chromia layer submitted to high‐pressure and high‐temperature treatments showed that internal compressive stresses develop during the growth, varying from −2.1 GPa when the scale is very thin to −2.4 GPa when the scale reaches a thickness of 0.6 µm. Relaxation phenomena seem to take place during isothermal oxidation. During cooling, thermal stresses are induced, which are purely elastic according to the perfect reversibility of cooling–heating cycles.