EXSITU AND INSITU DETERMINATION OF STRESS DISTRIBUTIONS IN CHROMIUM-OXIDE FILMS BY RAMAN MICROSCOPY

Raman microscopy has been used, in situ, to measure stress variations in alpha-Cr2O3 scale as it formed at 800-degrees-C on chromium metal. Stress in the growing scale was seen to decrease with increasing thickness. The stresses in adhering, buckled and spalled oxide scale, at room temperature, following oxidation at 800-degrees-C, have also been measured and are consistent with these features being the result of stress relief behaviour. The stress induced shift of the nu-1A1g vibrational mode of alpha-Cr2O3 has been determined as 2.8 +/- 0.8 kbar cm-1. Raman microscopy has also been used to determine the distribution of stress, with 1-mu-m spatial resolution in a chromia film formed on 20Cr-25Ni-Nb stainless steel. It has been demonstrated that compressive stress at the centre of grains is relieved at grain boundaries. Values of +19.5 kbar and -19.5 kbar were obtained for the hydrostatic parts of the stress tensors for grain and grain boundary respectively, by comparison with data obtained from chromia powder and single crystals in diamond anvil cell experiments. Results are presented in the form of Raman frequency shift profiles and a stress contour map of an entire grain and boundary.