Mapping residual stress using optical microprobe in alumina films formed by thermal oxidation of NiAl

Chromium ion luminescence spectroscopy with an optical microprobe has been used to map the residual compressive stress in alumina films grown by high temperature thermal oxidation of NiAl single crystals for 15 min at 1100 degrees C. The films were mainly a alumina containing small 'islands' of alpha alumina similar to 5 mu m in diameter, and the distribution of the two phases was determined by direct imaging of the luminescence. The stress in the alpha alumina was measured to +/- 20 MPa with a lateral resolution of similar to 1 mu m and mapped by scanning the optical probe over the specimen surface. The mapping speed was demonstrated to be capable of creating a 625 pixel map in 90 min. The stress is compressive and approximately equal to 3 GPa, but varies significantly with position. The mean residual stress is due to thermal expansion mismatch between the film and substrate, but the observed variations suggest that other local stress generation and relief processes are occurring.