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

Chromium ion luminescence spectroscopy with an optical microprobe has been used to map the residual compressive. stress in aluminafilms grown by high temperature thermal oxidation of NiAI single crystalsfor 15 min at 1100°C. Thefilms were mainly θ alumina containing small ‘islands’ of α alumina ∼5μm in diameter, and the distribution of the two phases was determined by direct imaging of the luminescence. The stress in the α alumina was measured to ± 20 MPa with a lateral resolution of ∼1 μm and mapped by scanning the optical probe over the specimen surface. The mapping speed was demonstrated to be capable ofcreating a 625 pixel map in 90 min. The stress is compressive and approximately equal to 3 GPa, but varies significantly withposition. The mean residual stress is due to thermal expansion mismatch between the film and substrate, but the observedvariations suggest that other local stress generation and relief processes are occurring.