BENEFICIAL-EFFECTS OF Y2O3 DISPERSION ON AL2O3 PROTECTIVE COATING

We have studied the mechanism by which despersed Y2O3 exerts beneficia l effects on the oxidation protection of an Al2O3 coating (film) on th e basis of the following models: 1) Y2O3 suppresses surface segregatio n of S by trapping it in the alloy matrix and accordingly suppresses s palling of the Al2O3 film; 2.1) The diffusion of alloying elements int o the Al2O3 film is retarded by dispersed oxide particles, probably th rough formation of a diffusion barrier at the Al2O3/alloy interface; 2 .2) The diffusion in Al2O3 film is retarded through incorporation of Y 2O3 into the film 3)Y2O3 acts as an adhesive at the Al2O3/alloy interf ace; 4) Y2O3 incorporated into the Al2O3 film suppresses the spalling by increasing the plasticity of the film. We reached the following con clusions: 1 is very important, 2 is also significant but 3 and 4 are n egligible.