High temperature micromechanical testing for meso/macroscale characterizations under controlled atmospheres
Résumé
The evaluation of the thermomechanical properties within high temperature coated systems for aeronautical turbine applications motivated the concomitant developments of (i) preparation techniques to extract tens-of-micrometre-thick with centimetre-size specimens, and (ii) dedicated mechanical test rigs under highly controlled atmospheres to test them at elevated temperatures. Owing to the high “surface/volume” ratio of micromechanical specimens, a particular attention has be paid on the testing atmospheres in order to limit surface reactivity at high temperature. Freestanding coatings, interdiffusion zones and substrates constitutive of turbine blades were thus tested up to 1100°C to document the gradient of meso/macroscopic mechanical and thermal properties, and in particular, the brittle-to-ductile transition behaviour of the coating material. This recent high temperature micromechanical development enables a better evaluation of the plastic behaviour of coating materials in comparison with previous microtensile devices. Such a database is necessary for materials selection and for feeding numerical models predicting both the high temperature mechanical behaviour and the service life of coated structural components.
Furthermore, size effects approach in reactive environments were also explored. Using ultrathin specimens amplify the contribution of the environment due to high “surface/volume” ratio and offer a limited content of reactive elements. The progressive consumption of the reactive element during oxidation allows breakaway phenomenon, also called “reservoir effect”, for short exposures at elevated temperature. The investigation of environmental degradation on the debit in mechanical performance of structural materials is thus possible using large but ultrathin specimens.