Multimaterials systems such as thermal barrier coatings used at high temperature in gas turbines are subject to severe thermomechanical loading. During service, complex thermomechanical stress is generated, resulting from thermal variations that establish through the thickness of the multimaterials system. The occurrence of such thermal gradients specifically provokes spallation, enhancing the overall damage of material surface.
Consequently, it is of utmost concern to investigate the cyclic oxidation and spallation of materials under imposed/controlled temperature gradient. In order to reproduce the conditions of materials utilization a novel, specifically dedicated cyclic-oxidation-equipment is designed and implemented. Based on a coupled experimental/numerical approach, the design mainly focuses on the development of specific specimen-holders able to impose a controlled and measurable thermal-gradient through the specimen thickness using appropriate geometry, dimension, insulation, materials and fluids circulation. The possibility of controlling the gradient through the specimens brings a very interesting advance in the way to characterize TBC by reproducing at the laboratory scale the real in-service condition of high temperature turboengine materials.
In addition, the oxidation/spallation testing equipment includes the possibility to monitor in situ, using various optical means such as high-resolution CCD, high-speed and infrared cameras, the evolution of the material surface while thermally cycled. This versatile real-time approach allows the identification and analysis of the spallation mechanisms at different microstructural and time scales.
Preliminary results showing the impact of a thermal gradient on the propensity of thick APS-YSZ (300µm-1500µm) to spall upon thermal cycling are presented. The presence of a gradient has major influence on the kinetics and extent of spallation and the morphology and typical critical size of individual spalls. For given thermal gradients, the influence of the material microstructure, namely the size and crystal orientation of grains and the morphology of grain boundaries, as well as the time-scale of spallation event, are discussed.