Environmental barrier coating (EBC) durability modeling using a progressive failure analysis approach

Abstract

The need for a protecting guard for the popular Ceramic Matrix Composites (CMCs) is getting a lot of attention from engine manufacturers and aerospace companies. This is because the CMC has a weight advantage over standard metallic materials and more performance benefits. They are also commonly porous material and this feature is somewhat beneficial since it allows some desirable infiltration. They further undergo degradation that typically includes coating interface oxidation as opposed to moisture induced matrix which is generally seen at a higher temperature. Variety of factors such as residual stresses, coating process related flaws, and casting conditions may influence the degradation of mechanical properties of CMC. The cause of such defects which cause cracking and other damage is that not much energy is absorbed during fracture of these materials. Therefore, an understanding of the issues that control crack deflection and propagation along interfaces is needed to maximize the energy dissipation capabilities of layered ceramics. These durability considerations are being addressed by introducing highly specialized form of environmental barrier coating (EBC) that is being developed and explored in particular for high temperature applications greater than 1100 °C 1-3 . The EBCs are typically a multilayer of coatings and are in the order of hundreds of microns thick. Thus, evaluating components and subcomponents made out of CMCs under gas turbine engine conditions are suggested to demonstrate that these materials will perform as required especially when subjected to extreme temperatures and harsh operating environment. The need exists to use advanced computational methods to assess risk associated with exposure to high temperature of EBC coated CMC specimens. In the work presented here, multiscale progressive failure analysis (PFA) approach was used to evaluate the damage growth in the coating and CMC after exposure time to cyclic and elevated temperatures. In each cycle, the specimen was heated to 1300 °C then maintained at that temperature for a period of time before cooling it down to room temperature. The PFA evaluation was carried out with the GENOA 4 software using integrated capability inclusive of: finite element structural analysis, micro-mechanics, damage progression and tracking, fracture mechanics, and life prediction. In this paper, reverse engineered constituent properties obtained from CMC lamina properties were used as input to PFA to evaluate the degradation of specimen strength during thermal cycling. The analysis results indicated that the damage initiated in the top coat of the EBC then propagated down to the bond before reaching the CMC. Life assessment of the CMC was carried out twice, once using micromechanics properties as input and another time using macromechanics properties. It was determined that the use macromechanics properties yielded a more conservative life prediction for the CMC specimen as compared to that obtained from the use of micromechanics with fiber and matrix properties as input. Residual stresses evaluated during cooldown supported the onset of damage in the top coat. All stages of damage evolution were captured with PFA including damage initiation and damage propagation. Details on the life prediction of EBC and CMC materials are discussed next.