Abstract
The thermal cyclic behavior of self-healing thermal barrier coatings (SH-TBC) is analyzed numerically to develop a lifetime prediction model. Representative microstructures are studied adopting a unit cell based multiscale modeling approach along with a simplified evolution model for the thermally-grown oxide layer (TGO) to study the evolution of damage and healing in a self-healing TBC system. The fracture and healing process is modeled using the cohesive zone-based healing model along with a crack tracking algorithm. The microstructural model includes splat boundaries and a wavy interface between the Top Coat and the Bond Coat, typical of Air Plasma Sprayed TBCs. A particle-based self-healing mechanism is accounted for with a random distribution of healing particles subjected to a numerically accelerated thermal cyclic loading condition. Lifetime extension of the self healing TBCs is quantified by conducting thermal cyclic analyses on conventional TBCs (benchmark system without self-healing particles). Parametric analyses on healing parameters such as crack filling ratio and strength recovery of the healed crack are also conducted. The results are presented in terms of the evolution of the crack pattern and the number of cycles to failure. For self-healing TBCs with a suitable healing reaction (i.e., cracks being partially filled and a minimal local strength after healing), an improvement in TBC lifetime is observed. In contrast, if the healing mechanism is not activated, the presence of the healing particles is actually detrimental to the lifetime of the TBC. Correspondingly, in addition to superior crack filling ratio and healed strength, significant improvement in lifetime is achieved for self healing TBCs with a higher probability of crack-healing particle interaction. This highlights the importance of a robust activation mechanism and a set of key material requirements in order to achieve successful self-healing of the TBC system.
Highlights
Thermal Barrier Coatings (TBC) are protective material systems applied onto the hot sections of gas turbine engines in aircraft and industrial applications
A typical TBC system is made of three distinct layers, namely, a Top Coat (TC) made of a ceramic material, a metallic Bond Coat (BC) layer and a Thermally Grown Oxide (TGO) layer formed during operation as a result of oxidation of the BC layer (Padture et al, 2002)
Distinct computational samples of TBC systems are subjected to thermal cycle simulations and the fracture patterns and number of cycles to failure are recorded
Summary
Thermal Barrier Coatings (TBC) are protective material systems applied onto the hot sections of gas turbine engines (e.g. turbine blades) in aircraft and industrial applications. These stresses, in turn, induce the formation of microcracks in the TBC layers, resulting in complete failure of the TBC system upon coalescence of the microcracks, typically referred to as spallation (Evans et al, 2001) This has a direct consequence on the lifetime of the TBC system warranting cost-intensive maintenance operations involving re-deposition of the coating. It is clear from the literature that obtaining an efficient selfhealing TBC design is difficult due to the various (non-linearly) interacting factors In this regard, there exists a demand to have a tool that can simulate the cracking and healing behavior of the TBC system, which in turn can be used to achieve an optimal configuration of self-healing TBC.
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