In various mechanical systems, such as: diesel engines and gas turbines, that generate or are exposed to extreme heat, thermal barrier coatings (TBC) are normally applied so as to protect various metal components. A typical TBC system is usually comprised of four layers, namely: a superalloy substrate (SUB), thermally grown oxide (TGO), metallic bond coat (BC), and ceramic top coat (TC). Each of these layers has distinct thermal-physical and mechanical properties, thereby making it difficult to comprehend the life prediction and failure mechanism of TBCs. The main preparation methods for TC are; atmospheric plasma sprayed (APS) and EB-PVD, where the former is the most cost effective and widely used technique. Generally, APS deposited coatings contain substantial pre-existing cracks corresponding to the non-bonded lamellar interfaces, a defect that favors propagation of delamination failure that predominantly initiates in the ceramic layer.
Literature has it that investigations on the residual stress states of TBCs models without crack can predicate the site of crack nucleation. Unfortunately, preceding studies have failed to consider stress relaxation and redistribution produced by crack propagation. To sum it up, presently, the failure mechanisms of TBCs are yet to be completely understood due to the complexities of its own structure and the uncertainties of material behavior.
Recently, a team of Xi’an Jiaotong University researchers Zhi-Yuan Wei (PhD candidate), Prof. Chang-Jiu Li, and led by Prof. Hong-Neng Cai presented a study in which the failure mechanisms of TBCs, involving the overall dynamic failure process including successive crack propagation, coalescence and spallation, were thoroughly examined. Remarkably, their investigations made use of the novel model coupling TGO growth and crack propagation techniques. Their work is currently published in the research journal, Ceramics International.
The researchers considered the residual stresses induced in the top coat and in the TGO as calculated during thermal cycling. The stresses in the top coat were then used to calculate strain energy release rates for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, was eventually examined using extended finite element method.
The authors observed that the tensile stress in the top coat increased continuously with an increase in an undulation amplitude. The team also noted that the strain energy release rates for top coat cracks accumulated with cycling, resulted in the propagation of crack toward the top coat/TGO interface. In addition, the TGO cracks were seen to nucleate at the peak of the TGO/bond coat interface and propagate toward the flank region of the TC/TGO interface. Both top coat cracks and TGO cracks successively propagated and finally linked-up leading to coating spallation. Three kinds of the failure mechanisms resulting in the spallation of ceramic coating were proposed, and all these mechanisms depend on the positions of TC crack.
In summary, the study by Prof. Hong-Neng Cai and his research team presented an in-depth examination of the failure mechanisms of TBCs, involving an overall dynamic failure process resulting from crack propagation and coalescence during thermal cycling. Astoundingly, the propagation and coalescence behavior of cracks predicted by their model were in accordance with experimental observations. Altogether, the results can be used to provide useful guidance for optimization strategy of TBC coating microstructure design for prolonged thermal cyclic lifetime.
Zhi-Yuan Wei, Hong-Neng Cai, Chang-Jiu Li. Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model. Ceramics International, volume 44 (2018) page 22556–22566Go To Ceramics International