Failure analysis of thermal barrier coating systems

The efficiency of aircraft engines can be improved significantly by increasing the temperature in the combustion chambers and the gas path in the high-pressure turbine sections. The temperatures in these areas of today's high thrust engines exceed the temperature capability of typical turbine alloys. Thermal Barrier Coating (TBCs) has played an important role in last three decades in increasing the temperature capability of gas turbine engines. The development of ceramic TBCs for high heat flux environments requires a fundamental understanding of fracture mechanisms in order to prevent the spalling of the coating from the substrate.; In the present work, a specific computational procedure for the problem is developed. The method we have employed is based on the singular integral equations - boundary collocation technique. The advantage of this method is its effectiveness and accuracy in evaluation of the principle fracture mechanics parameters. The developed integral equations are formed using basic singular solutions for dislocations interacting with the biomaterial interface. The principle singular solutions for periodic dislocations are derived in the form of complex valued analytic potentials.; Analyses of the thin TBC systems show that the thermal stresses provide sufficient driving force for cracks to propagate through the coating from the free surface toward the interface. The cracks branching above the interface tend to be deflected toward the interface due to the material combination of TBC systems and thermal expansion of the substrate. Cracks branching and growth along the interface appears to be a preferable stable path under these conditions.; Based on the numerical method and developed scheme, fracture mechanics characteristics of the thin and thick TBC system are investigated. Computations of the transient temperature fields of a two-layer system with cracks branching above or along the interface are carried out using finite difference method. The results show that the insulation effect of the branching cracks leads to complex temperature distributions around the cracks tips, and the insulation effect increases the stress intensity factors of the branching cracks. The investigation of the thick TBC systems shows that thicker thermal barrier coatings provide better protections.