Stress analysis of TBC coated nickel base superalloys for gas turbine applications with laser drilled holes under cyclic oxidation and TMF loading scenariosThursday (29.09.2016) 12:30 - 12:33 Part of:
Advanced gas turbine blades for industrial gas turbines are coated basically with MCrAlY Bond Coatings and ceramic Thermal Barrier Coatings (TBC) combined with cooling holes to protect the base material from high gas temperature. Cooling holes in rotating blades are typically drilled by Electro Chemical Machining (ECM) or Electro Discharge Machining (EDM) before application of TBC. Laser drilling of cooling holes offers significant advantages compared with the established techniques because the drilling process can be performed as the last manufacturing step after application of the ceramic TBC coating. Currently the main problem using laser drilling techniques is the fact, that the part of the molten material in the cooling holes remains at the cooling hole wall as a so-called recast layer, which exhibits small cracks as a result of tensile stresses during rapid solidification and cooling during processing. These cracks can act as nucleation cracks and grow during service of the turbine blade, especially under thermo-mechanical fatigue conditions. Using advanced laser drilling methods consisting of established fast material removal drilling techniques combined with a smooth laser ablation technique of the cooling hole walls, recast layer free cooling holes can be manufactured, which is one of the goals of the current research works. Critical areas of crack formation are mainly the interface between the bond coating and TBC as well as the interface between the bond coating and the base material.
To understand the influence of cyclic oxidation as well as TMF loading on crack initiation and lifetime of undrilled and drilled TBC coated components FEM simulation of stress evolution in a TBC coated superalloy IN 617 with NiCoCrAlY bond coat and APS ZrO2 – 8wt% Y2O3 have been performed in this work for undrilled and drilled specimen using literature data for the relevant physical and mechanical properties of TBC, bond coating as well as the superalloy. For the interface of TBC and bond coating 2 types of flat and wavy interface has been analyzed. These results will be compared in the future with the results of thermal cycling tests.