Abstract
Enhancement of stability, durability, and performance of thermal barrier coating (TBC) systems providing thermal insulation to aero-propulsion hot-section components is a pressing industrial need. An experimental program was undertaken with thermally cycled eight wt.% yttria stabilized zirconia (YSZ) TBC to examine the progressive and sequential physical damage and coating failure. A linear relation for parameterized thermally grown oxide (TGO) growth rate and crack length was evident when plotted against parameterized thermal cycling up to 430 cycles. An exponential function thereafter with the thermal cycling observed irrespective of coating processing. A phenomenological model for the TBC delamination is proposed based on TGO initiation, growth, and profile changes. An isostrain-based simplistic fracture mechanical model is presented and simulations carried out for functionally graded (FG) TBC systems to analyze the cracking instability and fracture resistance. A few realistic FG TBCs architectures were considered, exploiting the compositional, dimensional, and other parameters for simulations using the model. Normalized stress intensity factor, K1/K0 as an effective design parameter in evaluating the fracture resistance of the interfaces is proposed. The elastic modulus difference between adjacent FG layers showed stronger influence on K1/K0 than the layer thickness. Two advanced and promising TBC materials were also taken into consideration, namely gadolinium zirconate and lanthanum zirconate. Fracture resistance of both double layer and trilayer hybrid architectures were also simulated and analyzed.
Highlights
Thermal barrier coating (TBC) systems provide thermal insulation to gas turbine engine components exposed to high temperature and oxidizing environments for aero-propulsion.Conventionally, a thermal barrier coating (TBC) system is a two-layered coating consisting of eight percent yttria stabilized zirconia (YSZ) (TBC), and a bond coat (BC) enriched in aluminum over a Ni base superalloy substrate
Our current research focuses on physics-based modeling of the damage and failure process in thermally cycled TBC of gas turbine engine blades
With more thermal cycling the thermally grown oxide (TGO) thickens and the geometry changes to more complex shapes and curvatures
Summary
Thermal barrier coating (TBC) systems provide thermal insulation to gas turbine engine components exposed to high temperature and oxidizing environments for aero-propulsion. In functionally gradient materials (FGMs), weak and sharp interfaces between two dissimilar materials are substituted by a number of graded layers These layers vary in chemistry, properties, and microstructure, and progressively reduce mechanical and thermal stresses at the interface and enhance the performance. Thermo-mechanical response and fracture behavior of FG BC-TBC systems under high heat flux thermal loads carried out earlier evidenced surface cracks as well as TBC-BC interface cracks [8,9]. Our current research focuses on physics-based modeling of the damage and failure process in thermally cycled TBC of gas turbine engine blades. Physics-based damage analysis and modeling of failure mechanism(s) involved through experimental research Both kinetics of oxide growth and crack formation are considered as the primary and secondary damages, respectively. The attempt made here should be treated as the initial step towards more refined modeling for realistic situations
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