Reliability Assessment Of Thermal Barrier Coatings For Turbine Blade And Its Applications

Thermal barrier coatings(TBCs)is applied onto components at the hot end of aero-engine with a cooling structure.It can prolong service life of the components by lowering temperature of metallic base in the component significantly or it can improve performance of the aero-engine and reduce fuel consumption and pollutant emission by increasing working temperature of the aero-engine.It has been widely applied in military and civil aero-engines and ships as well as industrial gas turbine.Nevertheless,the TBCs works in a severe tough environment and it has to bear various loads,including erosion,high-temperature oxidization and corrosion.Therefore,the TBCs is inevitable to be peeled after a certain service period.Once the TBCs is peeled off,the bottom metal that bears forces may be failed quickly due to exposure to high-temperature fuel directly.This will surely cause accidents,and thereby influences life safety,production safety and national defense security.Therefore,studying the failure mechanism and predicting service life of the TBCs can improve its safety application extremely significantly.Nevertheless,TBCs is a kind of porous brittle material with abundant micropores and microcracks,which brings great dispersion and randomness of its strength and relative parameters.Moreover,the service life of the TBCs is dispersed greatly due to the great randomness of loads in service.Prediction of service life based on the theory of certainty analysis has fundamental defects since it cannot consider dispersion.Since strength and load randomness of the engineering structure and mechanical structure are far lower than those of the TBCs,safety design and test both apply the reliability evaluation method based on probability analysis rather than the safety factor method based on certainty method.In reliability evaluation method based on probability analysis,various variables that influence failure and service life are viewed as random variables and safety of the structure is assessed by the probability of failure.Therefore,it is very necessary to evaluate service safety of the TBCs according to reliability.On this basis,this study achieved some innovative results:(1)A failure probability assessment model that considers the dispersion of material properties,micro-structure and environment of TBCs was developed.A reliability assessment model and numerical simulation method of turbine blade TBCs were evaluated based on NESSUS and FEM software.A method which is used to refine dominant parameters affecting the reliability of TBCs was established by combining reliability sensitivity analysis and ? theorem.(2)A numerical simulation method of gas-solid flows that can simulate the state of motion of solid particles in high-temperature fuel gas between blade lattices was developed.An erosion rate model that considers material and mechanical parameters and microstructure of the TBCs as well as speed and impact angle of erosion particle was constructed.The state of motion of abundant solid particles in fuel gas between blade lattices was simulated by using the numerical simulation method of gas-solid flows and the failure effect of the whole TBCs on blade surface by abundant particles under real situations was disclosed.Results show that due to high-speed rotating of blades,particles impact onto the TBCs surface at a larger speed in relative to the coating,causing more serious erosion to working blades than guide blade.The region with serious erosion is at front of rear surface of lades and tail of the basin of blade.Besides,the erosion-induced failure criteria of a whole-blade TBCs were developed based on the erosion rate model,gas-solid two phase flow numerical method and local failure criteria,which were used in the reliability assessment.It was found that fracture toughness and thickness of the TBCs are two primary influencing factors of reliability.(3)A phase-field model that can simulate local failure of the TBCs on turbine blade which is caused by high-temperature oxidization was developed.It was used to simulate the failure process and analyze influences of relevant factors on failure.When the interface is uneven,cracks that cause peeling of the TBCs is initiated from the wave peak region in TGO layer.When the interface roughness is high,TGO grows more quickly at the wave peak.As a result,the TBCs fails more quickly.The expansion mode of the initiated cracks varies with the interface roughness.When the interface roughness is relatively high(A/L>1/4),cracks may be expanded to pass through the TGO-ceramic interface and enter into the ceramic layer,finally interacting with cracks initiated from the adjacent wave peak in the ceramic layer of the valley surface.Finally,the thermal barrier carrier peels off completely.When the interface roughness is relatively small(A/L<1/8),cracks initiated in the TGO layer may continue to expand in TGO and expand at the TGO-ceramic interface,but it will not enter into ceramics.When the interface roughness is 1/8<A/L<1/4,the cracks expand to near the interface and branches might be developed.Some crack branch may expand toward the ceramic layer and another crack branch may expand along the interface.Reliability and influencing factors of the TBCs in high-temperature oxidization environment were evaluated.Results demonstrate that although temperature difference on the turbine blade surface is only 150~200 K in service environment,the growth rate of TGO is different due to the uneven temperature.Therefore,the difference of failure probability of the TBCs at the different position of blade surface may be increased by dozens of times.For example,the local failure probability at the windward leading edge of blade can reach 70%,while the failure probability in the thick low-temperature region at the blade back is only 0.2%.It was found that elastic modulus and temperature variations are dominants parameters affecting the reliability of TBCs.(4)Based on the reliability evaluation theory,the reliability of a TBCs on the plate specimens and turbine blade with coatings were tested by using a thermal recirculation still or a simulation device of service environment.Test results were compared with theoretical assessment results,which verify reliability and practicability of the reliability assessment method.Moreover,for the convenience of engineering applications or from the perspective of users,a transformation method between the reliability assessment results and traditional prediction of service life was developed.A performance assessment method of the TBCs when the TBCs at different positions of turbine blade has different degrees of significance was proposed.