Delamination Of Thermal Barrier Coatings Research Articles

Modeling 3D TGO dynamic growth and stresses and the resulting modes of buckling failure in thermal barrier coatings

AbstractBased on experimental data, this study presents a model of the dynamic growth of a thermally grown oxide (TGO) in thermal barrier coatings (TBCs) during thermal cycling. The model was solved using the finite element method. The thermal stress results of the model were compared with experimental results to verify the accuracy of the model. The stress analysis revealed that the combined effect of radial tensile stress and circumferential compressive stress can lead to the delamination of TBCs in a buckling form, which can induce the formation of cracks in TBCs along the axial direction. This observation was consistent with the conclusions drawn from the experimental results. Crack initiation and propagation result from the action of the maximum shear and radial stresses. In addition, the relationship between buckling failure and microscopic cracks was investigated. Thermal cycling induces radial and circumferential cracks, leading to buckling, delamination, and ultimate coating failure as the radial and circumferential cracks interact at the interface.

Failure behaviors of EB-PVD thermal barrier coatings under isothermal oxidation and thermal cycling conditions

The thermal growth oxide (TGO) layer formed by oxidation of the bond coat (BC) layer is an important factor affecting the durability of thermal barrier coatings (TBCs). In this study, a series of isothermal oxidation and thermal cycling experiments were performed on samples containing TBCs at different temperatures. The initiation, propagation and connection process of cracks in the cross-section of the samples were studied, and the oxidation failure mechanism of the samples was analyzed. The results demonstrate that the diffusion of the Al element in NiCoCrAlYHf coating to the top coat (TC) layer promotes the formation and rapid thickening of TGO at TC/BC interface. With the consumption of Al, two sublayers of mixed oxide layer and Al2O3 layer are gradually formed in the TGO layer. The TC/TGO interface is the main initiation location of interface crack. In addition, the types and initiation locations of cracks caused by isothermal oxidation are different from those caused by thermal cycling. Under isothermal oxidation, the TC layer near the TC/TGO interface and the TC/TGO interface are prone to induce horizontal cracks, which finally lead to the delamination and spalling of TBCs. Under the thermal cycling, vertical cracks occur in the TC layer, extend towards the TC/TGO interface, and merge with the interface cracks, eventually resulting in the delamination failure of the TBCs.

Numerical Study on the Effect of Partial Delamination of Thermal Barrier Coating on the Cooling Effect of Turbine Blade

A localized delamination modeling method based on a thin-wall thermal resistance model is developed in this study. Numerical investigations are then conducted with this model to analyze the influence of factors such as the position, area, shape, and depth of localized delamination on blade cooling efficiency. Firstly, the simulation results of two coating modeling methods (the thin-wall thermal resistance model and the direct modeling method) are examined. Then, the overall insulation efficiency of the blade surface with a complete thermal barrier coating is analyzed based on the thin-wall thermal resistance model. Subsequently, the impact of different delamination factors on blade cooling effectiveness is explored. The research results indicate that compared to the direct modeling method, the thin-wall thermal resistance model has a simulation deviation of less than 7% and can effectively improve modeling and simulation efficiency while ensuring computational accuracy. The highest insulation efficiency of the thermal barrier coating is observed in the middle section of the blade’s back surface near the cold air inlet, reaching 8.3%. Under a constant delamination area, the shape of the localized delamination has a minor effect on blade cooling effectiveness. When more delamination occurs in areas with higher insulation efficiency, the impact on blade cooling effectiveness becomes more significant.

Study of the TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes

To protect superalloys under extremely high temperatures in aero-engine core components, thermal barrier coatings (TBC) are sprayed on the material surface and thousands of film cooling holes are drilled which produce a cooling air film. A large proportion of these holes must be drilled at inclined angles relative to the surface in order to provide optimal cooling effects. Laser drilling has been widely applied in the industrial fabrication of film cooling holes due to its high precision, high efficiency, flexibility in drilling angle, and ability to drill all kinds of materials. In particular, nanosecond lasers have attracted lots of interest because they can achieve a good balance between drilling quality and efficiency. However, TBC is prone to delaminate during laser drilling, which severely reduces the lifetime of aero-engine components. In this study, we investigate TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes in TBC-coated cobalt-based superalloy. It was found that the leading edges of the inclined holes suffer from more severe TBC delamination than the trailing edges. By studying the effects of laser focus position, pulse width, pulse energy, repetition rate and average power, we found that faster hole breakthrough and less heat accumulation were essential for the reduction of TBC delamination. Through multi-physical numerical simulations, it was demonstrated that the asymmetric heat transfer effect in laser inclined drilling causes much larger interfacial thermal stress on the leading edge than the trailing edge, qualitatively explaining our experimental observations. The results of this study contribute to the understanding of TBC delamination in laser drilling of inclined film cooling holes.

Estimation of thermal barrier coating fracture toughness using integrated computational materials engineering

The fracture toughness of thermal barrier coatings (TBC) is a critical mechanical property that governs damage resistance. Catastrophic delamination of TBC under erosion conditions occurs in TBC with low toughness. Prior research has explored indirect and complex experiments to measure TBC toughness, but the miniaturized nature of the multi-layered coating makes it difficult to quantify its intrinsic toughness. This paper integrates computational modeling and experimental approaches to estimate the TBC toughness and the substrate delamination strength. The results show that a typical newly fabricated yttrium stabilized zirconia coating under service conditions has a toughness estimated in the range of 0.1–0.5 MPa m1/2 and a toughness of thermally grown oxide layer in between 1.5 and 1.7 MPa m1/2. The analysis also determined that a thermally grown oxide with a fracture toughness above 2.0 MPa m1/2 would not delaminate under the service conditions. Overall, the approach demonstrates the value of integrated computational material approaches, which can save time and enhance predictive power.

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.

Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.

Investigation of calcium–magnesium-alumino-silicate (CMAS) resistance and hot corrosion behavior of YSZ and La2Zr2O7/YSZ thermal barrier coatings (TBCs) produced with CGDS method

Thermal barrier coatings (TBCs) commonly expose to oxidation, hot corrosion, and CaO-MgO-Al2O3-SiO2 (CMAS) attacks during the service condition. All of these attacks cause the spallation or delamination of TBCs from the bond coat. In this study, the effect of these attacks on YSZ and La2Zr2O7/YSZ TBC systems was investigated. YSZ and double layer La2Zr2O7/YSZ topcoats were deposited using electron beam physical technique (EB-PVD) on CoNiCrAlY bond coat produced by cold gas dynamic spray (CGDS) technique. CMAS tests at 1225 °C for 4, 8, 12, 16, 20, 24, 28, and 32 h and 5 h cyclic hot corrosion tests at 1000 °C were carried out on TBCs. Before and after the tests, TBCs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray diffraction (XRD) and image analysis software program. At the end of the tests, the formed phases, microstructural changes, and general failure mechanisms were investigated in detail for each test. The general result shows that double-layered TBCs provide significant contributions and durability against high-temperature corrosive attacks of conventional YSZ TBC. In addition, in this study, hot corrosion damage mechanism and CMAS attack significantly damaged the column morphology of the EB-PVD method, reducing the thermal insulation properties and strain tolerance of TBCs. As a result of the tests, spallation and phase changes occurred in the damaged TBC systems.

Nondestructive measurements of residual stress in air plasma‐sprayed thermal barrier coatings

AbstractPremature spallation of thermal barrier coatings (TBCs) is a critical issue during the service of gas turbines, and nondestructive evaluation is crucial to address this problem. Herein, a novel approach that indicates delamination by measuring the residual stress evolution of thermally grown oxide (TGO) for air plasma spraying (APS) TBCs is proposed and verified via the combination of photoluminescence piezo‐spectroscopy (PLPS) and X‐ray computed tomography. A mineral‐oil‐impregnating approach and a cold‐mount low‐shrinkage epoxy‐mounting approach are used to alleviate the signal attenuation by pores and microcracks in APS TBCs, improving the detectable PLPS signal and X‐ray transmission for stress measurement and delamination characterization, respectively. We have nondestructively measured the TGO residual stress mapping in APS TBCs and its evolution with oxidation. Furthermore, the evolution of TGO morphology and critical microcracks are obtained by X‐ray computed tomography. The synchronous evolution of TGO residual stress, TGO thickness, and critical microcracks as a function of oxidation time is obtained and correlated. The transition point, as experimentally identified, at which the TGO stress starts to drop, agrees well with the critical moment of microcrack coalescence. This directly verifies that the TBC delamination can be effectively indicated by residual stress evolution of TGO in APS TBCs.

Infiltration Behavior of CMAS in LZ-YSZ Composite Thermal Barrier Coatings

Calcium magnesium alumina-silicate (CMAS) corrosion on thermal barrier coatings (TBCs) is one of the reasons for the early delamination of TBCs. In this study, the effects of CMAS on the microstructure evolution of TBCs were investigated for yttria-stabilized zirconia (YSZ), lanthanum zirconate (LZ), and composite coating with a 50:50 volume ratio of LZ and YSZ (LZ-YSZ). The LZ showed the mitigated penetration of CMAS by the formation of the reaction products produced by the high reactivity between the LZ and the molten CMAS. The LZ-YSZ showed a denser microstructure on the surface, resulting in retarding the penetration of CMAS. Therefore, the LZ-YSZ composite TBC is expected to prolong the lifetime performance and protect against further corrosive degradation of TBCs in high-temperature environments. The infiltration behavior and reaction mechanism of molten CMAS were discussed based on the microstructures after the corrosion test.

Nondestructive Evaluation of Thermal Barrier Coatings Interface Delamination Using Terahertz Technique Combined with SWT-PCA-GA-BP Algorithm

Thermal barrier coatings (TBCs) are usually subjected to the combined action of compressive stress, tensile stress, and bending shear stress, resulting in the interfacial delamination of TBCs, and finally causing the ceramic top coat to peel off. Hence, it is vital to detect the early-stage subcritical delamination cracks. In this study, a novel hybrid artificial neural network combined with the terahertz nondestructive technology was presented to predict the thickness of interface delamination in the early stage. The finite difference time domain (FDTD) algorithm was used to obtain the raw terahertz time-domain signals of 32 TBCs samples with various thicknesses of interface delamination, not only that, the influence of roughness and the thickness of the ceramic top layer were considered comprehensively when modeling. The stationary wavelet transform (SWT) and principal component analysis (PCA) methods were employed to extract the signal features and reduce the data dimensions before modeling, to make the cumulative contribution rate reach 100%, the first 31 components of the SWT detail data was used as the input data during modeling. Finally, a back propagation (BP) neural network method optimized by the genetic algorithm (GA-BP) was proposed to set up the interface delamination thickness prediction model. As a result, the root correlation coefficient R2 reached over 0.95, the various errors—including the mean square error, mean squared percentage error, and mean absolute percentage error—were less than or equal to 0.53. All these indicators proved that the trained hybrid SWT-PCA-GA-BP model had excellent prediction performance and high accuracy. Finally, this work proposed a novel and convenient interface delamination evaluation method that could also be potentially utilized to evaluate the structural integrity of TBCs.

Damage Assessment and Fracture Resistance of Functionally Graded Advanced Thermal Barrier Coating Systems: Experimental and Analytical Modeling Approach

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.

Correlation of thermal characteristics and microstructure of multilayer electron beam physical vapor deposition thermal barrier coatings

Referring to the expected increasing amount of aviation until the year 2030, energy and fuel efficiency as well as harmful emissions are of prime importance to the technological and ecological advancement of aircraft engines. Minimizing fuel consumption and improving energy efficiency of jet engines can be reached by increased turbine entry temperatures. However, the permitted combustion temperatures are restricted by material-dependent maximum service temperatures. Nowadays, yttria stabilized zirconia (YSZ) is commonly used as thermal barrier coating (TBC), only withstanding a permanent surface temperature of T ≈ 1200°C. In recent years, research projects have shown that multilayer systems might be a solution to withstand higher combustion temperatures. In order to assess this potential of multilayer systems, quadruple TBC consisting of 7 wt.% YSZ and La2Zr2O7 were deposited on INCONEL® 600 by electron beam physical vapor deposition. The quadruple TBC were fundamentally characterized with regard to their microstructure as well as their thermal conductivity λ. Considering the requirements with regard to the application, the long term behavior under isothermal and thermal cycling load is of great importance. To classify this behavior, atmospheric annealing tests were performed at temperatures of T = 1200°C and T = 1300°C for t = 50 h as well as thermal cycling tests were conducted at a temperature of T = 1150°C for N = 1000 cycles. The results of the quadruple layer TBC were compared to the prior investigated single and double layered TBC coatings consisting of YSZ or 7YSZ/La2Zr2O7. The investigations showed no major influence of multilayer architecture regarding the thermal conductivity λ compared to conventional 7YSZ single layer architecture. Moreover, multilayer architecture leads to the increase of porosity inside the TBC. However, no strong correlation between porosity and thermal conductivity can be revealed. The analyses of the thermal cycled TBC show that TBC delamination is mainly caused by accelerated growth of thermally grown oxide and sintering effects. Furthermore, no improvement of the thermal cycling behavior due to quadruple multilayer architecture compared to 7YSZ single layer can be determined. The comparison of thermal cycling behavior regarding single, double and quadruple layer architecture emphasizes increased TBC lifetime of double layer TBC.

Damage Analysis of Thermal Barrier Coatings Subjected to a High-Velocity Impingement of a Solid Sphere

A delamination of thermal barrier coatings (TBC) applied to turbine blades in gas turbine could be caused by a high-velocity impingement of various foreign objects. It is important to accurately predict the size of interfacial crack for safety operation of gas turbine. In this study, in order to establish a practical equation for prediction of the length of interfacial crack, a high velocity impingement test and a finite element analysis (FEA) based on a cohesive model were conducted. As the result, the length of interfacial crack is linearly increased with the impact velocity. In addition, it was confirmed that it was accurately estimated by the FEA. The equation for prediction of the length of interfacial crack was formulated based on these results and the energy conservation before and after impingement. Finally, the applicability of the equation was demonstrated in a wide range of impact velocity through a comparison with the experimental results.

Comprehensive Numerical Simulation on Thermally Grown Oxide and Internal Stress Evolutions in Thermal Barrier Coatings

TBCs (Thermal Barrier Coatings) is deposited on gas turbine blades to protect the substrate from a combustion gas flow. One of the serious problems occurred in gas turbine is TBC delamination which is caused by startup, steady and stop operation in service. TBC delamination results from subjecting to both cyclic thermal stress and evolution of internal stress due to thermally grown oxide (TGO). In this study, the finite element code which can simulate thermal and internal stress fields generated in TBC was developed. The developed code involves the follows: inelastic constitutive equation for ceramic coating, bilinear-type constitutive equation for bond coating and Chaboche-type inelastic constitutive equation for the substrate, and mass transfer equation in consideration of oxygen diffusion and chemical reaction with aluminum. Thermal cycling simulation was conducted using the developed code. It was confirmed that maximum stress and its location in the ceramic coating/bond coating interface were matched with the associated experimental results.

Quantitative Evaluation of Partial Delamination in Thermal Barrier Coatings Using Ultrasonic C-scan Imaging

Adoption of thermal barrier coatings (TBC) will help to improve the durability and thermal efficiency of turbine engines. Delamination of TBC owing to thermally grown metal oxides is a critical issue that needs periodical evaluation to ensure safety and effective maintenance for the financial benefits of industry. In this paper, a methodical approach using ultrasonic C-scan technique for quantitative evaluation of partial delamination area in TBC specimens was investigated. Preliminary studies were conducted using acoustic simulations and the experiments were carried out on a set of coin shaped TBC specimens prepared using plasma spray technique and isothermally degraded at 1100 °C for 25 h, 50 h, 100 h, and 150 h. Ultrasonic C-scans were performed using pulse echo technique with incident ultra sound radiation inclined normal to the surface of base metal of TBC system. Delamination maps of respective specimens were formulated from ultrasonic signals through proper signal processing and analysis techniques and the degree of delamination was evaluated with the help of existed mathematical model. The simulation results shows good agreement with the experimental data and this approach shows ability to estimate the degree of delamination from base metal surface.

Delamination-free millisecond laser drilling of thermal barrier coated aerospace alloys

Millisecond (ms) pulsed laser drilling is currently state-of-the-art in producing acute angle film cooling holes over aero-engine and gas-turbine components made from uncoated nickel superalloys. After laser drilling, most of these components are coated with a high-temperature thermal barrier coating (TBC) to maintain the temperature of the component at a level appropriate for its application. It is desirable to produce holes over the TBC coated components; however, the current state-of-the-art ms laser drilling of acute angle holes over TBC coated materials results in a high level of coating delamination and hence is not used in manufacturing industries. The recent introduction of ms quasi-continuous wave (QCW) fiber lasers has had a significant impact on industrial laser drilling. This paper reports the results of a fundamental investigation carried out on ms QCW laser drilling of angular holes over TBC coated superalloys and provides a pragmatic solution to the issue of TBC delamination. In addition to the investigation on traditional percussion and trepanning laser drilling processes, a new method of drilling called “laser drilling post-laser TBC decoating” is evaluated with the aim of achieving delamination-free laser drilling of TBC coated aerospace alloy.

Low cycle fatigue performance of Ni-based superalloy coated with complex thermal barrier coating

Thermal barrier coatings (TBCs) are widely applied to protect high-temperature components against high temperatures in harsh environments. Nineteen cylindrical specimens of Inconel 713LC were manufactured using the investment castings technique, and 10 specimens were subsequently coated with a novel complex thermal barrier coating (TBC) system. The TBC system comprises a metallic CoNiCrAlY bond coat (BC) and a complex ceramic top coat (TC). The TC is a mixture of a near eutectic nanocrystalline ceramic made of zirconia (ZrO2), alumina (Al2O3), silica (SiO2) and conventional yttria stabilized zirconia (YSZ) ceramic in the ratio of 50/50 in wt%. Low cycle fatigue (LCF) tests were carried out in a symmetrical push-pull cycle under strain control at 900 °C. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves of the TBC-coated and uncoated material were assessed. Fatigue life curves in total strain representation showed transient behaviour. Fracture surfaces and polished sections parallel to the loading axis of the TBC-coated and uncoated specimens prior and after cyclic loading were observed by means of scanning electron microscopy (SEM) to study the degradation mechanisms during high-temperature LCF. TBC delamination was observed at the TC/BC interface, and rafting of precipitates occurred after high-temperature exposure. The microstructural investigations further the discussion of the differences in the stress-strain response and the fatigue life of the TBC-coated and uncoated superalloy.

An Effective Electrical Resonance-Based Method to Detect Delamination in Thermal Barrier Coating

This research proposes a simple yet highly sensitive method based on electrical resonance of an eddy-current probe to detect delamination of thermal barrier coating (TBC). This method can directly measure the mechanical characteristics of TBC compared to conventional ultrasonic testing and infrared thermography methods. The electrical resonance-based method can detect the delamination of TBC from the metallic bond coat by shifting the electrical impedance of eddy current testing (ECT) probe coupling with degraded TBC, and, due to this shift, the resonant frequencies near the peak impedance of ECT probe revealed high sensitivity to the delamination. In order to verify the performance of the proposed method, a simple experiment is performed with degraded TBC specimens by thermal cyclic exposure. Consequently, the delamination with growth of thermally grown oxide in a TBC system is experimentally identified. Additionally, the results are in good agreement with the results obtained from ultrasonic C-scanning.

Mechanistic Study on the Degradation of Thermal Barrier Coatings Induced by Volcanic Ash Deposition

Thermal stress generated on thermal barrier coatings (TBCs) by volcanic ash (VA) deposition was assessed measuring the tip deflection of a multilayered beam structure as a function of temperature. The TBC in this study was deposited onto the surface of a blade utilized in a land-based gas turbine which is composed of 8 wt.%Y2O3-ZrO2/CoNiCrAlY on a Ni-based superalloy. The VA-deposited TBC sample was heated at 1453 K, and the effect of VA deposition on TBC delamination was examined in comparison with a TBC sample without VA deposition as a reference. On the basis of the VA attack damage mechanism which was investigated via the tip deflection measurement and a comprehensive microstructure examination, a damage-coupled constitutive model was proposed. The proposed model was based on the infiltration of the molten VA inside pores and phase transformations of yttria -tabilized zirconia in the TBC system. The numerical analysis results, which were simulated utilizing the finite element code installing the developed constitutive model, showed us that VA attack on the TBC sample induced near-interfacial cracks because of a significant increasing in the coating stress.