Thermal Barrier Coatings Specimens Research Articles

High precision thickness evaluation of thermal barrier coating with high frequency eddy current testing method

Consisting of a top ceramic coating (TC) and a bond coating (BC) layer, thermal barrier coating (TBC) is a key component of the heavy duty gas turbine blade to protect the substrate material from high working temperature. Thinning of the top coating and the thermally grown oxide (TGO) layer formed between TC and BC cause TBC thickness reduction, and may significantly affect the in-service performance of TBC. High frequency eddy current testing (HF ECT) is considered as an efficient technique for nondestructive evaluation (NDE) of the TBC thickness, but its measurement accuracy is not satisfied up to now. Aiming to realize a high precision TBC thickness evaluation with HF ECT signals, a numerical method for simulating HF ECT of TBC was developed and implemented with consideration of the dielectric properties of TC and TGO material. Through numerical simulations with the developed code, an ECT probe of optimized configuration and robust against lift-off variation was designed and fabricated for the TBC thickness evaluation. A HF ECT measurement system is then setup to investigate the validity of the prototype ECT probe and sizing method experimentally. It was found that the thickness of the TC layer can be evaluated within 10 μm accuracy for the plate TBC specimens, which reveals the HF ECT method and new ECT probe are efficient for NDE of the TBC thickness.

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

AbstractThe stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce3+ ions (YAG:Ce3+)/yttrium oxide partially stabilized zirconia double‐ceramic‐layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce3+ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce3+ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.

Shakedown Analysis and Experimental Study of Thermal Barrier Coatings.

Interfacial stress–strain fields become complicated in thermal barrier coatings (TBCs) under cyclic thermal loading, which affects the stability and spalling failure of TBCs directly. The convex and concave interfacial structures of TBCs were approximated as a multilayer cylinder model, and an analytical method of TBCs for shakedown analysis was established. A series of 8-YSZ TBC specimens were prepared by the plasma spraying process, followed by isothermal and thermal shock tests. The results showed that the stability limit is significantly greater than the elastic limit, the limit for the convex model was higher than that in the concave model, the first failure occurs in the concave area, and the main failure mode of a thermal barrier coating is the appearance of cracks at the interface layer during a thermal shock test. For the coating samples prepared in this study, the stability limits were between 950 °C and 1050 °C, and the validity of the stability limit analysis model of a multilayer structure was verified.

Characterization of the stress distribution and evolution inside a thermal barrier coating after initial thermal cycles

AbstractThe residual stress introduced inside a thermal barrier coating (TBC) during manufacturing and service processes is one of the main causes of thermal barrier failure. The formation and evolution of internal stress in the TBC begin at the early stage of service, but studies on the mechanism of the distribution and evolution of the stress in the TBC during the initial thermal cycle are still lacking. To explore the evolution mechanism of the stress in the TBC interior, an experimental study on the regulation mechanism of the initial thermal cycle on the TBC internal stress was carried out in this paper. First, the internal stress of TBC specimens after thermal cycles was characterized based on photoluminescence spectroscopy (PL) and terahertz time‐domain spectroscopy (THz‐TDS) technologies, in which the homogenization of the near‐interface stress field was observed during the initial thermal cycle. Then, the evolution of the microstructure and phase structure of the TBC specimens was characterized. Finally, the phenomenological model of the evolution of the TBC internal structure was established, revealing that the initial thermal cycle regulated the microstructure of the top coating (TC) through phase transformation to realize the homogenization of the near‐interface stress field.

Novel thermal barrier coatings based on Mg2SiO4/8YSZ double-ceramic-layer systems deposited by APS

Novel thermal barrier coatings (TBCs) based on Mg 2 SiO 4 /8YSZ double-ceramic-layer (DCL) were prepared by atmospheric plasma spraying (APS). Thermal cycling tests of the TBC specimens were performed at 1373 K. Results indicate that the DCL coating has a much longer lifetime compared to the single layer Mg 2 SiO 4 coating. This is attributed to the fact that DCL Mg 2 SiO 4 /8YSZ TBCs further enhances the thermal insulation effect, improves sintering resistance and relieves thermal mismatch between ceramic layer and bond coat (BC). It can be concluded that this DCL coating structure design is ideal to optimize the performance of single layer Mg 2 SiO 4 TBC and shows the Mg 2 SiO 4 exhibiting a promising potential as new TBC materials. • The DCL coating has a much longer thermal cycling lifetime than that of the single layer coating of Mg 2 SiO 4 . • The designed DCL coating system is helpful to prolong the thermal cycling life. • The failure mechanism of DCL coating was studied.

Tailoring microstructure of double-layered thermal barrier coatings deposited by suspension plasma spray for enhanced durability

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs.

Measurement of Stress Optic Coefficient for Thermal Barrier Coating Based on Terahertz Time-Domain Spectrum

The residual stress introduced inside the thermal barrier coating (TBC) top coating during manufacturing and service processes is one of the main causes of thermal barrier failure. Therefore, a nondestructive and accurate measurement of the residual stress in top coating is essential for the evaluation of TBC life. The terahertz time-domain spectroscopy (THz-TDS) technique, which is based on the calibration or measurement of the stress optical coefficients of the measured materials, is applicable to the measuring of internal stress of nonmetal materials. In this work, to characterize the internal stress in TBC, the stress optic coefficient of the TBC top coating was measured by reflection-type THz-TDS. First, the mechanics model for the internal stress measurement in a TBC top coating was derived based on the photoelastic theory. Then, the THz time-domain spectra of TBC specimens under different loadings were measured in situ by a reflection-type THz-TDS system. Finally, the unimodal fitting, multimodal fitting and barycenter methods were used to carry out the data processing of the THz time-domain spectral-characteristic peaks. By comparing the processed results, the results using the barycenter method were regarded as the calibrated stress optical coefficient of the TBC due to the method’s sufficient accuracy and stability.

Method for Predicting Thermal Fatigue Life of Thermal Barrier Coatings Using TGO Interface Stress

Thermal barrier coatings (TBCs) applied to high-temperature components of gas turbines consist of a ceramic top coat, a metallic bond coat, and thermally grown oxide (TGO) generated between the top coat and bond coat. Because TBCs are subjected to repeated thermal stress at the coating interface under thermal fatigue conditions and eventually breakage, it is crucial to evaluate the thermal fatigue durability of TBCs according to the stress. In this study, coin-type TBC specimens were prepared by depositing commercial coating powders on Ni-based super alloys via the air plasma spray method, and the thermal fatigue life of the TBCs was experimentally evaluated. According to the test results and references, a finite-element analysis was conducted. The maximum stress of the TGO interface was evaluated according to the thickness and equivalent elastic modulus, and simulating the microstructure including the pores of the top coat. Using these relationships, a thermal fatigue life prediction equation considering the coating thickness (t), equivalent elastic modulus (E), and operating temperature (T) was derived, and subsequently verified.

Improvement of Oxidation Resistance and Adhesion Strength of Thermal Barrier Coating by Grinding and Grit-Blasting Treatments

The formation of thermally grown oxide (TGO), which is composed of alumina and mixed oxides, leads to the delamination of thermal barrier coating (TBC). In this study, to improve the oxidation resistance and adhesion strength of TBCs, grinding and grit-blasting treatments with alumina grits were applied to the surface of the bond-coat (BC) before deposition of the top-coat (TC). These treatments are expected to pre-form an alumina layer as an oxidation barrier and also optimize the TC/BC interfacial roughness. A high-temperature exposure test of TBC specimens grit-blasted with alumina grits of different sizes (B-TBC) revealed the growth of a continuous alumina layer in the B-TBC specimens in contrast to the formation of a complex TGO with alumina and mixed oxides in non-blasted TBC (S-TBC). Moreover, the area and thickness of TGO in the B-TBC specimens were much lower than those in the S-TBC. An indentation test was conducted to evaluate the TC/BC interfacial fracture toughness KIFC which confirmed a significantly higher KIFC of the B-TBC specimens than that of the S-TBC specimen. These results demonstrated that the grinding and grit-blasting treatments are effective in improving the oxidation resistance and adhesion strength of the TBC system.

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.

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.

Crack-Resistance Behavior of an Encapsulated, Healing Agent Embedded Buffer Layer on Self-Healing Thermal Barrier Coatings

In this work, a novel thermal barrier coating (TBC) system is proposed that embeds silicon particles in coating as a crack-healing agent. The healing agent is encapsulated to avoid unintended reactions and premature oxidation. Thermal durability of the developed TBCs is evaluated through cyclic thermal fatigue and jet engine thermal shock tests. Moreover, artificial cracks are introduced into the buffer layer’s cross section using a microhardness indentation method. Then, the indented TBC specimens are subject to heat treatment to investigate their crack-resisting behavior in detail. The TBC specimens with the embedded healing agents exhibit a relatively better thermal fatigue resistance than the conventional TBCs. The encapsulated healing agent protects rapid large crack openings under thermal shock conditions. Different crack-resisting behaviors and mechanisms are proposed depending on the embedding healing agents.

Constitutive parameters identification of thermal barrier coatings using the virtual fields method

Thermal barrier coatings (TBCs) are widely applied in thermal components to protect metallic components. Owing to the complex layered structure of TBCs and difficult preparation of coating, the mechanical characterization of TBCs should be of primary importance. With regard to TBCs, this study deals with the constitutive parameters identification of bi-material. Considering the complex construction and boundary of bi material, the virtual fields method (VFM) was employed in this study. A methodology based on the optimized virtual fields method combined with moire interferometry was proposed for the constitutive parameters identification of bi-material. The feasibility of this method is verified using simulated deformation fields of a two-layer material subjected to three point bending loading. As an application, the deformation fields of the TBC specimens were measured by moire interferometry. Then, the mechanical parameters of the coating were identified by the proposed method. The identification results indicate that Young’s modulus of the TBC top coating is 89.91 GPa, and its Poisson’s ratio is 0.23.

Sampling moiré method and its application to determine modulus of thermal barrier coatings under scanning electron microscope

The sampling moiré method has been widely utilized to determine the displacement and strain distributions of materials and structures at macroscales. The method exhibits a simple operation and low requirements regarding the specimen grating. Further, a single image is required for the phase analysis. The method is a promising method in the field of experimental mechanics. In this study, the mechanisms of moiré pattern generation are systematically studied using simulations. Two essential conditions for their generation are presented and the applicability of the control equation to moiré-spacing determination is discussed. To promote the application of the sampling moiré method in microdeformation measurements, three-point bending tests using thermal-barrier coating (TBC) specimens (bilayer beam structure) are performed under a scanning electron microscope (SEM). Using the soft-imprint technique, a high-frequency grating is fabricated on the side surface of the TBC specimen to generate sampling moiré patterns under the SEM. The microdisplacement and strain components are calculated using the sampling moiré method and their values are used to determine the TBC Young's modulus. To examine the reliability of the sampling moiré method, the measured strains are compared with those obtained with the geometric phase analysis (GPA). The results are in good agreement. Consequently, the sampling moiré method encourages further applications in microdeformation measurements and mechanical-property characterizations of other materials.

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.

CMAS behavior of yttrium aluminum garnet (YAG) and yttria-stabilized zirconia (YSZ) thermal barrier coatings

Calcium magnesium aluminosilicate (CMAS) that is formed from the ingested deposits in gas turbines degrades thermal barrier coatings (TBCs), especially for the most widely used material; yttria-stabilized zirconia (YSZ). In the present work, we examine the behavior of yttrium aluminum garnet (YAG) as an alternative material for TBCs. CMAS interaction studies were conducted by making composite pellets of YAG-CMAS and YSZ-CMAS powders. These pellets, after being subjected to heat treatment between 1100°C and 1500°C were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS), which showed YAG to be almost inert to CMAS whereas YSZ exhibited significant phase changes. To test the behavior of TBCs with YAG and 8YSZ as the topcoat material in a CMAS environment, cyclic furnace tests were conducted in which a controlled amount of CMAS was applied and then the samples were cycled to failure. In addition, to simulate the continuous accumulation of CMAS expected in service, a cyclic furnace test was devised in which a small dose of aqueous solution of CMAS was applied on TBC specimens at the start of every cycle until the samples were cycled to failure. In all these tests YAG TBCs outperformed YSZ in terms of durability. The mechanisms of CMAS attack are described and the relative resistance of YAG and YSZ is shown to be consistent with the Optical Basicity (OB) theory.

A prominent driving force for the spallation of thermal barrier coatings: Chemistry dependent phase transformation of the bond coat

The influence of substrate and bond coat chemistry on the degradation mechanism leading to the early spallation of thermal barrier coatings (TBCs) has not been well understood despite years of research effort. This is largely due to the sheer number of factors (i.e. interfacial rumpling and oxide growth kinetics) that all seem to contribute to the degradation of TBCs. To clarify the chemical effect, extensive characterizations and in-depth analysis near the oxide-bond coat interface, were carried out on the isothermally exposed TBC specimens. It is evident that the formation of γ′ along the grain boundaries can significantly enhance rumpling, while martensitic transformation during cooling creates out-of-plane stresses and causes crack nucleation at the oxide-bond coat interface. These partial phase transformations in the β bond coat system were determined to be a prominent driving force for the TBC spallation. To prevent the early spallation of TBCs, it is necessary to minimize the formation rate of γ′ and martensitic phases, which can be achieved by tailoring the inherent substrate/bond coat composition as elucidated in this paper.

TBC delamination life prediction by stress-based delamination map

The thermal barrier coating (TBC) is applied to gas turbine components to protect the hot-section components from extremely high temperature condition. Since metallic substrate cannot endure such severe condition of gas turbines, the delamination of the TBC can cause the failure of the whole system. Thus, the delamination life of the TBC is one of the most important issues for designing the hot-section components which operate at high temperature condition, and thermal stress between the coating layers is known as one of the major causes of the delamination. Especially, growth of thermally grown oxide (TGO) by Al-diffusion from bond coat is known as one of the main reasons to increase thermal stress between coating layers. Thus, in this study, the TGO growth behavior in the TBC is investigated, using the isothermally aged TBC specimens at high temperature conditions (900 - 1100°C), and thermal stress on TGO layer is investigated by finite element analysis, considering the TGO growth in the TBC. Finally, the test and FEA results are concluded as the TGO delamination map which can predict the delamination life time of TBC under isothermal condition.

Influence of Alkali Metal Oxide on CMAS Damage Progression in Air Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs), which typically comprised of a yittria-stabilized zirconia (YSZ) ceramic top coat, a bond coat and a metallic substrate, have been widely used to protect the hot-section metallic components of gas turbine engines. More recently at high temperatures exceeding 1200℃, the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals with the intake air and from unclean fuels. The chemical composition of CMAS is mainly comprised of CaO, MgO, Al2O3 and SiO2. In addition to that, alkali metal oxides can be interblended into CMAS constituents under some operating environments. In this study, the effect of alkali metal oxide (K2O) on CMAS damage progression on air plasma sprayed (APS) YSZ TBC specimens was investigated through simulating the CMAS damage at lab-scale employing synthetic CMAS products. Two types of synthetic CMAS products both with and without K2O were prepared. The synthetic CMAS was spread on the APSed TBC specimen, followed by isothermal exposure at different temperatures up to 1300℃ for various times. It was found that the infiltration of CMAS into the top coat was observed in the both CMAS compositions. However, the infiltration rate was significantly decreased due to the presence of K2O. In the case of CMAS without K2O, the reaction between CMAS and YSZ was significant resulting in the change in microstructural morphology, whereas it was little occurred on the CMAS including K2O. These results suggest that the addition of K2O to YSZ top coat can mitigate the CMAS damage progression.

Measurement of Refractive Index of Thermal Barrier Coating Using Reflection of Terahertz Waves and Variable Aperture

SUMMARYA method to measure the refractive index of the ceramic layer of a thermal barrier coating (TBC) using the intensity ratio between the reflection of terahertz waves from the ceramic surface and a reference metal plate is proposed. This method uses a variable aperture to change the relative proportions of specular refraction and scattering from the ceramic surface. The refractive index is calculated based on the specular reflectivity, which is obtained from the intensity ratio when the field of view is extrapolated to zero. Measurement of a TBC specimen resulted in a refractive index of 4.7, which was in good agreement with the refractive index of the ceramic material obtained by terahertz time‐domain reflectometry. In addition, the surface roughness of the ceramic surface was obtained based on the frequency characteristics of the reflections from the ceramic surface and a reference metal plate. A surface roughness of 14 μm was obtained, which matched the results of microscopic observation of a TBC specimen whose coating was applied using the same lot as the specimen used in the measurement.