Thermal Barrier Coating Layer Research Articles

Highly-durable plasma-sprayed Al2O3-YSZ/YSZ double ceramic layer TBCs against CMAS corrosion

The addition of Al2O3 inside thermal barrier coatings (TBCs) is highlighted effective on mitigating TBCs degradation induced by CMAS corrosion. In this study, the cost-effective 20 wt.%Al2O3-7YSZ/7YSZ double ceramic layer (DCL) TBCs were fabricated and their long-term performance resisting CMAS corrosion was investigated. Significant bending and CMAS infiltration occurred readily for the single layer 7YSZ coatings just after 5 h duration. The top sacrificial Al2O3-7YSZ layer of DCL coatings greatly constrained coating deflection and CMAS infiltration. This is due to the chemical reaction between alumina splats and CMAS, generating interconnected anorthites for effectively blocking CMAS infiltration and improving resistance to CMAS attack. However, it is highlighted that the role of alumina splats on mitigating CMAS degradation was found different between the short-term corrosion and long-term corrosion. In addition, interfacial delamination/cracking was not observed between the top Al2O3-7YSZ layer and bottom 7YSZ layer after 100 h CMAS corrosion, demonstrating superior interface integrity and durability.

Improved thermal shock resistance of GYYZO-YSZ double ceramic layer TBCs induced by induction plasma spheroidization

In thermal barrier coatings (TBCs) applications, conventional Y2O3 stabilized ZrO2 (7-8YSZ) rapidly deteriorates when exposed to temperatures above 1200 °C. Alternatively, Gd2O3-Yb2O3-Y2O3 co-stabilized ZrO2 (GYYZO) emerges as a promising TBC candidate for superior thermal insulation capability and phase stability up to 1300 °C. In this study, we have fabricated and evaluated 7-8YSZ TBCs, GYYZO-YSZ double ceramic layer (DCL) with calcined GYYZO powder and GYYZO-YSZ DCL with calcined and induction plasma spheroidized GYYZO powder in terms of thermal shock resistance. Under thermal shocks up to 1300 °C, YSZ quickly fails after 648 ± 60 cycles for insufficient high temperature capability. Benefiting from the DCL system, the lifespan of calcined GYYZO-YSZ DCL is successfully prolonged to 1582 ± 40 cycles, however the weak ceramic interface induces premature failure. By spheroidization, the lifetime of spheroidized GYYZO DCL is further increased to 2050 ± 70 cycles, as a result of the combined effect of coating densification, interface enhancement and introduction of vertical cracks.

Effect of microstructure on CMAS corrosion behavior of (Gd0.8Yb0.2)2Zr2O7/YSZ thermal barrier coatings prepared by EB-PVD

Three types of (Gd0.8Yb0.2)2Zr2O7 (GYbZ)/YSZ double ceramic layer thermal barrier coatings (TBCs) were prepared with different microstructures by electron beam physical vapor deposition. The CMAS corrosion performance of the three TBCs was studied at 1250 ℃ compared with YSZ-TBC. The results showed that the CMAS corrosion resistance strongly depends on their microstructures. TBCs consisting of smaller inter-column gaps and larger specific surface area exhibited superior CMAS corrosion performance. The reaction products of GYbZ-TBCs are mainly composed of Gd-apatite and fluorite c-ZrO2 based fluorite. The effect of microstructure on CMAS infiltration depth was discussed.

Methods for characterizing the interfacial adhesion in thermal barrier coating systems

Thermal barrier coatings (TBC) are widely used to protect the hot section structural components, such as aircraft gas turbine, against hot corrosion and large thermal gradients during the service life. The resistance to interfacial debonding (adhesion) is one of the key factors for predicting the TBC durability during operation. The goal of the study is analysis of the existing approaches to quantification of the interfacial adhesion for multi-layered thermal barrier coating systems and thermally-grown oxide on Ni-based superalloys and discussion of the problem regarding the reasons for a large spread of the adhesion characteristics. Theoretical prerequisites for determining the adhesion characteristics are considered to provide a theoretical background for quantification of the interfacial adhesion. The absence of national standard tests applicable to TBC is marked along with a significant scatter of the values obtained using international test-regulating documents and proprietary technologies. The values of the intensity of the strain energy release are found to vary from 0.3 to 230 J/m2 depending on the method for measuring adhesion, experimental details and type of the system considered. The factors that determine a large spread of the interfacial adhesion values were divided into two groups: 1 — intrinsic, attributed to the features of TBC layer formation and temperature affecting the value and distribution of residual strains and stresses and direction of the failure initiation; 2 — extrinsic, which are related to test conditions and technique of experimental data processing. A complex approach involving both intrinsic and extrinsic parameters is required to address the problem of interfacial adhesion quantification.

LaYZrO/YSZ double ceramic layer thermal barrier coatings by EB-PVD: Thermal performance, morphology and failure behavior

Thermal barrier coatings have been widely investigated over the last decade. Lanthanum zirconate is known for its high melting point and good thermal stability in aircraft engine environment. Owing to the distinctive thermophysical properties, Y doping in pyrochlore La2Zr2O7 might be a promising TBCs material as the top ceramic layer. It remains a challenge to completely understand the LaYZrO/YSZ double ceramic layer TBCs properties and microstructure evolution during thermal cyclic. In this paper, LaYZrO/YSZ double ceramic layer TBCs have been prepared by EB-PVD. The phase structure, thermal performance, microstructure and thermal cyclic lifetime have been researched. Thermal conductivity has been decreased compared with YSZ and La2Zr2O7, simultaneously, coefficient of thermal expansion has been increased compared with La2Zr2O7. The failure behavior of LaYZrO/YSZ double ceramic layer TBCs have been elucidated as typical TGO growth mode and specific sintering crack evolution mode. This study of properties and failure behaviors might provide the valuable investigation and thought for other advanced TBCs.

Experimental and Computational Investigation of Integrated Internal and Film Cooling Designs Incorporating a Thermal Barrier Coating

Abstract Few studies in the open literature have studied the effect of thermal barrier coatings (TBC) when used in combination with shaped hole film cooling and enhanced internal cooling techniques. The current study presents Reynolds-averaged Navier–Stokes (RANS) conjugate heat transfer simulations that identify trends in cooling design performance as well as experimental measurements of overall effectiveness using a flat-plate matched-Biot number model with a simulated TBC layer of 0.42D thickness, where D is the film cooling hole diameter. Coolant is fed to the film cooling holes in a co-flow configuration, and the results of both the smooth and rib-turbulated channels are compared. At a constant coolant flow rate, enhanced internal cooling was found to provide a 44% increase in spatially-averaged overall effectiveness, ϕ¯¯, without a TBC. The results show that the addition of a TBC can raise ϕ¯¯ on a film-cooled component surface by 47%. The optimum velocity ratio was found to decrease with the addition of enhanced cooling techniques and a TBC as the film provided minimal benefit at the expense of reduced internal cooling. While the computational results closely identified trends in overall system performance without a TBC, the model over-predicted effectiveness on the metal–TBC interface. The results of this study will inform turbine component design as material science advances increase the reliability of the TBC.

Numerical study on the influence of coolant temperature, pressure, and thermal barrier coating thickness on heat transfer in high-pressure blades

Several studies have focused on investigating the heat transfer in blades; however, these studies have primarily adopted numerical approaches, and the heat transfer in an actual turbine blade remains unclear thus far, owing to the complex topology of the blade. To address this gap in literature, this study investigates the influences of the thermal barrier coating (TBC) thickness and coolant on the mainstream hot gas pressure ratio (pc/pt) and temperature ratio (Tc/Tg) in blade film cooling, respectively, by simulating the conjugate heat transfer (CHT) in the first-stage cooling blade of an W501F engine. A complete blade design is used to obtain a realistic heat transfer profile. The results demonstrate equivalent high-thermal-stress regions between the simulation and industrial patterns. The leading tip (LT), trailing tip (TT), platform, and leading edge (LE) regions have higher thermal stresses than the other regions. Coolants with higher temperature are less effective in cooling the blade due to the nonlinear effects of compressible air. Although the coolant reduces the temperature across the blade with similar efficacy, the temperature variation is still significant; a decrease of 100 K in the coolant temperature results in a reduction of 58 K in the average blade temperature. At a fixed coolant temperature and standard pressure ratio pc/pt (approximately 1.07), the coolant film uniformly envelopes the entire LT surface. However, for the same blade design, the trailing passage lacks a coolant film at this pressure ratio. As the pressure ratio exceeds 1.50, the coolant flows to all regions in the trailing passage cooling the TT at an enhanced rate. However, as the ratio continues to increase, liftoff jet occurs and reduces the cooling performance at the LT, resulting in overheating in this region. The application of a 0.8 mm-thick TBC layer on the blade surface reduces the heat transfer coefficient by 35%, thus decreasing the heat transfer.

High-entropy (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 ceramic: A promising thermal barrier coating material

• (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 exhibits great high-temperature phase stability up to 1600 °C and excellent chemical compatibility with Al2O3 even at 1300 °C. ○ The grain growth rate of high entropy hafnate is more sluggish than that of Y2Hf2O7. ○ The thermal conductivity of (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 (0.73–0.93 W m −1 K −1 ) is much lower than those of components RE2Hf2O7 and many thermal barrier coating materials. ○ (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 can be as next generation thermal barrier coating material. Thermal barrier coating (TBC) materials perform an increasingly important role in the thermal or chemical protection of hot components in a gas turbine. In this study, a novel high entropy hafnate (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 was synthesized by solution combustion method and investigated as a potential TBC layer. The as-synthesized (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 possesses a pure single disordered fluorite phase with a highly homogeneous distribution of rare earth (RE) cations, exhibiting prominent phase stability and excellent chemical compatibility with Al 2 O 3 even at 1300 °C. Moreover, (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 demonstrates a more sluggish grain growth rate than Y 2 Hf 2 O 7 . The thermal conductivity of (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 (0.73-0.93 W m −1 K −1 ) is smaller than those of components RE 2 Hf 2 O 7 and many high entropy TBC materials. Beside, the calculated thermal expansion coefficient (TEC) of (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 (10.68 × 10 −6 /K, 1100 °C) is smaller than that of yttria-stabilized zirconia (YSZ). Based on the results of this work, (Y 0.2 Gd 0.2 Dy 0.2 Er 0.2 Yb 0.2 ) 2 Hf 2 O 7 is suitable for the next generation TBC materials with outstanding properties.

Review of Numerical Simulation of TGO Growth in Thermal Barrier Coatings

Thermally grown oxide (TGO) is a critical factor for the service life of thermal barrier coatings (TBC). Numerical simulations of the growth process of TGO have become an effective means of comprehensively understanding the progressive damage of the TBC system. At present, technologies of numerical simulation to TGO growth include two categories: coupled chemical-mechanical methods and mechanical equivalent methods. The former is based on the diffusion analysis of oxidizing elements, which can describe the influence of bond coat (BC) consumption and phase transformation in the growth process of TGO on the mechanical behavior of each layer of TBC, and has high accuracy for the thickness evolution of TGO, but they cannot describe the lateral growth of TGO and the rumpling phenomenon induced. The latter focuses on describing the final stress and strain state after the growth of a specific TGO rather than the complete growth processes of TGO. Based on the measured TGO thickness growth curve, simulations of thickening and lateral growth can be achieved by directly applying anisotropic volumetric strain to oxidized elements and switching elements properties from the BC to the TGO.

LaYZrO/YSZ Double Ceramic Layer Thermal Barrier Coatings by EB-PVD: Thermal Performance, Morphology and Failure Behavior

LaYZrO/YSZ Double Ceramic Layer Thermal Barrier Coatings by EB-PVD: Thermal Performance, Morphology and Failure Behavior

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.

Comparison of calcium–magnesium-alumina-silicate (CMAS) resistance behavior of produced with electron beam physical vapor deposition (EB-PVD) method YSZ and Gd2Zr2O7/YSZ thermal barrier coatings systems

Thermal Barrier Coatings (TBCs) are generally used in both industrial gas turbines and aircraft engines. In this study, metal powders with CoNiCrAlY content were coated on Inconel 718 nickel-based superalloy substrate materials using the cold gas dynamic spray (CGDS) method, which is a new generation thermal spray coating method. As a top coating, YSZ and Gd2Zr2O7/YSZ (GZO/YSZ), respectively, single and double layer ceramic layers were deposited with an innovative method, physical vapor deposition by electron beam (EB-PVD). All produced TBC systems were sprinkled with vermiculite powder containing CMAS minerals and subjected to CMAS corrosion tests and the effect of CMAS damage on the phase stability and microstructure of TBC systems was determined using elemental mapping analysis, X-ray diffraction (XRD) analysis, and scanning electron microscope (SEM). According to the results obtained, it has been determined that the double layer TBC system with GZO/YSZ is more resistant to CMAS damage than the single layer YSZ system.

Analysis of crack initiation and propagation in Thermal Barrier Coatings using SEM-Based geometrical model with extended finite element method

Thermal Barrier Coatings (TBC) are widely used to protect the metallic components that operate at harsh conditions of elevated temperatures and oxidizing environments. Thermally grown oxide (TGO) causes cracks formation in the top coat (TC) that may lead to spallation failure of TBC. This work investigates effect of pores and TGO thickness on crack initiation and propagation due to thermal mismatch between TBC layers. Image processing is used to convert an SEM image, including pores, into a finite element (FE) model. An FE model using XFEM implemented in ABAQUS was developed to investigate crack initiation and propagation for various TGO thicknesses considering the effect of plastic deformation of BC, TGO and substrate. Results show that presence of pores changes the critical sites for crack initiation from the TC/TGO interface to be around the pores within the TC. Crack initiation temperatures and crack lengths were found to be affected with both TGO thickness and pores.

Structural parameter study on stress intensity factors of interfacial crack in thermal barrier coatings

Cracks on the wave-shaped interface between the top coating (TC) layer and the bonding coating (BC) layer of thermal barrier coatings (TBCs) are unavoidable during manufacture and operation, which may reduce the strength of the TBCs under thermal shock. A model with curved crack on the wave-shaped interface is established, and the influences of structural parameters, including the porosity, the distribution of pores and thickness of the TC layer, the interface roughness and position of crack tip on the wave-shaped interface, on the stress intensity factors (SIFs) of the cracks during thermal shock are numerically studied with the finite element method (FEM). According to the microstructure of the TBCs, the shape of a pore is assumed to be elliptical, and the pores are randomly distributed in the TC layer. A program is developed to automatically generate the distribution of pores. The variation laws of the stress intensity factors (SIFs), KI and KII, of the curved interfacial crack under various structural parameters are obtained and analyzed, which may provide an instruction for the design of the TBCs.

Failure mechanism of thermal barrier coating on the first stage turbine blade

One of the main components in a gas turbine for the power plant is the turbine blade. The turbine blade lifetime depends on materials, operation, and maintenance. A prominence reason for accelerate turbine blade degradation is caused by the failure of the thermal barrier coating (TBC). In the present work, we studied the failure of TBC on the 1st stage turbine blade. A scanning electron microscope, optical light microscope, and X-Ray diffraction were used for samples examination. The analysis results showed that the TBC of first stage turbine blades is a plasma-water type sprayed and consists of 7 wt. % yttria-stabilized zirconia (YSZ). Mechanism of separation in the TBC layer is caused by thermal fatigue due to thermal differences expansion between the TBC and the thermally grown oxide (TGO), thereby causing tensile stress and compression on the TBC. The pressure causes micro-cracks on the top coat lead to the separation of the TBC top coat.

Influence of amorphous phase in LaMgAl11O19 on properties of LaMgAl11O19/YSZ thermal barrier coatings

Double ceramic layer thermal barrier coatings (DCL-TBCs) of LaMgAl11O19/YSZ (LMA/YSZ) were prepared by atmospheric plasma spraying (APS). Composition, microstructure and phase stability of coatings were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and differential scanning calorimetry (DSC). The amorphous phase content of LMA coating increased with increasing the plasma gun current from 350 A to 650 A. The DCL-TBCs of LMA/YSZ whose LMA coating was prepared with a plasma gun current of 600 A (600LMA/YSZ) had the lower amorphous phase content, an excellent thermal insulation performance and the longest thermal cycling lifetime, indicating that properties of LMA coating could be largely improved by adjusting the APS conditions.

Polarization effect on hole evolution and periodic microstructures in femtosecond laser drilling of thermal barrier coated superalloys

A high-pulse-energy femtosecond pulsed Ti:sapphire laser was employed to perform laser drilling of 2.4-mm-thick thermal barrier coated In718. The influences of polarization on hole profile and morphology of periodic structures on the sidewall of drilled holes were explored. Based on Fresnel formula, combined with the effects of multiple reflections and energy-trapping, a comprehensive analysis of the effect of polarization on polarization removal was proposed. The microstructures on the sidewall of drilled holes for different layers under different polarizations were compared. The fabricated structures on the thermal barrier coating (TBC) layer showed a strong dependence on the polarization state of the laser beam. The structures with ball-like profile were generated under circular polarization while pine needle-like periodic structures were produced under linear polarization. However, for both the bond coat and substrate layers, the microstructures of the sidewall were independent of the polarization state of the laser beam. For TBC layer the interaction between the resonance of charge carriers and the second harmonic of the surface contributed to a corrugated structure in harmony with the polarization state. Thereinto, the charge carriers may remember the polarization state of the light beam and maintain this information because of the low mobility.

Evolution of internal cracks and residual stress during deposition of TBC

Thermal barrier coating (TBCs) are ceramic coatings that are deposited on metallic substrates to provide high thermal resistance. Residual stress is among the critical factors that affect the performance of TBCs. It evolves during the process of coating deposition and in-service loading. High residual stresses result in significant cracking and premature delamination of the TBC layer. In the present study, a hybrid computational approach is used to predict the evolution of internal cracks and residual stress in TBC. Smooth particle hydrodynamics (SPH) is first used to model the deposition of yttria-stabilized zirconia (YSZ) layer that contains various interfaces and micropores on a steel substrate. Then, three-dimensional (3D) finite element analysis is utilized to predict the evolution of internal cracks and residual stress in the ceramic coating layer. It is found that multiple cracks emerge during the solidification of the coating layer due to the development of high tensile (quenching) stresses. The cracking density is higher at regions near the coating interface. It is also found that compressive (residual) stresses are developed when the deposited coating is cooled to room temperature. The residual stress state is equibiaxial and nonlinear across the thickness/width of the TBC layer. The residual stress profile predicted compares well with that of hole drilling experiments.

Microstructural characterization of modified plasma spray LZ/YSZ thermal barrier coating by laser glazing

One of the high potential material, lanthanum zirconate (LZ) is deposited on superalloy 625 due to its excellent phase stability, low thermal conductivity as well as the high melting point. However, microstructural defects were detected due to lamellar structure behaviour exhibited from the plasma-sprayed coating techniques. This paper investigated the effect of surface modification on LZ thermal barrier coating (TBC) by using laser glazing method. The double ceramic layer (DCL) TBCs are composed of NiCoCrAlYTa bond coating formed via the HVOF process while 8YSZ ceramic top coat and La2Zr2O7 as the third layer were produced by utilizing the APS technique. Nd:YAG pulsed laser has been used to modify the surface layer of plasma-sprayed LZ coatings. The microstructural morphologies of both as-sprayed and laser glazed samples were investigated using Scanning Electron Microscopy (SEM). The phases of the coatings were analysed with X-ray Diffractometry (XRD). The results showed that the laser glazing process reduced surface roughness attributed to the sealing of porosities and other structural defects of the coating. Finally, the impacts of laser energy input and laser distance on the characterization of the glazed layer are discussed.

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.