Graded Thermal Barrier Coatings Research Articles

Hot Corrosion Behavior of Plasma-Sprayed Gd2Zr2O7/YSZ Functionally Graded Thermal Barrier Coatings

The development of advanced thermal barrier coating (TBC) materials with better hot corrosion resistance, phase stability, and residual stresses is an emerging research area in the aerospace industry. In the present study, four kinds of TBCs, namely, single-layer yttria-stabilized zirconia (YSZ), single-layer gadolinium zirconate (GZ), bilayer gadolinium zirconate/yttria-stabilized zirconia (YSZ/GZ), and a multilayer functionally graded coating (FGC) of YSZ and GZ, were deposited on NiCrAlY bond-coated nickel-based superalloy (Inconel 718) substrates using the atmospheric plasma spray technique. The hot corrosion behavior of the coatings was tested by applying a mixture of Na2SO4 and V2O5 onto the surface of TBC, followed by isothermal heat treatment at 1273 K for 50 h. The characterization of the corroded samples was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to identify physical and chemical changes in the coatings. GIXRD was used to analyze the residual stresses of the coatings. Residual stress in the FGC coating was found to be −15.2 ± 10.6 MPa. The wear resistance of TBCs is studied using a linear reciprocating tribometer, and the results indicate that gadolinium zirconate-based TBCs showed better performance when deposited in bilayer and multilayered functionally graded TBC systems. The wear rate of as-coated FGC coatings was determined to be 2.90 × 10−4 mm3/Nm, which is lower than the conventional YSZ coating.

Adaptive Control of the Atmospheric Plasma Spray Process for Functionally Graded Thermal Barrier Coatings

Functionally graded coatings (FGCs) have a material composition continuously varying through the thickness but uniform in the surface parallel to the coated substrate. When used as a thermal barrier on a metallic substrate, the coating composition varies from an almost pure metal near the substrate to a pure ceramic adjacent to the outer surface exposed to a hot environment. Challenging issues in producing high quality FGCs in the presence of external disturbances with an atmospheric plasma spray process (APSP) include controlling the mean temperature, the mean axial velocity, and the positions of the constituent material particles when they arrive at the substrate to be coated. The unavoidable disturbances include fluctuations in the arc voltage and clogging of the powder in the delivery system. For a two-constituent coating, this work proposes using three modified robust model reference adaptive controllers based on the σ-modified laws and low frequency learning. One controller adjusts the current and flow rates of argon and hydrogen into the torch. The other two controllers adjust the distance of the two powder injector ports from the plasma jet axis and the average injection velocity of each powder. It is shown through numerical experiments that the three controllers implemented in an APSP consistently produce high-quality FGCs.

Thermal Cyclic Fatigue Behavior of Nanostructured YSZ/NiCrAlY Compositionally Graded Thermal Barrier Coatings

In this research, thermal cyclic fatigue (TCF) behavior of a compositionally graded layer (CGL) nanostructured coating was investigated and compared with the TCF behavior of a micro-CGL coating. The layers, as a thermal barrier coating (TBC), were made from nanostructured and/or conventional micro-sized yttria-stabilized zirconia (YSZ) and NiCrAlY powders. According to the composition of each layer, a certain ratio of YSZ/NiCrAlY powder was mixed and deposited by air plasma spray process on IN 738-LC as a substrate. In order to investigate the effect of temperature on the lifetime of coatings, TCF tests were conducted at two different high temperatures (900 and 1100 °C). These tests were done by holding the samples at the mentioned temperatures for 1 h and then fast cooling to 100 °C by compressed air for 10 min. Microstructural evaluation showed that there were some differences between the damage mechanisms of these two groups of compositionally graded TBCs. The results also showed that the average thermal cyclic lifetime of nanostructured compositionally graded TBCs is approximately 1.6 times higher than that of the micro-one. Moreover, it was indicated that the presence of much more spinel oxide regions formed at higher temperature has a remarkable effect on the lifetime of compositionally graded TBCs.

Analytical solution of quenching-induced deposition stress in functionally graded thermal barrier coatings

Abstract Deposition stress caused by quenching of splats in the deposition process of Functionally Graded Thermal Barrier Coatings (FG-TBCs) has a profound influence on its residual thermal stress. In this work, according to the progressive thermal spray process of FG-TBCs, a theoretical solution is developed for predicting deposition stress caused by quenching of splats in the deposition process of FG-TBCs. The verification of this theoretical solution has been done by reduceing to Tsui and Clyne's model. The deposition stress filed evolution and the total/final deposition stresses during the deposition process of FG-TBCs have been studied, and stress concentrations in FG-TBCs and conventional TBCs have been compared. Meanwhile, the effect of some geometric parameters, such as the thickness of Functionally Graded Coating (FGC) and the substrate, has also been discussed. The results show that, compared with the deposition stress arisen in conventional TBCs, the deposition stress concentration in the FG-TBCs has been significantly reduced; the magnitudes of deposition stresses are the same at the top surface of FGC; however, they decrease at the bottom with the FGC thickness increasing; the magnitudes of the deposition stresses in the FGC decrease with the decrease of substrate thickness. This work may provide further insights into the development and optimization of more efficient FG-TBCs for industrial applications.

A Penny-shaped Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Penny-shaped Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Normal Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Normal Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Penny-shaped Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Penny-shaped Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

Overview of Failure Modes of Pseudo Compositionally Graded Thermal Barrier Coatings and Remedies for Augmenting the Service Life of the Coated Components

Overview of Failure Modes of Pseudo Compositionally Graded Thermal Barrier Coatings and Remedies for Augmenting the Service Life of the Coated Components

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate under Transient Thermal Loading

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate under Transient Thermal Loading

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate under Transient Thermal Loading

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate under Transient Thermal Loading

Performance and Steady State Heat Transfer Analysis of Functionally Graded Thermal Barrier Coatings Systems

Thermal barrier coatings (TBC’s), typically 8 wt.% Yttria Stabilized Zirconia (8YSZ), in single layered configuration have been traditionally used in aerospace components to protect them from degradation at high temperatures and to improve the thermal efficiency of the system. This paper compares the performance of two types of TBC configurations: single layered and multilayered functionally graded materials (FGM). Aerospace alloy, Inconel 718 substrates, NiCrAlY bond coat (BC) and 8YPSZ top coat (TC) were the materials used. FGM configuration was used to improve the durability and life of the conventional TBC system by reducing the coefficient of thermal expansion (CTE) mismatch. The TBCs were subjected to thermal fatigue (thermal shock and thermal barrier test) in laboratory scale burner rig test and oxidation stability test in high temperature furnace upto 1000°C. The as-sprayed and thermal fatigue tested specimen were characterized by X-ray diffraction (XRD) analysis and Scanning Electron Microscope (micro-structure). Results are discussed in the light of suitability of coating configuration, thermal fatigue and spalling characteristics with reference to aerospace applications at temperatures in the 900°C to 1500°C range. Computational work was carried out comprising a simulation model involving the developed TBCs..

Ceramic Fibers Reinforced Functionally Graded Thermal Barrier Coatings

The long‐term durability is a considerable challenge for the use of thermal barrier coatings (TBCs). In order to solve this problem, introduction of yttria‐stabilized zirconia (YSZ) short fibers in functionally graded system is employed to strengthen the durability of TBCs. Four coatings are deposited on In738LC substrate by atmospheric plasma spray (APS). Thermal cycling behaviors and fiber toughening mechanisms of coatings are systematically studied. Result shows that the thermal cycling lifetime of graded TBCs with the addition of fibers can reach 406 ± 21 at 850 °C, which increases by 60% compared to that of typical APS YSZ TBCs. Moreover, the improvement of lifetime mainly attributes to the fiber breakage, the fiber‐matrix interface debonding, and the crack deflection.

A Normal Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Normal Crack in a Functionally Graded Thermal Barrier Coating Bonded to a Homogeneous Elastic Substrate under Transient Thermal Loading

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate Under Transient Thermal Loading

A Crack Parallel to the Interface Between a Functionally Graded Thermal Barrier Coating and a Homogeneous Elastic Substrate Under Transient Thermal Loading

Heat Transfer Process in Jet Turbine Blade with Functionally Graded Thermal Barrier Coating

In the jet engine the temperature of exhaust gases should be as high as possible, from the point of view of its efficiency. The value of this temperature is limited by toughness of the turbine blades material. Superalloy Inconel 625, which is commonly used in aerospace industry, indicates 13% less yield point in 800OC than in 25OC. The temperature of exhaust gases can reach 1500OC. The blade material has to be protected due to this fact. The one possibility of turbine blade protection is using of thermal barriers coatings (TBC). The coating has a very low thermal conductivity and therefore it protects against the thermal shock failure of the substrate material. The TBC can be manufactured as: 1) monocrystalline, 2) layered structures (e.g. [1-3]) or 3) as a functionally graded material (e.g. [4-7]). The differences between the properties of blade material and TBC can lead to significant increase of the high shear stresses in the substrate-TBC interface.In this paper numerical analyses of cooled turbine blade with various kinds of functionally graded thermal coatings were performed. The main aim was to find the optimal material properties distribution of the functionally graded TBC to avoid damage initiation and growth between TBC and substrate. In the calculations the effect of temperature on material properties both mechanical and thermal was taken into consideration.

Microstructure and oxidation behavior of conventional and pseudo graded NiCrAlY/YSZ thermal barrier coatings produced by supersonic air plasma spraying process

In this paper, the microstructure and oxidation behavior of conventional and pseudo graded NiCrAlY/yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) is reported. Both conventional and graded TBCs were produced by supersonic air plasma spraying process. In the pseudo graded TBCs, no interface between bond-coat, graded region and top-coat was observed. The ceramic (YSZ) top-coat in both conventional and graded TBCs was found to exhibit a single phase tetragonal-prime structure. Isothermal oxidation results confirmed the formation of thermally grown oxides (TGO) layer in both conventional and graded TBCs. In graded TBCs, the dispersed metallic phase was also found to be oxidized in the graded region. For oxidation time ≥50h, the TGO thickness in the conventional TBCs was found to be lower than that of the graded TBCs. Further, in graded TBCs, the vertical and parallel cracks were formed in the YSZ coating during oxidation for 5h. The cracks were enlarged during oxidation at 1200°C for 200h. After oxidation (at 1200°C for 200h), the parallel cracks in the graded TBC penetrated into the graded/layered region. The evolution of cracks in the graded TBCs led to a constant residual stress (0.62±0.2GPa) in the TGO. However, owing to the presence of few cracks in the conventional TBCs, the residual stress in the TGO increased with oxidation time. After oxidation at 1200°C for 200h, the conventional TBCs have higher residual stress (2.1±0.3GPa) in the TGO as compared to the graded TBCs.

Micromechanical Simulation of Thermal Cyclic Behavior of ZrO2/Ti Functionally Graded Thermal Barrier Coatings

This study numerically investigates cyclic thermal shock behavior of ZrO2/Ti functionally graded thermal barrier coatings (FG TBCs) based on a nonlinear mean-field micromechanical approach, which takes into account the time-independent and dependent inelastic deformation, such as plasticity of metals, creep of metals and ceramics, and diffusional mass flow at the ceramic/metal interface. The fabrication processes for the FG TBCs have been also considered in the simulation. The effect of creep and compositional gradation patterns on micro-stress states in the FG TBCs during thermal cycling has been examined in terms of the amplitudes, ratios, maximum and mean values of thermal stresses. The compositional gradation patterns highly affect thermal stress states in case of high creep rates of ZrO2. In comparison with experimental data, maximum thermal stresses, amplitudes and ratios of thermal stresses can be effective parameters for design of such FG TBCs subject to cyclic thermal shock loadings.

Compositionally Graded Thermal Barrier Coating by Hybrid Thermal Spraying Route and its Non-isothermal Oxidation Behavior

The present study concerns a detailed investigation of the characteristics and oxidation resistance property of a duplex and compositionally graded thermal barrier coating on Inconel 718. The duplex coating consists of a CoNiCrAlY bond coat layer sprayed on to sand-blasted Inconel 718 substrate (by high velocity oxy-fuel spraying) followed by deposition of a yttria-stabilized zirconia (YSZ) top coat by plasma spraying. The compositionally graded coating consists of several layers deposited by plasma spraying of pre-mixed CoNiCrAlY and YSZ powders in the weight ratios of 70:30, 50:50, 30:70, and 0:100 varying from the bond coat to the top surface, respectively. A detailed investigation of the microstructure, composition, and phases in the coating and its non-isothermal oxidation behavior from room temperature to 1250°C was performed. Oxidation proceeds by three stages in the as-received Inconel 718 and the compositionally graded coating, but by two stages in the duplex coating with a maximum activation energy for oxidation in the compositionally graded coating at high temperature (stage III). The kinetics and mechanism of oxidation were established.

Water-Quenching Thermal Shocking Property of Sm<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>/NiCoCrAlY Function Grade Thermal Barrier Coatings

In the present manuscript, the influence of interface location, coating structure (layer number) and layer-material composition on water-quenching thermal-shocking property of Sm2Zr2 O7/NiCoCrAlY functional graded thermal barrier coatings (TBCs) were investigated using finite element method. Results show that radial thermal stress at surface of top-layer have the highest value. Radial thermal stress in TBCs decrease gradually with increase of layer number, and function graded TBCs have better thermal-shocking resistance compared with other TBCs. 1357 function graded TBCs have the most excellent thermal shocking resistance in these investigated TBCs in the current manuscript.

Influence of Substrate Condictions on Thermal-Shocking Property of Sm<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>/NiCoCrAlY Graded Thermal Barrier Coatings

The effect of substrate conditions, including material type, thickness and radius on thermal shocking properties of plasma sprayed Sm2Zr2O7/NiCoCrAlY functional graded thermal barrier coatings were analyzed by ANSYS—a finite element method software. Results show that thermal stresse in coatings increase with increasing of thermal expansion coefficients of substrates. The effect of substrate thickness can be ignored when it is greater than 20mm. Thermal stresses increase with radius increasing, however, they are almost not effected when substrate radius is over 18mm.