Advanced Thermal Barrier Coatings Research Articles

Condensation behavior of gaseous phase during transported in the near-substrate boundary layer of plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition (PS-PVD) has exhibited the potential ability to prepare columnar structures for advanced thermal barrier coatings (TBCs). The coating structure is nominally affected by operating parameters, but it is controlled by the type of deposition unit actually and essentially. In order to realize the columnar structure deposited by gaseous phase units, the transition behavior of gaseous phase units to clusters must be fundamentally understood. This work investigated the transport process of gaseous phase units in the PS-PVD near-substrate boundary layer along with the condensation behavior. The Monte Carlo method was used to examine the transport process and condensation behavior of gaseous phase units under different scale boundary layers. Simulation results show that it is easier to form more numerous larger clusters at the edge of the plasma jet than at the center. Based on the understanding of the changes in deposition unit caused by the condensation of gaseous phase in the near-substrate boundary layer of PS-PVD, an outlook towards TBCs with different structures is presented. And it is in good agreement with the experimental data.

Synthesis and characterization of ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) nanoparticles for advanced TBCs

Multicomponent rare-earth oxide-doped zirconia-based thermal barrier coatings (TBCs) are attractive alternatives for yttria-stabilized zirconia (YSZ) in advanced TBC applications. In this study, a zirconia-based nanopowder doped by multiple rare-earth oxides ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) was synthesized using sol-gel and calcination method. The product was then characterized by X-ray diffraction (XRD), simultaneous thermal analysis (STA), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and dilatometric test. According to STA results, the proper calcination temperature of (La1-xGdx)2Zr2O7 nanopowders could be considered > 800 °C. The results indicated that Gd2Zr2O7 exhibits a defect fluorite-type structure, and with the incorporation of larger La3+, it transformed into an ordered pyrochlore-type structure. The microstructure and morphology of the product were studied by field emission scanning electron microscopy (FESEM), revealing the formation of (La1-xGdx)2Zr2O7 nanoparticles with the mean size below 100 nm. Also, the thermal expansion coefficient of (La1-xGdx)2Zr2O7 nanoparticles was determined 8–12.5 × 10−6 K−1 at 1000 °C.

Sol-gel synthesis and characterization of ZrO2-25wt.%CeO2-2.5wt.%Y2O3 (CYSZ) nanoparticles

This study is intended to improve the high-temperature properties of micro-CYSZ: ZrO2-25 wt%CeO2-2.5 wt%Y2O3 in order to apply it in new generation of thermal barrier coatings (TBCs) through substituting micro-CYSZ with nano-CYSZ. Nano-CYSZ was synthesized by sol-gel method followed by calcination for 3 h at 1000 °C, and subsequently underwent a heat treatment at 1300 °C for 50 h. Results obtained from Fourier-transform infrared spectroscopy (FTIR) and thermogravimetry/differential scanning calorimetry (TGA/DSC) indicated the formation of Zr–Y–Ce bonds, and proper calcination at 1000 °C, respectively. The microstructure and morphology of the product was studied by field emission scanning electron microscopy (FESEM), revealing the formation of CYSZ nanoparticles with the mean size of 40 nm. X-ray diffraction (XRD) and Raman analysis also confirmed the formation of non-transformable (t') ZrO2 phase (ca2<1.01) as well as the stability of this phase after heat treatment. Results suggest nano-CYSZ as a promising choice for advanced TBCs in the new aero-engine and power generation applications.

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.

Synthesis and phase stability of zirconia-lanthania-ytterbia-yttria nanoparticles; a promising advanced TBC material

ABSTRACT Advanced thermal barrier coatings (TBCs) developed by incorporating multicomponent rare earth oxide dopants into zirconia are promising alternative to replace yttria-stabilized zirconia (YSZ) thermal barrier coatings. In this study, a zirconia-based nanopowder coating doped by multiple rare earth oxides (YLaYbZr: 5.86 Mol.%Y2O3-1.99 Mol.%La2O3-1.98 Mol.%Yb2O3-ZrO2) was synthesized using co-precipitation technique which is advantageous in terms of simplicity and cost-effectiveness. The product was then characterized using X-ray diffraction (XRD) method and field emission scanning electron microscopy (FESEM). The stability of YLaYbZr compound was studied after a heat treating the product at 1300°C for 50 h. The results indicated that the initially obtained powder was a metastable tetragonal (t-prime) zirconia. Rietveld refinement of the XRD data from YLaYbZr powder after 50 h of heat treatment at 1300°C confirmed stabilization of zirconia in the t-prime phase with around 15 wt.% monoclinic impurity. Furthermore, FESEM results (before and after heat treatment) indicated orderly particles of uniform shape and size with a small tendency toward agglomeration.

Plasma spray–physical vapor deposition toward advanced thermal barrier coatings: a review

Plasma spray–physical vapor deposition (PS–PVD) is a unique technology that enables highly tailorable functional films and coatings with various rare metal elements to be processed. This technology bridges the gap between conventional thermal spray and vapor deposition and provides a variety of coating microstructures composed of vapor, liquid, and solid deposition units. The PS–PVD technique serves a broad range of applications in the fields of thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), oxygen permeable films, and electrode films. It also represents the development direction of high-performance TBC/EBC preparation technologies. With the PS–PVD technique, the composition of the deposition unit determines the microstructure of the coating and its performance. When coating materials are injected into a nozzle and transported into the plasma jet, the deposition unit generated by a coating material is affected by the plasma jet characteristics. However, there is no direct in situ measurement method of material transfer and deposition processes in the PS–PVD plasma jet, because of the extreme conditions of PS–PVD such as a low operating pressure of ~ 100 Pa, temperatures of thousands of degrees, and a thin and high-velocity jet. Despite the difficulties, the transport and transformation behaviors of the deposition units were also researched by optical emission spectroscopy, observation of the coating microstructure and other methods. This paper reviews the progress of PS–PVD technologies considering the preparation of advanced thermal barrier coatings from the perspective of the transport and transformation behaviors of the deposition units. The development prospects of new high-performance TBCs using the PS–PVD technique are also discussed.

Dominant effects of 2D pores on mechanical behaviors of plasma sprayed ceramic coatings during thermal exposure

Realizing engineering application of new candidate ceramic materials is highly required for the development of advanced thermal barrier coatings (TBCs). In addition to the optimization of intrinsic properties of ceramic materials, a feasible way is to tailor the structure of coatings based on the understanding of structure-properties evolution during thermal exposure. Herein, the unique sintering behavior and the consequent effect on spallation of plasma sprayed coatings made of a candidate material, i.e., La2Zr2O7 (LZO), were investigated by experiments and simulation. Experimental results showed that significant changes occurred for the porosity, 2D pore density and hardness during thermal exposure. The 2D pores are the essential microstructural characteristics and are dominantly responsible for the changes of mechanical property. Simulation results suggested that the healing of 2D pores increase the driving force for crack extension. This is the main cause for the spallation of ceramic top coat during thermal cyclic test. Finally, some structural designs towards long lifetime of TBCs were discussed. The dominant effects of 2D pores on mechanical properties and failure of TBCs provide fundamental understanding to the structural tailoring of advanced TBCs for future applications.

Research Progress in Preparation and Development of Excellent Bond Coats for Advanced Thermal Barrier Coatings

Research Progress in Preparation and Development of Excellent Bond Coats for Advanced Thermal Barrier Coatings

Multicomponent Entropy-Stabilized Zirconates with Comprehensive Properties for Advanced Thermal Barrier Coating

Thermal barrier coating materials, commonly ceramics, are deposited on superalloy surfaces to improve their service temperature. However, the intrinsic brittleness and low thermal expansion coefficient of ceramic materials pose a great challenge to developing new materials to replace yttria-stabilized zirconia. Herein, entropy engineering is employed to fabricate multicomponent rare earth zirconates with a large thermal expansion coefficient, excellent mechanical properties and ultralow thermal conductivity, without sacrificing other properties. This work provides a new insight into developing novel ceramics with a combination of mechanical and thermophysical properties, which would be significant in material development for thermal barrier coatings, thermoelectric fields, etc.

Investigation of ternary rare earth oxide-doped YSZ and its high temperature stability

Due to the limited temperature capability of widely used yttria partially stabilized zirconia (YSZ), considerable effort has been directed towards developing promising thermal barrier coating materials for applications above 1200 °C. La2O3-Gd2O3-Yb2O3-Y2O3-doped zirconia (LGYYSZ), as a promising ceramic for advanced thermal barrier coatings, was designed and prepared. The LGYYSZ ceramics consist of tetragonal double-prime phase and cubic phase, and no new phase appears after long-term annealing. The thermal expansion coefficients of LGYYSZ are comparable to the value of YSZ. The values of the thermal conductivities of 0.5LGYYSZ and 1.0LGYYSZ are 1.34 W/(mK) and 1.21 W/(mK) at 1000 °C, respectively, which are approximately 30–40% lower than that of YSZ. Due to the rare earth ions with different radii/masses and the compound effect of multiple point dislocations, a significant improvement of the sintering resistance of LGYYSZ ceramics was observed above 1400 °C with La3+ dopant.

Increasing thermal and mechanical properties of thermal barrier coatings by suspension plasma spraying technology

Suspension plasma spraying technology is a promising method to produce advanced thermal barrier coatings using nanopowders. A liquid feeder system is needed to overcome the limitations of nanosized powders spraying – low interta of the particles. The process is capable of producing coatings with different types of structure – from very dense and compact to columnar with high volume of vertical intercolumnar voids. Consequently, the coating thermal and mechanical properties can be higher than sprayed using conventional technologies.

Stress states in plasma-sprayed thermal barrier coatings upon temperature cycling: Combined effects of creep, plastic deformation, and TGO growth

Abstract To ascertain material parameter effects on the stress states is beneficial to comprehend the crack growth behavior and delamination mechanism in thermal barrier coatings (TBCs). In this work, numerical models are established to explore the combined effects of material parameters including creep, plastic deformation, and thermally grown oxide (TGO) growth on the stress states upon temperature cycling. For all layers, thermal-physical properties reliant on temperature are incorporated into the model. The process of bond coat (BC) oxidation, namely TGO growth, is materialized by changing material properties with cycles. Based on the principle of a single variable, the residual stress states are explored using many different material combinations. The results indicate that the tensile stress in the ceramic top coat (TC) decreases with the increase in the TGO lateral strain distribution gradient. Increasing the BC yield strength or decreasing the TGO growth stress can reduce the tensile stress in TC if there is no creep in the model. When BC yield strength is relatively high (≥150 MPa), BC creep will strengthen the TC tensile stress. TGO creep can decrease the tensile stress in TC irrespective of TGO growth stress and BC creep. When TGO creep rate is higher than 10 B tgo , an exceedingly small tensile stress can always be achieved. This work could provide significant theory direction for material selection and composition control towards advanced TBCs with prolonged lifetime.

Stress states and crack behavior in plasma sprayed TBCs based on a novel lamellar structure model with real interface morphology

Abstract To ascertain the crack growth behavior and coalescence mechanism in thermal barrier coatings (TBCs) is beneficial for understanding the failure of TBCs and proposing the probable optimization methods. In this work, a novel lamellar structure model with real interface morphology is developed to explore the crack growth behavior and the failure mechanism of TBCs during thermal cycling. Three typical defects which include pore, inter-splat crack, and intra-splat are incorporated in the model. To simulate the oxidation process of the bond coat (BC) realistically, The oxidation growth process is simulated via changing the BC properties to thermally grown oxide (TGO) properties layer by layer. The effects of the lateral growth strain distribution through TGO thickness on the stress states are executed. Moreover, the influences of BC creep on the crack growth and coating lifetime are further elaborated. The results show that the larger the lateral growth strain gradient, the smaller the residual tensile stress. The irregular interface morphology results in the redistribution of residual stresses. Although the pores and cracks can alleviate the tensile stress near the valley, large stress concentration will occur near them. At the early phase of thermal cycling, the cracks grow steadily. After more cycles, the cracks propagate rapidly and merge with others. The simulated delamination path is in agreement with the experiment results. Not only does BC creep change the crack coalescence mechanism, it also decreases the thermal cyclic lifetime of TBCs. The coating optimization method proposed in this study provides another option for developing advanced TBCs with longer lifetime.

Durable TBCs with self-enhanced thermal insulation based on co-design on macro- and microstructure

During thermal exposure of thermal barrier coatings (TBCs), sintering causes degradation of the thermal barrier performance, as well as spallation. However, it is difficult to simultaneously enhance the thermal insulation and extend the lifetime, because the dominant factors (e.g., micropores and the macroscopic thickness) may have opposite effects on these two performances. In this study, a co-design of the macro- and microstructure is proposed to enhance multiple performance characteristics of TBCs. The microstructural design is achieved by using a co-spraying method, to form a hybrid layered coating. During thermal exposure, new pores are formed because the contractions of dense splats and loose porous nanoheaps are reversed. Consequently, the thermal barrier performance is self-enhanced by 40% with respect to that of mono-layered coatings. The macrostructural design is realized by tailoring the thickness according to the equivalent thermal insulation. As a result, the lifetime is extended by 35%. The mechanisms responsible for the simultaneously enhanced performance characteristics are discussed. This study is expected to contribute to structural tailoring of advanced TBCs with both high thermal insulation and long lifetimes.

Achieving self-enhanced thermal barrier performance through a novel hybrid-layered coating design

The thermal insulation and durability of thermal barrier coatings (TBCs) are mainly affected by sintering-induced healing of 2D micropores, which is inevitable under high temperature conditions. In this study, we designed and prepared novel hybrid-layered TBCs. During thermal exposure, the degree of degradation in thermal conductivity is observed to decrease from 80 to 100% for conventional coatings to ~20% for the novel coatings. For a detailed understanding, the evolution of the hybrid-layered TBCs can be divided into two stages: during stage I (0−10 h), ultrafast healing of 2D micropores occurs, mainly caused by the multiple contacts between the counter-surface. At this stage, the thermal and mechanical properties also increase sharply. During stage II (after 10 h), some new 2D mesopores are formed. Compared with the 2D micropores, the newly formed 2D mesopores have a much larger aspect ratio that increases the ratio of the effective area for thermal insulation from 10 to 30% to 60%, which accounts for the ~50% self-enhancement in the thermal barrier performance. This self-enhancing behavior is expected to prolong the lifetime and increase the performance of the TBCs, which is the main objective of using advanced TBCs in next-generation applications.

Superior cyclic life of thermal barrier coatings with advanced bond coats on single-crystal superalloys

Advanced thermal barrier coatings are essential to further increase the efficiency of gas turbine engines. One limiting factor of the TBC lifetime is the temperature dependent formation of the thermally grown oxide (TGO) during thermal exposure resulting in critical stress levels at the top coat-bond coat interface. Oxide dispersion strengthened (ODS) bond coats demonstrated slower oxygen scale growth during thermal exposure in comparison to standard bond coats.In this study for the first time TBC samples on single-crystal substrates (comparable to CMSX4) with thin ODS Co-based flash coats on the same Co-based bond coat (Amdry 995) and a porous atmospherically plasma sprayed (APS) yttria stabilized zirconia (YSZ) topcoat were manufactured by thermal spray techniques and evaluated with respect to their thermal cyclic behavior. As the major performance test cyclic burner rigs, which can establish thermal conditions similar to those in gas turbines, were applied.TBC samples with the new material combination show superior performance compared to previous samples. Cross sections of the samples were analyzed by scanning electron and laser scanning microscopy. Lifetime data and failure mode of the samples are discussed with respect to material properties such as thermal expansion coefficients, microstructural changes and TGO growth.

Phase stability of ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 compound at 1100 °C and 1300 °C for advanced TBC applications

Phase instability of conventional zirconia compounds (YSZ: Yttria Stabilized Zirconia) at temperatures above 1200 °C, is one of the important reasons of thermal barrier coatings (TBCs) destruction in new generation of turbines. In this study, ZGYbY: ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 powder, as a new material for TBC applications, was synthesized through a chemical co-precipitation method to enhance the phase stability of the YSZ at higher temperatures. A cyclic method with 5 h cycles at 1100 °C and 1300 °C for 50 h was employed to investigate the phase stability of YSZ and ZGYbY compounds. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis confirmed the phase stability of both of powders during thermal cycling at 1100 °C for 50 h. The results of thermal cycling at 1300 °C for 50 h indicated that ZGYbY powder exhibits excellent phase stability due to full retention of t-prime zirconia phase and restriction of tetragonal to monoclinic phase transition upon cooling. However, in the case of YSZ powder, after thermal cycling at similar condition, it decomposed to two new phases including cubic and monoclinic zirconia with 62 wt% and 38 wt%, respectively.

Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.

Thermo-Physical Properties Measurement of Advanced TBC Materials with Pyrochlore and Perovskite Structures

Thermal barrier coatings (TBCs) serve as thermal insulator in the hot region of an aircraft engine. Besides this, it also protects the underlying metal surface from the harsh corrosive and eroding environment. The associated lower thermal conductivity of TBC ceramic materials plays an important role in the improvement of thermal efficiency of the engine in term of increased combustion temperature and power. The thermal conductivity of the conventional yttria stabilized zirconia (YSZ) and three advanced ceramic materials with perovskite (CaZrO3) and pyrochlore structure (La0.75Nd0.25)2Zr2O7 &amp; Nd2Ce2O7) have been determined using differential scanning calorimetry (DSC). With thin metallic disk on the ceramic samples of different heights were heated / scanned using a standard DSC apparatus. The results were evaluated for the thermal conductivity measurement using well established procedure /calculations. The analyzed results were compared with that of other techniques given by other researchers and found to be in good agreement with an error of 10-15%. The result of coefficient of thermal expansion (CTE) that was measured using a dilatometer up to 1273°K has also given.

Sintering resistance of advanced plasma-sprayed thermal barrier coatings with strain-tolerant microstructures

Sintering is one of the key issues in the high temperature service of thermal barrier coatings (TBCs), considering the continuously increasing operation temperature of gas-turbine for higher energy efficiency. Based on the conventional processing method of air plasma spraying (APS), suspension plasma spraying (SPS) technique has been developed recently, in order to improve the strain tolerance of TBCs. This strain tolerance of plasma-sprayed TBCs is largely effected by the sintering behavior, which is presently not fully understood. In this work, evolution of mechanical properties, in terms of Young’s modulus and viscosity, is systematically investigated by in-situ three-point bending test at 1200 °C on free-standing coatings, including micro-cracked APS, segmented APS, vertically cracked SPS and columnar structured SPS TBCs and correlated to the microstructural evolution. Based on experimental results, power law relations are proposed for the sintering induced mechanical evolution, which deepen the understanding of the sintering behavior of plasma-sprayed TBCs.