Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

Evaluation of the phase stability,and mechanical and thermal properties of Ba(Sr1/3Ta2/3)O3 as a potential ceramic material for thermal barrier coatings

Ba(Sr_1/3Ta_2/3)O₃ (BST) ceramic was synthesized by a solid-state reaction method. The phase stability, microstructural evolution, and mechanical and thermal properties of the BST ceramic were investigated and characterized to evaluate the potential application of BST as a top coating material for thermal barrier coatings (TBCs). The results show that BST can maintain a stable hexagonal perovskite structure up to 1600 °C. Anisotropic growth of the grains above 1400 °C was observed. Its low elastic modulus and high fracture toughness suggest a high damage tolerance for the BST ceramic. In addition, the moderate coefficient of thermal expansion and superior heat insulation capability of the BST ceramic provide this ceramic the potential to serve as a top coating material of TBCs at higher temperature.     

Thermal cycling behavior and bonding strength of single-ceramic-layer Sm2Zr2O7 and double-ceramic-layer Sm2Zr2O7/8YSZ thermal barrier coatings deposited by atmospheric plasma spraying

The single-ceramic-layer (SCL) Sm₂Zr₂O₇ (SZO) and double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZO)/8YSZ thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying on nickel-based superalloy substrates with NiCoCrAlY as the bond coat. The mechanical properties of the coatings were evaluated using bonding strength and thermal cycling lifetime tests. The microstructures and phase compositions of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that both coatings demonstrate a well compact state. The DCL SZO/8YSZ TBCs exhibits an average bonding strength approximately 1.5 times higher when compared to the SCL SZO TBCs. The thermal cycling lifetime of DCL SZO/8YSZ TBCs is 660 cycles, which is much longer than that of SCL 8YSZ TBCs (150 cycles). After 660 thermal cycling, only a little spot spallation appears on the surface of the DCL SZO/8YSZ coating. The excellent mechanical properties of the DCL LZ/8YSZ TBCs can be attributed to the underlying 8YSZ coating with the combinational structures, which contributes to improve the toughness and relieve the thermal mismatch between the ceramic layer and the metallic bond coat at high temperature.     

Progress update on extending the durability of air plasma sprayed thermal barrier coatings

Air plasma sprayed thermal barrier coatings (TBCs) are widely used in gas turbines to provide thermal insulation for the metallic engine components. During service, the multi-layered and multi-material systems undergo thermal and mechanical degradation. The degradation mechanisms include sintering, phase transformation, residual stress, oxidation, erosion and CMAS attack. The degradation leads to the initiation and propagation of cracks at or near the interface between the topcoat and bond coat, eventually merging into large-scale delamination and resulting in failure of the TBCs. Recent progress in the development of methods for mitigating the detrimental impact of these failure mechanisms via composition and processing modifications has been reviewed. Meanwhile, the applications of newly-emerging materials with superior properties have also been discussed. The review emphasises the relationships between composition, microstructure and properties of TBCs, which is beneficial for the exploration of the advanced TBCs with higher durability.     

In situ characterization of high temperature elastic modulus and fracture toughness in air plasma sprayed thermal barrier coatings under bending by using digital image correlation

The evolutions of elastic modulus and fracture toughness are the key factors affecting the failure mechanism and durability of thermal barrier coatings (TBCs). Combined the high temperature three-point bending with the digital image correlation (DIC) method, the variations of high temperature elastic modulus and fracture toughness of air plasma sprayed TBCs with temperature are determined. The surface and interfacial cracking information can be monitored real-time by DIC system. The results show that when the temperature rises from 30 °C to 800 °C, the elastic modulus and fracture toughness of TC decrease from 20.3 GPa to 13.1 GPa and from 1.31 MPa m^1/2 to 1.16 MPa m^1/2, respectively. And the interfacial fracture toughness increases from 83.7 J/m² to 156.3 J/m². These results are consistent with the available values determined in literatures, which ensures the validity of this method.     

Correlations Between Microstructure and Mechanical Properties of Air Plasma‐Sprayed Thermal Barrier Coatings Exposed to a High Temperature

An indentation method is used to study the variations in Young's modulus, hardness and fracture toughness of air plasma‐sprayed thermal barrier coatings at a high temperature. The coatings were exposed to 1100°C during 1700 h. A sudden increase in Young's modulus for the first 600 h was observed, while the hardness increased after 800 h as a consequence of sintering. Conversely, there was a reduction of 25% in fracture toughness after 1700 h, evidencing the thermal barrier coating degradation. The evolution of these mechanical properties was correlated with microstructural changes. After 1700 h, the thermally grown oxide thickness reached 6.8 μm, the volumetric percentage of porosity was reduced from 6.8% to 4.7% and the amount of monoclinic phase increased to 23.4 wt%. These characteristics are closely related to the stress distribution in the top coat, which promotes cracks nucleation and propagation, compromising the coating durability.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Mechanical properties of lanthanum zirconate-based composite thermal barrier coatings

Lanthanum zirconate is a promising candidate material for thermal barrier coating (TBC) applications due to its low thermal conductivity and high temperature phase stability. However, its application is limited by thermal durability caused by low fracture toughness and low coefficient of thermal expansion. We recently developed LZ/8YSZ composite TBC systems using blended LZ and 8YSZ powders, which have demonstrated excellent thermal cycling performance. In this study, the mechanical properties of the composite TBCs were characterised using both nanoindentation and Vicker’s microhardness tests. The nanoindentation results show that both Young’s modulus and nanohardness increase with increasing 8YSZ content, suggesting the mechanical properties can be tailored by changing the volume ratio of 8YSZ. The ratios of Young’s modulus to nanohardness remain constant, ～18, irrespective to the coating’s composition. The microhardness results show the same dependence with 8YSZ content, which is confirmed by the analytic models based on composite theory.     

Improve durability of plasma-splayed thermal barrier coatings by decreasing sintering-induced stiffening in ceramic coatings

In this study, a newly-tailored plasma-sprayed (PS) yttria-stabilized zirconia (YSZ) ceramic coating towards enhanced strain tolerance and sintering resistance was developed to improve the durability of TBCs. The thermal shock life was found to be markedly prolonged by more than four times. Failure mechanisms and sintering behavior of the newly-structured and conventional TBCs were systematically investigated through microstructural and mechanical analyses. Conventional TBCs suffered a premature spallation due to rapid sintering-induced stiffening of the ceramic top coat. In contrast, the new coating exhibits an enhanced sintering resistance whereby preserving a good strain tolerance over time. Specifically, its elastic modulus after thermal exposure remains comparable to the as-sprayed states. The effect of ceramic top coat stiffness on cracking behavior of TBCs was clarified by a corresponding cohesive zone finite element modeling. This study provides a new option for improving TBCs durability and the results could benefit the increased integrity of TBCs.     

Multilayer,Multimaterial Thermal Barrier Coating Systems: Design,Synthesis, and Performance Assessment

Thermal barrier coatings (TBCs) are increasingly playing a vital role in enhancing efficiency and performance of gas turbine engines. As engine operating temperatures rise, yttria‐stabilized zirconia (YSZ), the currently principal TBC material, reaches its operational limits. Gadolinium Zirconate (GDZ)‐based pyrochlore oxides are now emerging contenders, not only due to their lower thermal conductivity, but also their ability to resist attack by silicate deposits. However, GDZ cannot be directly substituted for YSZ due to its incompatibility with the thermally grown alumina layer, therefore requiring to be a component of multilayer system. Although industry has already adopted these materials in various applications, a number of fundamental issues arise with respect to layered‐coating design, their properties and processing dependence. In this study several multilayer architectures, based on the YSZ–GDZ system, have been developed and tested for durability under furnace thermal cycling conditions. Coating designs considered optimization of microstructure and properties of individual layers based on their location within the top‐coat thickness to address competing interests of thermal conductivity, compliance, and resistance to silicates. A large variance in durability was observed in coatings made with different multilayer designs. The results and the associated failure mechanisms are rationalized through preliminary evaluation of elastic energies at the failure locations and corresponding energy release rates. The results point to new strategies in the design and manufacturing of optimal multilayer coatings.     

Substrate-constrained effect on the stiffening behavior of lamellar thermal barrier coatings

Thermal exposure would compromises the compliance and thermal insulating performance of thermal barrier coatings (TBCs). However, most publications were based on free-standing coatings in which the stress resulting from substrate is essentially different from TBCs on superalloy substrate. In this paper, the constrained effect of substrate on the ceramic top-coat of plasma sprayed lamellar TBCs was investigated. Results showed that the structural changes evolve from micro-scale to macro-scale during thermal exposure. In a relatively shorter thermal exposure stage, the inter-splat pores became narrowed, whereas the intra-splat cracks became widened. Consequently, the healing kinetics of inter-splat pores was much faster than that of the intra-splat cracks. In a relatively longer thermal exposure stage, some macroscale cracks appeared in coating surface owing to the gradually stiffening coatings. As a result, the microscale intra-splat cracks near the macroscale cracks were healed rapidly. In brief, the substrate constraint induced structural changes were stage sensitive.     

CMAS corrosion resistance in high temperature and rainwater environment of double-layer thermal barrier coatings odified by rare earth

In order to explore the difference of CMAS corrosion resistance in high temperature and rainwater environment of single-layer and double-layer thermal barrier coatings (TBCs), and further reveal the mechanism of CMAS corrosion resistance in above environment of double-layer TBCs modified by rare earth, two TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer ZrO₂–Y₂O₃ (YSZ) and double-layer La₂Zr₂O₇(LZ)/YSZ, respectively. Subsequently, CMAS corrosion resistance tests at 1200 °C and rainwater environment of two TBCs were carried out. Results demonstrate that after high temperature CMAS corrosion for the same time, due to phase transformation, the volume of YSZ ceramic coating in single-layer TBCs shrank and surface cracks formed, which would lead to coating failure. When LZ ceramic coating of double-layer TBCs reacted with CMAS, compact apatite phases and fluorite phases formed, the penetration of CMAS into ceramic coating was inhibited effectively. Raman analysis and calculation results show that both of the surface residual stress of ceramic coating in two TBCs were compressive stress, and the residual stress of ceramic coating in double-layer TBCs were smaller than that of single-layer TBCs. Atomic force microscopy of TBCs after CMAS corrosion show that surface of double-layer TBCs was more uniform and compact than that of single-layer TBCs. The electrochemical properties in simulated rainwater of two TBCs after high temperature CMAS corrosion showed that double-layer TBCs possessed higher free corrosion potential, lower corrosion current and higher polarization resistance than those of single-layer TBCs. Consequently, the presence of LZ ceramic coating effectively improved CMAS corrosion resistance in high temperature and rainwater environment of double-layer TBCs.     

Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing,Microstructure, and Thermal Aging

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd₂Zr₂O₇ which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.     

Industrial Sensor TBCs: Studies on Temperature Detection and Durability

This article describes recent developments of the thermal barrier sensor concept for non-destructive evaluation (NDE) of thermal barrier coatings (TBCs) and on-line condition monitoring in gas turbines. Increases in turbine entry temperature in pursuit of higher efficiency will make it necessary improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation it will also be necessary to devise techniques for on-line conditions monitoring and NDE. Thermal barrier sensor coatings, which consist of a ceramic doped with rare-earth activator to provide luminescence, may be a possible solution. The thermo-luminescent response of such materials has been shown to be suitable for surface and sub-surface temperature measurement and possibly for material phase determination. Herein we describe a number of steps in the development of the sensor coating technology. For the first time sensor coatings have been successfully produced using a production standard air plasma spray (APS) process. Microscopic analysis of the coatings showed them to be similar to standard TBCs and thermal cycle testing of the coatings to destruction showed them to exhibit durability similar to that of standard TBCs suggesting that the addition of rare earth dopants to produce sensor coatings does not change the material structure or the longevity of coatings. Calibration of the coatings using the lifeteime decay response mode showed them to have a dynamic range for temperature measurement extending to just under 1000°C. However, it should be noted that newer compositions have been shown to respond up to 1300°C. Finally, a study of surface temperatures and film cooling has been conducted in a research combustor using APS sensor coatings and some preliminary results are presented.     

A review on failure mechanism of thermal barrier coatings and strategies to extend their lifetime

Thermal barrier coating (TBC) system is an essential technology in many fields associated to high temperatures. The main function of these TBCs is to protect the metallic parts against high temperatures over 1000 °C. However, degradation occurs both in thermal and mechanical performances during service. Thus, understanding the underlying degradation and failure mechanisms of TBCs is significant to assess and further enhance the durability and reliability of TBCs. Regarding the durability of TBCs, this paper reviews different failures mechanisms of TBCs caused by residual stresses, phase transformations, sintering, hot corrosion attack and oxidation. Subsequently, some methods are summarized to alleviate the undesirable effects of the causes, so as to extend the lifetime of TBCs. Regarding the thermal barrier performance of TBCs, the neoteric advances to resist degradation in thermal conductivity of TBCs are reviewed. In addition, some new ceramic materials with superior intrinsic properties are introduced for ultra-high temperature applications. In brief, this review correlates the microstructure and properties of TBCs for finer interpretation and degradation-resistant design on their thermal and mechanical properties, which would benefit the advanced TBCs in future engineering applications.     

Cracking behavior and delamination mechanism of lamellar structured TBC with localized mixed oxides

Mixed oxide (MO) with localized growth feature and high growth rate remarkably affects the lifetime of thermal barrier coatings (TBCs), which indicates that clarifying the ceramic cracking mechanism induced by MO is critical for developing new coatings with high durability. Two kinds of TBC models involving spherical and layered mixed oxides are created to explore the influence of MO growth on the local stress state and crack evolution during thermal cycle. The growth of α-Al₂O₃ is also included in the model. The undulating interface between ceramic coat and bond coat is approximated using a cosine curve. Dynamic ceramic cracking is realized by a surface-based cohesive interaction. The ceramic delamination by simulation agrees with the experimental observation. The effects of MO coverage ratio and growth rate on the TBC failure are also discussed. The results show that the MO growth causes the local ceramic coat to bear the normal tensile stress. The failure mode of coating is turned from α-Al₂O₃ thickness control to MO growth control. Once the mixed oxide appears, local ceramic cracking is easy to occur. When multiple cracks connect, ceramic delamination happens. Suppressing MO formation or decreasing MO growth can evidently improve the coating durability. These results in this work can provide important theoretical guidance for the development of anti-cracking TBCs.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Grinding of hard and brittle ceramic coatings: Force analysis

In this work, a grinding force model has been proposed and a correlation between the analytical model and the experimental grinding forces obtained during finishing of plasma sprayed ceramic coatings has been investigated. Thermally sprayed ceramic coatings have low fracture toughness and develop micro brittle fracture under moderate mechanical loading. In this investigation, grinding of air plasma sprayed alumina, zirconia, and titania coatings were studied theoretically and experimentally. It was observed that the ground surface contained micro-cracks and debris of irregular fragmented chips owing to the micro-brittle fracture. A grinding force model is proposed to incorporate the fracture behavior of the ceramic coating. This was substantiated through experimental values showing low grinding forces.     

Process-Controlled Plasma-Sprayed Yttria-Stabilized Zirconia Coatings: New Insights from Ultrasmall-Angle X-ray Scattering

A multicomponent microstructure model is applied in ultrasmall-angle X-ray scattering studies of two groups of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs). One group was sprayed from a single powder feedstock using controlled processing conditions. The other group included three different feedstock morphologies (obtained from different manufacturing methods), each with a similar particle size distribution and sprayed under the same average controlled processing conditions. The microstructure is quantitatively related to the feedstock morphology and processing conditions. Relationships are explored among these microstructures and the coating properties (e.g., thermal conductivity, elastic modulus). The degree of microstructural anisotropy is demonstrated to be pore-size dependent, being more pronounced for larger pores, and more sensitive to feedstock morphology ( powder processing ) than to spray processing. The microstructure analysis indicates two broad distributions of interlamellar pores, which combined, account for 70%–80% of the pore volume. The total porosity is found to increase with decreasing particle temperature or velocity. For all coatings, a negative linear relationship exists between thermal conductivity and total porosity. Comparison of the new analysis is made with earlier small-angle neutron scattering results, and implications are considered for a more general application of this metrology in TBC microstructure design.     

Sintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation

This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 h/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions.