coatings

Review Research Progress of Failure Mechanism of Thermal Barrier Coatings at High Temperature via Finite Element Method

Zhong-Chao Hu 1,2, Bin Liu 3,*, Liang Wang 2,*, Yu-Hang Cui 1, Yan-Wei Wang 1, Yu-Duo Ma 1, Wen-Wei Sun 1 and Yong Yang 1,* 1 School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China; [email protected] (Z.-C.H.); [email protected] (Y.-H.C.); [email protected] (Y.-W.W.); [email protected] (Y.-D.M.); [email protected] (W.-W.S.) 2 Integrated Computational Materials Research Centre, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China 3 Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin 150001, China * Correspondence: [email protected] (B.L.); [email protected] (L.W.); [email protected] (Y.Y.)



Received: 16 June 2020; Accepted: 20 July 2020; Published: 25 July 2020


Abstract: In the past decades, the durability of thermal barrier coatings (TBCs) has been extensively studied. The majority of researches emphasized the problem of oxidation, corrosion, and erosion induced by foreign object damage (FOD). TBCs with low thermal conductivity are usually coated on the hot-section components of the aircraft engine. The main composition of the TBCs is top-coat, which is usually regarded as a wear-resistant and heat-insulating layer, and it will significantly improve the working temperature of the hot-section components of the aircraft engine. The application of TBCs are serviced under a complex and rigid environment. The external parts of the TBCs are subjected to high-temperature and high-pressure loading, and the inner parts of the TBCs have a large thermal stress due to the different physical properties between the adjacent layers of the TBCs. To improve the heat efficiency of the hot-section components of aircraft engines, the working temperature of the TBCs should be improved further, which will result in the failure mechanism becoming more and more complicated for TBCs; thus, the current study is focusing on reviewing the failure mechanism of the TBCs when they are serviced under the actual high temperature conditions. Finite element simulation is an important method to study the failure mechanism of the TBCs, especially under some extremely rigid environments, which the experimental method cannot realize. In this paper, the research progress of the failure mechanism of TBCs at high temperature via finite element modeling is systematically reviewed.

Keywords: thermal barrier coatings (TBCs); finite element method; thermal-mechanical; TGO (thermally growth oxide); failure mechanism

1. Introduction Since the application of gas turbines in related industrial fields in the 20th century, the research of gas turbines has performed a vital function in promoting the development of national energy. The efficiency of gas turbines has been affected by gas temperature and compressor compression ratio [1]. With the demand for combustion efficiency and thrust weight ratio of gas turbines becoming stricter and stricter, hot-section components have undergone development from alloy to single-crystal nickel-based superalloy materials under the continuous exploration of researchers since the 1940s. The working temperature was also raised from 760 to 1150 ◦C. Nevertheless, they are still unable to

Coatings 2020, 10, 732; doi:10.3390/coatings10080732 www.mdpi.com/journal/coatings Coatings 2020, 10, 732 2 of 25 meet the performance requirements of high temperature resistance, high strength, corrosion resistance, and so on. It needs coating protection to avoid damage of the substrate. With the increase in working temperature, it is more and more difficult to further raise the working temperature of gas turbine material. Thermal barrier coatings (TBCs) and high-pressure air cooling are required to reduce the working temperature of the aircraft (Figure1). However, the assembly of cooling equipment not only increases the load of the aircraft, but also brings potential safety hazards, which limits the flight distance. Therefore, plasma-sprayed TBCs have become one of the crucial means to reduce the working temperature of the hot-section component of aero-engines and gas turbines. TBC technology has become a core technology to provide cooling to aircraft engines and turbine blades. The existence of TBCs enables working parts to withstand higher temperatures and protects them from wear, oxidation, and erosion. The application of advanced TBCs on the hot-section components of gas turbines can ensure that the substrate works at high temperatures, which will significantly improve the thermal efficiency of the aircraft [2,3]. With the development of TBC technology and its wide recognition, it has attracted the attention of quite a few scholars. Carter et al. [4] illustrated the common failure mechanisms found in gas turbine blades, many factors that shorten the lifetime of gas turbine were considered, such as mechanical damage, high temperature damage, high temperature exposure, creep failures, etc. Clarke et al. [5] emphasized that the improvement of thrust-weight ratio and reliability of gas turbines is closely related to the development of thermal barrier coating technology. There is an acute need for high-temperature protective coatings in the fields of aerospace, energy, and the nuclear industry. Many scholars have carried out various research works on TBCs [2,6–14]. It has been found that the failure caused by TBCs is unavoidable under the conditions of high temperature and long-term usage. During thermal cycling, the failure of atmospheric plasma-sprayed thermal barrier coatings (APS-TBCs) usually occurs in the surface layer near the interface. This is due to the thermal mechanical stress caused by the mismatch of the thermal expansion coefficient between the adjacent layers in the TBCs during thermal cycling. The existing method of APS leads to crack propagation at the interface. With the release of energy in the process of grain growth, transverse or longitudinal cracks are formed. The uneven distribution of cracks will lead to the delamination of coatings during cooling, the coatings on the surface of hot-section component will peel off, thus affecting the service life of gas turbines [15]. Reliability and durability are two key factors determining the service lifetime of TBCs [14]. Reliability requires that the TBCs have strong bonding strength, high thermal insulation, low residual stress, and outstanding high-temperature oxidation resistance. The durability of TBCs requires that the TBCs have a long service lifetime under actual working conditions [13,14]. Distribution of temperature field and residual stress are two important aspects of TBCs under actual application conditions. Effective thermal conductivity or thermal insulation effect is usually calculated by the distribution of temperature field across the whole coatings. The temperature distribution of TBCs can be calculated via finite element method (FEM). Residual stress is also essential to TBCs, and it will affect the failure modes and service lifetime of TBCs. In fact, residual stress will occur in the TBCs during the manufacturing process. Additionally, under the conditions of thermal shock and high-temperature oxidation, residual stress can be also induced, which makes the coating peel off directly. It is a common phenomenon that the coating peeling restricts the wide application of TBCs. TBCs usually operate under alternate cooling and heating conditions. Therefore, improving thermal shock resistance is a direct and effective way to prolong the lifetime of TBCs. With the renewal of the engine, the higher thrust weight ratio leads the TBCs to the direction of ultra-high temperature, low thermal conductivity, and long lifetime. Correspondingly, the research on the structure system and preparation technology of ultra-high temperature TBCs, as well as the characterization of advanced TBCs in a complex working environment need to be further explored [16]. However, the preparation of TBCs with excellent performance via experimental method is a complex process, and there are many unknown factors, which need further exploration. The finite element simulation is helpful to optimize the preparation process and the coating structure, save research time and cost, and make the research work more efficient. Finite element simulation can help us to find the optimal process and the coating structure under specific Coatings 2020, 10, 732 3 of 25 target conditions [17]. Before designing excellent TBCs, the layered structure, micro-structure, and Coatings 2020, 10, x FOR PEER REVIEW 3 of 25 relatedCoatings manufacturing 2020, 10, x FOR PEER technology REVIEW of various TBCs should be understood. The moving process of3 theof 25 spray gun in plasma-sprayed TBCs is denoted in Figure2. The following is the research status of the processprocess of ofthe the spray spray gun gun in inplasma-sprayed plasma-sprayed TBCs TBCs is denotedis denoted in inFigure Figure 2. 2.The The following following is theis the research research dominating factors affecting the failure modes and life evaluation of TBCs. statusstatus of ofthe the dominating dominating factors factors affecting affecting the the failure failure modes modes and and life life evaluation evaluation of ofTBCs. TBCs.

FigureFigure 1. Hot-section1. Hot-section component component model model of of thermal thermal barrier barrier coatings. coatings. ( a )( aThe ) The interior interior appointments appointments of of thethe turbine; turbine; (b )( b Yttria) Yttria stabilized stabilized zirconia zirconia (YSZ) (YSZ) thermal thermal barrier barrier coatings; coatings; (c )( thermalc) thermal barrier barrier coatings coatings (TBC)(TBC) coated coated on on the the turbineturbine turbine blade.blad blade. Adaptede. Adapted with with permission permission fromfrom from [[13];13 [13];]; Copyright Copyright 2016 2016 Elsevier. Elsevier.

FigureFigure 2. (2.a )( aThe ) The microstructure microstructure of ofthe the as-sprayed as-sprayed coating coating ( b )( bRaster ) Raster pattern pattern in inthe the x– yx –planey plane for for one one cyclecycle of of gun gun movement movement [[18].18 [18].].

2. Structure and Preparation Technology of TBCs 2. 2.Structure Structure and and Preparation Preparation Technology Technology of of TBCs TBCs 2.1. Structure Model of TBCs 2.1.2.1. Structure Structure Model Model of ofTBCs TBCs TBCs have been widely used in many industrial fields related to energy and power because TBCsTBCs have have been been widely widely used used in inmany many industrial industrial fields fields related related to toenergy energy and and power power because because of of of their excellent high-temperature resistance, high thermal insulation, and corrosion resistance. theirtheir excellent excellent high-temperature high-temperature resistance, resistance, high high thermal thermal insulation, insulation, and and corrosion corrosion resistance. resistance. TBCs TBCs TBCs are an important inorganic coating, which consists of the ceramic oxide layer and bond-coat. areare an an important important inorganic inorganic coating, coating, which which consists consists of ofthe the ceramic ceramic oxide oxide layer layer and and bond-coat. bond-coat. While While While spraying on the base of superalloy material, the influence of high temperature on the hot-section sprayingspraying on on the the base base of ofsuperall superalloyoy material, material, the the influence influence of ofhigh high temperature temperature on on the the hot-section hot-section components has been reduced, thus the service temperature is reduced and the working efficiency of componentscomponents has has been been reduced, reduced, thus thus the the service service temp temperatureerature is isreduced reduced and and the the working working efficiency efficiency of of gas turbine has been improved [19]. The common TBCs can be divided into three types: double-layer gasgas turbine turbine has has been been improved improved [19]. [19]. The The common common TBCs TBCs can can be be divided divided into into three three types: types: double-layer double-layer structure coatings, multi-layer structure coatings, and functional gradient coatings, as shown in structurestructure coatings, coatings, multi-layer multi-layer structure structure coatings, coatings, and and functional functional gradient gradient coatings, coatings, as asshown shown in in Figure3. FigureFigure 3. 3. Coatings 2020, 10, 732 4 of 25 Coatings 2020, 10, x FOR PEER REVIEW 4 of 25

Figure 3. Main structural systems of thermalthermal barrierbarrier coatings.coatings. (a) TypicalTypical double-layerdouble-layer thermalthermal barrierbarrier coating; (b) multi-layermulti-layer thermal barrier coating; (c) Electron beam-physical vapor deposition (EB-PVD) gradient thermal barrier coating.

2.1.1. Double-Layer Structure Coating 2.1.1. Double-Layer Structure Coating As for the classical double-layer structure coating, TBCs are composed of a ceramic layer and As for the classical double-layer structure coating, TBCs are composed of a ceramic layer and bond-coat. The thickness of the ceramic layer is generally less than 300 µm, and the thickness of the bond-coat. The thickness of the ceramic layer is generally less than 300 μm, and the thickness of the bond-coat is generally less than 150 µm. The ceramic surface made of oxide ceramics has the functions bond-coat is generally less than 150 μm. The ceramic surface made of oxide ceramics has the functions of heat insulation and corrosion resistance. In addition, the bond-coat consisting of metal alloys can of heat insulation and corrosion resistance. In addition, the bond-coat consisting of metal alloys can enhance the bonding strength between the substrate and the ceramic surface and weaken the mismatch enhance the bonding strength between the substrate and the ceramic surface and weaken the of thermal expansion between the two layers, and at the same time enhance the high-temperature mismatch of thermal expansion between the two layers, and at the same time enhance the high- oxidation resistance of the substrate [20]. temperature oxidation resistance of the substrate [20]. 2.1.2. Multi-Layer Structure Coating Model 2.1.2. Multi-Layer Structure Coating Model Multi-layer structure coatings are composed of a ceramic layer, bond-coat, and multi-layer Multi-layer structure coatings are composed of a ceramic layer, bond-coat, and multi-layer immediate layer with different functions, in which the immediate layer can include a self-healing layer immediate layer with different functions, in which the immediate layer can include a self-healing and/or an insulation layer. layer and/or an insulation layer. Double-layer TBCs are a widely used coating structure model because of their high thermal Double-layer TBCs are a widely used coating structure model because of their high thermal insulation, high interfacial bonding strength, and relatively low preparation cost. However, the thermal insulation, high interfacial bonding strength, and relatively low preparation cost. However, the and physical properties of the ceramic layer and the bond-coat are quite different. During cooling from thermal and physical properties of the ceramic layer and the bond-coat are quite different. During the high temperature, residual stress is easy to induce in the coating, and cracks occur easily at the cooling from the high temperature, residual stress is easy to induce in the coating, and cracks occur interface, which eventually contributes to the ceramic layer falling off and the service lifetime would not easily at the interface, which eventually contributes to the ceramic layer falling off and the service reach the expected value. Moreover, the service process of TBCs at high temperature and the complex lifetime would not reach the expected value. Moreover, the service process of TBCs at high environment of high-pressure results in many unfavorable factors which can accelerate the failure of temperature and the complex environment of high-pressure results in many unfavorable factors the TBCs, such as thermal stress, mechanical stress, chemical reaction of coatings at high temperature, which can accelerate the failure of the TBCs, such as thermal stress, mechanical stress, chemical and thermal corrosion. In order to prolong the service lifetime of coatings and reduce the failure of the reaction of coatings at high temperature, and thermal corrosion. In order to prolong the service coating under various factors, the concept of multi-layer TBCs has been proposed. Compared with lifetime of coatings and reduce the failure of the coating under various factors, the concept of multi- double-layer TBCs, multi-layer TBCs add several layers with different functions between the ceramic layer TBCs has been proposed. Compared with double-layer TBCs, multi-layer TBCs add several layer and the bond-coat. The interlayer coating can be a sealing layer, a heat insulation layer, a thermal layers with different functions between the ceramic layer and the bond-coat. The interlayer coating stress control layer, or a non-diffusion layer, etc [21]. Different multi-layer structure coatings have can be a sealing layer, a heat insulation layer, a thermal stress control layer, or a non-diffusion layer, different bond-coats, and their properties are also different [22]. For example, the blocking layer that etc [21]. Different multi-layer structure coatings have different bond-coats, and their properties are has been fabricated by Al2O3 can reduce the diffusion of oxygen atoms, prevent oxidation reaction in also different [22]. For example, the blocking layer that has been fabricated by Al2O3 can reduce the the coating, and slow down the oxidation failure of the bond-coat. diffusion of oxygen atoms, prevent oxidation reaction in the coating, and slow down the oxidation failure of the bond-coat. Coatings 2020, 10, 732 5 of 25

2.1.3. Functional Graded Thermal Barrier Coatings Model The structural characteristics of functional graded coatings are as follows: the composition of bond-coat and ceramic material between the coatings from metallic substrate to ceramic surfaces exhibit continuous gradient changes [23]. In view of the premature failure of ceramic coatings, functional gradient coatings were proposed by Japanese scholars in the 1990s. The structure and mechanical properties of functionally graded coatings are continuous, with the composition of the coatings due to the gradient change between the ceramic layer and the metallic substrate. This continuous structure can effectively alleviate the thermal stress between the ceramic layer and the metallic substrate, and effectively improve the thermal shock resistance of the coating [24]. Functionally graded TBCs have excellent thermal shock resistance compared with double-layer TBCs, but the preparation is difficult and the repeatability limits the practical application of functionally graded TBCs. Besides the above three coatings, there are also coatings structure models such as thick coatings and double ceramic TBCs. Thick coatings have a large thickness and good thermal insulation, which can be used to improve the combustion temperature of fuel, such as double ceramic TBCs. In addition, they play an important role in improving fuel combustion temperature and reducing exhaust emissions. Nevertheless, the increase in thickness also leads to the increase in temperature gradient and thermal stress in the coating. The increase in thermal stress and the temperature gradient will cause failure of the coating far away from the substrate interface before the interface cracking. This design of a double ceramic layer structure combines the advantages of two different ceramic materials and complements each other’s shortcomings. The structure of double ceramic coatings is to add an intermediate layer with a small difference in thermal expansion coefficient between the zircon coating and the transition layer. Wang et al. [14] discovered that the thermal shock resistance of La2Zr2O7/YSZ double ceramic coatings has significantly improved compared to that of single La2Zr2O7 coatings. La2Zr2O7/8YSZ double ceramic coatings were fabricated by Xu via Electron Beam-Physical Vapor Deposition (EB-PVD) [25]. Besides, the thermal cycle test at 1373 K in an air furnace indicates that the double ceramic layer TBCs have a higher thermal cycle lifetime than that of single ceramic layer TBCs (LZ and YSZ). The research shows that this design can greatly prolong the thermal cycle lifetime of coatings and significantly increase the service temperature of coatings. It will be one of the effective ways to flourish ultra-high temperature TBCs in the future [26].

2.2. Preparation Technology of Thermal Barrier Coatings The preparation methods of TBCs include electron beam-physical vapor deposition, (EB-PVD) [27], plasma spray physical vapor deposition (PS-PVD) [28], and atmospheric plasma spray (APS) [29], as shown in Table1. Coatings 2020, 10, x FOR PEER REVIEW 6 of 25 CoatingsCoatings 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 6 6of of 25 25 CoatingsCoatingsCoatings 2020 2020, 10,, 1010, x, , xFOR 732 FOR PEER PEER REVIEW REVIEW 6 6of of 25 25 6 of 25 Coatings 2020, 10, x FOR PEER REVIEW 6 of 25 Table 1. Comparison of EV-PVD, PS-PVD, APS. TableTable 1. 1. Comparison Comparison of of EV-PVD, EV-PVD, PS-PVD, PS-PVD, APS. APS. YSZ Microstructure Failure MechanismTableTableTable 1. 1. Comparison Comparison 1. Comparison of of EV-PVD, EV-PVD, Finite of EV-PVD, Element PS-PVD, PS-PVD, PS-PVD,Model APS. APS. APS. Experiment Model Reference YSZYSZ Microstructure Microstructure Failure Failure Mechanism Mechanism Table 1. Comparison Finite Finite of EV-PVD, Element Element PS-PVD, Model Model APS. Experiment Experiment Model Model Reference Reference YSZYSZYSZ Microstructure Microstructure Microstructure Failure Failure Mechanism Mechanism Finite Finite Element Element Model Model Experiment Experiment Experiment Model Model Model Reference Reference Reference YSZ Microstructure Failure Mechanism Finite Element Model Experiment Model Reference Typical characteristic with ◆ In the process of thermal shock TypicalTypical characteristic characteristic with with ◆◆ In In the the process process_ In the of of processthermal thermal ofshock shock columnar grain, the ◆cooling, cracks are easy to form from EB-PVB TypicalcolumnarTypical characteristic characteristic grain, the with with cooling,◆ In In the the cracks process processthermal are of easyof shockthermal thermal to cooling,form shock shock from [3,8,27–33] columnar Typicalgrain,Typical the characteristic characteristiccooling, with with cracks◆ In arethe processeasy to ofform thermal from shock [3,8,27–33] EB-PVBEB-PVB columnaradjacent columnar columnar grain, grain, the the grains cooling,porescooling, and cracks cracks propagatecracks are are areeasy easy along easy to to form totheform form interface,from from [3,8,27–33] EB-PVBEB-PVB adjacent adjacent columnar columnarcolumnarcolumnar grains grains grain, grain, thepores thepores and and propagate cooling,propagate cracks along along are the easythe interface, interface, to form from [3,8,27–33] [3,8,27–33] EB-PVBare leanedEB-PVB with each other leading to prematurefrom pores failure and propagate of TBCs. [3,8,27–33] [3,8,27–33] adjacentareadjacent leaned columnar columnar withadjacentadjacent each grains columnargrains other columnar poresleadingpores grains grains and and to propagatepremature porespropagate and propagatealong failure along the theof along interface,TBCs. interface, the interface, are leaned with each other leading to alongpremature the interface, failure of TBCs. areare leaned leaned withare withare leaned each eachleaned other withother with each leadingeachleading other other to to premature leadingpremature to prematurefailure failure of of failureTBCs. TBCs. of TBCs. leading to premature failure of TBCs.

★ The gap andF The pore gap are and the poremain are defects ★ The gap and pore are the main defects which★ The exist gap ★the inand The the main pore gap columns. defects andare porethe With which main are the defects main defects Typical characteristic with ★which★ The The gapexist gap and exist inand the pore pore in columns. the are are columns. the the Withmain main With thedefects defects PS-PVD Typical characteristic with immersionwhich exist ofwhich in high-speed the exist columns. in the corrosives, columns. With the Withthe the [3–8,28,30–33] feather-likeTypical characteristic columnarTypical characteristic with whichwhich with exist exist in in the the columns. columns. With With the the [3–8,28,30–33] PS-PVDPS-PVD Typical TypicalPS-PVD characteristic characteristicTypical characteristic with with immersionimmersion with ofthe of high-speed high-speed immersion corrosives, ofcorrosives, the the [3–8,28,30–33] [3–8,28,30–33] PS-PVDfeather-like columnar coating willimmersion be corroded of high-speed quickly, corrosives, the [3–8,28,30–33] [3–8,28,30–33][3 –8,28,30–33] PS-PVDPS-PVD feather-likefeather-like columnarfeather-like columnar columnarimmersioncoatingimmersion will ofhigh-speed beof high-speed high-speedcorroded corrosives, quickly, corrosives, corrosives, the the the feather-likefeather-like columnar columnar resultingcoating willincoating the be failure corroded will beof corrodedthe quickly, coating. quickly, coatingcoating will will coatingbe be corroded corroded will bequickly, quickly, corroded resultingresulting in in resultingthe the failure failure in theof of thefailure the coating. coating. of the coating. resultingresulting in in quickly,the the failure failure resulting of of the the coating. in coating. the failure of the coating.

N A continuous crack ▲ A continuous▲network A continuous crack can network form crack cannetwork can Exhibit lamellarExhibitExhibit structure lamellar lamellar structure▲ structure▲ A A continuous continuous crack crack network network can can Exhibit lamellar structure ▲form▲ accordingformaccording toaccording the toconnectivity the to the connectivity of of characteristic.Exhibit lamellarcharacteristic. characteristic.Micro-pores structure Micro-poresform A A accordingcontinuous continuous to crack thecrack connectivity network network can can of Exhibitcharacteristic.Exhibit lamellar lamellar Micro-pores structure structure inter-splatform according crackinter-splatconnectivity andto the intra-splatcrack connectivity of and inter-splat intra-splat crack, of crack, APS andcharacteristic. micro-cracksAPSMicro-pores and Micro-pores aremicro-cracks and forminter-splatform are according according crack to andto the the intra-splat connectivity connectivity crack, of of [8,13,16,34–38][8,13,16,34–38] APScharacteristic.characteristic. Micro-pores Micro-pores whichinter-splat will resultwhich crackcrack andinwilland the result intra-splatintra-splat crack in thegrowth crack crack, growth [8,13,16,34–38][8,13,16,34–38] APSAPS andand micro-cracks micro-cracksmicro-cracksdistributed are are areat randominter-splatinter-splat in the crack crack and and intra-splat intra-splat crack, crack, [8,13,16,34–38] distributed at random in the which will resultcrack, in which the crack will resultgrowth in [8,13,16,34–38] [8,13,16,34–38] APSAPS anddistributedand micro-cracks micro-cracks distributedat random are are in at the random alongwhich in the will lamellaalong result the interface in lamella the crack orinterface through growth or through ceramicdistributed atceramic random in thewhichalongwhich thewill will lamella resultthe result crack ininterface in the the growth crack crack or alonggrowththrough growth distributedceramicdistributed theat at random ceramicrandom in in the the lamella along theinterface. lamella interface. interface or through ceramic alonglamellaalong the theinterface. lamella lamellathe lamella interface interface interface or or through through or ceramicceramic lamella interface. lamellalamella interface. interface.through lamella interface.

Coatings 2020, 10, 732 7 of 25

2.2.1. EB-PVD EB-PVD is a commonly used technology for preparing TBCs. Its working principle can be depicted as follows: after the vacuum chamber is vacuumed to a certain degree, the electron gun will shoot an electron beam, bombard the evaporated materials in the water-cooled crucible, and the evaporated materials will condense to the preheated substrate surface in the form of atoms or molecules, forming a columnar structure of coating on the substrate. The pores between the columns and the interstices in the column provide favorable conditions for the transfer of heat and oxygen, resulting in a decrease in heat insulation performance and the easy oxidation of the coating structure, which is conducive to alleviating the thermal stress caused by the difference in thermal expansion coefficient of the coating, and improve the anti-thermal fatigue and thermal shock resistance of the materials [16,38,39]. However, it cannot be ignored that due to the existence of a large number of pores perpendicular to the coating surface and the intragranular gap between the adjacent columnar grains, it will not be conducive to the thermal insulation of the coating. In the process of thermal shock, cracks can easily form pores and propagate along the interface, leading to premature failure of TBCs. Evans et al. [40] investigated the composition of the coatings and their thermal properties using EB-PVB. Van sluytman et al. [39] prepared ZrO2–Y2O3–Ta2O5 TBCs. It was found that the coating sintered and contracted under the constant oxidation temperature of 1450 ◦C, resulting in the generation of residual stress in the TBCs, leading to premature peeling failure of the coating. Therefore, as for the TBCs prepared via EB-PVB, the separation of different layers can lead to the final failure.

2.2.2. PS-PVD Plasma spraying-physical vapor deposition (PS-PVD) is a new multi-functional film and coating preparation technology based on low-pressure plasma spraying. This technology combines the technical advantages of APS and EB-PVD and can be used for vapor deposition in the form of spraying. PS-PVD uses a high-power plasma spray gun (180 kW) to work under ultra-low pressure (150 Pa). Under this condition, the plasma jet of PS-PVD expands rapidly, making its length up to 2000 mm and diameter up to 400 mm. It can realize gas-liquid-solid multiphase coating deposition and obtain non-line-of-sight deposition [28]. Rare earth doped zirconia (ZrO2: 1.7Y2O3–1Gd2O3–1Yb2O3 at.%) TBCs were fabricated by Deng [28] via PS-PVD. There was a large quantity of pseudo-columnar crystal structures in the coatings. The pseudo-columnar crystal size in PS-PVD is obviously large and resistant to fracture. However, the gaps and pores are the main defects which exist in the columns. With the immersion of high-speed corrosives, the coating will be corroded quickly, resulting in the failure of the coating.

2.2.3. APS Atmospheric plasma spraying (APS) [22,41] can produce the plasma arc which is driven by direct current as a heat source. Actually, high-energy is the typical characteristic. Additionally, powder is suspended in a carrier gas (inert protective gas). The coating is formed by impinging powder particles on the surface of the substrate during spraying. The microstructure of the coating is related to the properties of the spraying material, spraying parameters, and surface temperature of the substrate. In order to prevent a turbine blade from being corroded, plasma spraying technology was applied to prepare 8YSZ coating by Mack [42]. The formation and growth of cracks under thermal cycling were elucidated by combining scanning electron microscopy (SEM) with the X-ray diffraction (XRD) technique, and the corrosion velocity was observed by calcium-magnesium-aluminum-silicate (CMAS) deposition. It was found that in a short period of time, there was an obvious CMAS attack on grain boundaries, but the interaction between YSZ and invaded CMAS was without direct influence on the pore structure and inner surface density. The densification of YSZ microstructures was observed only at the later stage if a marvelous amount of CMAS was immersed. Wang et al. [13,14] prepared 8YSZ TBCs by APS. The effects of horizontal and vertical cracks on the stress around the TGO (Thermally Coatings 2020, 10, 732 8 of 25

Growth Oxide) layer during thermal cycling were studied. Dong et al. [43] fabricated YSZ coatings by APS, finding that TGO with different thickness was prepared by controlling the isothermal oxidation time of the bond-coat. Thermal cycling tests were carried out for different thicknesses of TGO at 1150 ◦C, the duration of each cycle was 240 s. The results indicated that the YSZ coating has strong adhesive strength, and excellent thermal shock resistance. However, the influence of failure is related to TBC structure, the typical microstructure feature of APS-TBCs depends on its typical lamellar structure, the low bonding strength is due to lots of void defects in the APS-TBCs. Generally, the pores, the inter-splat cracks, and intra-splat cracks existing in YSZ coating would affect the quality of APS-TBCs. Besides, the pores and inter-splat cracks distribution affect the interface bonding rates, which are attributed to the process of APS. In APS-TBCs, a continuous crack network can be formed according to the connectivity of interlaminar and intralaminar cracks, which will result in crack initiation and propagation along the lamellar or through-lamellar direction. A host of research results have showed that the cracks exist in the top-coat layer or near the TC (top-coat)/TGO interfaces in APS-TBCs. During the thermal shock, these cracks can propagate along the stratiform interface or through lamella under complex stress states [44–46]. The spallation of coating frequently take place owing to the crack initiation along the layer interface, which is accountable for the failure of APS-TBCs. As the spread of these cracks is random and irregular, it is very urgent to determine the crack propagation behavior and coalescence mechanism [36].

3. The Factors Affecting the Service Lifetime of TBCs

3.1. TGO Hundreds of different types of coatings are used to protect various engineering materials from corrosion, wear, and erosion, and to provide lubrication and insulation. TBCs are regarded as the coating with the most complex structure among coatings. They must work in the high-temperature environment of aircraft and industrial gas turbines. Due to the diffusion and reaction of oxygen and aluminum, an additional layer called TGO (thermally-grown oxide) is formed at the interface between TC and BC (Figure4). For the whole high-temperature oxidation, the oxides were primarily Al 2O3 and a small amount of Cr and Co oxides at the TC/BC (bond-coat) interface [47]. During the development of TGO, the local accumulation of TGO has a profound effect on the stress distribution near the interface. Understanding the formation and stress distribution of TGO is the basis for predicting TBC’s lifetime, which plays an important role in preventing further oxidation and corrosion of TBCs [11,48]. Due to the diversity of reaction mechanisms in the oxidation process of TGO, many studies have illustrated that TGO grows rapidly at the initial stage of oxidation. This is attributed to the following reason: with an increase in temperature, Al2O3 can be produced freely, resulting in the uneven growth of the thickness of TGO and an increase in the roughness of TGO, and the accumulation of residual stress with the increase in thickness, which results in growth stress in the thin layer of TGO [3]. The results reveal that coating failure usually occurs at the TGO/BC interface. This phenomenon is mainly due to the large stress produced by the growth of TGO layer and its interface [49]. Coatings 2020, 10, 732 9 of 25 Coatings 2020, 10, x FOR PEER REVIEW 9 of 25

FigureFigure 4. TheThe crackingcracking modes modes of TBCsof TBCs with thewith TGOs the (thermally-grownTGOs (thermally-grown oxides)of oxides) different of thicknesses: different thicknesses:(a) 1.3 µm, ( ab)) 1.3 2.7 μµm,m, (b (c) )2.7 3.9 μm,µm, (c) ( d3.9) 5.0 μm,µ m,(d) (5.0e) 6.5μm,µ (m,e) 6.5 (f) μ 7.0m, µ(fm,) 7.0 (g μ)m, 7.7 (gµ)m. 7.7 Reprintedμm. Reprinted with withpermission permission from from [43]; [ Copyright43]; Copyright 2012 2012 Elsevier. Elsevier.

OxidationOxidation is isaccompanied accompanied by bygrowth growth strains strains and andassociated associated stresses; stresses; the strain the represents strain represents the overall the volumeoverall increase volume increaseupon converting upon converting the alloy theto Al alloy2O3—when to Al2O the3—when growth the strain growth is large strain enough, is large the enough, stress suppressesthe stress suppresses the internal the TGO internal formation, TGO forming formation, a critical forming stress. a critical Once this stress. value Once is exceeded, this value it is may exceeded, cause interfacialit may cause crack interfacial generation crack and generationpropagation and along propagation the TC/TGO/BC along interface, the TC/TGO and/ BCthe interface,service lifetime and the is affected.service lifetimeAs shown is ainffected. Figure As 5, shownEvans [3] in analyzed Figure5, Evansthe intrinsic [ 3] analyzed failure of the TBCs, intrinsic and failurethe growth of TBCs, of TGO and layerthe growth is the key of TGOissue layerof coating is the du keyrability. issue ofHowever, coating durability.the thickness However, and morphology the thickness of TGO and vary morphology with the increaseof TGO in vary service with time the increase at high temperature. in service time Therefore, at high temperature. it is necessary Therefore, to study the it is evolution necessary of to TGO study layer the duringevolution thermal of TGO exposure. layer during Influenced thermal by the exposure. diversity Influenced of the high-t byemperature the diversity oxidation of the high-temperature mechanism, the oxidationoxidation diversity mechanism, of TBCs the oxidationis extremely diversity complex, of including TBCs is extremely the composition complex, of includingmaterials, thethe compositionpreparation process,of materials, and the the thickness preparation of coatings. process, andTherefore, the thickness different of coatings.thicknesses Therefore, and shapes diff erentof thethicknesses TGO layer andare formed,shapes ofwhich the TGO is of great layer significance are formed, to which stress is distribution of great significance around the to TGO stress layer distribution and to the around life prediction the TGO oflayer TBCs. and During to the the life thermal prediction shock of cycle, TBCs. growth During of the TGO thermal is observed, shock and cycle, the growth multilayer of TGO accumulation is observed, of TGOand theis observed multilayer in the accumulation protrusion of of the TGO surface is observed roughness in the of the protrusion TBCs. The of accumulation the surface roughness of TGO results of the inTBCs. the cracking The accumulation of the top of layer TGO [50]. results This in phenom the crackingenon ofis themore top obvious layer [50 for]. This TBCs phenomenon fabricated byis moreAPS [43,51,52]obvious for(Figure TBCs 5). fabricated According by to APS previous [43,51 primary,52] (Figure simulation5). According work [53], to previouswe can also primary observe simulation a similar phenomenonwork [53], we that can the also growth observe rate of a similarTGO near phenomenon the peak region that is the higher growth than rate that ofnear TGO the nearvalley the region. peak Cheregion et al. is [54] higher investigated than that that near TGO the valley grew unevenly region. Che at the etal. peaks [54] and investigated valleys of that the TGOrough grew top coat/TGO unevenly interfaceat the peaks after andisothermal valleys exposure of the rough at 1050 top °C. coat TGO/TGO grew interface exponentially after isothermal near the peaks exposure and valleys, at 1050 and◦C. TGOTGO was grew thick exponentially at the peaks, near where the peaksthere are and large valleys, tensile and stresses, TGO was uniformity, thick at theand peaks,unevenness where at there the peaks.are large There tensile is a significa stresses,nt uniformity,difference in and the distribution unevenness of at st theress peaks. between There uneven is a and significant uniform diTGO.fference For unevenin the distributionTGO, the maximum of stress out-of-p betweenlane uneven stress andof peak uniform and valley TGO. increase For uneven 200%, TGO, and the maximum maximum tensileout-of-plane stress along stress the of peakinterface and of valley the TGO/surface increase 200%, layer and is the smaller maximum than that tensile of uniform stress along TGO. the Moreover, interface comparedof the TGO with/surface TBCs layer which is smallerwere serviced than that under of uniforma uniform TGO. temperature Moreover, field, compared in the case with where TBCs TBC which is subjectedwere serviced to a thermal under gradient,a uniform thetemperature gradient will field, affect in the the growth case where rate and TBC stress is subjected distribution to a of thermal TGO. Thus,gradient, it is of the significant gradient importance will affect the to growthconsider rate the andfailure stress modes distribution of TBCs under of TGO. uniformThus, and it is non-uniformof significant temperatureimportance tofields. consider Ranjbar-Far the failure et al. modes [55] consider of TBCsed under a nonhomogeneous uniform and non-uniform temperature temperature distribution. fields. The resultsRanjbar-Far indicated et al. the [55 residual] considered stress adistribution nonhomogeneous was significantly temperature affected distribution. by thermal The gradient. results Based indicated on thethe TCF residual experiments, stress distribution Schulz et al. was[56] observed significantly that the affected spallation by thermal of the TBCs gradient. is mainly Based correlated on the with TCF TGOexperiments, formationSchulz that is etinfluenced al. [56] observed by uniform that temperat the spallationure field, of theand TBCs the failur is mainlye of the correlated EB-PVD TBCs with TGOwas causedformation by TC/TGO that is influenced interfacial crack by uniform propagation, temperature which easily field, results and the in failurethe spallation of the and EB-PVD buckling TBCs of wasthe TCcaused layer. by To TC investigate/TGO interfacial the effect crack of propagation,the thermal gr whichadient easily on the results TBC failure in the spallationmodes, the and growth buckling of the of TGOthe TC layers layer. was To taken investigate into account the e ffbyect Shi of [4]. the The thermal results gradient indicated on that the the TBC thermal failure gradient modes, affected the growth the verticalof the TGOstress layers initiation was and taken propagation. into account Meanwhile, by Shi [4 the]. The interfacial results cracking indicated behavior that the was thermal dominated gradient by TGOaffected growth the verticalstress. The stress thermal initiation protective and propagation. performance Meanwhile, and oxidation the interfacialresistance crackingof the TBCs behavior were influenced. Coatings 2020, 10, 732 10 of 25 was dominated by TGO growth stress. The thermal protective performance and oxidation resistance of the TBCs were influenced. Coatings 2020, 10, x FOR PEER REVIEW 10 of 25

Coatings 2020, 10, x FOR PEER REVIEW 10 of 25

Figure 5. Schematics indicating the TGO growth modes and its implications for the development of Figure 5. Schematics indicating the TGO growth modes and its implications for the development of growth stresses. Reprinted with permission from [8]; Copyright 2001 Elsevier. growthFigure stresses. 5. Schematics Reprinted indicating with permission the TGO growth from [modes8]; Copyright and its implications 2001 Elsevier. for the development of Donggrowth et stresses. al. [43] Reprinteddiscussed with the inpermissionfluence of from TGO [8 ];with Copyright different 2001 thic Elsevier.knesses on the failure behavior Dong et al. [43] discussed the influence of TGO with different thicknesses on the failure behavior of the TBCs. The results of thermal cycling tests of APS-TBCs with different initial thicknesses of TGO Dong et al. [43] discussed the influence of TGO with different thicknesses on the failure behavior of theindicated TBCs. Thethat with results the ofincrease thermal in the cycling thickness tests ofof TGO, APS-TBCs the thermal with cycling different life decreases initial thicknesses with the of of the TBCs. The results of thermal cycling tests of APS-TBCs with different initial thicknesses of TGO TGOincrease indicated in the that power with function, the increase and there in is the a critic thicknessal thermal of cycling TGO, thelife which thermal decreases cycling significantly life decreases withwithindicated the increasethe increase that inwith thein theTGO power increase thickness. function, in the As thicknessshown and there in ofFigure TGO, is a critical6, the moreover, thermal thermal thecycling typical cycling life failure decreases life whichmodes with decreaseswere the significantlyaffectedincrease byin with the TGO thepower thickness, increase function, inmaking and TGO there the thickness. iscrack a critic generation Asal thermal shown and cycling in Figurepropagation life 6which, moreover, along decreases the the TC/TGO/BCsignificantly typical failure with the increase in TGO thickness. As shown in Figure 6, moreover, the typical failure modes were modesinterface. were aTherefore,ffected by the TGO thickness thickness, of TGO making has a sign theificant crack effect generation on the thermal and propagation cycle life of TBCs, along the affected by TGO thickness, making the crack generation and propagation along the TC/TGO/BC TC/TGOand/ BCthe interface.uneven distribution Therefore, and the accumulation thickness of TGOof TGO has will a significant also affect etheffect service on the lifetime thermal of cyclethe life interface. Therefore, the thickness of TGO has a significant effect on the thermal cycle life of TBCs, coatings. To further investigate the effect of the service lifetime of TBCs, the thermal expansion of TBCs,and andthe uneven the uneven distribution distribution and accumulation and accumulation of TGO of will TGO also will affect also the aff serviceect the lifetime service of lifetime the of mismatch should be considered. the coatings.coatings. ToTo furtherfurther investigate the the effect effect of of th thee service service lifetime lifetime of ofTBCs, TBCs, the thethermal thermal expansion expansion mismatchmismatch should should be considered.be considered.

Figure 6. Stress components and equivalent plastic strain (PEEQ) at the ambient temperature

considering the interface crack: (a) Mises stress, (b) S11, stress in x direction, (c) S22, stress in y Figuredirection,Figure 6. Stress 6. andStress components (d ) componentsPEEQ, andequivalent equivalent and equivalentplastic plastic strain. plastic strain Reprinted (PEEQ)strain with(PEEQ) atthe permission ambientat the ambientfrom temperature [57]; temperature Copyright considering the interface2017considering ASME. crack: the (interfacea) Mises crack: stress, (a ()b Mises) S11, stressstress, in(b)x S11,direction, stress in (c )x S22, direction, stress ( inc) yS22,direction, stress in and y (d) PEEQ,direction, equivalent and plastic(d) PEEQ, strain. equivalent Reprinted plastic with strain. permission Reprinted from with [ 57permission]; Copyright from 2017 [57]; ASME.Copyright 2017 ASME.

Coatings 2020, 10, 732 11 of 25 Coatings 2020, 10, x FOR PEER REVIEW 11 of 25

3.2.3.2. Thermal Thermal Expansion Expansion Mismatch Mismatch TheThe thermal mismatch stressesstresses nearnear thethe TGOTGO layer layer occur occur during during thermal thermal cycling, cycling, which which is is due due to tothe the great great diff erencesdifferences in physical, in physical, thermal, thermal, and mechanical and mechanical properties properties of adjacent of adjacent layers. Thelayers. damage The damageand failure and of failure TBCs of are TBCs related are torelated the mismatch to the mi ofsmatch the thermal of the thermal expansion expansion coefficient. coefficient. The cracks The cracksgenerated generated by the residualby the residual stress may stress nucleate may nucleate and expand and toexpand the interface, to the interface, which will which affect will the affect stress thestate stress in the state TBCs in [the54,58 TBCs]. Jiang [54,58]. et al. Jiang [57] have et al. pointed [57] have out pointed a large localout a tensile large local stress tensile in the TGOstress coating in the TGOduring coating cooling, during and acooling, crack occurs and a atcrack the peakoccurs value at the of peak the TGO value interface. of the TGO In addition, interface. Fan In addition, et al. [59] Fanhave et discussed al. [59] have the ediscussedffect of TGO the on effect the multipleof TGO surfaceon the multiple cracking behaviorsurface cracking in APS-TBCs. behavior The in driving APS- TBCs.force ofThe the driving periodic force surface of the crack periodic and surface the mismatch crack and of the mismatch crack propagation of the crack path propagation were discussed path wereby using discussed the extended by using finite the elementextended method finite el (XFEM)ement method and the (XFEM) periodic and boundary the periodic conditions. boundary The conditions.stress distribution The stress was distribution calculated and was the calculated crack propagation and the crack path propagation was simulated path by was XFEM simulated (Figure by7). XFEMThe results (Figureindicate 7). The that results the elastic indicate modulus that the of theelastic TGO modulus layer plays of the a significant TGO layer role plays in controlling a significant the rolestrain in energy controlling release the rate. strain The energy related fracturerelease rate. mechanism The related was mainly fracture controlled mechanism by thewas mismatch mainly controlledbetween the by topthe mismatch coating and between the bond-coat, the top coating which and can the be bond-coat, used as a which guide can for be the used well-designed as a guide forstrain-resistant the well-designed APS-TBCs. strain-resistant APS-TBCs.

FigureFigure 7. StressStress distribution distribution for for the the TGO TGO with with different different elastic elastic modulus modulus and and surface surface crack crack locations: locations: (a) peak, ETGO = 400 GPa, (b) middle, ETGO = 400 GPa, (c) valley, ETGO = 400 GPa, (d) peak, ETGO = 40 GPa, (a) peak, ETGO = 400 GPa, (b) middle, ETGO = 400 GPa, (c) valley, ETGO = 400 GPa, (d) peak, ETGO (e) middle, ETGO = 40 GPa, (f) valley, ETGO = 40 GPa. In these cases, the crack length was selected to be = 40 GPa,(e) middle, ETGO = 40 GPa, (f) valley, ETGO = 40 GPa. In these cases, the crack length 20was mm, selected other to parameters be 20 mm, other were parameters considered were to be considered same. Reprinted to be same. with Reprinted permission with permissionfrom [59]; Copyrightfrom [59]; Copyright2012 Elsevier. 2012 Elsevier.

HuangHuang et et al. al. [60] [60] considered considered the the ef efffectect of of interface interface roughness roughness on on st strainrain energy energy release release rate rate (Strain (Strain EnergyEnergy Release Release Rate) Rate) and and surface cracking behavior behavior in in APS-TBCs APS-TBCs by by XFEM. XFEM. The The driving forces forces of multiplemultiple surface crackscracks inin the the coating coating/substrate/substrate system system were were predicted predicted and and presented. presented. It was It was found found that thatthe interfacialthe interfacial roughness roughness has a significanthas a significant effect on ef thefect surface on the roughness, surface roughness, interfacial stressinterfacial distribution, stress distribution,and propagation and modespropagation of cracks. modes Sfar of et cracks. al. [61] establishedSfar et al. [61] a TBCs established failure model, a TBCs the failure residual model, stress the of residualthe TC region stress at of the the peak TC region of TC/ TGOat the interface peak of TC/TGO due to thermal interface mismatch due to failurethermal was mismatch studied, failure which was was studied,used to evaluatewhich was the interfaceused to evaluate toughness the or interface to measure toughness the initiation or to ofmeasure cracks. Thethe initiation stress concentration of cracks. Thenear stress the TGO concentration layer was discussed near the TGO when layer the vertical was di crackscussed was when above the the vertical peak value crack of was the TGOabove layer the peakby establishing value of the the TGO TBCs layer failure by establishing model. Compared the TB withCs failure the horizontal model. Compared crack, the with vertical the crack horizontal could crack,be partially the vertical released, crack resulting could in be stress partially concentration released, nearresulting the TGO in stress layer. concentration When horizontal near cracks the TGO exist, layer.the maximum When horizontal tensile stress cracks is located exist, the at the maximum peak of the tensile TGO stress/BC interface. is located When at the there peak is ofavertical the TGO/BCcrack, interface.the stress When concentration there is tendsa vertical to appear crack, near the stre thess crack concentration tip. The eff ecttends of horizontalto appear cracksnear the and crack vertical tip. Thecracks effect on theof horizontal stress around cracks TGO and is obviously vertical cracks different. on Thethe stress existence around of vertical TGO cracksis obviously can significantly different. Thereduce existence the maximum of vertical tensile cracks stress can significantly concentration reduce in the the TGO maximum layer. Tolpygo tensile stress et al. [concentration5] observed that in thethe TGO thermal layer. mismatch Tolpygo of et 7YSZal. [5] coatingobserved occurred that the duringthermal the mismatch cycling of furnace 7YSZ coating test, and occurred horizontal during and theinterfacial cycling cracks furnace inclined test, and near horizontal the TGO layer,and interfacial resulting cracks in coating inclined failure. near In the order TGO to study layer, the resulting failure inmechanism coating failure. caused In byorder TGO, to study Wang the et failure al. [12, 13me,16chanism,62,63] caused have done by TGO,a generous Wang etof al. research [12,13,16,62,63] on crack have done a generous of research on crack propagation in previous reports. Including the study of horizontal cracks, vertical cracks, internal cracks, or interfacial cracks in coatings, the results indicated Coatings 2020, 10, 732 12 of 25 propagation in previous reports. Including the study of horizontal cracks, vertical cracks, internal cracks, or interfacial cracks in coatings, the results indicated that the stress concentration is related to the location of vertical and horizontal cracks, as shown in Figure8, and the crack length also a ffects the failure of TBCs [5,13,14], especially for the thick thermal barrier coatings (TTBCs) fabricated by APS, vertical cracks (also known as segmentation cracks) are distributed in the top-coat (ceramic layer); usually, the length of segmentation cracks is more than half of the thickness of the coating. For the thickness of the top coating, the existence of segmentation cracks will reduce the stress concentration of TTBCs, thereby increasing the strain tolerance of the coating [58,64,65]. Therefore, it is useful to ensure a significant improvement in thermal shock resistance. Although segmentation cracks are not conducive to reduce thermal conductivity, the thickness of TBCs is enough, which will ensure that TBCs still have a high thermal insulation effect [5]. In fact, the stress level and distribution play a crucial role in controlling the failure of TBCs during thermal shock. The special microstructure of TBCs affects the stress distribution and maximum stress level of TBCs. In particular, there were many segmentation cracks in the TC layer, which released more or less energy and caused the stress concentration. Besides, the geometric characteristics of segmentation cracks will affect the failure behavior of materials. Although the existence of cracks can increase the strain tolerance and improve the thermal shock resistance of materials, the propagation of segmentation cracks is still the main cause of coating failure. Dong et al. [66] studied the distribution of temperature on a YSZ-free surface during crack coalescing via FEM, then the influence of sintering of YSZ induced by heat-transfer overlapping on energy release rate was quantificationally evaluated. Fan et al. [67] investigated the effect of the periodic surface cracks on interfacial fracture of TBC systems via cohesive zone model (CZM). The results indicated that the spacing of surface cracks has a significant effect on the initiation and propagation of cracks in TBC systems. For short interfacial cracks, it can be concluded from the simulation results that suitable high surface crack density can improve the durability of the TBC system. The delamination behavior of the interface often occurs when the vertical crack propagates to the interface. Therefore, controlling the delamination behavior is very important for delaying the failure of TBCs. Bake et al. [68] discovered the initiation and propagation of cracks near the interface, and the direction of crack propagation tended to the BC/TGO interface. The results indicate that the morphology and deformation of TGO play an important role in the nucleation and propagation of cracks. Wei et al. [69] analyzed that an increase in the transverse growth strain of TGO would lead to premature peeling of coatings, and the bond-coat and creep of TGO have only a slight shrinkage effect. If the stress of TGO is low, the cracking of ceramics occurs. However, growth of the TGO tends to increase the interfacial stress within the TBC system and would affect the surface crack density. Zhu et al. [70] found that cracks easily propagate from the surface to the interface with the surface crack density decreasing, while the cracks generate and propagate along the surface with high surface crack density. The saturated crack density decreases with the increase in ceramic coating thickness. Furthermore, the surface crack density has a significant effect on the initiation and propagation of interface cracks. The interfacial delamination length decreases with an increase in surface crack density, and the interfacial delamination would not occur before reaching the critical surface crack density. It can delay the delamination of the coating and improve the thermal shock resistance and durability of the coating with reducing the thermal matching and increasing the interfacial adhesion strength of the coating. The TGO growth rate, which may accelerate the failure of TBCs, was affected via oxidation temperature and time. Experimental simulation was confirmed by Trunova [71]. The failure modes and lifetime of the APS-TBC were influenced, the large extent on the thermal load conditions were taken into account. It promotes delamination cracking in the TBC close to the TBC/TGO interface during thermal cycling. The failure crack path to transfer to TGO was affected during the introduction of high temperature residence time, and caused the final failure to be almost completely within TGO. The experimental results indicated that the observed damage evolution was mainly caused by the formation of cracks, the connection of single cracks, and crack propagation. Within a certain residence time, the degradation of the TBC system is affected by the degradation process connected with the adhesion layer, and damage accumulation is influenced Coatings 2020, 10, 732 13 of 25

Coatingsby TGO 2020 growth, 10, x FOR stress, PEER leading REVIEW to a reduction in cycle lifetime. In addition, the toughness value13 of 25 is affected by cracks during the thermal cycle; it is possible to learn from the available research findings propagationthat several crackpaths, propagation such as crack paths, bridging, such aswill crack enhance bridging, fracture will toughness enhance fracture [72]. Considering toughness [the72]. influenceConsidering of high the influence temperature of high and temperature TBC exposure and ti TBCme, exposurea model of time, interface a model fracture of interface toughness fracture of thermaltoughness barrier of thermal coating barrier is proposed. coating The is proposed. model is ex Thepressed model in is the expressed form of an in theArrhenius form of type an Arrhenius showing temperature-dependenttype showing temperature-dependent characteristics, characteristics, and also shows and the also dependence shows the of dependence the density of of the microcracks density of distributedmicrocracks along distributed the interface along the of interfacethe coating. of the As coating. the experimentallyAs the experimentally measured measured microcrack microcrack density exhibitsdensity exhibitsthermal thermalcycle-dependent cycle-dependent behavior, behavior, therefore, therefore, the proposed the proposed interface interface crack toughness crack toughness model wasmodel used was to used explain to explain the experimentally the experimentally obtained obtained toughness toughness value. value. Kiyohiro Kiyohiro et al. et[73] al. observed [73] observed that thethat area the areaand thickness and thickness of TGO of TGOin blasted in blasted TBC (B-TBC TBC (B-TBC)) specimens specimens is much is much lower lower than that than of that the of non- the blastednon-blasted TBC TBC(S-TBC). (S-TBC). An Anindentation indentation test test was was perf performedormed to toevaluate evaluate the the TC/BC TC/BC interface interface fracture fracture toughnesstoughness ( KIFC), ,confirming confirming that that the the KKIFCIFC ofof the the B-TBC B-TBC specimen specimen was was sign significantlyificantly higher higher than than that that of of thethe S-TBC specimen. These These results results indicate indicate that that gr grindinginding and and sandblasting sandblasting effectively effectively improve the oxidation resistance and adhesion strength of the TBC system. Therefore, Therefore, the the microcracks microcracks caused caused by by thermalthermal mismatch mismatch failure failure are are an important factor fo forr evaluating interfacial toughness or predicting crackcrack initiation.

Figure 8. CrackCrack propagation propagation paths paths in in the the TBCs TBCs for for diffe differentrent crack positions positions and and TGO TGO with with different different

elasticelastic modulus: modulus: ( (aa)) peak, peak, EETGOTGO = =400400 GPa, GPa, (b ()b middle,) middle, ETGOETGO = 400= 400 GPa, GPa, (c) (valley,c) valley, ETGOETGO = 400= 400GPa, GPa, (d) peak,(d) peak, ETGOE TGO= 40= GPa,40 GPa, (e) (middle,e) middle, ETGOETGO = 40= 40GPa, GPa, and and (f ()f )valley, valley, EETGOTGO == 4040 GPa. GPa. Reprinted Reprinted with permission from [59]; [59]; Copyright 2012 Elsevier.

3.3. Roughness Roughness of of the the Interface Interface The effect effect of interface roughness on the evolutio evolutionn of TGO thickness and growth stress was also investigated,investigated, which which showed showed the the effects effects of of growth stress evolution in the thermally grown oxide on thethe failure failure mechanism mechanism of of TBCs. TBCs. Duri Duringng thermal thermal load, load, the the failure failure of of plasma-sprayed plasma-sprayed TBCs TBCs frequently frequently occurs in the top coatings near the interface. Th Therefore,erefore, it it is is necessary to to study the non-uniform growth mechanism mechanism of of TGO TGO on on the the rough rough interface. interface. Inhomogeneity Inhomogeneity of ofthe the temperature temperature field field results results in inhomogeneityin inhomogeneity and and roughness roughness of ofTGO. TGO. With With the the in increasecrease of of the the roughness, roughness, the the thickness thickness of of TGO increasesincreases unevenly unevenly during during the the growth growth of of TGO. TGO. The The rough interface not only causes inhomogeneous growth of TGO,TGO, butbut also also a ffaffectsects the the stress stress distribution distribution of TBCsof TBCs (Figure (Figure9). A 9). lot A of lot research of research has been has donebeen doneby Evans by Evans [3,7,8], [3,7,8], and it wasand foundit was thatfound the that durability the durability and stability and stability are limited are by limited delamination by delamination along the alonginterface the betweeninterface the between TGO and the theTGO bond-coat. and the bond-coa The interfacialt. The interfacial cracks at BC cracks/TGO at/YSZ BC/TGO/YSZ tend to produce tend toenergy produce release. energy As release. rumpling As isrumpling suppressed, is suppressed, the mode delaminationthe mode delamination is related tois related the rate to of the energy rate ofrelease. energy Actually, release. Actually, as the energy as the release energy rate release is equal rate is to equal the type to the II toughnesstype II toughness of the interface,of the interface,it was itfound was thatfound the that lower the limit lower of TGOlimit leadingof TGO toleading delamination to delamination can be predicted. can be predicted.However, However, this method thisof methodpredicting of predicting the critical the thickness critical has thickness not been has realized not been due realized to type due II toughness to type II beingtoughness well-known being well- and knowndifficult and to measure. difficult Meanwhile,to measure. ShenMeanwhile, et al. [74 Shen] observed et al. [74] that observed the thickness that ofthe TGO thickness in the valleyof TGO area in the valley area decreased with the increase of roughness, and the TGO obviously increased with the increase of the interface roughness at peak; after 1000 h oxidation, the maximum tensile stress and compressive stress gradually increase, with the increase of the roughness, the interfacial fracture begins to increase in ten degrees. In the past decades, many researchers have discussed the Coatings 2020, 10, 732 14 of 25 decreased with the increase of roughness, and the TGO obviously increased with the increase of the Coatingsinterface 2020 roughness, 10, x FOR PEER at peak; REVIEW after 1000 h oxidation, the maximum tensile stress and compressive14 stressof 25 gradually increase, with the increase of the roughness, the interfacial fracture begins to increase in ten relationshipdegrees. In theof crack past decades,propagation many between researchers the surfac havee discussedof TBCs and the the relationship stress distribution of crack propagationof the TGO [75].between Guo et the al. surface [76] considered of TBCs andthe effects the stress of inte distributionrfacial roughness of the TGO and [thickness75]. Guo of et TBCs al. [76 on] considered the shear mechanicalthe effects ofproperties interfacial of EB-PVD-TBC roughness and interfaces. thickness It ofwas TBCs found on that the the shear residual mechanical compressive properties stress of wasEB-PVD-TBC related to interfaces.interfacial roughness It was found and that the the thickn residualess of compressive TBCs via Raman stress spectroscopy. was related to The interfacial stress intensityroughness of andTBCs the increases thickness with of the TBCs increase via Raman of TGO. spectroscopy. Therefore, large The stress stress intensity often occurs of TBCs at the increases thicker TGOwith layer. the increase of TGO. Therefore, large stress often occurs at the thicker TGO layer.

FigureFigure 9. 9. ProfileProfile of of the the top top surface surface of of the the TGO TGO layer layer af afterter 500 500 h h oxidation oxidation under under operating operating conditions: conditions: ((aa)) the the main main view view and and ( (bb)) the the top top view. view. Reprinted Reprinted with with permissi permissionon from from [77]; [77]; Copyright Copyright 2019 2019 Elsevier. Elsevier.

TheThe interfacial interfacial roughness roughness also also has has an an obvious obvious effect effect on on the the residual residual stress stress of of TGO TGO and and the the lifetime lifetime ofof TBCs, thethe largelarge residual residual compression compression in thein thermallythe thermally grown grown oxide nearoxide the near interface the interface [8]. In addition [8]. In , addition,the thermal the activation thermal activation creep process creep is deformedprocess is underdeformed high-temperature under high-temperature oxidation. Wang oxidation. et al. [Wang16,78] etcharacterized al. [16,78] characterized the failure behavior the failure of APS-TBCsbehavior of under APS-TBCs the three-point under the bending three-point (3PB) bending test using (3PB) acoustic test usingemission acoustic (AE) technology.emission (AE) The technology. results indicated The theresults changes indicated in acoustic the changes emission in parameters acoustic emission (number parametersof acoustic emission(number events, of acoustic amplitude emission of acoustic events, emission, amplitude and of energy acoustic of acoustic emission, emission) and energy as well of as acousticstress versus emission) strain as during well as thermal stress versus load. Thestrain change during of thermal curve has load. a good The correspondence.change of curve has The a results good correspondence.of AE analysis and The cross-section results of AE observation analysis and indicated cross-section that main observation cracks tend indicated to propagate that main towards cracks the tendTC/BC to propagate interface. Thetowards actual the failure TC/BC of interface. APS-TBCs The is dueactual to failure the shedding of APS-TBCs of the metallicis due to coatingthe shedding on the ofsubstrate, the metallic and thecoating propagation on the substrate, of horizontal and cracksthe propagation along the interface of horizontal of the cracks substrate along/bond-coat the interface under ofbending the substrate/bond-coat moment. Sun et al. under [79] compared bending dimoment.fferent samples Sun et al. using [79] real compared three-dimensional different samples morphologies using realvia finitethree-dimensional element modeling, morphologies and pointed via finite out thatelement this modelingmodeling, method and pointe is and out effective that this tool modeling to obtain methodvaluable is research an effective progress tool of to stress obtain distribution valuable re insearch TBCs. Theprogress results ofof stress finite distribution element analysis in TBCs. indicated The resultsthat the of residual finite element stress analysis of TBCs indicated is driven that by the the shear residual stress stress and of the TBCs axial is driven stress of by La the2Zr shear2O7 (LZO).stress The failure of TBCs originates from the corner and propagates parallel to the inherent layered structure and the axial stress of La2Zr2O7 (LZO). The failure of TBCs originates from the corner and propagates parallelof APS-TBCs. to the inherent The actual layered failure structure mode of and APS-TBCs. cross-sectional The actual SEM failure morphology mode and verified cross-sectional the failure SEMmechanism. morphology Importantly, verified thethe localfailure stress mechanism. changes significantlyImportantly, duethe local to the stress inhomogeneous changes significantly growth of dueTGO. to Finitethe inhomogeneous deformation would growth not of be TGO. neglected Finite in thedeformation oxidation would process. notTherefore, be neglected a theoretical in the oxidationmodel of oxidationprocess. Therefore, stress based a theoretical on finite deformation model of oxidation was established, stress based and the on e fffiniteect of deformation a rough interface was established,on stress state and was the studied. effect Besides,of a rough the localinterface accumulation on stress ofstate TGO was will studied. have a profound Besides, impactthe local on accumulationthe stress distribution of TGO will near have the interface, a profound and impact the stress on fieldthe stress of the distribution rough interface near will the alsointerface, change and the thegrowth stress rate field of of TGO the atrough different interface locations will (Figurealso change 10). Inthe a growth word, the rate rough of TGO interface at different leads tolocations uneven (Figuregrowth 10). of TGO, In a word, and uneven the rough growth interface leads leads to increased to uneven roughness growth ofof TGO.TGO, The and roughness uneven growth of TBCs leads has toa significantincreased roughness impact on of stress TGO. distribution, The roughness the failureof TBCs mechanism, has a significant and service impact lifetime. on stress distribution, the failure mechanism, and service lifetime. Coatings 2020, 10, 732 15 of 25 Coatings 2020, 10, x FOR PEER REVIEW 15 of 25

Figure 10. (a–c) shows the distribution of the normal stress σ at room temperature after 100 cycles Figure 10. (a–c) shows the distribution of the normal stress σ22at room temperature after 100 cycles in thein the defect-free defect-free model, model, porous porous model, model, and and lamellar lamellar model, model, respectively. respectively. Similarly, Similarly, (d–f) shows(d–f) shows the shear the σ stressshear σstress12 distribution distribution in the correspondingin the corresponding models. Reprintedmodels. Reprinted with permission with permission from [36]; Copyrightfrom [36]; 2019Copyright Elsevier. 2019 Elsevier.

3.4. Foreign Object Damage TBCs used on many hot-section components of aero-turbines are faced with various adverse conditions inin the the process process of of service. service. One One of theof specialthe special conditions conditions that shouldthat should be paid be attention paid attention is foreign is objectforeign damage object damage (FOD). In(FOD). this case,In this the case, hard the foreign hard foreign matter matter particles particles often found often infound the airin the path air of path the operatingof the operating aero-turbine, aero-turbine, which are which impacted are impacted by the leading by the edge leading of the edge High of Pressure the High Turb Pressure (HPT) blade,Turb even(HPT) if blade, sucha even single if such impact a single will occur, impact lead will to occur, crack lead propagation to crack propagation of the local TBCs, of the thuslocal eliminating TBCs, thus thermaleliminating protection thermal onprotection the surface on the of thesurface hottest of th parte hottest of the part whole of the engine. whole FODengine. particles FOD particlesinhaled orinhaled released or released in the gas in the turbine gas turbine may collide may collide with coated with coated components components and cause and thecause failure the failure of TBCs of (FigureTBCs (Figure 11). Rotating 11). Rotating blades blades are most are vulnerablemost vulnerable to impact to impact damage damage because because their high their rotational high rotational speed resultspeed inresult large in particlelarge particle/TBC/TBC impact impact velocity, velocity, although although the velocity the velocity associated associated with thewith particle the particle itself isitself relatively is relatively low [low[40].40]. Lots Lots of studies of studies have have shown shown that that the the conditions conditions leading leading to to FOD FOD in in jetjet enginesengines are complex, and the impact angle, impact velocity, particleparticle size,size, particleparticle composition, and material temperature are jointly aaffected.ffected. TBCs were affected affected by the foreign body damage (FOD) particles via finitefinite element method, considering a diameter of 4 mm, the incident angle of foreign body and blade temperature aaffectingffecting thethe damagedamage analysisanalysis duringduring impactimpact processprocess waswas studied.studied. It was found that the impact perpendicular to the surface is the most dangerousdangerous [[80].80]. Rahaman et al. [[81]81] observed that a modulated TBC structure can better withstand impact damage. As As shown in Figure 10,10, the resultsresults showed that cracks mainly occurred along the lamellarlamellar interface, the visible delamination of TBC observed nearnear thethe edgeedge of of coating. coating. Larsen Larsen [82 [82]] provided provided that that the the depths depths of FODof FOD damage damage date date follow follow the Weibullthe Weibull probability probability distribution. distribution. Although Although a few measurementa few measurement depths exceededdepths exceeded 1 mm, most 1 mm, damages most weredamages in the were range in the of range 0 to 400 of µ0 m.to 400 According μm. According to previous to previous studies studies [82], it [82], was it found was found that these that these FOD eventsFOD events would would not directly not directly lead to lead failure, to thefailure, type th ofe damage type of thatdamage they representthat they shouldrepresent be consideredshould be inconsidered any design in any criteria design developed criteria todeveloped deal with to FOD. deal Moreover,with FOD. theMoreover, information the information provided represents provided therepresents most widely the most known widely data known about data the size about of actualthe size foreign of actual body foreign damage body to fansdamage and compressorto fans and blades.compressor More blades. research More is needed research to is understand needed to understand how FOD may how a ffFODect component may affect failurecomponent under failure high under high cycle fatigue conditions. An experimental simulation was conducted by Peters et al. [83], studying the role of such foreign body damage affecting the fatigue crack growth threshold and large Coatings 2020, 10, 732 16 of 25

Coatings 2020, 10, x FOR PEER REVIEW 16 of 25 cycle fatigue conditions. An experimental simulation was conducted by Peters et al. [83], studying the role of such foreign body damage affecting the fatigue crack growth threshold and large and small and small crack early crack growth in fan blade alloys. The FOD was simulated by the high-velocity crack early crack growth in fan blade alloys. The FOD was simulated by the high-velocity (the sizes of (the sizes of these hard particles are in the millimeter regime, with impact velocities determined these hard particles are in the millimeter regime, with impact velocities determined primarily by the primarily by the blade speed and in the range of 100–350 m/s) impact of steel spheres on a flat surface, blade speed and in the range of 100–350 m/s) impact of steel spheres on a flat surface, which was found which was found to markedly reduce the fatigue strength, primarily due to earlier crack initiation. to markedly reduce the fatigue strength, primarily due to earlier crack initiation. To further investigate To further investigate the effect of the service lifetime of TBCs, the CMAS corrosion of TBCs should the effect of the service lifetime of TBCs, the CMAS corrosion of TBCs should be considered. be considered.

Figure 11. Image of the leading edge of an engine-runengine-run high-pressure turbine blade showing severe foreign object damage. Note Note the the large large gray region which indicates bond coat exposed by completecomplete . removal of the TBC. Reprinted with permissionpermission fromfrom [[40];40]; CopyrightCopyright 20122012 Elsevier.Elsevier

3.5. CMAS Corrosion of Thermal Barrier Coating The faultsfaults ofof aircraft aircraft in in flight flight are are mostly mostly due todue the to high the temperaturehigh temperature of turbine of inlet.turbine The inlet. siliceous The siliceousminerals minerals (dust, sand, (dust, and sand, volcanic and volcanic ash) in theash) air in arethe mostlyair are mostly composed composed of CaO, of MgO, CaO, AlMgO,2O3 ,Al SiO2O23,, SiOand2, other and other mixed mixed oxides oxides (CMAS). (CMAS). They areThey sucked are sucked into an into aircraft an aircraft engine engine before CMASbefore CMAS and engine and enginecomponents components occur at occur 1240~1260 at 1240~1260◦C. In the°C. reaction,In the re theaction, molten the molten CMAS willCMAS adhere will toadhere the TC to surfacethe TC surfaceand penetrate and penetrate into the columnarinto the columnar gap of the gap molten of the phase. molten Then, phase. CMAS Then, solidifies CMAS into solidifies hard zone into during hard zonecooling, during which cooling, changes which the mechanical changes the properties mechanical of topproperties coat. Erosion of top ofcoat. the Erosion coating of by the both coating ingested by particles,both ingested from particles, the operating from environment the operating and environment particles as itand degrades, particle iss as a perennial it degrades, source is a of perennial concern, sourcea great of deal concern, of research a great has deal been of doneresearch already has been [4,6]. done Cai etalready al. [84 ][4,6]. discovered Cai et al. that [84] the discovered permeation that of theCMAS permeation into TC changesof CMAS the into stress TC distributionchanges the aroundstress distribution the tissue and around induces the thetissue formation and induces of cracks. the formationMoreover, theof cracks. permeation Moreover, of CMAS the would permeation lead to materialof CMAS discontinuity, would lead resulting to material in a slightlydiscontinuity, higher resultingstress level in a around slightly the higher microstructure stress level around of the melted the microstructure CMAS/TC interface, of the melted CMAS CMAS/TC permeable interface, layer, CMASand TC permeable/BC interface layer, (Figure and 12TC/BC). interface (Figure 12). Secondly, creep of aero-engine components will occur during long-term operation at high temperature, and the deformation may contact or squeeze other components, resulting in reduced efficiency of the turbine. Therefore, the serious consequences of failure caused by CMAS are receiving more and more attention. Wu et al. [85] found that the TC coating at the interface between CMAS and YSZ was partially dissolved in CMAS, which induced the phase transformation of YSZ coating from tetragonal phase to monoclinic phase, and further resulted in the porosity and thermal conductivity of the coating decreasing. Su et al. [86] simulated the effect of CMAS penetration on the delamination crack of EB-PVD-TBCs at interface by the finite element method. The immersion of CMAS into the EB-PVD-TBCs columnar gap increased the in-plane modulus of the top coat. With the increase of the in-plane modulus of top coat, the tensile stress level of delamination cracks above the interface decreased. When the shear stress level decreases, the delamination crack tends to increase when the delamination crack extends to the bending interface. However, once the crack extends to a flat Coatings 2020, 10, 732 17 of 25 edge, CMAS penetration will begin to enhance its growth and the spallation of the coating will occur (Figure 13). Zhang et al. [87] pointed out that CMAS penetrates into TBCs, and produces transient thermal stresses due to different thermal expansion coefficients. It is easy to cause high in-plane tension when CMAS penetrates into TBCs. The stress produced by the rapid cooling of the top coat promotes the propagation of vertical cracks from the top surface to the bottom of the top coat. At the same time, the accumulation of tensile stress in the outer plane makes the horizontal cracks easily appear at the interface between the penetrating and non-penetrating areas of CMAS. Yang et al. [88] found that the thicker the CMAS deposited, the easier cracks formed at the YSZ/BC interface during cooling. Kim et al. [89] discovered that vertical cracks and holes appeared in TBCs during thermal shock cycles, which had a significant impact on thermal fatigue life. Vertical cracks effectively adapted to the volume change of coatings during heating/cooling cycles, resulting in thermal fatigue life of TBCs increased significantly. Additionally, the uniformly distributed pores in the YSZ coating have a more significant effect on the thermal fatigue of the TBCs by reducing the elastic modulus of the coating. Park et al. [90] discussed the effect of substituting cerium for yttrium stabilizer on the thermal corrosion properties of TBCs from a microscopic point of view. The results showed that even within one hour of the thermal corrosion test, the penetrating salt was close to the interface of the bond-coat, and the reaction between the penetrating salt and the tetragonal stabilizer in zirconia occurred, with the transformation from tetragonal phase to monoclinic phase. Unexpected failures may occur under thermal shock conditions in corrosive environments, especially in YSZ-TBC systems [89]. Therefore, the failure caused by CMAS is closely related to the thickness of deposition and cracks. Cracks easily appear at the interface between the top layer and CMAS during cooling, the difference of thermal expansion coefficient leads to the generation of residual stress, which causes the failure of the coating. Additionally, the composition of the material, the thickness of the coating, and the defects in the coating will affect the high temperature corrosionCoatings 2020 rate, 10, ofx FOR the PEER hot-section REVIEW components. 17 of 25

Figure 12. (a) Distribution of shear stress σ12, stress in the y-axis direction σ22, and max in-plane Figure 12. (a) Distribution of shear stress σ12, stress in the y-axis direction σ22, and max in-plane principal stress σ around the microstructure with and without CMAS at different cooling times, principal stress σMPS around the microstructure with and without CMAS at different cooling times, ((bb)) magnificationmagnification ofof zonezone II withwith CMASCMAS andand zonezone IIII withoutwithout CMASCMAS shownshown inin (a(a).). ReprintedReprinted withwith permissionpermission fromfrom [[84];84]; CopyrightCopyright 20192019 Elsevier.Elsevier.

Secondly, creep of aero-engine components will occur during long-term operation at high temperature, and the deformation may contact or squeeze other components, resulting in reduced efficiency of the turbine. Therefore, the serious consequences of failure caused by CMAS are receiving more and more attention. Wu et al. [85] found that the TC coating at the interface between CMAS and YSZ was partially dissolved in CMAS, which induced the phase transformation of YSZ coating from tetragonal phase to monoclinic phase, and further resulted in the porosity and thermal conductivity of the coating decreasing. Su et al. [86] simulated the effect of CMAS penetration on the delamination crack of EB-PVD-TBCs at interface by the finite element method. The immersion of CMAS into the EB-PVD-TBCs columnar gap increased the in-plane modulus of the top coat. With the increase of the in-plane modulus of top coat, the tensile stress level of delamination cracks above the interface decreased. When the shear stress level decreases, the delamination crack tends to increase when the delamination crack extends to the bending interface. However, once the crack extends to a flat edge, CMAS penetration will begin to enhance its growth and the spallation of the coating will occur (Figure 13). Zhang et al. [87] pointed out that CMAS penetrates into TBCs, and produces transient thermal stresses due to different thermal expansion coefficients. It is easy to cause high in- plane tension when CMAS penetrates into TBCs. The stress produced by the rapid cooling of the top coat promotes the propagation of vertical cracks from the top surface to the bottom of the top coat. At the same time, the accumulation of tensile stress in the outer plane makes the horizontal cracks easily appear at the interface between the penetrating and non-penetrating areas of CMAS. Yang et al. [88] found that the thicker the CMAS deposited, the easier cracks formed at the YSZ/BC interface during cooling. Kim et al. [89] discovered that vertical cracks and holes appeared in TBCs during thermal shock cycles, which had a significant impact on thermal fatigue life. Vertical cracks effectively adapted to the volume change of coatings during heating/cooling cycles, resulting in thermal fatigue life of TBCs increased significantly. Additionally, the uniformly distributed pores in the YSZ coating have a more significant effect on the thermal fatigue of the TBCs by reducing the elastic modulus of the coating. Park et al. [90] discussed the effect of substituting cerium for yttrium stabilizer on the thermal corrosion properties of TBCs from a microscopic point of view. The results showed that even within one hour of the thermal corrosion test, the penetrating salt was close to the interface of the bond-coat, and the reaction between the penetrating salt and the tetragonal stabilizer in zirconia Coatings 2020, 10, x FOR PEER REVIEW 18 of 25 occurred, with the transformation from tetragonal phase to monoclinic phase. Unexpected failures may occur under thermal shock conditions in corrosive environments, especially in YSZ-TBC systems [89]. Therefore, the failure caused by CMAS is closely related to the thickness of deposition and cracks. Cracks easily appear at the interface between the top layer and CMAS during cooling, the difference of thermal expansion coefficient leads to the generation of residual stress, which causes the failure of the coating. Additionally, the composition of the material, the thickness of the coating, andCoatings the2020 defects, 10, 732 in the coating will affect the high temperature corrosion rate of the hot-section18 of 25 components.

Figure 13. ((aa)) Low Low magnification magnification backscattered backscattered electron electron image image of of region region with with delamination delamination cracks, cracks, (b) backscattered electron image showing ph phasease contrast at interface TBCTBC/CMAS,/CMAS, ( c) backscattered electron imageimage showingshowing phase phase contrast contrast in CMASin CMAS infiltrated infiltrated crack, crack, and (andd) backscattered (d) backscattered electron electron image imageshowing showing CMAS CMAS infiltrated infiltrated crack. Notecrack. that Note the that intercolumnar the intercolumnar gaps on gaps both on sides both of sides the crack of the remain crack remainaligned, aligned, indicating indicating that delamination that delamination is strictly is mode strictly I. Reprinted mode I. Reprinted with permission with permission from [31]; Copyrightfrom [31]; Copyright2018 Springer 2018 Nature. Springer Nature. 4. Conclusions and Prospects 4. Conclusions and Prospects The performance of the material is dependent on the microstructure of the material. • The performance of the material is dependent on the microstructure of the material. The dual- The dual-ceramic coating has higher thermal insulation performance and can greatly prolong the ceramic coating has higher thermal insulation performance and can greatly prolong the thermal thermal cycle life of the coating, improving the service temperature of the coating significantly. cycle life of the coating, improving the service temperature of the coating significantly. It is one It is one of the effective ways to develop ultra-high temperature TBCs in the future. of the effective ways to develop ultra-high temperature TBCs in the future. • At present, the factors affecting the service lifetime of TBCs, such as TGO, thermal mismatch, • At present, the factors affecting the service lifetime of TBCs, such as TGO, thermal mismatch, and high temperature corrosion, are being studied. The The damage damage of of TBCs caused by CMAS at high temperature is is emphasized. emphasized. However, However, it itshou shouldld not not be be ignored ignored that that the the uneven uneven growth growth of ofTGO TGO will will cause cause the theaccumulation accumulation of residual of residual stress stress and affect and atheffect distribution the distribution of interfacial of interfacial stress. stress.Additionally, Additionally, the mechanism the mechanism of the influence of the influence of the distribution of the distribution of horizontal of horizontal and vertical and vertical cracks crackson the onstress the around stress around TGO during TGO during thermal thermal cycling cycling is revealed is revealed by finite by finiteelement element simulation. simulation. The Thethermal thermal shockshock resistance resistance of the of coating the coating can be can further be further improved improved by controlling by controlling the density the density of the of thesegmentation segmentation cracks. cracks. With thethe developmentdevelopment of of space space technology, technology, the the urgent urgent need need to achieveto achieve space-space space-space integration integration has hasbecome become the focusthe focus of development of development in the in research the research and development and development of supersonic of supersonic missiles, missiles, space rockets, space rockets,and space and shuttle space vehicles.shuttle vehicles. In the extreme In the extrem environmente environment of long-term of long-term hypersonic hypersonic cruise, cruise, the severe the severefriction friction between between the wing the and wing the atmosphere and the atmosphere creates extremely creates high extremely temperatures. high temperatures. Additionally, Additionally,the engine intake the engine is subjected intake to is extremely subjected highto extremely thermal loadshigh thermal and mechanical loads and loads mechanical during flight.loads duringThe traditional flight. The high-temperature traditional high-temperature resistant ceramic resistant materials ceramic would materials not meet would the existing not meet needs. the existingThe development needs. Theof adevelopment new generation of a of new ultra-high genera temperaturetion of ultra-high TBCs (workingtemperature temperature TBCs (working above 1500 ◦C) is also urgent. However, due to the influence of ultra-high temperature, the current material processing equipment, and processing technology are also greatly limited. How to design and prepare ultra-high temperature protective materials with good oxidation resistance, ablation resistance, thermal shock resistance, and maintain a certain ultra-high strength has become an essential technology to be solved urgently for new aircraft. In the complex ultra-high temperature service environment, due to the limitations of experimental conditions, there are few reports on the theoretical model of residual thermal stress experienced by ultra-high temperature ceramic coatings at different temperatures. Coatings 2020, 10, 732 19 of 25

In addition, the effect of the change of CMAS erosion on hot-section components of aviation aircraft can be simulated by finite element calculations due to the change of CMAS corrosion. The concurrent mode of catastrophic failure caused by the deposition probability of inhaled particles and the relationship between different failures has not been fully studied. While the previous reports mainly focused on the simulation of TBCs residual stress during thermal spraying and actual service, the influence of cracks on the service lifetime of coatings should not be neglected. Besides, how to reduce the experimental cost through simulation and optimize the process also faces great challenges, Liu et al. [91] simulated the preparation process of PS-PVD to produce TBC by the available finite element software ANSYS Fluent 16.0. The deposition quality of TBC was adjusted by altering the size of the free plasma jet, spraying distance, and feeding powder rate, which provide a basic method for further research on the transport of gas-phase materials for TBC in the boundary layer. However, the spraying environment pressure was considered to be constant at 100 Pa and the substrate temperature was constant at 1000 ◦C. PS-PVD is seriously affected by the track of environment pressure and temperature. Particles in the process of spraying are not taken into account in the actual environment, which may lead to local temperature change caused by collision and splashing, then affect the atmospheric pressure of the environment. In turn, the quality of the coating is affected, including the formation of pores and cracks. Therefore, further research will make an effort to optimize the preparation process of TBCs by simulation based on the working conditions. At present, there are few studies on the lifetime prediction of TBCs. Therefore, under different application conditions, there is still much work to be done on the lifetime prediction of TBCs. In order to achieve this goal, the simulation of crack propagation is an important method and approach to predict the lifetime of TBCs. Although the virtual crack closure technique (VCCT), extended finite element method (XFEM), and cohesive zone model (CZM) are used to solve the problems of crack propagation, simulation is limited when there are multiple branching or connecting cracks, as shown in Table2. There are many random micro-pores and micro-cracks in TBCs. Crack growth in this kind of irregular structure is a very important research area. In addition, for the crack growth at the blade of the engine, the traditional way is to eliminate the residual stress generated by the crack by cold processing, such as shot peening, or to prevent the crack from appearing by adding TiO2 and other substances at the crack. However, the occurrence of cracks is often accompanied by the release of energy. How to effectively control the cracks to spread in a favorable direction is also worth studying. There are no effective methods and procedures to solve these problems. Therefore, the heat transfer and failure of TBCs are still challenging issues which should be further investigated in the future. CoatingsCoatings 2020 2020, 10, 10, x, x FOR FOR PEER PEER REVIEW REVIEW 2020 of of 25 25 Coatings 2020, 10, 732 20 of 25

TableTable 2. 2. Comparison Comparison of of VCCT, VCCT, XFEM, XFEM, and and CZM. CZM. Table 2. Comparison of VCCT, XFEM, and CZM. MethodsMethods of of ComputationalComputational DescriptionDescription of of the the Model Model Advantages Advantages Disadvantages Disadvantages References References Note Note Methods of ComputationalComputational Description of the Model Advantages Disadvantages References Note Mechanic Description of the Model Advantages Disadvantages References Note Mechanic Mechanic

★★ It FIt is isIt extraordinarily extraordinarily is extraordinarily suitable suitable suitable to to count tocount the the _ An initial crack should be VirtualVirtual Crack Crack energyenergycount release release the energy rate rate in in release the the process process rate in of of the crack crack ◆◆ An An initial initial crack crack should should be be predefined predefined predefined before the [37,92,93][37,92,93] ClosedClosed growthgrowthprocess based based of crackon on th the growthe thought thought based that that onthe the the bbeforeefore the the simulation simulation of of crack crack propagation propagation Virtual Crack Closed TechniqueClosed growth based on the thought that the simulationbefore the ofsimulation crack propagation of crack propagation Calculate J Technique energythought required that the is equal energy to required the work is of ◆The propagation path of the crack[37 ,should92,93] Calculate be CalculateJ integration J (VCCT) Technique energy required is equal to the work of _◆TheThe propagationpropagation pathpath ofof thethe crack should be equal to the work of marking the crack integrationintegration (VCCT)(VCCT) markingmarking the the crack crack closed, closed, when when the the crack crack crackalsoalso defined shoulddefined bebefore before also the definedthe simulation simulation process process closed, when the crack propagates a propagatespropagates a a tiny tiny displacement. displacement. before the simulation process propagatestiny displacement. a tiny displacement.

▊▊ItIt is is notIt not is essential notessential essential to to define define to define an an initial initial an crack crack initial crack ▊▊CanCan solve solve the the problems problems of of crack crack Can solve the problems of crack propagationpropagation with with non-continuous non-continuous propagation with non-continuous H▼When▼WhenWhen the the the crack crack crack propagates propagates propagates to to to a a [94-97][94-97] Calculate Calculate ExtendedExtended Finite Finite characteristicscharacteristics characteristics acomplicatedcomplicated interface, interface, it it is is not not very very[ 94effective effective–97] Calculate propagationpropagation propagation of of the the Extended Finite ElementElement ▊▊The propagation path of the cracks is alsocomplicated interface, it is not very effective propagation of the Element ▊TheThe propagation propagation path path of the of thecracks cracks is alsovery effective to model the of the crack at the inner of the (XFEM) toto model model the the problems problems of of the the interfacial interfacial crackcrack at at the the inner inner (XFEM)(XFEM) notnotis essential essential also not to essentialto be be de defined,fined, to be not not defined, dependent dependent not problems of the interfacial top coat fracturefracture ofof the the top top coat coat onon thedependent the inner inner details details on the of of inner geometrical geometrical details ofstructure structure fracture onlyonlygeometrical dependent dependent structure on on the the external external only dependent shape shape of of the the

structurestructureon the body externalbody shape of the structure body ▲▲ ItN It canIt can can solve solve solve the the the problem problem problem of of ofthe the the energy energy energy dissipation based on the dissipationdissipation based based on on the the degradation degradation of of degradation of interface stiffness Many parameters should be CohesiveCohesive Zone Zone interfaceinterface stiffness stiffness ■■ManyMany parameters parameters should should be be set set [98–102[98-102]][98-102] Cohesive Zone ModelCohesive (CZM) Zone interfaceN It is stiffness not essential to refine the mesh setMany parameters should be set [98-102] Model (CZM) ▲▲ It is not essential to refine the mesh ■■The computational cost is high interfacialinterfacial fracture fracture Model (CZM) ▲ Itduring is not theessential simulation to refine process, the mesh andthe ■TheThe computationalcomputational cost cost is is high high interfacial fracture during the simulation process, and the crack duringcrack the is simulation not necessary process, to be and the crack isis not prefabricatednot necessary necessary to to be be prefabricated prefabricated

Coatings 2020, 10, 732 21 of 25

Author Contributions: Conceptualization, Y.-H.C., Y.-W.W., W.-W.S. and Y.-D.M.; Writing—Original Draft Preparation, Z.-C.H.; Writing—Review and Editing, B.L., L.W., and Y.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Nos. 51701050, 51671208, 51672067), National NSAF (Grant No. U1730139), Training Program of the Major Research Plan of the National Natural Science Foundation of China (No. 91960107) and Natural Science Foundation of Heilongjiang Province (JJ2016ZR1110, JJ2018QN0578). This work was supported by Key Laboratory of Superlight Materials & Surface Technology (Harbin Engineering University), Ministry of Education. And this work was also supported by the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2017295), Natural Science Foundation of Shanghai (No. 19ZR1479600). And this work was also supported by the National Natural Science Foundation of Hebei Province (E2018202034). Conflicts of Interest: We declare that we have no conflict of interest.

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