Sintering‐induced delamination of thermal barrier coatings by gradient thermal cyclic test

Lifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma‐sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering‐induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high‐temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering‐induced delamination of the top coat. Based on the above‐described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.     

Laser thermal gradient testing of air plasma sprayed multilayered,multi-material thermal barrier coatings

Air plasma sprayed (APS) thermal barrier coatings (TBCs) are a widely used technology in the gas turbine industry to thermally insulate and protect underlying metallic superalloy components. These TBCs are designed to have intrinsically low thermal conductivity while also being structurally compliant to withstand cyclic thermal excursions in a turbine environment. This study examines yttria-stabilized zirconia (YSZ) TBCs of varying architecture: porous and dense vertically cracked (DVC), which were deposited onto bond-coated superalloys and tested in a novel CO₂ laser rig. Additionally, multilayered TBCs: a two-layered YSZ (dense + porous) and a multi-material YSZ/GZO TBC were evaluated using the same laser rig. Cyclic exposure under simulative thermal gradients was carried out using the laser rig to evaluate the microstructural change of these different TBCs over time. During the test, real-time calculations of the normalized thermal conductivity of the TBCs were also evaluated to elucidate information about the nature of the microstructural change in relation to the starting microstructure and composition. It was determined that porous TBCs undergo steady increases in conductivity, whereas DVC and YSZ/GZO systems experience an initial increase followed by a monotonic decrease in conductivity. Microstructural studies confirmed the difference in coating evolution due to the cycling.     

The correlation of the scattering coefficient and morphology-based microstructure in yttria-stabilized zirconia coatings

The microstructure dependence of thermal radiative properties is critically needed to the design and operation of the thermal barrier coating systems. In this study, different yttria-stabilized zirconia coating layers are fabricated by air plasma spray with three average porosities 5.9%, 14.5%, and 23.3% at coating thickness from 283 to 955 µm. The room-temperature, spectral directional–hemispherical transmittance, and reflectance are measured over the wavelength range from 1.35 to 2.5 µm. The radiative properties of absorption and scattering coefficients are reduced by using a hybrid method of the discrete ordinate method and the Kubelka–Munk four-flux method. Using the image processing tools developed in-house, the porosity and pore size distribution (PSD) are obtained from SEM images for each coating. A numerical algorithm is used to convert the two-dimensional PSD into a three-dimensional PSD assuming that all pores are spheroid. The scattering coefficient is directly computed by the Mie theory based on the PSD. The new approach provides a predictive model of radiative properties based on the PSD, which is extracted from the coating cross-section images. Comparison of radiative properties obtained by the direct Mie theory computation and those obtained by the reduction from spectral measurement is made and discrepancy is discussed.     

Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing,testing under thermal gradient loading,and future challenges

Cr₂AlC layers with thickness up to 100 µm were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr₂AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr₂AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed.     

Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings

Nanostructured 4SYSZ (scandia (3.5 mol%) yttria (0.5 mol%) stabilized zirconia) and 5.5 SYSZ (5 mol% scandia and 0.5 mol% yttria) thermal barrier coatings (TBCs) were deposited on nickel-based superalloy using NiCrAlY as the bond coat by plasma spraying process. The thermal shock response of both as-sprayed TBCs was investigated at 1000 °C. Experimental results indicated that the nanostructured 5.5SYSZ TBCs have better thermal shock performance in contrast to 4SYSZ TBCs due to their higher tetragonal phase content and higher fracture toughness of this coating     

Comparison of thermal shock resistances of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of the current study is evaluation and comparison of thermal shock behavior of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, the nanostructured and conventional YSZ coatings were deposited by atmospheric plasma spraying (APS) on NiCoCrAlY-coated Inconel 738LC substrates. The thermal shock test was administered by quenching the samples in cold water of temperature 20–25 °C from 950 °C. In order to characterize elastic modulus of plasma-sprayed coatings, the Knoop indentation method was employed. Microstructural evaluation, elemental analysis, and phase analysis were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffractometry (XRD) respectively. The results revealed that failures of both nanostructured and conventional TBCs were due to the spallation of ceramic top coat. Thermal stresses caused by mismatch of thermal expansion coefficients between the ceramic top coat and the underlying metallic components were recognized as the major factor of TBC failure. However, the nanostructured TBC, due to bimodal unique microstructure, presented an average thermal cycling lifetime that was approximately 1.5 times higher than that of the conventional TBC.     

Plasma sprayed nanostructured GdPO4 thermal barrier coatings: Preparation microstructure and CMAS corrosion resistance

Nanostructured GdPO₄ thermal barrier coatings (TBCs) were prepared by air plasma spraying, and their phase structure evolution and microstructure variation due to calcium–magnesium–alumina–silicate (CMAS) attack have been investigated. The chemical composition of the coating is close to that of the agglomerated particles used for thermal spraying. Nanozones with porous structure are embedded in the coating microstructure, with a percentage of ~30%. CMAS corrosion tests indicated that nanostructured GdPO₄ coating is highly resistant to penetration by molten CMAS at 1250°C. Within 1 hour heat treatment duration, a continuous dense reaction layer forms on the coating surface, which are composed of P–Si apatite based on Ca_2+xGd_8?x(PO₄)_x(SiO₄)_6?xO₂, anorthite and spinel phases. This layer provides effective prevention against CMAS further infiltration into the coating. Prolonged heat treatment densifies the reaction layer but does not change its phase composition.     

Hot corrosion studies of nanostructured gadolinium zirconate thermal barrier coatings

Improvement of hot corrosion resistance is one of the important parameters governing the lifetime and efficiency of the thermal barrier coatings (TBCs). In this study, the Gadolinium Zirconate (GZ) was synthesized by ball milling method and deposited by Electron Beam-Physical Vapour Deposition (EB-PVD) on Ni-based superalloy substrate with NiCrAlY as an intermediate bond coat. The effect of nanostructured GZ TBCs on hot corrosion resistance were studied under three different salt mixture environments viz; SM1, SM2 and SM3 in isothermal condition at 900 °C for 12 h. The results indicated that EB-PVD coated nanostructured GZ TBCs have improved the hot corrosion resistance and performed well under SM1 and SM3 conditions with minimal weight gain and without any spallation, whereas, the TBC suffered severe spallation under of SM2 salt condition with higher weight gain among the other two conditions. The formation of microcracks along the columnar gaps of the topcoat were found in the SM2 condition, have allowed the molten salts infiltration up to the coating interface. The formation of dense corrosive products GdVO₄ and m-ZrO₂ phases were identified after hot corrosion in SM1 and SM3 condition, which were absent in SM2 condition.     

Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Effects of Processing Parameters on the Deposition of Yttria Partially Stabilized Zirconia Coating During Suspension Plasma Spray

The process–microstructure relationship in suspension plasma spray (SPS) of yttria partially stabilized zirconia (YSZ) has been studied experimentally. An ethanol‐based suspension with a powder loading of 25 wt% was plasma sprayed with radial injection under four different plasma conditions, to examine the effects of plasma gas composition (Ar/He ratio), secondary gas (Ar/He and Ar/H₂), and the nozzle diameter of the plasma gun. The suspension feeding rate was optimized firstly and coatings were prepared for microstructural observation. Capturing of in‐flight particles into water as well as collection of splats formed on heated flat metal substrates were utilized in order to better understand the more complicated intermediate process steps in SPS. It was found that a plasma jet with higher momentum allowed a higher suspension flow rate and both columnar and deep vertically cracked structure could be created depending on the plasma parameters as well as the substrate surface roughness.     

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).     

Protocol for selecting exemplary silicate deposit compositions for evaluating thermal and environmental barrier coatings

A protocol for selecting representative silicate compositions for comparative testing of gas turbine coating materials is presented. It begins with a curated dataset of compositions of engine deposits and naturally occurring siliceous debris including volcanic ashes, sands, and dusts. The compositions are first reduced to the five major oxides—those of Ca, Mg, Fe, Al, and Si—and then distilled further using principal component analysis and k-means clustering. The process ultimately yields four classes of possible deposits with common chemical characteristics. Each class is represented by a composition centroid and a range in Ca:Si ratios. Key thermophysical properties of the possible deposits are calculated and related to the glass network connectivity, characterized by the Si:O ratio. Finally, deposits from each of these classes are compared in terms of their reactions with prototypical thermal and environmental barrier oxides, with due consideration of the effects of composition variations within each deposit class. The protocol is, in principle, adaptable to datasets compiled by OEMs and researchers in gas turbine coatings.     

Failure mechanisms of calcium magnesium aluminum silicate affected thermal barrier coatings

Depth profiles of the phase composition of two examples of calcium magnesium aluminum silicate (CMAS) affected thermal barrier coatings (TBCs) from an aero gas turbine engine were obtained using a monochromatic and collimated beam of synchrotron radiation. One TBC was deposited by plasma spray and the other by electron beam physical vapor deposition. These examples were complemented with an X‐ray diffraction (XRD) study of mixtures of TBC zirconia powder and sand heated in a furnace. The XRD results were compared with electron backscatter images and energy dispersive spectroscopy studies of the cross sections and mixtures. It was found that when liquid, the CMAS enhances mass transport leading to the densification of the zirconia, which then leads to spalling because of the increased residual stresses generated on cooling. Even without spalling densification will reduce a TBC's ability to thermally insulate. The enhanced mass transport can also lead to destabilization of the zirconia if yttrium ions preferentially transfer to the liquid or greater stabilization if calcium or magnesium ions transfer from the liquid to the zirconia. Zircon also precipitates when the zirconium from the TBC reacts with the silicon in the liquid CMAS.     

Thermal shock resistance of thermal barrier coatings modified by selective laser remelting and alloying techniques

Aiming to improve the thermal shock resistance of thermal barrier coatings (TBCs), the plasma-sprayed 7YSZ TBCs were modified by selective laser remelting and selective laser alloying, respectively, in this study. A self-healing agent TiAl₃ was introduced into the 7YSZ TBCs by selective laser alloying to fill cracks during thermal cycling. The thermal shock experiments of the plasma-sprayed, laser-remelted, and laser-alloyed TBCs were conducted by a means of heating and water-quenching method. Results revealed that some segmented microcracks were distributed on the surface of the laser-remelted and the laser-alloyed zones, showing a dense columnar crystal structure. After thermal shock tests, the numbers of segmented microcracks on the laser-remelted coating increased, whereas, in the laser-alloyed condition, some irregular particles formed, leading to the decreased numbers of segmented microcracks. The laser-alloyed coating exhibited the best thermal shock resistance, followed by the laser-remelted condition, with the thermal shock lifetime 3.3 and 2.7 times higher than that of the as-sprayed coating, respectively. On the one hand, both columnar grains and segmented microcracks in the laser-treated zone could effectively improve the strain tolerance of coatings. On the other hand, the oxidation products of TiAl₃ under high-temperature condition could seal the microcracks to postpone the crack connection. Thus, the thermal shock resistance of the laser-treated coatings was significantly improved.     

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl₁₁O₁₉ (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer TBCs.     

Introducing segmentation cracks in air plasma-sprayed thermal barrier coatings by controlling residual stress

Segmentation cracks are crucial for enhancing the strain tolerance and decreasing the propensity of delamination for thermal barrier coatings (TBCs). In this study, segmentation cracks were prepared in air plasma-sprayed TBCs by controlling the residual stress. The evolution of the stress in the coating was characterized via photoluminescence piezospectroscopy using trace α-Al₂O₃ impurities as stress sensor. Tensile stress (~170 MPa) formed in the as-deposited coating was converted into compressive stress through further thermal exposure. The relationship between the formation of the segmentation cracks and stress in the coating was investigated. It was demonstrated that the segmentation cracks could be formed when a critical coating thickness is achieved. The critical coating thickness and spacing of the segmentation cracks dependent on the tensile stress in the as-deposited coating, and they could be manipulated by controlling the deposition and substrate temperatures. In addition, the evolution of the microstructure and phase composition of the yttria-stabilized zirconia coating was examined.     

Characteristics and thermal cycling behavior of plasma-sprayed Ba(Mg1/3Ta2/3)O3 thermal barrier coatings

Ba(Mg_1/3Ta_2/3)O₃ (BMT) powders were synthesized by the solid state reaction method. BMT thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS). The phase composition and microstructure of the BMT coatings were characterized. The thermal cycling behavior of the BMT coatings was investigated by the water quenching method from 1150 °C to room temperature. The results reveal that BMT powders have an ordered hexagonal perovskite structure, whereas the as-sprayed coating of BMT has a disordered cubic perovskite structure because of the different degree of structural order for different treatment conditions. During thermal cycling testing, the entire spalling of coatings occurred within the BMT coating near the bond coat. This is attributed to the following reasons: (1) the growth of a thermally grown oxides (TGO) layer, which leads to additional stresses in the coatings; (2) the coefficient of thermal expansion mismatch between the BMT coating and bond coat, which develops enormous stress in the coatings; (3) the precipitation of Ba₃Ta₅O₁₅ due to the evaporation of MgO during the spraying process, which changes the continuity of the coatings.     

Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia

Gadolinium zirconate (Gd₂Zr₂O₇, GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3 mol% yttria partially-stabilized zirconia (YSZ) incorporation. The fracture toughness of the composite of 90 mol% GZ-10 mol% YSZ (GZ–YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ–YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock lifetime of GZ–YSZ composite coating was improved, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ–YSZ composite coatings were discussed.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.     

Scale-progressive healing mechanism dominating the ultrafast initial sintering kinetics of plasma-sprayed thermal barrier coatings

The thermal insulating performance of plasma-sprayed thermal barrier coatings (PS-TBCs) depends dominantly on inter-splat pores, which would be inevitably healed during high temperature exposure. The sintering kinetics of TBCs appears to be highly stage-sensitive. However, the ultrafast sintering kinetics during the initial sintering stage is not yet well understood. In this study, the sintering behavior of PS-TBCs was investigated in a scale-progressive (from nano- to micro-scale) way. Moreover, a novel healing mechanism suitable for lamellar TBCs was proposed based on a combined-effect of material and pore structure. Regarding the changing behavior of material, nano-scale roughening can be found at the as-deposited smooth pore surface after thermal exposure. Regarding the 2D featured inter-splat pores, the roughening behavior facilitates multiple contacts between the counter-surfaces of inter-splat pores. As a result, micro-scale bridge-connection can be observed at the healed parts. This multiple contacts mechanism caused by scale-progressive healing behavior significantly accelerated the matter transfer, resulting in ultrafast sintering kinetics at the initial sintering stage.