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.     

Preparation of plasma sprayed nanostructured GdPO4 thermal barrier coating and its hot corrosion behavior in molten salts

Nanostructured GdPO₄ coatings, designed as the outer layer of double-ceramic-layer thermal barrier coatings (DCL-TBCs), were produced by air plasma spraying (APS). The coatings have close chemical composition to that of the agglomerated particles used for thermal spray. Nanozones with porous structure are embedded in the coating microstructure, having a percentage of ~30%. Hot corrosion tests of the coatings were carried out in V₂O₅ and Na₂SO₄+V₂O₅ salts at 900 °C for 4 h. Results indicate that dense reaction layers, consisting of GdVO₄ and Gd₄(P₂O₇)₃, form on the coating surfaces, which could suppress further penetration of the molten salts. In the V₂O₅ molten salt, the reaction layer is thicker and less molten salt trace could be found beneath the layer.     

Effect of splat-interface discontinuity on effective thermal conductivity of plasma sprayed thermal barrier coating

The thermal barrier coating obtained by atmospheric plasma spraying (APS TBCs) has a distinct lamellar microstructure, in which the splats discontinuous interfaces running parallel to the metal/ceramic interface contribute largely to the reduction in the effective thermal conductivity of APS TBCs. The dependency of such contribution on the topological structure of the interface discontinuity is investigated in the present work. Firstly, the concept of discontinuity of splats interfaces was defined to quantify the splats discontinuous interfaces revealed by microscopic observations. Then, the microstructure model with a random distribution of discontinuous interfaces was established by utilizing the finite element simulation method to investigate the effect of interlayer discontinuity on thermal conductivity of the APS TBCs. Finally, an optimal topological structure of the interface discontinuity was found to be responsible for the lowest effective thermal conductivity of the APS TBCs and typical parametrical tendencies demonstrated.     

Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy

The thermal shock behavior of a thermal barrier coating (TBC) prepared by plasma spraying at 1100 °C was investigated. The TBC consisted of a double layer structure of 8YSZ/CoCrAlYTaSi. The morphology, microstructure, phases and the elemental distribution of the TBCs were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), X-ray diffraction (XRD) and electron probe micro-analysis (EPMA). The characterization results showed that the film consisted primarily of metastable tetragonal phases (t′), and a large number of micro-cracks were present in the 8YSZ crystals. Following eighty-six thermal shock cycles of the specimens a large areal spallation was observed on the 8YSZ coating. The decreased concentration of yttrium at the coating interfaces weakened the inhibition of crystal growth and the phase transition of the Al₂O₃. The growth of TGO (Thermal growth oxide) and the diffusion into the 8YSZ coating produced deformation and stress in the ceramic coating. Tantalum appeared to absorb the oxygen that diffused into the coatings and delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, which was beneficial to prolonging the life of the TBC.     

Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating

The crystallization behavior of the amorphous phase of the plasma sprayed LaMgAl₁₁O₁₉ (LaMA) coating during thermal aging processes has been investigated. Results indicate that LaMA coating exhibits much similar microstructure and thermal properties such as close coefficient of thermal expansion and specific heat capacity etc. to the sintered LaMA bulk after aging at 1673 K for 20 h. On the other hand, a solid state reaction seems to occur to reform the ideal magnetoplumbite-type LaMA phase coupled with the formations of the La-rich aluminate intermediate phases. When the aging temperature is held between 1273 K and 1473 K, nanosized platelet-like grains as well as sub-grains with high aspect ratios are present. The phase stability has been investigated through the chemical compositions and X-ray diffraction analysis. The recrystallization mechanism of the amorphous LaMA coating has been explored by tracing the microstructure evolutions during thermal aging process.     

Superposed structure of double-ceramic layer based on YSZ/LaMgAl11O19 thermal barrier coating

The superposed structure of double ceramic layer (SDCL) could be an effective means to develop long-life thermal barrier coating (TBC) at high temperatures. In this study, YSZ/LaMgAl₁₁O₁₉ TBC system with double-ceramic layer (DCL) and SDCL structures were prepared on nickel-based superalloy substrates by atmospheric plasma spraying. The thermal cycling behavior of the coatings was investigated using a furnace at 1000 °C and burner-rig facility at 1375 ± 25 °C on the coating surface. Results showed that the thermal cycle life of the SDCL structure was increased by 7.2% for the furnace and 13.2% for the burner-rig facility compared with that of the DCL structure. The relatively long thermal cycle life of the SDCL structure was attributed to the blocking of the propagation of cracks in the LMA layers by the YSZ ceramic layer and the release of residual thermal stresses by the formation of cracks in the LMA layers.     

Hot corrosion behavior of BaLa2Ti3O10 exposed to calcium-magnesium-alumina-silicate at elevated temperatures

Calcium-magnesium-alumina-silicate (CMAS) attack has been regarded as one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this study, CMAS corrosion behavior of BaLa₂Ti₃O₁₀, a novel TBC material, is investigated at 1300?°C and 1350?°C for 0.5?h, 4?h, 12?h and 24?h. Results reveal that BaLa₂Ti₃O₁₀ has high resistance to molten CMAS infiltration, attributable to the formation of a dense reaction layer. X-ray diffraction, scanning electron microscope, energy dispersive spectroscope, transmission electron microscope confirm that the layer consists of apatite, celsian and perovskite phases. With increased corrosion duration, the layer retains good phase stability and the thickness increases. The formation of corrosion products and the reaction layer are discussed according to a dissolution-reprecipitation mechanism and the optical basicity theory.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Review of suspension and solution precursor plasma sprayed thermal barrier coatings

As one of the promising methods that can be employed to fabricate high-performance thermal barrier coatings (TBCs), suspension plasma spraying (SPS) or solution precursor plasma spraying (SPPS) has received significant attention in academic research. Enhanced performances have been shown in the SPS-/SPPS-coatings due to their special microstructures, such as uniformly distributed micro-pores, vertical cracks or columnar structures. Since there are more complexities than conventional plasma spraying methods, many works have been devoted to study the mechanism and properties of SPS-/SPPS-coatings during the past decades. In this work, the latest development of SPS or SPPS is reviewed in order to discuss some key issues in terms of preparation of suspension or solution precursor, injection mode of liquid phase, interaction between liquid and plasma jet, microstructure of as-sprayed coatings and corresponding deposition mechanism. Meanwhile, the potential application of SPS or SPPS in some new-type TBCs is introduced at the end of this paper.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

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).     

Influence of SiC fiber on thermal cycling lifetime of SiC fibers /YSZ thermal barrier coatings by atmospheric plasma spraying

The initiation and propagation of cracks under thermal stresses easily is one of the problems limiting the thermal cycling lifetime of thermal barrier coatings (TBCs). In order to improve the thermal cycling lifetime, SiC fibers were introduced to yttria stabilized zirconia (YSZ) coating deposited on In738LC substrate by atmospheric plasma spray (APS). Phase composition, thermal cycling behaviors and fiber toughening mechanisms of coatings were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal cycling test. Results showed that the thermal cycling lifetime and fracture toughness of SiC fibers/YSZ coatings could reach 442?±?13 and 1.54?±?0.19?MPa $\sqrt{m}$ respectively, which were 1.6 times and 1.3 times higher than that of conventional TBCs. There are two stages of fiber reinforced during thermal cycling, and the first is crack deflection and termination, the second is fiber debonding, pull-out, breakage and bridging. Meanwhile, SiC fibers could prevent the stress-activated ZrO₂ martensitic transformation by reducing the stress in the lattice.     

Atmospheric plasma sprayed 7%-YSZ thick thermal barrier coatings with controlled segmentation crack densities and its thermal cycling behavior

Yttria-stabilized zirconia (YSZ)-coatings are deposited on Ni-based superalloy IN738 by atmospheric plasma spraying (APS). For the first time, controlled segmentation crack densities are manually developed in the coatings, even after the APS deposition. This method allows to user to control segmentation densities as well as cracks depth, which could be designed as per coating thickness and required application. Thermal cycling test shows promising strain tolerance behavior for the segmented coatings, whereas coating without segmentation could not sustain even for its first thermal cycle period. Further, microstructural studies reveal that a very thin layer of TGO was formed and obvious no coating failure or spallation was observed after thermal cycling test at 1150 °C for 500 cycles.     

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.     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Structure-property relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades

Novel lanthanum-cerium oxide/8?wt% yttria partially stabilized zirconia (LC/8YSZ) thermal barrier coatings (TBCs) were deposited by supersonic atmospheric plasma spraying. The thermal insulation temperature and thermal shock resistance of LC/8YSZ double-ceramic-layer TBCs (DCL-TBCs) were quantitatively evaluated by a burner rig test. The results showed that the thermal insulation temperature increased with the increase of LC layer thickness in DCL-TBCs. When the thickness ratio between LC layer and 8YSZ layer was close to 1:1, the DCL-TBCs had the highest thermal shock resistance. LC/8YSZ thickness ratio significantly affected the energy release rate and the stress induced by thermal gradient or sintering. The sintering stress was found to be the main reason that caused the delamination of LC layer, however, the stress induced by thermal gradient resulted in the spallation of YSZ layer.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Yb2O3 and Gd2O3 doped strontium zirconate for thermal barrier coatings

Yb₂O₃ (10 mol%) and Gd₂O₃ (20 mol%) doped SrZrO₃ was investigated as a material for thermal barrier coating (TBC) applications. The thermal expansion coefficients (TECs) of sintered bulk Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 were recorded by a high-temperature dilatometer and revealed a positive influence on phase transformations of SrZrO₃ by doping Yb₂O₃ or Gd₂O₃. The results for the thermal conductivities of Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 indicated that both dopants can reduce the thermal conductivity of SrZrO₃. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 showed lower Young's modulus, hardness and comparable fracture toughness with respect to YSZ. The cycling lifetimes of Sr(Zr_0.9Yb_0.1)O_2.95/YSZ and Sr(Zr_0.8Gd_0.2)O_2.9/YSZ double layer coatings (DLC), which were prepared by plasma spraying, were comparable to that of YSZ at operating temperatures <1300 °C. However, the cycling lifetime of Sr(Zr_0.9Yb_0.1)O_2.95/YSZ DLC was 25% longer, whereas Sr(Zr_0.8Gd_0.2)O_2.9/YSZ DLC had a shorter lifetime compared to the optimized YSZ coating at operating temperatures >1300 °C.     

Microstructure assessment of suspension plasma spraying coatings from multicomponent submicronic Y-TZP/Al2O3/SiC particles

In this research, Suspension Plasma Spraying (SPS) technique was used for the thermal deposition of a multicomponent mixture made up of an Y-TZP/Al₂O₃ matrix with SiC particles. Two suspensions of Y-TZP and Al₂O₃ with different SiC particles content (6?wt% and 12?wt%) were tested as feedstocks in the SPS process. Three stand-off distances were varied in order to assess coating microstructure and evaluate the presence of SiC in the final coatings. Coatings were characterised in terms of porosity, microstructure and phase distribution. The estimate of the amount of SiC in the coating was carried out by XRD technique.Findings showed typical cauliflower-like SPS microstructure which intensifies with stand-off distance. Coatings porosity varied significantly between 8% and 25% whereas minimum porosity was found for the intermedium stand-off distance of 40?mm.Microstructure analysis also revealed the presence of SiC particles in the coatings which was confirmed by EDX analysis, overall XRD tests as well as TG analysis. Finally, evaluation of SiC content in the final coatings by means of XRD analysis showed that most of SiC particles (c.a 80%) of the feedstocks were preserved in the final coatings.