The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl₂O₄) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO₃), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO₃ intermediate phase was not observed and La₄Al₂MgO₁₀ was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.     

Preoxidation of bond coat in IN-738LC/NiCrAlY/LaMgAl11O19 thermal barrier coating system

The low thickness of thermally grown oxide (TGO) layer and presence of amorphous phase in the as-sprayed LaMgAl₁₁O₁₉ (LaMA) coating reduce the thermal cycling lifetime of thermal barrier coatings (TBCs). In the present study, the as-sprayed Ni-22Cr-10Al-1.0Y bond coat was preoxidized at 1060?°C to produce a continuous oxide scale prior to subsequent deposition of the ceramic top coat. The optimum time of peroxidation treatment and thickness of the continuous aluminum oxide layer were estimated 15?h and 2?µm respectively. The oxidized layer due to the preoxidation treatment of bond coating reduces the amorphous phase in as-sprayed LaMA coating and increases the microhardness of LaMA coating from approximately 600 to 900HV. Also, preoxidation of the NiCrAlY bond coating increases adhesion strength of the LaMA top coating, even slightly more than the adhesion strength of the as-spray 8YSZ coating. The LaMA coatings have a lower hardness in compared with the 8YSZ coating (~ 1010Hv), which results a better elastic behavior.     

Mg2SiO4 as a novel thermal barrier coating material for gas turbine applications

Forsterite-type Mg₂SiO₄ was investigated systematically for thermal barrier coating (TBC) applications. Results showed that Mg₂SiO₄ synthesized by solid-state reaction possessed the good phase stability up to 1573 K. The thermal conductivity of Mg₂SiO₄ at 1273 K was lower ˜20% than that of yttria stabilized zirconia (8YSZ). Mg₂SiO₄ also presented moderate thermal expansion coefficients, which increased from 8.6 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (473˜1623 K). Mechanical properties including hardness, fracture toughness, and Young’s modulus of Mg₂SiO₄ were comparable to those of 8YSZ. The sintering results indicated a promising low-sintering activity of Mg₂SiO₄. Mg₂SiO₄ samples were subjected to water quenching test at 1573 K and showed a superior thermal shock resistance compared to 8YSZ. Mg₂SiO₄ coating with stoichiometric composition was produced by atmospheric plasma spraying. The thermal cycling test result showed that Mg₂SiO₄ coating had a lifetime more than 830 cycles at 1273 K, which is desirable for TBC applications.     

Effect of neck formation on the sintering of air-plasma-sprayed thermal barrier coating system

Sintering neck is a featured microstructure that may have significant effect on the sintering behaviour of air-plasma-sprayed thermal barrier coating system (APS TBCs). Based on experimental observations, a multi-necking wedge-shaped model for the sintering of APS TBCs was proposed by considering the sintering stress as surface tension and by employing the thermal-elasto-viscoplastic constitutive relation. Deformation pattern, stress distribution, sintering induced shrinkage, stiffening behaviour and temperature field were analysed by using finite element method. It is shown that the formation of sintering neck significantly affects thermal and mechanical properties related to sintering. Mechanisms of thermal and mechanical degradation induced by sintering were further elucidated.     

Effect of environmental pressure on the microstructure of YSZ thermal barrier coating via suspension plasma spraying

Suspension plasma spraying (SPS) as a potential technique to prepare thermal barrier coatings (TBCs) has been attracting more and more attention. However, most reports on SPS were carried out in the atmosphere. Given the unique features of in-flight particles and plasma jets under low pressure, the resulting coatings are expected to be different from those under atmospheric pressure. In this article, yttria-stabilized zirconia (YSZ) thermal barrier coatings were prepared using suspension plasma spraying under different environmental pressures. The results show that as the environmental pressure decreased, the column-like structural coating turned into a vertical crack segmented structure, as well as a dramatic decrease in surface roughness. More nanoparticle agglomerates were formed in the coating under lower environmental pressures. The real porosity of the coating increased with a decrease in environmental pressure.     

Effect of TGO thickness on the thermal barrier coatings life under thermal shock and thermal cycle loading

Effect of thermally grown oxide (TGO) thickness on thermal shock resistance of thermal barrier coatings (TBCs) and also their behavior under a cyclic loading (including aging at maximum temperature) was evaluated experimentally. In order to form different thicknesses of TGO, coated samples experience isothermal loading at 1070?°C for various periods of times. Heat-treated samples were heated to 1000?°C and cooled down rapidly in water from the substrate side using a mechanical fixture. The life of samples was investigated as a function of TGO thickness. Furthermore, by performing an experiment the simultaneous effect of the TGO growth and thermal expansion mismatch– on the failure of thermal barrier coatings was evaluated. The results demonstrated that the presence of TGO with a thickness of 2–3?µm has a positive effect on the resistance against thermal shock.     

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.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.     

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.     

Influences of interface morphology and thermally grown oxide thickness on residual stress distribution in thermal barrier coating system

Calculation of residual stress with finite element method is a basic work in failure mechanism investigation in thermal barrier coating (TBC) system because the residual stress is main driving force for crack nucleation and propagation. In this work, a complicated cosine curve with gradually increasing amplitude was used to simulate interface morphologies between layers so as to study the residual stress behavior during the cooling process in air plasma spraying TBC system by finite element method. The substrate, thermally grown oxide (TGO) and top coat (TC) are considered to be elastic and bond coat (BC) elastic-perfectly plastic. The material properties are all temperature dependent. The stress result comparison between models with and without substrate shows the effect of substrate on the residual stress distribution around layers interfaces should not be ignored as the substrate influences the value of normal residual stress as well as the stress distribution along undulating interfaces. Then the model with substrate was used to study the residual stress evolution along interfaces during cooling down from the temperature of 1000 °C to room temperature. The influences of the thickness of TGO and the amplitude and wavelength of interface on the residual stress distributions near interfaces were considered. The results show that these influences are very complicated. Meanwhile, it's found that the hybrid roughness parameter containing information for height and spacing is more suitable to describe the interface complicacy. The results facilitate understanding the failure mechanism relevant to interface morphology and TGO thickness.     

Effect of microstructure on the performance of Zr6Ta2O17 ceramics as thermal barrier coatings

In this study, Zr₆Ta₂O₁₇ ceramics with porous, fine-grained, and coarse-grained structures were obtained via in situ solid-state reactions, and their mechanical characteristics were examined. The significantly low thermal conductivity of dense Zr₆Ta₂O₁₇ ceramics (1.0 W m⁻¹ K⁻¹) was due to the grain boundary gap caused by superstructured grains. A calcium–magnesium–alumina–silicate (CMAS) corrosion experiment demonstrated that the formation of an interlocking structure composed of ZrO₂, CaTa₂O₆, and ZrSiO₄ prevented the penetration of CMAS impurities, thereby revealing the application potential of porous ceramics. In dense Zr₆Ta₂O₁₇ ceramics, the low-volume diffusion induced by an entropy-stable structure is conducive for corrosion resistance; however, the grain boundary is vulnerable to attacks by CMAS, which can be mitigated by the formation of a coarse crystal structure, thereby effectively improving the corrosion performance. This work provides a critical perspective on the thermal barrier coating design of A₆B₂O₁₇ (A = Zr, Hf; BNb, Ta) ceramics.     

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.     

Effect of functionally graded structure design on durability and thermal insulation capacity of plasma-sprayed thick thermal barrier coating

This paper aimed to design and optimize the structure of a thick thermal barrier coating by adding graded layers to achieve a balance between high thermal insulation capacity and durability. To this end, conventional TBC, conventional TTBC, and functionally graded TTBCs were deposited on the superalloy substrate by air plasma spraying. To determine the quality of the bond strength of the coatings, the bonding strength was measured. The durability of coatings was evaluated by isothermal oxidation and thermal shock tests. Then, at a temperature of 1000 °C, the thermal insulation capacity of the coatings was carried out. The microstructure of the coatings was characterized by a scanning electron microscope. The results showed that the thickness of the TGO layer formed on the bond coat in the conventional TBC and TTBC under the oxidation test at 1000 °C after 150 h was 2.79 and 2.11 μm, respectively, whereas, in the functionally graded TTBC samples, no continuous TGO layer was observed as a result of internal oxidation. The functionally graded TTBC presented higher durability than conventional TTBC due to improved bonding strength, thermal shock resistance, and the lack of a TGO layer at the bond/top coat interface. Also, the thermal insulation capacity of the functionally graded TTBC (with 1000 μm thickness of YSZ coating) was better than TTBC.     

Effect of substrate curvature on residual stresses and failure modes of an air plasma sprayed thermal barrier coating system

A set of aerofoil shaped air plasma sprayed thermal barrier coated (APS-TBC) specimens were adopted in this paper to investigate the stress distributions in the ceramic top coat (TC) and the thermally grown oxide (TGO), the mechanism of local crack generation and propagation at the TC/BC (bond coat) interface. The failure mode of the TBC system, the distribution of asperities at TC/BC interface, thickness of the TC and BC, and the TC microstructure were found to be influenced by substrate curvature. Residual stress was therefore measured across the thickness of the TC, along the undulating TGO and mapped at locations of asperities where failure tended to occur to interpret the initiation of local failure. The role of the TGO was investigated via its chemical bonding with the TC and the decohesion occurring at the TGO/BC interface. The crack propagation at the interface has been discussed with respect to the macro-failure of the TBC system.     

Simulation of residual stress in thick thermal barrier coating (TTBC) during thermal shock: A response surface-finite element modeling

The current study demonstrates a well-designed response surface methodology (RSM), based on the generated dataset of finite element method (FEM) to establish an integrated model for simulation of residual stress distribution in a thick thermal barrier coating (TTBC). In this study, typical TTBCs were applied on Hastelloy X Nickel-based superalloy using air plasma spray technique followed by thermal cycling. The recorded stress data of Raman spectroscopy was employed to verify the proposed FEM model. A relatively good agreement was obtained between predicted residual stresses and measured ones. Verified FEM model was used to carry out the parametric studies to evaluate the effects of such various parameters as interface amplitude, wavelength, thermally grown oxide thickness and preheating temperature on the stress distribution in the TTBC during the thermal cycling. The computed data were subsequently used for the development of RSM model. In conclusion, experimentally verified numerical data was used to construct a statistical model based on RSM and successfully used to predict the residual stress distribution field in TTBC during thermal cycling. The obtained results of hybrid FEM- RSM model were in acceptable conformity with Raman spectroscopy measurements.     

High-entropy thermal barrier coating of rare-earth zirconate: A case study on (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 prepared by atmospheric plasma spraying

We report a double-ceramic-layer (DCL) thermal barrier coating (TBC) with high-entropy rare-earth zirconate (HE-REZ) as the top layer and yttria stabilized zirconia (YSZ) as the inner layer sprayed on Ni-based superalloy by atmospheric plasma spraying. La₂Zr₂O₇ (LZ) was selected as a reference for the HE-REZ. Thermal cycling test results demonstrate that the HE-REZ/YSZ DCL coating exhibited obviously improved thermal stability when compared to the LZ/YSZ DCL coating. The reasons for the improvement of the thermal shock resistance are considered to be the anti-sinterability of the HE-REZ ceramics during the thermal cycling test attributed to the sluggish diffusion effect and as well as the better match in the coefficient of thermal expansion of HE-REZ coating with the YSZ inner layer. In addition, the HE-REZ coating maintains fluorite structure after thermal cycling test. This study makes one step forward in the development and application of high-entropy rare-earth zirconate ceramic thermal barrier coatings.     

Composition effects on elastic,thermal and corrosion properties of multiple-RE silicate (Ho1/4Er1/4Yb1/4Lu1/4)2SiO5 as a promising thermal and environmental barrier coating material

(Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂SiO₅ is synthesized and characterized for the application of a promising multifunctional thermal and environmental barrier coating (TEBC) material. X-ray diffraction and scanning electron microscopy analysis indicate that a X2-type multiple-RE silicate (4RE_1/4)₂SiO₅ is formed with homogeneous distribution of the four rare earth species. Dense bulk sample exhibits excellent phase stability up to 1400 °C. Key properties including Young’s modulus, thermal conductivity and thermal expansion coefficient show interesting composition effects. Specially, (Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂SiO₅ demonstrates higher elastic stiffness, lower thermal conductivity, lower thermal expansion coefficient and good resistances to molten CMAS and water vapor corrosions. These results confirm the strategy of multiple-RE engineering that may provide optimal property of advanced TEBCs.     

Effect of partial crystallization of an amorphous layer on the mechanical properties of ceramic/metal-glass coating by thermal spraying

On the basis of successful preparation of amorphous-ceramic composite coating by atmospheric plasma spraying, a nanostructure bond-coat was prepared in-situ by inducing partial crystallization of an amorphous layer through heat treatment. The unit mechanics contribution rate (UMCR) was proposed to evaluate the mechanical properties of the coating, and the interference of the substrate to the evaluation of the coating mechanical properties was removed. In this work, the mechanical properties of the coating were systematically evaluated at the macroscopic, mesoscopic and microscopic scales through three-point bending (3 PB), microhardness and nanoindentation tests, respectively. Results show that the nanoparticle-ceramic coatings formed by heat treatment have a higher hardness and Young's modulus. The mechanical properties at the micro scale were obviously better than those at the macro scale. A partial crystallization was observed in the amorphous bond-coats in the process of heat treatment, and a large number of nanoparticles and solid solution were formed in the nanoparticle-ceramic coatings, effectively hindering the crack growth and improving the coating performance.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.