Application Of Thermal Barrier Coatings Research Articles

DFT analysis of mechanical, thermal, half-metallic, and thermoelectric properties of perovskites PrMnO3 and NdMnO3 ortho-manganite

In this study, we present a theoretical investigation of the structural, mechanical, thermal, electronic, magnetic and thermoelectric properties of orthorhombic rare earth perovskites RMnO3 (R=Pr- Nd). We used density functional theory (DFT) and the full-potential linearized augmented-plane wave method (FP-LAPW) as implemented in the WIEN2K package. We found that our proposed compounds are mechanically stable and elastically anisotropic materials. By examining the thermal characteristics of these compounds, we discovered that they exhibit low thermal conductivity, this feature has practical applications in thermal barrier coatings (TBC). Therefore, within both GGA and GGA + U approximations, we found that these materials are half-metal ferromagnetic (HMF) and maintaining their half-metallicity under stress, this suggests that orthorhombic rare earth perovskites RMnO3 might be appropriate for spintronic devices. Finally, we explored the thermoelectric properties as a function of temperatures using Boltzmann transport theory as implemented in the BoltzTrap code.

High-temperature oxidation and diffusion studies on selected Al–Cr–Fe–Ni high-entropy alloys for potential application in thermal barrier coatings

In this work, the properties of two high-entropy alloys (HEAs) Al25Cr20Fe20Ni35 and Al20Cr20Fe20Ni40 (at.%) were compared for potential application as bond coats in thermal barrier coatings (TBCs). For this reason, both their oxidation resistance under isothermal and thermal shock conditions, as well as diffusion phenomena occurring at the substrate-HEA diffusion couple interface, were investigated. It was determined that both alloys demonstrate good phase stability and oxidation resistance during prolonged exposure to air atmosphere at 1000°C. In the initial 100 h period, selective aluminum oxidation is responsible for the protective scale growth on both materials under both isothermal and thermal shock conditions. However, only the Al20Cr20Fe20Ni40 high entropy alloy maintains a single Al2O3 layer for over 1000 h of oxidation with cyclic temperature changes. On the other hand, diffusion studies indicate that mainly iron and chromium travel between the ferritic steel substrate and the HEAs. Aluminum diffusion is seemingly limited by the unexpected formation of a thin aluminum nitride layer at the interface between the materials. From all these results it can be concluded that the Al20Cr20Fe20Ni40 alloy shows more promise for application in TBCs based on high entropy materials than Al25Cr20Fe20Ni35.

Effects of Induction Plasma Spheroidization on Properties of Yttria-Stabilized Zirconia Powders for Thermal Barrier Coating Applications.

In this study, the induction plasma spheroidization (IPS) technique was adopted to improve the microstructure and properties of the traditional agglomerated ZrO2-7wt%Y2O3 (YSZ) powders used in thermal barrier coating (TBC) applications. Compared with agglomerated YSZ powders, IPS-treated powder has a more desirable microstructure, and the overall performance of the spray powders for TBC preparation is significantly improved. Specifically, IPS-treated powder has a dense, solid, defect-free, and chemically uniform microstructure, and its apparent density, flowability, and powder strength are significantly improved, which is believed to substantially enhance the coating performance when prepared with this IPS-treated powder.

Interfacial Stability between High-Entropy (La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 and Yttria-Stabilized Zirconia for Advanced Thermal Barrier Coating Applications

(La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (HEZ) has shown considerable promise as a novel thermal barrier coating material for temperatures exceeding 1300 °C. This study systematically investigates the interfacial stability of (La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 with yttria-stabilized zirconia (YSZ), which is of paramount importance for its application in double-layer thermal barrier coatings. Our findings highlight that rare earth elements with a smaller radius diffuse more easily into the YSZ lattice, resulting in a broader diffusion zone. Simultaneously, the incorporation of rare earth elements into the YSZ lattice inhibits tetragonal-to-monoclinic phase transformation. Compared to La2Zr2O7/YSZ, HEZ/YSZ demonstrates superior high-temperature stability, which could be attributed to the higher fracture toughness and lower thermal expansion coefficient of HEZ, the absence of t-m transformation and the formation of a continuous gradient diffusion layer that minimizes interface stress. This study offers a practical strategy for designing materials for durable double-layer thermal barrier coating systems.

Reliability Evaluation of EB-PVD Thermal Barrier Coatings in High-Speed Rotation and Gas Thermal Shock

The uncertain service life of thermal barrier coatings (TBCs) imposes constraints on their secure application. In addressing this uncertainty, this study employs the Monte Carlo simulation method for reliability evaluation, quantifying the risk of TBC peeling. For reliability evaluation, the failure mode needs to be studied to determine failure criteria. The failure mode of high-speed rotating TBCs under gas thermal shock was studied by combining fluid dynamics simulations and experiments. Based on the main failure mode, the corresponding failure criterion was established using the energy release rate, and its limit state equation was derived. After considering the dispersion of parameters, the reliability of TBCs was quantitatively evaluated using failure probability and sensitivity analysis methods. The results show that the main mode is the fracture of the ceramic layer itself, exhibiting a distinctive top-down “step-like” thinning and peeling morphology. The centrifugal force emerges as the main driving force for this failure mode. The failure probability value on the top side of the blade is higher, signifying that coating failure is more likely at this location, aligning with the experimental findings. The key parameters influencing the reliability of TBCs are rotation speed, temperature, and the thermal expansion coefficient. This study offers a valuable strategy for the secure and reliable application of TBCs on aeroengine turbine blades.

Elevating the comprehensive thermal properties of rare-earth cerates through high-entropy design

In the rapidly evolving aerospace sector, the quest for sophisticated thermal barrier coating (TBC) materials has intensified. These materials are primarily sought for their superior comprehensive thermal characteristics, which include a low thermal conductivity coupled with a high coefficient of thermal expansion (CTE) that synergizes with the substrate. In our study, we adopt a solid-state method to synthesize a series of high-entropy rare-earth cerates: La2Ce2O7 (1RC), (La1/2Nd1/2)2Ce2O7 (2RC), (La1/3Nd1/3Sm1/3)2Ce2O7 (3RC), (La1/4Nd1/4Sm1/4Eu1/4)2Ce2O7 (4RC), and (La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2Ce2O7 (5RC), all sintered at 1,600 °C for 10 h. We thoroughly examine their phase structure, morphology, elemental distribution, and thermal properties. Our in-depth analysis of the phonon scattering mechanisms reveals that 4RC and 5RC exhibit exceptional thermal properties: high CTEs of 13.00 × 10−6 K−1 and 12.77 × 10−6 K−1 at 1,400 °C, and low thermal conductivities of 1.55 W/(m·K) and 1.68 W/(m·K) at 1,000 °C, respectively. Compared to other TBC systems, 4RC and 5RC stand out for their excellent thermal characteristics. This study significantly contributes to the development of high-entropy oxides for TBC applications.

A novel dual-phase high-entropy (La0.2Nd0.2Gd0.2Er0.2Yb0.2)2Zr2O7 ceramic for thermal barrier coatings applications: Preparation, microstructure, and thermo–physical properties

High-entropy rare earth zirconates (RE2Zr2O7) have attracted significant research attention for thermal barrier coatings (TBCs) application due to their superior performance. However, currently used high-entropy RE2Zr2O7 shows only the modest improvements, thus limiting their overall performance enhancement. Therefore, novel high-entropy RE2Zr2O7 (La0.2Nd0.2Gd0.2Er0.2Yb0.2)2Zr2O7 (HEC) was synthesized in this study by chemical co-precipitation method, which was characterized by dual-phase and significant differences in ionic radius and atomic mass. Compared with single-component RE2Zr2O7 (RE = La, Nd, Gd, Er, and Yb) ceramics, HEC demonstrates significantly superior performance, including higher hardness (11.705 GPa), fracture toughness (1.693 MPa m1/2), and thermal expansion coefficient (11.1926 × 10−6/K, 1400 °C), and lower thermal conductivity (1.33–1.43 W/(m•K), 26–1000 °C). The comprehensive performance improvement can be attributed to the high-entropy effect, dual-phase nature, and significant differences in ionic radius and atomic mass. This study provides valuable insight into the development of high-entropy RE2Zr2O7 and suggests their significant potential in TBCs.

Hot corrosion behaviour of mullite thermal barrier coatings for marine diesel engines

Mullite's inherent qualities have made it a potential material for the application of thermal barrier coatings (TBCs) for diesel engine components. Hot corrosion at 600–800 °C can cause TBC degradation, thus significantly affecting the performance of engine components and reducing their service life. This work examines the hot corrosion behaviour of atmospheric plasma-sprayed (APS) mullite coating over NiCrAlY bond coat on mild steel substrates. The coated specimen surface was covered with a mixture of Na2SO4 (sodium sulphate) and V2O5 (vanadium pentoxide) in the form of paste and heated in a muffle furnace at 700 °C for up to 300 h. SEM, EDS, and XRD characterisations were used to investigate the mechanism of hot corrosion. Coatings remained intact after corrosion tests; however, it had reacted with corrosive salts, particularly sodium sulphate. This was evidenced by the removal of amorphous silica, followed by the formation of nosean as a major phase. During the reaction between sodium sulphate and mullite coating, vanadium pentoxide was found to be acting as a flux and mineraliser.

Thermal–Fluid–Solid Coupling Analysis on the Effect of Cooling Gas Temperature on the Fatigue Life of Turbine Blades with TBCs

The application of thermal barrier coatings (TBCs) can increase the blade’s operating temperature and fatigue life. Previous studies have neglected the effects of cooling gas temperature variations on the temperature field, stress field, and fatigue life of blades and TBCs. For this reason, this paper considers the inhomogeneity of the high-temperature gas inlet temperature, the internal cooling gas temperature, the periodicity of the external flow field, etc., and establishes a finite element model of the gas turbine blade with TBCs. Then, the life of the blade and the TBCs is predicted based on the Ncode 2020R2. Finally, the effect of the cooling gas temperature on the temperature, stresses, and life of the blade and the TBCs is analyzed. The results show that the fatigue life of the TBCs is lower than that of the blade, and the low-life region of the TBCs is located at the leading edge of the blade, which is consistent with the TBCs shedding region of the real blade and verifies the accuracy of the life prediction method in this paper. The fatigue life of the blade and TBCs firstly increases and then decreases with the increase in the cooling gas temperature, and the trend of the stress changes in the opposite direction. When the cooling gas temperature is increased from 573 K to 873 K, the minimum life of the blade is increased by 358.5%, and that of the TBCs is increased by 138.7%. The conclusions can provide guidance for the design of long-life turbine blades with TBCs.

CaO–MgO–Al2O3–SiO2 (CMAS) corrosion behaviour of LaMgAl11O19/GdPO4 thermal barrier coating materials

CaO–MgO–Al2O3–SiO2 (CMAS) corrosion poses serious hidden dangers for the application of thermal barrier coatings (TBCs). In this study, LaMgAl11O19 (LMA) and GdPO4 were mixed at molar ratios of 2:1, 1:1 and 1:2 to prepare LMA/GdPO4 materials, and the CMAS corrosion behaviours of these materials were investigated at 1300°C–1500 °C for 20 h and 40 h. It was demonstrated that temperature was the main factor influencing the corrosion behaviours and products. The materials were damaged at 1300 °C by the crystallization of CMAS melts to form CaAl2Si2O8. In contrast, the materials were corroded by CMAS melts via the reaction between CMAS and GdPO4 at 1500 °C. These results indicate that the addition of GdPO4 to LMA can improve the resistance of the LMA material to CMAS corrosion.

Low thermal conductivity and thermal radiation of La2Zr2O7 ceramics with hierarchical structure above 1000 K

The hierarchically porous La2Zr2O7 (LZO) microsphere fabricated by the electro-spraying assisted phase inversion (ESP) method is a promising feedstock for thermal barrier coatings (TBCs), due to its low thermal conductivity, quite high near-infrared reflectivity and high temperature structure stability. However, the thermal properties of hierarchically porous LZO ceramics above 1000 K are still unclear, limiting its application in TBCs. The effect of hierarchical structure on thermal conduction and thermal radiation was investigated in comparison with mono-sized pores, using the ESP microspheres (LZO-ESP) and other two carbon fillers to modulate the pores. The results showed that LZO-ESP samples exhibited ultra-low thermal conductivity (1.28 W m−1 K−1 at 1173 K), which was 30% lower than that of the reference samples of similar porosity. Heat flux simulation revealed that hierarchical structure can induce more disorder domains than the randomly distributed mono-sized pores. The disorder domains with high tortuosity of heat flux played a predominant role in the reduction of thermal conductivity. Additionally, the hierarchical structure significantly reduced thermal radiation by 36% compared with dense sample due to the decrease in transmissivity.

Trends in mechanical, anisotropic, electronic, and thermal properties of MAX phases: a DFT study on M2SX phases

Based on the density functional theory, the structural, mechanical and thermal properties of 27 S-based M2SX (M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; X = B, C, N) phases have been investigated systematically. The calculated formation enthalpies (ΔH) reveal that W2SC and W2SN are thermodynamically unstable phases due to the positive ΔH, and the V2SN, Nb2SN, and Mo2SC are mechanically unstable phases with C12 > C11. The calculated phonon spectrum indicates that 16 phases are dynamically stable phases except M2SN (M=V, Nb, Ta), M2SX (M=Cr, W; X = B, C, N), Mo2SC, and Mo2SN phases. Among the 16 stable phases, the Ti2SB, V2SB, Ta2SB and Ta2SC are predicted in this work, for the other 12 stable phases, the physical properties have been reported more or less. From the calculated results, V2SC, Nb2SC, and Ta2SC are resistant to compression and thermal shock, and which exhibit potential application in thermal barrier coatings. All the stable phases are more compressible along the a-axis, which means that c-axis is stiffer owing to strong atomic bond. Electronic structures indicate the stable phases are all metallic, and the Mo2SB and Ta2SC can be applied in thermoelectric material with very low lattice thermal conductivity. All the present results could motivate exploration and research on novel S-based MAX phases in the future.

Non-destructive evaluation of corrosion degree of thermal barrier coatings using fluorescence

With the increasing application of thermal barrier coatings (TBCs) in marine environments, the failure judgment of TBCs is becoming increasingly important. This paper introduces a method for non-destructive testing of the degree of corrosion of TBCs under alternating high and low temperatures and a Cl−-enriched environment. A Y2O3-8 wt.% ZrO2 + 1 wt% Tb2O3 (YSZ+Tb) coating (where YSZ is yttria-stabilised zirconia) was prepared by atmospheric plasma spraying, and its green fluorescence attributed to the 5D4→7F5 transition of Tb3+ ions was investigated. Moreover, the fluorescence intensity of the YSZ+Tb coating was reduced after corrosion due to a change in the Tb3+ crystal field structure caused by phase transition of the ceramic layer. The relationship between the fluorescence and degree of corrosion of the coating in alternating high and low temperatures was established.

Alternative Materials for Performant TBCs: Short Review

Thermal Barrier Coating (TBC) is a thermal insulation, which enables the coated substrate material to work above its melting temperature. The TBCs have been used to enhance the performance of the gas turbine for aeronautic and energy applications. Yttria stabilized zirconia YSZ (ZrO2 +7-8 wt.% Y2 O3 ) is a topcoat ceramic which is applied for more than 40 years to gas turbine components. YSZ has a high toughness and a good temperature capability up to about 1200 °C, higher operating temperatures is required for enhanced efficiency of gas turbine. Alternative materials for TBC application were developed during the last years allowing a higher temperature capability and lower thermal conductivity combined to higher toughness and thermochemical stability a of the TBCs.

Understanding the ultralow lattice thermal conductivity of monoclinic RETaO4 from acoustic‐optical phonon anti‐crossing property and a comparison with ZrO2

AbstractThe low thermal conductivity (κ) has a significant impact on the application of thermal barrier coatings (TBCs). Rare‐earth tantalates (RETaO4) are one kind of the most promising TBC materials with low thermal conductivity. However, the underlying mechanism of low κ in RETaO4 has remained a mystery. In this work, the thermal transport properties of monoclinic (m)‐RETaO4 (RE = Y, Eu, Gd, Dy, Er) compared with ZrO2 are conducted to reveal the mechanism of low lattice thermal conductivity in the former compounds using highly accurate phonon Boltzmann transport equation combined with first‐principles calculations. The predicted κ is in good agreement with experimental data, which proves that this work is convincing. The result shows that ErTaO4 has the lowest κ (1.37 W m−1 K−1 at 1600 K), which is much lower than ZrO2 (2.49 W m−1 K−1 at 1600 K). It is found that the strong anharmonicity and large scattering rate in m‐RETaO4 are mainly derived from strong ionic bonding in the crystal structure, and strong anti‐crossing property of acoustic‐optical phonon branches in phonon dispersion. Both mechanisms can effectively reduce the phonon group velocity and increase the phonon scattering rates of m‐RETaO4, leading to lower κ than ZrO2. Fortunately, two descriptors, including distortion degree and stretching force constant, are suggested to be used to quickly screen the doping or multicomponent RETaO4 with relatively lower κ, which could also be extended to other potential TBCs systems, that is, rare‐earth silicate, rare‐earth cerate, and so on.

Low thermal inertia thermal barrier coatings for spark ignition engines: An experimental study

Application of thermal barrier coatings in spark ignition engines have historically been avoided due to the knock penalty associated with higher surface temperatures induced by the ceramic layer. However, advances in low thermal inertia coatings (i.e. temperature swing coatings) that combine low thermal conductivity with low volumetric heat capacity can prevent excessively high surface temperatures during the intake stroke and reduce or avoid knock while improving performance and efficiency. In this study, a novel coating material was tested in a low compression ratio spark ignition engine to evaluate the performance of an advanced low thermal inertia coating during steady and cold-start conditions. A total of four pistons with this novel material was tested, with an additional piston coated with gadolinium zirconate. Spark timing sweeps demonstrated a maximum 0.8% relative thermal efficiency gain with a thin coating of the novel material. The novel material coated above 200-microns showed a deterioration in performance and efficiency due to charge heating increasing knock propensity. Cold-start tests demonstrated that charge heating is beneficial for reducing unburned hydrocarbons and particle matter emissions.

Bimodal microstructure toughens plasma sprayed Al2O3-8YSZ-CNT coatings

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al2O3 and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al2O3) to ∼13.9 GPa in MR of reinforced Al2O3-YSZ composite. Further, with the addition of CNT in Al2O3-8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam0.5 (for μ-Al2O3) to a maximum of 5.4 MPam0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO2 (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al2O3, nanostructured 8YSZ and CNTs in the μ-Al2O3 (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al2O3 matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications.

A robotic polishing trajectory planning method for TBCs of aero-engine turbine blade using measured point cloud

PurposeThe application of thermal barrier coatings (TBCs) allows aero-engine blades to operate at higher temperatures with higher efficiency. The preparation of the TBCs increases the surface roughness of the blade, which impacts the thermal cycle life and thermal insulation performance of the coating. To reduce the surface roughness of blades, particularly the blades with small size and complex curvature, this paper aims to propose a method for industrial robot polishing trajectory planning based on on-site measuring point cloud.Design/methodology/approachThe authors propose an integrated robotic polishing trajectory planning method using point cloud processing technical. At first, the acquired point cloud is preprocessed, which includes filtering and plane segmentation algorithm, to extract the blade body point cloud. Then, the point cloud slicing algorithm and the intersection method are used to create a preliminary contact point set. Finally, the Douglas–Peucker algorithm and pose frame estimation are applied to extract the tool-tip positions and optimize the tool contact posture, respectively. The resultant trajectory is evaluated by simulation and experiment implementation.FindingsThe target points of trajectory are not evenly distributed on the blade surface but rather fluctuate with surface curvature. The simulated linear and orientation speeds of the robot end could be relatively steady over 98% of the total time within 20% reduction of the rest time. After polishing experiments, the coating roughness on the blade surface is reduced dramatically from Ra 7–8 µm to below Ra 1.0 µm. The removal of the TBCs is less than 100 mg, which is significantly less than the weight of the prepared coatings. The blade surface becomes smoothed to a mirror-like state.Originality/valueThe research on robotic polishing of aero-engine turbine blade TBCs is worthwhile. The real-time trajectory planning based on measuring point cloud can address the problem that there is no standard computer-aided drawing model and the geometry and size of the workpiece to be processed differ. The extraction and optimization of tool contact points based on point cloud features can enhance the smoothness of the robot movement, stability of the polishing speed and performance of the blade surface after polishing.

Preparation and self-healing performance of spherical α-Al2O3@MoSi2 particles using the precipitation method

α-Al2O3 was coated on the powder surface using the precipitation method to improve the pre-oxidation resistance of molybdenum disilicide in thermal barrier coatings (TBCs). The coating effects of four different aluminum sources (Al(NO3)3·9H2O) to MoSi2 mass ratios were evaluated by viscosity and micro-morphology. The shell-forming effects on the powder size and post-treatment were thoroughly analyzed to meet the requirements for practical application in TBCs. The results showed that the precipitation method was superior to the sol-gel process in terms of the shell thickness obtained. Optimal shell-forming was generated at 1200 °C in an inert atmosphere. A bonding layer (Al6Si2O13) would be formed at the core-shell interface, further preventing oxidation penetration. The self-healing particles MoSi2@α-Al2O3 can effectively seal micro-cracks (width below 300 nm) because of the fluidity of SiO2 at working temperatures.

Biaxial stress in La2Ce2O7/YSZ thermal barrier coatings for oxidation evolution

High temperature oxidation could cause the spalling and failure of thermal barrier coatings (TBCs). La2Ce2O7(LCO)/YSZ double-layer TBCs were investigated with high temperature oxidation at 1100 °C, 1150 °C and 1200 °C, respectively. Interfacial crack propagation and TGO growth behavior were comparatively investigated, and the failure of double-layer TBCs is the result of the joint action of interface transverse crack propagation and TGO growth. Due to the difference in thermal expansion coefficients, the YSZ layer is subjected to biaxial stress, which contains the TGO growth stress and the thermal mismatch stress during the oxidation process. The transverse crack propagation was developed at the LCO/YSZ interface and the YSZ/BC interface, eventually. The thermal barrier coatings TGO thickened abnormally at 1000 °C and 1150 °C, while delamination at the LCO/YSZ interface resulted in coating failure at 1200°C. The thermal mismatch stress at the LCO/YSZ interface increased mainly due to the thermal expansion coefficient between LCO and YSZ. Transverse microcracks at the LCO/YSZ interface gradually propagated, resulting in delamination failure to LCO/YSZ. Therefore, the failure mechanism of biaxial internal stress has important guiding significance for the application of double-layer TBCs.