YSZ Thermal Barrier Coatings Research Articles

Hot-Corrosion Behavior of Gd2O3–Yb2O3 Co-Doped YSZ Thermal Barrier Coatings in the Presence of V2O5 Molten Salt

In this study, thermal barrier coatings (TBC) consisting of 3.5 mol% Yb2O3-stabilized ZrO2 co-doped with 1 mol% Gd2O3 and 1 mol% Yb2O3 (referred to as GdYb-YSZ) were fabricated by means of air plasma spraying. The as-fabricated coatings exhibited a metastable tetragonal (t′) structure. The hot-corrosion behavior of the GdYb–YSZ TBCs was investigated at 700, 800, 900, and 1000 °C for 10 h in the presence of V2O5 molten salt. During the corrosion tests, the t′ phase transformed into a monoclinic (m) phase; nevertheless, it was still detected on the corroded surfaces. The amount of t′ phase decreased with increasing corrosion temperature. The corrosion products formed on the GdYb-YSZ TBCs in V2O5 comprised Yb, Gd-doped YVO4, and m-ZrO2, irrespective of the temperature of corrosion. However, higher temperatures changed the morphologies of the Yb- and Gd-doped YVO4 corrosion products. The GdYb–YSZ TBCs exhibited improved corrosion resistance to V2O5 molten salt when compared to YSZ TBCs, and the related mechanism is discussed in detail in this paper.

Phase transformation failure in YSZ TBCs induced by component-dependent CMAS corrosion

Generally, CMAS corrosion is a key factor leading to the spalling failure in thermal barrier coatings (TBCs). Figuring out the component-dependent failure mechanism is the premise to improve the service life of TBCs. Here, CMAS corrosion behaviors with different Ca/Si components have been investigated through analyzing microstructure evolution in 8-YSZ coatings by EB-PVD. Capillary force from the gap between columnar crystals could accelerate the infiltration speed of the molten CMAS, leading to a fully populated coating with lower strain tolerance in 4 h. Subsequently, CMAS infiltration in the coatings led to preferential dissolution of yttrium ions and precipitation of yttria lean YSZ, and secondary CMAS phases. The corrosiveness of CMAS could be further tailored by optical basicity and viscosity. TBC failure could be enhanced in high-Ca case, resulting in the morphology evolution from columnar crystals to coarsening grain structures. Raman spectra indicates CMAS corrosion with high-Ca content will facilitate the phase transformation of t-YSZ to harmful monoclinic phase. In addition, DFT calculation demonstrates Ca doping into 8-YSZ will make monoclinic phase more stable and generate a 4.1 % volume expansion from the view point of energy. Consequently, the combination of the above low strain tolerance, new phases and volume expansion will accelerate the harmful morphology evolution. This work provides direct insight into the failure mechanism of CMAS corrosion and paves the way to design of new TBCs.

Comparison of microstructure and adhesion strength of YSZ and MSZ thermal barrier coatings in the presence and absence of Al2O3 interface layer

Comparison of microstructure and adhesion strength of YSZ and MSZ thermal barrier coatings in the presence and absence of Al2O3 interface layer

Prediction of Microstructure and Mechanical Properties of Atmospheric Plasma-Sprayed 8YSZ Thermal Barrier Coatings Using Hybrid Machine Learning Approaches

The preparation of thermal barrier coatings (TBCs) is a complex process involving the integration of physics and chemistry, mainly involving the flight behavior and deposition behavior of molten particles. The service life and performance of the TBCs were determined by various factors, especially the preparation process parameters. In this work, to set up the quantitative characterization model between the preparation process parameters and the performance characteristic parameters, the ceramic powder particle size, spraying power and spraying distance were treated as the model input parameters, the characteristic parameters of microstructure properties represented by the porosity, circularity and Feret’s diameter and the mechanical property represented by the interfacial binding strength and macrohardness were treated as the model output. The typical back propagation (BP) model and extreme learning machine (ELM) model combined with flower pollination algorithm (FPA) optimization algorithm were employed for modeling analysis. To ensure the robustness of the obtained regression prediction model, the k-fold cross-validation method was employed to evaluate and analyze the regression prediction models. The results showed that the regression coefficient R value of the proposed FPA-ELM hybrid machine learning model was more than 0.94, the root-mean-square error (RMSE) was lower than 2 and showed better prediction accuracy and robustness. Finally, this work provided a novel method to optimize the TBCs preparation process, and was expected to improve the efficiency of TBCs preparation and characterization in the future.

Cracking Behavior of Atmospheric Plasma-Sprayed 8YSZ Thermal Barrier Coatings during Thermal Shock Test

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service is usually related to the cracking behavior. In this study, plasma-sprayed TBCs were prepared with two kinds of agglomerated sintered yttria-stabilized zirconia (YSZ) powders with different particle sizes. The evolution of mechanical properties and crack propagation behavior of the coatings during the whole life stage were studied by a thermal shock test. The effect of powder particle size on the cracking behavior of the TBCs during thermal shock was analyzed from the aspect of pore structure, mechanical properties, and stress state of the coatings. The crack propagation and coalescence in the direction parallel to the substrate in the coating is the main factor leading to the spalling failure of the coating during thermal shock. Although the coating prepared by fine YSZ has higher fracture toughness, the lower strain tolerance will increase the cracking driving force on the crack tip of the coating during thermal shock, and the cracks in the coating propagate merge at a faster rate during thermal shock. The larger porosity and pore size of the coating prepared by coarse YSZ help the coating suffer less thermal stress during thermal shock. Although the existence of pores reduces the fracture toughness of the coating to a certain extent, the increase of strain tolerance reduces the crack growth rate in the coating, so the coating has a longer life.

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

Hot Corrosion Resistance of 8YSZ Thermal Barrier Coating Modified by Laser Alloying

Hot Corrosion Resistance of 8YSZ Thermal Barrier Coating Modified by Laser Alloying

Nanomechanical properties of plasma-sprayed nanostructured and conventional 8YSZ thermal barrier coatings

Nanostructured thermal barrier coatings (TBCs) have become a noteble research field for the development of high-performance thermal protection coatings. The mechanical properties are essential for TBCs, which determine the functionality, reliablity and durability of coatings. In this article, the nanostructured 8YSZ (n-8YSZ) and conventional 8YSZ (c-8YSZ) coatings were fabricated by atmospheric plasma spraying (APS). The goal of the work is to do an evaluation and comparison of the nanomechanical properties of n-8YSZ and c-8YSZ coatings. The microstructure of coatings was observed by scanning electron microscopy and the nanomechanical properties was characterized using nano-indenter. Results indicate that the microstructure of the n-8YSZ coating exhibits bi-modal distribution while the microstructure of the c-8YSZ coating shows mono-modal distribution. The elastic indentation work ( W e ) is 11.952 ± 1.243 nJ and the plastic indentation work ( W p ) is 20.266 ± 5.594 nJ for the n-8YSZ coating. Meanwhile, the W e is 11.753 ± 0.634 nJ and the W p is 16.508 ± 3.000 nJ for the c-8YSZ coating. The microstructure of the coating has little effect on W e , but much on W p . The dissipation ratio of plastic deformation work ( η ) is 0.621 ± 0.046 for the n-8YSZ coating and 0.579 ± 0.042 for the c-8YSZ coating. Therefore, it can be predicted that the n-8YSZ coating is superior to the c-8YSZ coating in terms of erosion performance.

Al2O3-modified 7YSZ thermal barrier coatings for protection against volcanic ash corrosion

To prevent volcanic ash corrosion using thermal barrier coatings (TBCs), a novel method using Al2O3-modification was proposed to reduce molten CMAS wettability on TBCs surface through a lotus effect. Plasma spray-physical vapor deposition (PS-PVD), a third-generation method for TBCs fabrication, was adopted to deposit 7YSZ TBCs with a feather-like microstructure. Then, Al2O3 modification was introduced to fabricate a dense Al2O3 overlay with nano/micro-sized grains on the TBCs surface. The wetting ability of CMAS on APS/EB-PVD/PS-PVD 7YSZ TBCs was comparatively in situ observed at 1250 °C for 3600 s. The results indicated that the dense Al2O3 overlay inhibited penetration of molten CMAS. Additionally, the micro/nano dual-sized structure, which is similar to the papillary structure of a lotus leaf, reduced molten CMAS wettability on the TBCs surface. The results demonstrated that the Al2O3-modified TBCs had better CMAS corrosion resistance than the as-deposited APS/EB-PVD/PS-PVD TBCs.

Solid Particle Erosion Behavior of La2Ce2O7/YSZ Double-Ceramic-Layer and Traditional YSZ Thermal Barrier Coatings at High Temperature

Thermal barrier coatings (TBC) used for turbine blades are indispensable for the most advanced aero-engines due to their excellent thermal insulation performance. Solid particle erosion (SPE) at high temperatures is one of the most critical factors in TBC failure. The high-temperature SPE failure behavior of TBC on circular sheets and turbine blades was investigated in this paper at erosion angles 60° and 90°. The high-temperature thermal shock behavior of TBC was also studied as the control group. The SPE failure mechanism of TBC is attributed to the spallation and thickness decrease of TBC. The formation of thermally grown oxide is the main reason for the TBC spallation, while the thickness decrease of TBC is due to the impaction of solid particles by near-surface cracking. The erosion angle is critical to the failure behavior of TBC, and TBC is more susceptible to SPE at an erosion angle of 60° than that at 90° because of the additional shear stress. Furthermore, a La2Ce2O7/YSZ double-ceramic-layer TBC was designed and deposited on turbine blades. The experimental results indicate that this type of double-layer TBC has more excellent performance under SPE than traditional YSZ TBC.

Finite element analysis of TGO thickness on stress distribution and evolution of 8YSZ thermal barrier coatings

AbstractAccording to the experimental research results of the thermally grown oxide (TGO) layered growth during the pre‐oxidation process of 8 wt.% yttria‐stabilized zirconia thermal barrier coating (TBC), a two‐dimensional sinusoidal TC/bonding coat (BC) curve interface model of the longitudinal section of TBCs based on finite element simulation was constructed; the thickness and composition of the TGO layer relative to the TC/BC curve interfacial stress distribution and its evolution during the thermal cycling process were studied. The results show that when the TGO layer uses α‐Al2O3 as the main oxide (black TGO), the thicker the black TGO layer, the more uniform the stress distribution of the TC/BC interface. When the TGO layer is dominated by spinel‐structured Co and Cr oxides (gray TGO), the stress “band” of the TC/BC interface is destroyed; it shows the alternating phenomenon of tensile stress zone and compressive stress zone, and after the rapid random growth of TGO, the concentrated tensile stress increased by a large jump. Affected by the thickness of the prefabricated black TGO layer, there is a limit peak in the thickness of the black TGO layer, the normal stress at the TC/BC boundary is minimized, and the magnitude of the stress change is also minimized.

Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO2-YTaO4 (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO4, while its effects on thermal/mechanical properties are elucidated from microstructures. Y3+ is substituted via Ce3+, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W∙m−1∙K−1, 900 °C) is realized in 9 mol% CeO2-YTaO4, and the thermal expansion coefficients are increased to 10.2 × 10−6 K−1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.

Shakedown Analysis and Experimental Study of Thermal Barrier Coatings.

Interfacial stress–strain fields become complicated in thermal barrier coatings (TBCs) under cyclic thermal loading, which affects the stability and spalling failure of TBCs directly. The convex and concave interfacial structures of TBCs were approximated as a multilayer cylinder model, and an analytical method of TBCs for shakedown analysis was established. A series of 8-YSZ TBC specimens were prepared by the plasma spraying process, followed by isothermal and thermal shock tests. The results showed that the stability limit is significantly greater than the elastic limit, the limit for the convex model was higher than that in the concave model, the first failure occurs in the concave area, and the main failure mode of a thermal barrier coating is the appearance of cracks at the interface layer during a thermal shock test. For the coating samples prepared in this study, the stability limits were between 950 °C and 1050 °C, and the validity of the stability limit analysis model of a multilayer structure was verified.

Condition Monitoring of a Three-Cathode Cascaded Plasma Spray Torch Regarding Process Reliability.

The TriplexPro™-210 plasma spray torch (Oerlikon Metco) is a three-cathode plasma generator. It became a kind of workhorse for the wide range of tasks handled at the Jülich Thermal Spray Center (JTSC). Compared to conventional single-cathode torches, the cascaded design of the nozzle suggests low fluctuations of the arc and thus high stability. However, after a certain time, degradation sets in even with such a torch, impairing the reliability of the process. It is therefore important to detect indications of performance loss in time and not only during the inspection of the deposited layer. In this study, standard samples of YSZ thermal barrier coatings were sprayed regularly over a period of two years. Operational data and feedstock characteristics were collected and correlated with the area-specific mass deposition. It turned out that the measured substrate surface temperature showed a distinct correlation. Searching for the reasons for the temperature variations, several process parameters could be ruled out as they are monitored by calibrated sensors, controlled, and their time course is recorded by the control unit. Moreover, there are other parameters, which can have a considerable impact such as the robot alignment or the substrate cooling conditions. However, the purposeful experimental variation of such variables resulted in a variability of the mass deposition being considerably smaller than observed over the two years. Thus, it can be concluded that torch degradation had a pronounced effect, too. The substrate surface temperature can be used as indicator for the torch status and the reliability of the spray process.

Evolution of the microstructure and mechanisms of performance degradation in EB-PVD YSZ thermal barrier coatings corroded by volcanic ash at 1150 °C

Volcanic ash (VA)/calcium-magnesium-alumina-silicate (CMAS) corrosion is a crucial issue causing the failure of thermal barrier coatings. This study focuses on the degradation process of electron-beam physical vapor deposition yttria-stabilized zirconia by natural VA at 1150 °C. The phase transformation and chemical reaction during VA attack were measured in real-time by in-situ high-temperature X-ray diffraction. The mechanical properties and the thermal conductivity of the as-deposited and VA corroded coatings were investigated. The correlation between these microstructural changes and the coatings’ properties are discussed. The degradation mechanisms are expected to guide the development of potential VA/CMAS mitigation methods.

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO2-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO2 reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al2O3 layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO2 doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.

Failure Analysis of Thermal Corrosion Cycling of EB-PVD YSZ Thermal Barrier Coatings Exposed to Molten NaCl

NaCl is the most abundant salt in seawater, thermal barrier coatings (TBCs) face the challenge of chloride salt hot corrosion in the marine environment. In this work, we focus on evaluating the thermal corrosion cycling of 9 wt.% Y2O3-ZrO2 (YSZ) exposed to NaCl molten salt. YSZ was prepared by electron beam–physical vapor deposition (EB-PVD). The result showed that the thermal cycling life of YSZ coating in the presence of NaCl molten salt was significantly shortened. The shortening of the service life of the coating can be ascribed to the collective effects of the thickening of thermal growth oxide and the degradation of the bond coating.

Novel thermal barrier coatings based on Mg2SiO4/8YSZ double-ceramic-layer systems deposited by APS

Novel thermal barrier coatings (TBCs) based on Mg 2 SiO 4 /8YSZ double-ceramic-layer (DCL) were prepared by atmospheric plasma spraying (APS). Thermal cycling tests of the TBC specimens were performed at 1373 K. Results indicate that the DCL coating has a much longer lifetime compared to the single layer Mg 2 SiO 4 coating. This is attributed to the fact that DCL Mg 2 SiO 4 /8YSZ TBCs further enhances the thermal insulation effect, improves sintering resistance and relieves thermal mismatch between ceramic layer and bond coat (BC). It can be concluded that this DCL coating structure design is ideal to optimize the performance of single layer Mg 2 SiO 4 TBC and shows the Mg 2 SiO 4 exhibiting a promising potential as new TBC materials. • The DCL coating has a much longer thermal cycling lifetime than that of the single layer coating of Mg 2 SiO 4 . • The designed DCL coating system is helpful to prolong the thermal cycling life. • The failure mechanism of DCL coating was studied.

Microstructural Characterization, Residual Stress, and Mechanical Properties of Plasma Sprayed 8YSZ Thermal Barrier Coatings

Microstructural Characterization, Residual Stress, and Mechanical Properties of Plasma Sprayed 8YSZ Thermal Barrier Coatings

Simultaneous measurement of thickness and sound velocity of porous coatings based on the ultrasonic complex reflection coefficient

The ultrasonic pulse-echo method is widely adopted in measuring coating thickness via parameter inversion of the reflection coefficient. However, the ultrasonic application to thermal barrier coatings (TBCs, as typically applied on aero-engine blades) remains a challenge, as the porous structure significantly attenuates sonic propagation, which is also difficult to characterize. In this article, a method that effectively addresses this issue without the need of the wave attenuation coefficient is presented. Specifically, the complex ultrasonic reflection coefficient can help calculate the coating-induced phase shift, which is found to linearly vary against the ultrasonic wave frequency. The slope of this linear function, depending on the structural porosity, enables simultaneous measurements of both the sound velocity and the thickness of the coating. Moreover, the effectiveness of the proposed method is verified by acoustic finite element simulations and experimental measurements of 8YSZ thermal barrier coatings.