ZrO2 Thermal Barrier Coatings Research Articles

Preparation and high-temperature performance of Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings

As a promising material for the advanced thermal barrier coatings (TBCs), scandium oxide (Sc2O3) and yttrium oxide (Y2O3) co-stabilized zirconia (ScYSZ) has been given increasing attention during the past decade. In this study, three types of TBCs including YSZ, ScYSZ and ScYSZ-YSZ double ceramic layer (DCL) coatings were deposited by a wide-velocity range high-energy plasma spraying technology. The thermal physical property, phase stability and thermal cycling performance of TBCs were comparatively studied. The results suggested that the thermal conductivity from 200 °C to 1000 °C of ScYSZ coating was reduced by approximately 30 % compared to the YSZ coating. After exposure at 1400 °C for 1000 h, the ScYSZ coating still kept the initial tetragonal phase structure, while the tetragonal phase was completely disappeared in the YSZ coating after 300 h at 1400 °C. Notably, the ScYSZ-YSZ DCL coating demonstrated the highest thermal cycling life within the temperature range of 1320∼1370 °C, exhibiting a remarkable 43 % increase over the single ScYSZ coating and doubling that of the single YSZ coating. The outcomes of this study hold significant promise for practical applications in the realm of high-performance TBCs.

Effect of Sc2O3 doping on YSZ TBCs: Morphologies, phase composition, mechanical properties, and high-temperature oxidation resistance

Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology, and then a high-temperature oxidation experiment was carried out at 1100 °C. Whereupon the morphologies, phase composition, and mechanical properties of YSZ ceramic coating and ScYSZ ceramic coating after different high-temperature oxidation time, as well as the high-temperature oxidation resistance of YSZ TBCs and ScYSZ TBCs, were comparatively studied. The results showed that after high-temperature oxidation, ScYSZ ceramic coating presented slighter cracking and exhibited better sintering resistance. After high-temperature oxidation, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was still a single T' phase, and their interplanar spacing became larger; nevertheless, the interplanar spacing of ScYSZ ceramic coating was smaller than that of YSZ ceramic coating. After high-temperature oxidation, ScYSZ ceramic coating had higher hardness, smaller elastic modulus, and smaller fracture toughness. In addition, ScYSZ TBCs exhibited better high-temperature oxidation resistance according to the morphology, equivalent thickness, high-temperature oxidation kinetics, and high-temperature oxidation thermodynamics of TGO.

Comparison of corrosion behaviors and wettability of CMAS on Ta2O5-Y2O3 co-stabilized ZrO2 and YSZ thermal barrier coatings

Comparison of corrosion behaviors and wettability of CMAS on Ta2O5-Y2O3 co-stabilized ZrO2 and YSZ thermal barrier coatings

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc2O3 and Y2O3 co-doping system on the thermal shock resistance of ZrO2 thermal barrier coatings, Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc2O3 and Y2O3 facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.

Effects of shot peening on microstructure, thermal performance, and failure mechanism of thermal barrier coatings

AbstractShot peening might be a potential technology to optimize the interface microstructure, plays a critical role on failure behaviors, of thermal barrier coatings (TBCs). It remains a significant challenge to understand the influence of shot peening on microstructure, oxidation resistance, and thermal shock life. In this work, the Y2O3‐stabilized ZrO2 TBCs have been deposited by EB‐PVD. The phase, microstructure, thermal performance, and failure mechanism of TBCs have been systemically investigated after shot peening. The shot peening process can improve the planeness of interface and reduce the formation of the cauliflower‐liked microstructure in TBCs. After shot peening, the TBC coatings exhibit relatively good isothermal oxidation resistance and high thermal shock life due to the optimization of TGO growth and the thermal stability. The phase transformation, TGO growth, and cracks extension might give rise to the failure of TBCs. This work might guide the investigation of the improvement of interface microstructure and failure behaviors.

Thermal cycling behavior of EB-PVD rare earth oxides co-doping ZrO2-based thermal barrier coatings

Novel ceramic topcoat of Gd2O3–Yb2O3–Y2O3 co-stabilized ZrO2 (GYbYSZ) thermal barrier coatings were fabricated via EB-PVD technique. The phase structural stability, phase constituent, chemical composition, morphology and cyclic oxidation of the thermal barrier coatings (TBCs) were systematically studied. Based on the XRD results, the GYbYSZ ceramics has not undergone phase transformation upon long-term annealing at 1373 K and 1523 K. Although the chemical content of the GYbYSZ ceramic coat deviates from the stoichiometric value, the coating is mostly composed of cubic phase, which is accord with the XRD pattern of the original ingot. A pyramidal-like morphology appears in the microtexture of the column tips and the measured diameters of the pyramids are about 2.5~4 μm. After thermal cycling, the surface of the coating presents a multi-layer structure, which is followed by layer-by-layer spallation. The failure zone of the ceramic coats is possible to occur the interior of the thermally grown oxide (TGO) layer, or within the top ceramic coat at the interface of bond coat/TGO layers. The degradation of GYbYSZ TBCs is primarily attributed to the accumulation and relaxation of residual stress, propagation of vertical through microcracks, the growth rumpling of TGO layer, the ridges of grain boundary and the abnormal oxidation of bond coat.

Y–Er–ZrO2 thermal barrier coatings by EB-PVD: Thermal conductivity, thermal shock life and failure mechanism

Rare earth stabilized ZrO2 thermal barrier coatings (TBCs) have attracted much attention due to their low thermal conductivity and high thermal stability under high temperature. The investigation of thermal shock life and failure mechanism of rare earth stabilized ZrO2 TBCs still remain a challenge. This work focuses on structure, thermal conductivity, thermal shock life and failure mechanism of the rare earth stabilized ZrO2 advanced TBCs. The advanced ceramic material exhibits low thermal diffusivity and thermal conductivity. Furthermore, the advanced TBCs also exhibit good thermal shock life at 1100 °C. After thermal shock test, the broken regions occur on the crack area within ceramic coating, TGO and interface. The TBCs interface instabilities play a key role on the failure mechanism of rare earth stabilized ZrO2 TBCs.

Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance

Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ∼18 µm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 °C for 4 h in V2O5 molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.

Effects of laser glazing on CMAS corrosion behavior of Y2O3 stabilized ZrO2 thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) is a serious threat to thermal barrier coatings (TBCs). The goal of this study is to tailor YSZ coating microstructures by laser for alleviating CMAS attack. A smooth laser glazed layer with a columnar microstructure and some open channels is produced on the coating surface. Under molten CMAS conditions, the glazed layer remains phase stability and has little evidence of dissolution. Exposed to thermal shock tests with CMAS deposits, the glazed layer keeps structural integrity. However, the open channels also act as the paths for CMAS penetration, which brings destructive damages to the coating beneath the glazed layer.

Corrosion behavior of Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings with CMAS attack

Abstract Traditional viewpoints state that defects, such as pores or cracks, that generally exist in plasma sprayed thermal barrier coatings (TBCs) can provide fast infiltration channels for molten calcia-magnesia-alumino-silicate (CMAS). In contrast with this viewpoint, results from this study suggest that the effect of defects on the CMAS infiltration of the yttria stabilized zirconia (YSZ) coating is limited, even though the dense YSZ bulk material still cannot inhibit the infiltration of CMAS. However, a few additions of Sc2O3 can significantly improve the CMAS resistance of the YSZ coating owing to the lower solubility of Sc3+ in the CMAS. Our findings provide key insights for the development of CMAS-resistant TBCs.

Acoustic emission monitoring and failure behavior discrimination of 8YSZ thermal barrier coatings under Vickers indentation testing

Acoustic emission (AE) is a dynamic non-destructive detecting technique, and could be a promising application for failure detection of plasma sprayed coatings. This work presents a thorough study for detecting and classifying different failure modes under Vickers indentation testing of 8 wt% Y2O3-stabilized ZrO2 (8YSZ) thermal barrier coatings (TBCs) by using acoustic emission technique. Cluster analysis and wavelet packet transform (WPT) were used to characterize the acoustic emission signals released from as-sprayed TBCs during indentation testing. Back-propagation (BP) neural network was used to study the effect of thermal exposure treatment on damage evolution of 8YSZ coatings. The results of cluster analysis showed that there were three types of AE signals associated with distinct failure modes for 8YSZ thermal barrier coatings under Vickers indentation testing. Based on wavelet packet transform, the dominant frequency bands of these three types of signals were concentrated at levels 0–156.25 kHz, 156.25–234.375 kHz and 312.5–390.625 kHz, respectively. Combined with scanning electron microscope (SEM) micrographs of residual Vickers impression, it can be found that 8YSZ coatings have undergone three processes: elastic-plastic deformation, micro-cracks propagation and debonding at the splat boundaries during the indentation testing. The BP neural network identification results showed that after high temperature thermal exposure treatment, the strain tolerance and service durability of 8YSZ thermal barrier coatings decreased while the more fracture signals are generated during the indentation process. The high temperature oxidation environment accelerated the degradation of the plasma sprayed 8YSZ thermal barrier coatings.

Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

The microstructural features of high-temperature sintered and CaO-MgO-Al2O3-SiO2 (CMAS) corroded air plasma sprayed Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr2Y2O7. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

AbstractThe reactivity between rare‐earth (RE‐) oxide stabilized ZrO2 or HfO2 thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1−x) ZrO2‐xY2O3 coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO2 system. In general, samples with nominal compositions ZrO2‐9Dy2O3, HfO2‐7Dy2O3, ZrO2‐8Y2O3, HfO2‐6Er2O3, ZrO2‐9.5Y2O3‐2.25Gd2O3‐2.25Yb2O3, and ZrO2‐30Y2O3 exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.

Effect of Microstructure on the Thermal Conductivity of Plasma Sprayed Y2O3 Stabilized Zirconia (8% YSZ)

In this paper, the effect of microstructure on the thermal conductivity of plasma-sprayed Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) is investigated. Nine freestanding samples deposited on aluminum alloys are studied. Cross-section morphology such as pores, cracks, m-phase content, grain boundary density of the coated samples are examined by scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD). Multiple linear regressions are used to develop quantitative models that describe the relationship between the particle parameters, m-phase content and features of the microstructure such as porosity, crack-porosity, and the length density of small and big angle-cracks. Moreover, the relationship between the microstructure and thermal conductivity is investigated. Results reveal that the thermal conductivity of the coating is mainly determined by the microstructure and grain boundary density at room temperature (25 °C), and by the length density of big-angle-crack, monoclinic phase content and grain boundary density at high temperature (1200 °C).

Evolution of mechanical properties of thermal barrier coatings subjected to thermal exposure by instrumented indentation testing

The evolution of mechanical properties, namely Young's modulus, hardness, fracture toughness and residual stresses of air plasma-sprayed 8wt% Y2O3-stabilized ZrO2 thermal barrier coatings (TBCs) subjected to thermal exposure at 1100°C, was evaluated based on an instrumented indentation testing system. It is found that the variation of Young's modulus and hardness with the increase of thermal exposure time can be divided into two regimes: a rapid initial increase, followed by a more gradual increase. Conversely, fracture toughness decreased from the initial value of 1.48–1.05MPam1/2 after 350h, indicating the degradation of TBCs. And the corresponding residual stress in top coat varied from the deposition stress of −29.9MPa to a maximum of −56.0MPa within 100h, and then decreased to −15.0MPa after 350h. The evolution of mechanical properties was correlated with microstructural changes in the top coat. Microstructure observations revealed that thermally grown oxide (TGO) formed at the interface between ceramic coat and bond coat, and was predominantly comprised of Al2O3 layer, along with some oxide clusters of (Cr, Al)2O3, (Co, Ni)(Cr, Al)2O4 and NiO. The growth of TGO followed a parabolic law with a rate constant of 0.13236μm2/h. After 350h, porosity in 8YSZ coatings was reduced from 8.39% to 5.79% as a result of activated diffusion-controlled sintering.

Effect of heat treatment on deformation and mechanical properties of 8 mol% yttria-stabilized zirconia by Berkovich nanoindentation

The effect of thermal treatment on the elasto-plastic transition and mechanical properties of air plasma-sprayed 8mol% Y2O3-stabilized ZrO2 (8YSZ) thermal barrier coatings was studied by nanoindentation test at ultra-low loads with a Berkovich indenter. The area contact function of the indenter was calibrated repeatedly under nano-scales, and the indenter tip radius was estimated under different indentation depths, respectively. Owing to the heterogeneous and porous microstructure, the scatter of all collected experimental data was analyzed by Weibull statistic method. It is interesting to observe that the hardness exhibits an apparent reverse indentation size effect under very small depths. The Young's modulus of 8YSZ varies with ranging from 213 to 246GPa due to the sintering effect. True hardness of 8YSZ increases from as-received 72.9GPa to a top value 79.7GPa under 100 thermal cycles, and then slightly decreases from this value to 75.5GPa under 175 thermal cycles. The pure elastic and elasto-plastic indentation curves were obtained by adjusting the indentation load magnitude. The elasto-plastic transition and resolved shear stress fields were discussed carefully from the use of energetic models and Hertzian contact theory.

Hot corrosion behavior of Sc2O3–Y2O3–ZrO2 thermal barrier coatings in presence of Na2SO4 + V2O5 molten salt

Sc2O3, Y2O3-stabilized ZrO2 (ScYSZ) thermal barrier coatings (TBCs) were prepared on nickel-based superalloys by air plasma spraying. Hot corrosion behavior of the ScYSZ TBCs was investigated in presence of Na2SO4+V2O5 salt at 1000°C. Results show that the ScYSZ coating exhibits superior chemical stability and improved phase stability in comparison with traditional YSZ coating. During corrosion, the destabilization of ScYSZ is attributed to the deficiency of stabilizers resulted from the synergic mineralization effect of vanadium-containing compounds. The tetragonal-to-monoclinic phase transformation of zirconia can induce the formation of cracks in ScYSZ coating, which is responsible for the degradation of the TBCs.

Phase stability and thermal conductivity of ytterbia and yttria co-doped zirconia

Rare earth oxides doping has been extensively investigated as one of the effective methods to lower thermal conductivity of 4.55mol% Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs). In the present work, 5–6mol% Yb2O3 and Y2O3 co-doped ZrO2 ceramics were synthesized by solid reaction sintering at 1600°C. The phase stability of the samples after heat treatment at 1500°C was investigated. Yb2O3 and Y2O3 co-doped zirconia, especially when Yb2O3/Y2O3≥1, contained less monoclinic phase than single Yb2O3 or Y2O3 phase doped zirconia, indicating that co-doped zirconia was more stable at high temperature than YSZ. The thermal conductivity of the 3mol% Yb2O3+3mol% Y2O3 co-doped ZrO2 was 1.8Wm−1K−1 at 1000°C, which was more than 20% lower than that of YSZ.

Microstructural evolution of 7wt.% Y2O3–ZrO2 thermal barrier coatings due to stress relaxation at elevated temperatures and the concomitant changes in thermal conductivity

The purpose of this study was to evaluate the combined effect of stress and temperature on the microstructure of air plasma-sprayed 7 wt.% Y{sub 2}O{sub 3}-ZrO{sub 2} thermal barrier coatings, and relate microstructural changes to the thermal conductivity, k{sub th}. To simulate TBC service conditions, stand-alone tubes of YSZ were stress relaxed, starting from a compressive stress of 60 MPa, at temperatures of 1000 C or 1200 C. The duration of the stress relaxation test was either 5 min or 3 h. Detailed scanning electron microscopy (SEM) and Porod's specific surface area (SSA) analysis of small angle neutron scattering (SANS) results were used to determine which void systems, either interlamellar pores or intralamellar cracks, contributed to the observed relaxation of stress in the coatings. SEM investigations revealed closure of intralamellar cracks located perpendicular to the stress direction. For thinner YSZ coatings, SANS measurements indicated a statistically significant reduction in the total SSA and SSA associated with intralamellar cracks after stress relaxation at the times, temperatures, and stress investigated compared to those samples that were exposed to identical times and temperatures, but no stress. The SSA associated with the interlamellar pores was not significantly smaller in YSZ coatings stress relaxed frommore » 60 MPa at 1200 C for 3 h compared to as-sprayed coatings. The thermal conductivity of the coatings was strongly influenced by stress, with increases in kth observed after only 5 min at 60 MPa and 1200 C. Reductions in the total SSA were directly linked to increases in k{sub th}.« less

Numerical study on the thermo-stress of ZrO2 thermal barrier coatings by high-intensity pulsed ion beam irradiation

This paper studies numerically the thermo-mechanical effects of ZrO2 thermal barrier coatings (TBCs) irradiated by a high-intensity pulsed ion beam in consideration of the surface structure. Taking the deposited energy of ion beams in TBCs as the source term in the thermal conduction equation, the distribution of temperature in TBCs was simulated. Then, based on the distribution, the evolution of thermal stress was calculated by the finite element method. The results show that tensile radial stress formed at the valley of TBC surfaces after irradiation by HIPIB. Therefore, if cracks happen, they must be at valleys instead of peaks. As for the stress waves, no matter whether through peak or valley position, tensile and compressive stresses are present alternately inside TBCs along the depth direction, and the strength of stress decreases with time.