Zirconia-based Thermal Barrier Coatings Research Articles

High temperature infiltration behavior and reaction characteristics of Colima volcanic ashes on gadolinium zirconate coatings deposited by atmospheric plasma spraying

AbstractVolcanic ashes are considered a serious threat to the aircraft industry. At high temperatures, they inflict severe thermochemical damage to the typical 7 wt.% yttria-stabilized zirconia thermal barrier coatings that protect the aircraft turbine. There is a need to evaluate alternative materials with excellent resistance to infiltration of molten siliceous particles, such as gadolinium zirconate. In this work, free-standing thermal barrier coatings of gadolinium zirconate were manufactured by atmospheric plasma spraying, varying deposit parameters to obtain different relative densities to evaluate the infiltration of molten Colima volcanic ashes for 1 h and 10 h at 1250 °C. The infiltration depth and the reaction products resulting from each interaction, were studied by different characterization techniques. In general, the coatings show high resistance to the infiltration of the volcanic ashes, reaching infiltration depths between 50 µm and 80 µm after 10 h of infiltration time. In this sense, gadolinium zirconate coatings are excellent candidates against the infiltration of Colima volcanic ashes compared to the typical yttria-stabilized zirconia-based thermal barrier coatings.

Recognizing Artifacts in Air Plasma Spray Thermal Barrier Coating Microstructures

Abstract Identifying metallographic TSC artifacts, specifically in air plasma spray zirconia-based thermal barrier coatings.

Calcium-Magnesium-Aluminum-Silicate (CMAS) Corrosion Resistance of Y-Yb-Gd-Stabilized Zirconia Thermal Barrier Coatings

It was published that zirconate coatings containing Gd/Yb and yttria-stabilized zirconia (YSZ) coatings with high Y content had good calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance. However, there are few reports on CMAS resistance of zirconia-based thermal barrier coatings (TBCs) doped with Y, Gd and Yb. This article intends to explore the role of Y, Gd and Yb in resisting CMAS corrosion of zirconia-based TBCs. The purpose is to optimize the rare earth modification of zirconia-based TBCs. 8YSZ and ZrO2-9.5Y2O3-5.6Yb2O3-5.2Gd2O3 free-standing TBCs were produced by air plasma spraying (APS) with commercial powders. The CMAS corrosion behavior and failure mechanism of these coatings were studied by heat treatment at 1350 °C. The microstructure and composition were characterized. The results showed the CMAS deposited on the surface of 8YSZ/YSZ-Yb-Gd coatings gradually penetrated into coatings. ZrSiO4 and m-ZrO2 were observed in the shallow layer of coatings. The diffusion rate of Y3+ in coatings into CMAS was greater than that of Gd3+ and Yb3+. Compared with YSZ coatings, CMAS penetrated deeper in YSZ-Yb-Gd coatings after the same corrosion time. The spallation of YSZ-Yb-Gd coatings occurred earlier than 8YSZ. In general, adding a small amount of Yb and Gd did not improve the CMAS resistance of YSZ TBCs.

Hot-corrosion resistance and phase stability of Yb2O3–Gd2O3–Y2O3 costabilized zirconia-based thermal barrier coatings against Na2SO4 + V2O5 molten salts

The hot corrosion behavior of plasma-sprayed Yb2O3-Gd2O3-Y2O3 co-doped ZrO2 (YGYZ) coating against Na2SO4 + V2O5 molten salts was investigated and its detailed hot corrosion mechanism was suggested. The chemical reactivity order between the stabilizers was proposed based on the electronic structure and Lewis acid-base mechanism. After hot corrosion, the YGYZ coating exhibited the maintenance of tetragonality ratio and relatively lower extent of destabilization less than 40% of conventional yttria-stabilized ZrO2 coating. These great chemical and phase stabilities imply that YGYZ is promising candidate for high-temperature thermal barrier material.

Synthesis and characterization of ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) nanoparticles for advanced TBCs

Multicomponent rare-earth oxide-doped zirconia-based thermal barrier coatings (TBCs) are attractive alternatives for yttria-stabilized zirconia (YSZ) in advanced TBC applications. In this study, a zirconia-based nanopowder doped by multiple rare-earth oxides ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) was synthesized using sol-gel and calcination method. The product was then characterized by X-ray diffraction (XRD), simultaneous thermal analysis (STA), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and dilatometric test. According to STA results, the proper calcination temperature of (La1-xGdx)2Zr2O7 nanopowders could be considered > 800 °C. The results indicated that Gd2Zr2O7 exhibits a defect fluorite-type structure, and with the incorporation of larger La3+, it transformed into an ordered pyrochlore-type structure. The microstructure and morphology of the product were studied by field emission scanning electron microscopy (FESEM), revealing the formation of (La1-xGdx)2Zr2O7 nanoparticles with the mean size below 100 nm. Also, the thermal expansion coefficient of (La1-xGdx)2Zr2O7 nanoparticles was determined 8–12.5 × 10−6 K−1 at 1000 °C.

Crack-Growth Behavior in Thermal Barrier Coatings with Cyclic Thermal Exposure

Crack-growth behavior in yttria-stabilized zirconia-based thermal barrier coatings (TBCs) is investigated through a cyclic thermal fatigue (CTF) test to understand TBCs’ failure mechanisms. Initial cracks were introduced on the coatings’ top surface and cross section using the micro-indentation technique. The results show that crack length in the surface-cracked TBCs grew parabolically with the number of cycles in the CTF test. Failure in the surface-cracked TBC was dependent on the initial crack length formed with different loading levels, suggesting the existence of a threshold surface crack length. For the cross section, the horizontal crack length increased in a similar manner as observed in the surface. By contrast, in the vertical direction, the crack did not grow very much with CTF testing. An analytical model is proposed to explain the experimentally-observed crack-growth behavior.

Application of Indentation for Estimating the Thermal Stability of Plasma Thermal Barrier Coatings

The physicomechanical properties of zirconia-based thermal barrier coatings, which are deposited by air-plasma method on a heat-resistant VKh4A high-chromium nickel alloy and are subjected to heat treatment under various conditions imitating real exploitation processes of the coatings, are studied. To estimate the fracture strength of the coatings (critical stress intensity factor K1c) and Young’s modulus, Vickers microindentation is used. This method is shown to allow one to estimate the effect of a heat load and to find the factors affecting the properties of the coatings.

Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress Toward Models and Materials Solutions

Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the requisite research needs, are discussed.

Effects of Purity and Phase Content of Feedstock Powder on Thermal Durability of Zirconia-Based Thermal Barrier Coatings

The thermal durability of thermal barrier coatings (TBCs) obtained using feedstock powders with different purity and phase content was investigated by cyclic thermal testing, including the effects on the sintering and phase transformation behaviors. Three kinds of 8 wt.% yttria-stabilized zirconia, namely regular purity (8YSZ), high purity (HP), and no monoclinic phase (nMP), were employed to prepare top coats by atmospheric plasma spraying on a NiCoCrAlY bond coat using a high-velocity oxy-fuel system. Use of 8YSZ, HP, and nMP for plasma spraying affected the microstructure and lifetime of the TBC in furnace cyclic testing (FCT) at 1100 °C and the sintering rate during annealing at 1400 °C for 50, 100, 200, and 400 h. In FCT, the TBC formed from nMP showed the longest durability, while that formed from HP showed lifetime performance similar to that obtained with regular-purity 8YSZ. The TBC obtained with nMP also exhibited the lowest monoclinic phase transition rate, followed by those obtained using HP and 8YSZ.

New deformability characteristics of zirconia-based plasma-sprayed thermal barrier coatings at low loads

A new approach to determining the residual stresses and the energy characteristics of the deformability of zirconia-based thermal barrier coatings is described. This approach consists in the formation of a deformation hysteresis in the elastic range of deformation during four-point static bending and in estimating the energy density of relaxation of the elastic strain in the coatings.

Phase composition and thermal conductivity of zirconia-based thermal barrier coatings

Atmospheric plasma spraying of powder materials has been used to produce thermal barrier coatings (TBCs) based on ZrO2 stabilized with 7 wt % Y2O3, including coatings doped with neodymium and samarium oxides, for state-of-the-art and next-generation high-temperature gas turbine engines. Doping with neodymium and samarium oxides has been shown to reduce the thermal conductivity of the TBCs by 10–20%. At the same time, changes in the phase composition, crystal structure parameters, and microstructure of the TBCs during heat treatment at the service temperature lead to an increase in the thermal conductivity of all the coatings by 50–70%.

Isothermal Oxidation Behavior of Supersonic Atmospheric Plasma-Sprayed Thermal Barrier Coating System

In this work, Y2O3 stabilized zirconia-based thermal barrier coatings (TBCs) were deposited by conventional atmospheric plasma spraying (APS) and high efficiency supersonic atmospheric plasma spraying (SAPS), respectively. The effect of Al2O3 layer stability on the isothermal growth behavior of thermally grown oxides (TGOs) was studied. The results revealed that the Al2O3 layer experienced a three-stage change process, i.e., (1) instantaneous growth stage, (2) steady-state growth stage, and (3) depletion stage. The thickness of Al2O3 scale was proved to be an important factor for the growth rate of TGOs. The SAPS-TBCs exhibited a higher Al2O3 stability and better oxidation resistance as compared with the APS-TBCs. Additionally, it was found that inner oxides, especially nucleated on the top of the crest, continually grew and swallowed the previously formed Al2O3 layer, leading to the granulation and disappearance of continuous Al2O3 scale, which was finally replaced by the mixed oxides and spinel.

Micro-chemical and -morphological features of heat treated plasma sprayed zirconia-based thermal barrier coatings

Zirconia-based plasma-sprayed coatings are extensively used in jet and land-based engines as thermal barrier coatings (TBCs) for protecting and insulating gas turbine metal components from the extreme temperature in the hot gas extending the engine life capabilities and service performances as well as reducing fuel consumption. Zirconia-based thermal barrier coatings stabilized with yttria and ceria were prepared by means of atmospheric plasma spray (APS) and thermal treated at different temperatures. The resulting fractured heated surfaces have been studied by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) combined with energy dispersive spectrometry (EDS) and secondary ion mass spectrometry (SIMS) in order to study the surface micro-chemical composition and morphology. The results disclose the variation of the stabilizing oxide amount, the occurrence of valence state modifications of cerium, impurity segregation phenomena and sintering. High temperature sintering influenced the porous microstructure leading to structural changes of the surface. This information confirmed that chemical and morphological aspects in plasma sprayed TBCs must be known in order to understand and predict relationships between the parameters of plasma spray process and TBC features, properties and performances for a better design of reliable TBCs.

Influence of Interface Roughness, Substrate and Oxide-Creep on Damage Evolution and Lifetime of Plasma Sprayed Zirconia-based Thermal Barrier Coatings

Abstract The stress state in thermal barrier coating (TBC) systems for gas turbine blades during thermal cycling is mainly governed by (i) the thermal and mechanical mismatch between the ZrO2-base topcoat and the Ni-base substrate alloy, (ii) the roughness profile at the interface between MCrAlY-bondcoat (BC) and TBC, and (iii) plastic deformation and creep in the BC and in the oxide scale growing at the interface between TBC and BC. In the present study, the influence of (ii) and (iii) on the thermal cycling lifetime was studied using a simplified model system consisting of (a) FeCrAlY substrates with different creep strength, i.e. a conventional and an oxide dispersion strengthened alloy, (b) oxide layers with different grain size, i.e. a coarse grained version applied by oxidation and a nanocrystalline version applied by sputtering and (c) a plasma sprayed ZrO2-TBC stabilized by 8% Y2O3. A trend of decreasing lifetime with increasing creep strength of the FeCrAlY was observed. Infrared pulse thermography analysis of delamination crack growth during thermal cycling tests showed that in case of low creep strength substrates, small delaminations grow continuously, link to each other and finally lead to spallation of the TBC, whereas in the case of high creep strength substrates fast crack growth and spallation occurs as soon as one delamination exceeds a diameter of approx. 2 mm. A fracture mechanics model for delamination crack growth is able to roughly describe this behavior. The influence of oxide creep seems to be less pronounced than that of substrate creep. However, the highest lifetimes were observed for the combination of Fecralloy substrate and nanocrystalline oxide layer. The influence of roughness on lifetime remains unclear due to a wide lifetime scatter.

Microindentation of plasma zirconia-based thermal barrier coatings

A technique of evaluating the micromechanical characteristics (namely, microhardness, microdurability, microbrittleness and static crack resistance coefficient) using microindentation as applied to plasma zirconia-based thermal barrier coatings is presented.

Microstructure and mechanical properties of zirconia-based thermal barrier coatings with starting powder morphology

The effects of starting powder morphology on the microstructure and mechanical properties of thermal barrier coatings (TBCs) prepared by an air plasma spray (APS) were investigated after thermal exposure of the TBCs. Three kinds of powder were prepared by a spray drying process. They showed mixed morphologies and microstructures, consisting of the following ratios of deformed hollow type to filled spherical type: 30:70 vol.% in water solvent (WS-1), 70:30 vol.% in methanol solvent (MS), and fully filled spherical type in water solvent (WS-2). Partial cracking was observed at the interface in the cases of the MS and WS-2 powders after 400 h. After 800 h, the defects such as interlamellar and vertical cracks were newly reproduced with the partial cracking at the interface in all samples tested. In the case of WS-1, there were relatively thin and uniform “splat” boundaries/interlamellar cracks and vertical cracks. The adhesive strength of TBCs with MS was higher than with the WS powders, because of the relatively dense microstructure. The hardness values depended on the plane of the TBC section and surface. Higher values were recorded on both planes of MS. The toughness values on the surface were higher than on the section. The highest toughness values on the section were achieved with MS. The feedstock powder morphology and microstructure affected the microstructure evolution and mechanical properties of the TBCs.

Chemical aspects of plasma spraying of zirconia-based thermal barrier coatings

Zirconia-based thermal barrier coatings (TBCs) of nominal chemical composition 8 wt.% Y 2O 3–ZrO 2 and 25.5 wt.% CeO 2–2.5 Y 2O 3–ZrO 2 were prepared by atmospheric plasma spray and low-pressure plasma spray by selecting different deposition parameters. The surface chemical composition has been investigated by X-ray photoelectron spectroscopy in order to study the variation of surface chemical composition induced by the plasma-spraying process as a function of deposition parameters. The results reveal the occurrence of chemical–physical reactions such as stabilizing oxide depletion and enrichment, reduction to lower valence states, impurity segregation phenomena and the formation of new species. The chemical information was confirmed by differential thermal analysis measurements, which indicates that chemical aspects in plasma spraying are relevant and should be considered in designing reliable TBCs for maximum performance in aerospace applications.

Anisotropic mechanical properties and contact damage in air-plasma-sprayed thermal barrier coatings with bond coating nature and thermal exposure condition

The anisotropic mechanical properties and contact damage of air-plasma-sprayed (APS) zirconia-based thermal barrier coatings (TBCs) have been investigated using Vickers and Hertzian indentation tests as functions of the nature of the bond coating and the degree of thermal exposure. The hardness values of the TBC systems are dependent on the applied load at relatively low loads, and became saturated at a load of 30 N, independent of the nature of the bond coating or the degree of exposure. The values of the top coating obtained on the top surface from the Vickers indentation tests were higher than those on the sectional plane, indicating that there is an anisotropic strain behavior due to the microstructure. The regions near to the interface of the top coating and the thermally grown oxide (TGO) layer show higher values after thermal exposure, whereas the values of the APS bond coating increased and the indentation values of the high-velocity oxygen fuel (HVOF) sprayed bond coating slightly decreased after thermal exposure, owing to resintering and element deficiency during thermal exposure, respectively. In contact damage tests, the TBC system with the HVOF bond coating showed less damage than the TBC system with the APS bond coating. The shape of the damage was different between the two systems. After thermal exposure, the damage was reduced in both TBC systems, and the cracking or delamination formed at the regions near to the interface of the top coating and the TGO layer in both TBC systems.

Effect of bond coat nature and thickness on mechanical characteristic and contact damage of zirconia-based thermal barrier coatings

Mechanical characterization and contact damage of zirconia-based thermal barrier coatings (TBCs) have been investigated using nanoindentation and Hertzian indentation tests. Two types of TBC systems with different bond coat thicknesses of 60±20 and 280±20 μm were prepared using two different processes: air plasma spraying (APS) and high-velocity oxygen fuel (HVOF). Top coats were coated onto each bond coat using the APS process. The TBC system with the bond coat formed using the HVOF process shows a step-like decrease in mechanical properties on passing from the substrate to the top coat, and the bond coat formed using the HVOF process indicates higher values of E and H than that prepared using the APS process. The mechanical properties directly affect indentation stress–strain curves of the TBC systems, and the stress–strain curves of the TBC system with the APS bond coat are significantly lower than those with the HVOF process. The bond coat thickness plays an important role in limiting the effect of damage, indicating that the thin bond coat enhances contact damage and transmits the damage to the substrate, whereas the thick bond coat limits the damage to within the bond coat at a relatively low load of P=500 N. As the indentation load and the number of cycles increase, the contact damage is enhanced and becomes more severe in the TBC system with the APS bond coat. The fracture modes in the top coat and between the top coat and the bond coat of the TBC system are dependent on the bond coat nature, including the accumulation of damage and its development at high loads of 1000 and 1500 N. The TBC system with the APS bond coat shows a bigger and more intensive damage region.

Effect of thermal fatigue on mechanical characteristics and contact damage of zirconia-based thermal barrier coatings with HVOF-sprayed bond coat

Mechanical characteristics, such as hardness, elastic modulus and indentation stress–strain curves, and contact damage of a thermal barrier coating (TBC) system with a top coat prepared using an air-plasma spraying (APS) process and a bond coat using a high-velocity oxygen flow (HVOF) process have been investigated using the nanoindentation and Hertzian indentation tests, as a function of the thermal fatigue condition. The bond coat and the top coat deposited on the substrate make the TBC system soft, showing lower stress–strain curves than that of the substrate. Thermal fatigue does not affect the stress–strain curves, except for thermal fatigue for 500 h. However, the thermally grown oxide (TGO) layer thickness is dependent on the exposure time under thermal fatigue, showing a nominal thickness of approximately 4 μm after thermal fatigue for 500 h, independent of the number of thermal fatigue cycles. The values of hardness, H, in each component are not greatly affected by thermal fatigue, except for thermal fatigue for 500 h, whereas the value of elastic modulus, E, in the bond coat is dominantly affected by thermal fatigue with a smaller increase for the other components—top coat and substrate. The H/ E ratio for the top coat is higher than those for the bond coat and the substrate, indicating that resintering of the top coat occurs during thermal fatigue. The top coat acts as a protection layer for contacts, resulting in reduced damage to the substrate. As the exposure time is increased in the thermal fatigue experiments, the damage to the top coat is inhibited with less crack coalescence. The higher stiffness in the bond coat induces a cracking or delamination at the interface between the bond coat and the substrate, whereas thermal fatigue increases the mechanical properties, especially E, of the bond coat and enhances the damage tolerance of the TBC system.