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.     

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO₂ doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO₂-Si composite coatings with different HfO₂ contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO₂ structures. During steam oxidation, HfSiO₄ phase was formed at the SiO₂/HfO₂ interface by solid-state reaction between them. The HfSiO₄ or HfO₂/HfSiO₄ mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO₂-Si coating with isolated HfO₂ particles. However, the HfO₂-Si coating with inter-connected HfO₂ framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO₂ framework structure played a dominating role in determining the steam oxidation resistance of the HfO₂-Si coating, and the connected HfO₂ framework and TGO network provided a rapid diffusion path for oxidants (H₂O, O^2? and OH^?) and deteriorated the oxidation resistance.     

Isothermal oxidation and TGO growth behaviors of YAG/YSZ double-ceramic-layer thermal barrier coatings

In this study, yttrium aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coatings (DCL TBC) and yttria-stabilized zirconia (YSZ) single-ceramic-layer thermal barrier coatings (SCL TBC) were deposited by atmosphere plasma spray (APS) on the Inconel 738 alloy substrate, and isothermal oxidation tests were performed to investigate the formation and growth behavior of thermally grown oxide (TGO). Results showed that the Al₂O₃ TGO thickness of both TBC groups increased by increasing the isothermal oxidation time，and then slowly decreased with the appearance and growth of the adverse multilayer structure comprising CoCr₂O₄, (Ni,Co)Al₂O₄, NiCr₂O₄, and NiO mixed oxides. However, since the significant inhibition effect of the YAG coating to oxygen ionic diffusion, the mixed oxides appearance time and TGO growth behaviors were delayed in the DCL TBC. As a result, the TGO thickness of the DCL TBC was always smaller than that of the SCL TBC in the entire oxidation process. And the Al₂O₃ layer thickness proportion in the total TGO of the DCL TBC was greater than or equal to that of the SCL TBC after oxidation for the same period. The results of weight gain showed that compared with the SCL TBC, the parabolic oxidation rate of the DCL TBC was decreased approximately 35%. Consequently, the DCL TBC has better high-temperature oxidation resistance than the SCL TBC.     

Microstructure and luminescence properties of YSZ-based thermal barrier coatings modified by Eu2O3

In order to study the variation of rare earth oxides during thermal failure of thermal barrier coatings, Eu2O3-doped YSZ coatings with 0.5 mol%, 1.0 mol% and 1.5 mol% were prepared by explosive spraying. SEM, XRD, EDS and microhardness tester were used to analyze the effect of different rare earth oxide doping content on the morphology, composition and mechanical properties of the coatings. The results showed that with the increase of rare earth oxide doping content, the porosity of the coatings decreased, and the microhardness and fracture toughness increased. When the doping amount of rare earth oxide is 1.0 mol%, the bonding strength and thermal cycle times of the coating are the highest, 33.4 Mpa and 185 times respectively. With the increase of the doping amount of rare earth, the luminous intensity of the sprayed coating increases. After thermal shock test, the luminous intensity of Eu2O3-doped YSZ coatings at 592 and 608 nm decreased to a certain extent.     

Microstructure and thermal shock resistance of Nd2O3-doped YSZ-based thermal barrier coatings

To study the impact of rare earth oxide doping on the thermal failure of thermal barrier coatings, 0.5 mol%, 1.0 mol% and 1.5 mol% Nd₂O₃-doped YSZ coatings were prepared by explosive spraying. SEM, XRD, EDS and microhardness testing were used to analyse the effect of different rare earth oxide doping contents on the morphology, composition and mechanical properties of the coatings. With an increase in the Nd₂O₃ doping content, the porosity of the coatings was reduced. The decrease in the porosity increased the compactness of the coatings and improved the microhardness and fracture toughness. The bonding strength and thermal shock resistance of the coatings were the highest among the samples herein when the rare earth doping content was 1.0 mol%, and the values were 37.6 MPa and 200 times, respectively. The thermal shock failure mode of the coating was mainly due to the exfoliation of the inner layer of the ceramic layer. The luminous intensity of the coating increased with increasing rare earth oxide doping content, and the emission spectrum of the Nd₂O₃-modified YSZ coating after the thermal shock test produced a new emission peak at 594 nm, which decreased at 708 nm.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

Numerical study of residual stresses in environmental barrier coatings with random rough geometry interfaces

To clarify the role of the coating interface geometry and thermally grown oxide (TGO) layer in the failure of environmental barrier coatings (EBCs) and to further understand the cracking and spalling mechanisms of coatings, in this study, the thermomechanical properties of the multilayer coating system (Yb₂SiO₅/Yb₂Si₂O₇/Si), the morphology of the coating interface and the influence of the oxide layer on the local stresses during cooling were considered based on a random rough interface geometry model. The results showed that the rough geometry increased the magnitude of residual stresses at the interface and that the stress distribution away from the interface was less affected than the coating without roughness. The cracks on the outer surface of the Yb₂SiO₅ layer initiate in the valley region and spread with a stress value independent of the TGO thickness, and this failure may occur by cracking under tensile stress. The overall stress intensity at the TOP/EBC interface was lower than that at the upper surface of the TOP layer. The presence of TGO increased the magnitude of residual stresses in the BC and EBC layers, which caused cracks at the TGO/BC and TGO/EBC interfaces to occur at opposite locations. The phase change of the TGO layer from β-cristobalite to α-cristobalite cause a rapid increase in the overall level of coating stress, which may be a direct factor in coating failure. The calculation results provide a theoretical basis for the coating design and manufacturing process.     

Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

In this work, impedance spectroscopy (IS) has been used to evaluate the degradation of thermal barrier coatings (TBCs) due to oxidation at 1150^oC. The spallation of TBCs was found to be induced by the development of thermally grown oxides (TGO) produced from the oxidation of TBCs. The change in the electrical properties of the TGOs was found to be related to the change in the microstructure and microchemistry of the TGO. The resistivity of the TGO due to oxidation from 10 to 1000 h decreased rapidly, which corresponded to the increase in porosity in the TGO and the compositional change of the TGO from α-Al₂O₃ to a mixture of α-Cr₂O₃ and (Ni,Co)(Cr,Al)₂O₄ spinel. The slow decrease in the resistivity of the TGO from oxidation for 1000–2000 h indicated that there was little change in the composition of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of α-Al₂O₃ in the TGO caused a rapid oxide growth during oxidation and led to the spallation of TBCs.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Exploration of the oxidation behavior and doping ratio of the Si–HfO2 bond layer used in environmental barrier coatings

Hafnia doping is expected to improve the performance of the silicon-bond layer of environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites. The optimal doping ratio, distribution of HfO₂, and oxidation mechanism of the bond layer have not yet been fully addressed. A prototype Si–HfO₂ bond layer with a designed HfO₂-rich area was used to examine its oxidation behavior. A random dispersion model was developed to calculate the optimal HfO₂ doping ratio and its appropriate distribution state. The simulation results recommended that 20–30 vol% is the optimal doping ratio, where HfO₂ is well dispersed inside Si without forming networks. This enables HfO₂ to react with and consume SiO₂ without accelerating oxygen diffusion inside the bond layer. This was confirmed by oxidation experiments on Si–xHfO₂ tablets, in which the thinnest thermally grown oxide was achieved for the 20 vol% HfO₂-doped Si tablet. Both the microstructure design and material composition selection are highly important to further boost the performance of the EBCs.     

Effect of TGO evolution and element diffusion on the life span of YSZ/Pt–Al and YSZ/NiCrAlY coatings at high temperature

In this work, the growth of thermally grown oxides (TGO) on Pt–Al and NiCrAlY bond coats and the element diffusion behavior were investigated. During oxidation, TGO initiated at YSZ/Pt–Al interface developed from a α-Al₂O₃ mono-layer to a α-Al₂O₃+NiO/α-Al₂O₃ double-layer with the increase of thermal cycling temperature. While for YSZ/NiCrAlY coating, after exposed at 1100 °C for 240 h, a double-layered TGO was formed at the interface of NiCrAlY/substrate. It is composed of an upper layer of α-Al₂O₃, Cr₂O₃ and NiCr₂O₄ mixture and a bottom layer of α-Al₂O₃. After the coating was thermal cycled at 1200 °C for 96 h, a triple-layered TGO was generated containing a bottom layer of α-Al₂O₃, a middle layer of Al₂O₃ and Cr₂O₃, and an upper layer of mixed α-Al₂O₃, Cr₂O₃ and NiCr₂O₄. The multi-layered structure of TGO is caused by the difference of element diffusion rate and formation energy of oxides. It facilitates the alternative accumulation and release of stress. Thus, the consequent service life of YSZ/Pt–Al coating is better than that of YSZ/NiCrAlY coating.     

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

The novel bi-layer environmental barrier coatings (EBCs) with HfO₂-SiO₂/Yb₂Si₂O₇ structure (70HfO₂-30SiO₂/Yb₂Si₂O₇: 70HS/YbDS, 50HfO₂-50SiO₂/Yb₂Si₂O₇: 50HS/YbDS, molar ratios) was tested in 90%H₂O–10%O₂ conditions between room temperature and 1475 °C in an Al₂O₃ tube furnace, then its performance was evaluated. The YbDS layer was contaminated by alumina impurities under steam conditions. After 22 cycles, the 70HS/YbDS completely separated from the SiC substrate, while the 50HS/YbDS and SiC did not separate, even though cracks formed at the 50HS/SiC interface and the TGO layer. Furthermore, the thermally grown oxide (TGO) layer formed at the HfO₂-SiO₂/SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO₂-SiO₂/TGO interface and TGO interior, which was the culprit leading to the failure of EBCs. These results demonstrated that the 50HS/YbDS EBCs have the potential to protect SiC in steam conditions at 1475 °C.     

Microstructure evolution of Al-modified 7YSZ PS-PVD TBCs in thermal cycle

The PS-PVD method was used to prepare 7YSZ thermal barrier coatings (TBCs) and NiCrAlY bond coatings on a DZ40 M substrate. To prevent oxidation of the coating, magnetron sputtering was used to modify the surface of TBCs with an Al film. To explore the stability of TBCs during thermal cycling, water quenching was performed at 1100 °C, and ultralong air cooling for 16,000 cycles was performed. The results showed that before water quenching and air cooling, the top surface structure of the 7YSZ TBCs changed. After water quenching, the surface of the Al film was scoured and broken, the surface peeled off layer-by-layer, and cracks formed at the interface between the thermally grown oxide and NiCrAlY. During air cooling of the thermal cycle, the Al film reacted with O₂ in the air to form a dense Al₂O₃ top layer that coated the cauliflower-like 7YSZ surface and maintained the feather-like shape. At the same time, the TGO layer between 7YSZ and NiCrAlY grew and cracked. The two thermal cycles of water quenching and air cooling led to different failure mechanisms of TBCs. Water quenching failure was caused by layer-by-layer failure of the 7YSZ top coat, while air cooling failure occurred due to the internal cracking of the TGO layer at the 7YSZ/NiCrAlY interface and the failure of the TGO/NiCrAlY interface.     

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

Shot 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.     

Oxidation behavior and electrical properties of metal interconnects with Ce-doped Ni–Mn spinel coatings

To improve the high-temperature oxidation resistance and electrical conductivity of ferritic stainless steels, protective Ce-doped NiMn₂O₄ spinel coatings were fabricated on the surface of SUS430 steel by electrophoretic deposition (EPD). The phase structure and microstructure of Ce-doped NiMn₂O₄ in both powder and coating forms were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The high-temperature oxidation of the NiMn₂O₄ spinel coating before and after Ce doping in the air at 800 °C for 168 h was studied by weight gain experiments. The area-specific resistance (ASR) of coatings was measured by a standard four-probe method. It was found that the Ce-doped NiMn₂O₄ spinel powder displayed a stable structure, high crystallinity, fine grain size, and decreased agglomeration when the Ce content was fixed at 0.05 mol?L^?1. The oxidation kinetics of NiMn₂O₄-coated SUS430 steel before and after Ce doping obeyed a parabolic law with parabolic rate constants of 4.58 × 10^?15 g² cm^?4 s^?1 and 1.83 × 10^?15 g² cm^?4 s^?1, respectively. When oxidized at 800 °C for 50 h, the ASR value of the coated samples before and after Ce doping stabilized at about 15.2 mΩ?cm² and 14.5 mΩ?cm², respectively. This work demonstrated that the Ce-doped NiMn₂O₄ spinel coating improved the high-temperature oxidation resistance and the electrical conductivity of metal interconnects.     

Solution precursor thermal spraying of gadolinium zirconate for thermal barrier coating

Gadolinium zirconate (GZ) is an attractive material for thermal barrier coatings (TBCs). However, a single layer GZ coating has poor thermal cycling life compared to Yttria Stabilized Zirconia (YSZ). In this study, Solution Precursor High Velocity Oxy-Fuel (SP-HVOF) thermal spray was used to produce a double layer GZ/YSZ TBC and compared the thermal cycling performance with the single layer YSZ TBC. The temperature behaviour of the solution precursor GZ was studied, and single splat tests were carried out to obtain an optimised spray parameter. In thermal cycling tests, the single-layer YSZ reached 20 % failure at 85 ± 5 cycles, whereas the double-layer GZ/YSZ was at 70 ± 15 cycles. The single-layer failed at the topcoat/TGO interface, whereas the double-layer failed at GZ/YSZ interface and topcoat/TGO interface. Moreover, Gd diffusion occurred near the GZ/YSZ interface, resulting in porosities in the GZ layer.     

Oxidation behaviors and mechanisms of Yb2O3-doped silicon coatings fabricated by vacuum plasma spray

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb₂O₃ could react with SiO₂ TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb₂O₃ were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb₂Si₂O₇, could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb₂O₃-doped Si coatings were analyzed based on microstructure and phase composition observations.     

MnxCo3-xO4 spinel coatings: Controlled synthesis and high temperature oxidation resistance behavior

A controllable method for the synthesis of Mn_xCo_3-xO₄ (x = 0.4, 0.8, 1.0, 1.2) spinel coatings with unique microstructures and high-temperature oxidation resistances for solid oxide fuel cells is crucially desired. Herein, we report the synthesis and analysis of synthesized coatings with different phase compositions of ferritic stainless steel interconnects. The coatings are fabricated using a sol-gel dip-coating method. Multiple experimental results confirm that the Mn_xCo_3-xO₄ coatings exhibit the advantages of having fine grains and uniform density. Furthermore, the as-prepared Mn_1.2Co_1.8O₄ coating has a low coefficient of thermal expansion (11.98 × 10^-6 K^-1), matching that of ferritic stainless steel, which effectively improves the anti-stripping property of the materials. In particular, oxidation tests illustrate that the oxidation weight gain of this coating decreased by 84.4% compared with bare ferritic stainless steel after 3600 h. The area-specific resistance (ASR) results indicate that the ASR value of the Mn_1.2Co_1.8O₄-coated sample reduced by about 70% compared with those of the ferritic stainless steel samples, which demonstrates its desirable high-temperature oxidation resistance property. It is hoped that this work stimulates new research ideas for the development of spinel oxide coating.     

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu³⁺) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu³⁺ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu³⁺ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu³⁺ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu³⁺ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu³⁺ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.     

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.