Cracking behavior and delamination mechanism of lamellar structured TBC with localized mixed oxides

Mixed oxide (MO) with localized growth feature and high growth rate remarkably affects the lifetime of thermal barrier coatings (TBCs), which indicates that clarifying the ceramic cracking mechanism induced by MO is critical for developing new coatings with high durability. Two kinds of TBC models involving spherical and layered mixed oxides are created to explore the influence of MO growth on the local stress state and crack evolution during thermal cycle. The growth of α-Al₂O₃ is also included in the model. The undulating interface between ceramic coat and bond coat is approximated using a cosine curve. Dynamic ceramic cracking is realized by a surface-based cohesive interaction. The ceramic delamination by simulation agrees with the experimental observation. The effects of MO coverage ratio and growth rate on the TBC failure are also discussed. The results show that the MO growth causes the local ceramic coat to bear the normal tensile stress. The failure mode of coating is turned from α-Al₂O₃ thickness control to MO growth control. Once the mixed oxide appears, local ceramic cracking is easy to occur. When multiple cracks connect, ceramic delamination happens. Suppressing MO formation or decreasing MO growth can evidently improve the coating durability. These results in this work can provide important theoretical guidance for the development of anti-cracking TBCs.     

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.     

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.     

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

According 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.     

Surface coating of α-Al2O3 nanoparticles with poly(vinyl alcohol) as biocompatible coupling agent for improving properties of bio-active poly(amide-imide) based nanocomposites having l-phenylalanine linkages

Nanocomposites (NCs) containing metal oxide nanoparticles (NPs) as fillers are used in a wide range of applications and in various fields. Poly(vinyl alcohol) (PVA) is an attractive polymer because of its many desirable applications and characteristics. In this investigation, at first, the surface of alumina (α-Al₂O₃) NPs was modified with PVA as a biocompatible modifier. Then, the optically active poly(amide-imide) (PAI) nanostructure was prepared by using molten tetrabutylammonium bromide as a molten ionic liquid and triphenyl phosphite as the condensing agent. Finally, the modified α-Al₂O₃ (α-Al₂O₃-PVA) NPs were incorporated into the PAI matrix for the preparation of PAI/Al₂O₃-PVA NCs (PAPNCs). To investigate effect and nature of coating on the surface of the α-Al₂O₃ NPs and preparation of PAPNCs, the samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. The surface morphology examination demonstrated the monodispersed characteristics of α-Al₂O₃ NPs after surface modification with the PVA and incorporation into the PAI matrix.     

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.     

Corrosion behavior of air plasma spraying zirconia-based thermal barrier coatings subject to Calcium–Magnesium–Aluminum-Silicate (CMAS) via burner rig test

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y₂O₃ on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y₂O₃, 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO₂ in TBCs because of the higher diffusion rate and solubility of Y³⁺ in CMAS than Zr⁴⁺. At the same time, 38YSZ/8YSZ + Y₂O₃ reacts with CAMS to form Ca₄Y₆(SiO₄)₆O/Y_4·67(SiO₄)₃O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y₂O₃ layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y₂O₃ dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties.     

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.     

Synergistic effect of CeO2 and Al2O3 nanoparticle dispersion on the oxidation behavior of MCrAlY coatings deposited by HVOF

In this study, the as-received and nano-scaled oxide dispersion strengthened (ODS) MCrAlY coatings were deposited using high-velocity oxy-fuel (HVOF) spraying process. The high-energy planetary ball-milling process was utilized to prepare CeO₂ and Al₂O₃ nanoparticles. ODS-NiCoCrAlY feedstock powders were also developed using the ball-milling process. The various formulations of Al₂O₃ and CeO₂ nanoparticles (0.5 and 1.0 wt%) were chosen to apply different types of ODS-NiCoCrAlY coatings. The microstructure of the as-received and ODS coatings were evaluated by field emission scanning electron microscope (FESEM) as well as the commercial and ODS powders. Furthermore, the microhardness of different compositions of ODS coatings was accordingly investigated and the obtained results were compared with as-received coating. On account of the measurement of oxidation kinetics, the freestanding as-received and ODS coatings were exposed to air at 1000 °C up to 500 h and the thickness growth rate of the α-Al₂O₃ oxide layer was simultaneously examined. The results exemplified that NiCoCrAlY+1.0 wt% nano-CeO₂+0.5 wt% nano-Al₂O₃ coating had a better oxidation resistance and lower oxide scale growth rate under the synergistic effects of both CeO₂ and Al₂O₃ nanoparticles.     

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

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

Fast formation of a black inner α-Al2O3 layer doped with CuO on Al–Cu–Li alloy by soft sparking PEO process

Forming high-temperature α-Al₂O₃ phase under soft sparking is an intriguing phenomenon in plasma electrolytic oxidation (PEO) of Al alloys, which contradicts the low energy input of the process. In this study, α-Al₂O₃ doped with black CuO is formed beneath an amorphous white outer layer on Al–Cu–Li alloy by PEO in a dilute silicate electrolyte under soft sparking. In comparison, reddish coatings with dominating γ-Al₂O₃ are formed under the conventional plasma discharges, although blackish inner layer with α-Al₂O₃ can also be exposed by heavily polishing the samples. In order to know the underlying mechanism, temperatures at the coating surface and the underlying substrate have been monitored by a thermocouple under the conventional and soft sparking PEO regimes, respectively. Interestingly, high temperatures are detected in the case of soft sparking rather than PEO with strong discharges. The formation of CuO, quartz, and cristobalite within the soft sparking coating also supports the existence of high temperature. Hence, the formation of α-Al₂O₃ under soft sparking can be resolved to the conventional thermal activation mechanism, without the need of seeking other plausible explanations. Thermal condition evaluation for soft sparking PEO suggests that values of the effective thermal conductivity during PEO process for the outer layer and the barrier layer at the coating/substrate interface might be lower than ∼0.05 and ∼0.0017 W m⁻¹ K⁻¹, respectively. It is believed that the amorphous structure of the outer and barrier layers effectively blocks the heat dissipation, facilitating the formation of a highly wear-resistant inner layer with α-Al₂O₃, CuO, and the other high-temperature species under soft sparking.     

Effect of substrate morphology on the deposition behavior of α-Al2O3 films by room temperature granule spray in vacuum process

In this study, we investigated the influence of surface morphology of a hard substrate on the deposition behavior of α-Al₂O₃ films coated by a granule spray in vacuum (GSV) process at room temperature. Three types of α-Al₂O₃ films were prepared by chemical vapor deposition (CVD) on a WC-Co base material: (006) textured film with a uniform and small mold-type morphology, (110) and (012) textured films with large grains and a relatively flat morphology, and flat (006) textured film after polishing. These CVD α-Al₂O₃ films were further coated with nano-grained α-Al₂O₃ films by the GSV process. The deposition behavior, adhesion strength, and residual surface stress were found to be strongly influenced by the surface morphology of the hard substrate. For the (006) textured CVD α-Al₂O₃ film with steep crests and troughs on the surface, the GSV α-Al₂O₃ film was deposited uniformly, possibly because the powder can be directly anchored by the molding effect and subsequent hammering effect. Such different deposition characteristics also led to a variation in the surface residual stress.     

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.     

A Novel α-Al2O3 Diesel Particulate Filter with Alkali Metal-Based Catalyst for Diesel Soot Oxidation

A diesel particulate filter (DPF) substrate was fabricated from alkali-resistant α-Al₂O₃ to make a practical use of alkali metal catalysts for diesel soot oxidation. The fundamental properties of the α-Al₂O₃-DPF, including its particle filtration efficiency, pressure drop, and thermal durability were comparable to those of the conventional DPFs. The diesel soot oxidation activity of the catalyzed α-Al₂O₃-DPF with a washcoat of alkali metal-based catalyst was, even after thermal aging, much higher than that of the conventional catalyzed-DPF with platinum group metal catalysts.     

Preparation of meso-macroporous α-alumina using carbon nanotube as the template for the mesopore and their application to the preferential oxidation of CO in H2-rich gases

Meso-macroporous α-Al₂O₃ was successfully prepared by using acid-treated carbon nanotube as mesoporous forming agent and polystyrene foam as the template for the macropore. A series of Ru/meso-macroporous α-Al₂O₃ catalysts were prepared by the incipient wetness impregnation method and applied to the preferential oxidation of CO (CO-PROX) in H₂-rich gases. SEM, N₂ adsorption–desorption, H₂-TPR techniques and TEM were employed to characterize the catalysts. The results indicate that the specific surface area was markedly elevated by introducing the mesopores, which led to the higher dispersion of ruthenium nanoparticles on the surface of α-Al₂O₃. The meso-macroporous α-Al₂O₃ supported ruthenium showed very high activity and selectivity for CO-PROX.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Transformation of anodic aluminum oxide to nanoporous α-Al2O3, ruby and Ti-sapphire micropatterns

Anodic aluminum oxide (AAO) stands in the forefront of the most important materials in modern nanotechnology. Its regular nanoporous structure is widely used for template assisted nanopatterning, optical applications, membrane science and plenty more. Our study presents an enhancement on the potential application of AAO by exploiting its inherent nature of polymorphism. Amorphous AAO is selectively transformed to α-Al₂O₃ by a laser-induced photo-thermal process. Carbon nanotubes (CNT) are employed as a sacrificial laser light absorber to accomplish the phase transformation of AAO to α-Al₂O₃. Doping of the evolving α-Al₂O₃ with Cr or Ti enables preparation of free designable photoluminescent surface patterns. The superior properties of α-Al₂O₃ are utilized to create hierarchically structured systems for optical, biomedical and lithographic applications.     

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.     

Microstructure and oxidation resistance of a NiCrAlY/Al2O3-sprayed coating on Ti-19Al-10Nb-V alloy

A Ti₃Al (α₂) alloy (Ti-19Al-10Nb-3V, wt%) coated by air plasma spraying (APS) with Ni22Cr10AlY and Ni22Cr10AlY/Al₂O₃ has been tested against cyclic oxidation in air at 850 °C for 10 cycles of 10 h each. Effects of pre-oxidation and cyclic oxidation treatments, both in air, on the microstructure of the single α₂/Ni22Cr10AlY and double coated α₂/Ni22Cr10AlY/Al₂O₃ specimens were investigated by X-ray diffraction (XRD), SEM-EDS and Raman spectroscopy. Several distinct results appear through the cyclic oxidization of the pre-oxidized α₂/Ni22Cr10AlY, namely, elemental diffusion where Ti and Al diffuses from one side and Ni and Cr from the other side, oxygen is found to be present at the support-bond coat interface and a partial reaction of the superficial NiO with α-Al₂O₃ to form the non-protective NiAl₂O₄ takes place. Conversely, pre-oxidation treatment of the double coated α₂/Ni22Cr10AlY/Al₂O₃ specimens causes transient alumina polymorphs obtained by APS to convert to α-Al₂O₃ and hence to ensure the lowest cyclic oxidation rate seen by a k_p value of 2.62×10⁻¹² g² cm⁻⁴ s⁻¹. Good adhesion to support of all the coated specimens is recorded following the cyclic oxidization treatment.     

In-depth composition profile of anodic oxide films on aluminium studied by auger electron spectroscopy

In-depth composition profiles of the anodic oxide films grown on Al (111), (110) and (100) single crystal electrodes in ammonium borate were tudied by Auger elecron spectroscopy with Ar ion etching. The oxide film had the composition at Al₂O₃ as referenced to an authentic α-Al₂O₃ single crystal. The Auger electron energy and peak shapes of Al and O in the oxide film agreed with those of α-Al₂O₃ crystal. Regardless of the crystal orientation of the substrate Al and in-depth profile, the oxide film has the chemical composition of Al₂O₃. The Auger signal peak of boron was approx. $\text{1}\text{50}$ of that of oxygen throughout the sputtered distance.