Thermal insulation performance is a measurement of the thermal protection offered by the thermal barrier coatings (TBCs) to the substrate, therefore, it is essentially important to compare different double ceramic layer (DCL) TBCs on the premise of the same thermal resistance. In this study, a series of LZO/YSZ DCL-TBCs, with the equivalent thermal insulation to 500 µm thick YSZ TBCs, were prepared, and their lifetimes were evaluated by thermal gradient cyclic test at the top coat surface temperature of 1300°C. Result show that, the lifetime of DCL-TBCs was more than doubled compared to 500 µm thick YSZ TBCs, when 100µm thick YSZ coating was substituted by LZO coating. In addition, the lifetime of DCL-TBCs decreased with the increase of LZO substitutional ratio. X-ray diffraction analysis revealed that LZO maintains the pyrochlore structure after thermal cyclic test. Microstructure examination demonstrated that, with the increase of LZO substitutional ratio, the delamination position transferred from near top/bond coating interface to near LZO/YSZ interface and finally to the inside of LZO coating. Therefore, this study would shed light to further coating structure optimization towards the next generation advanced DCL-TBCs.

The non-parabolic isothermal oxidation kinetics of low pressure plasma sprayed MCrAlY bond coat was investigated. To qualitatively explain the abnormal growth phenomenon of thermally grown oxides (TGO), the changes that occurred to their microstructure during the oxidation process were studied. Based on these observations, a modified model was developed to understand and quantitatively predict the non-parabolic oxidation and growth kinetics of TGO. This modified model, which fits well with experimental results, provides a novel method to quantitatively predict the long-term growth behaviour of TGO, and thereby benefits the development of long-life and highly reliable thermal barrier coatings.

Thermal cycle lifetime is essentially important to the application of thermal barrier coatings (TBCs) on the premise of the same thermal resistance. In this study, equivalent thermal insulation conception is introduced to the design of dense vertical crack (DVC) structured TBCs and the lamellar structured TBCs, to fairly compare the lifetime of TBCs with different structure. DVC-structured TBCs with the equivalent thermal insulation to lamellar YSZ TBCs were prepared, and their lifetimes were evaluated by thermal gradient cyclic test. Cross-sectional morphology and phase constitution before and after failure were examined by scanning electron microscope and X-ray diffraction, respectively. The failure mode was analysed. This study would shed light to further coating structure optimization.

Inter-lamellae bonding within thermal sprayed coatings is one of the most important factors influencing the properties and performance of coatings. It has been revealed that there exists a critical bonding temperature for a molten ceramic splat to form the bonding to the same splat surface. The erosion behaviors of thermal sprayed coatings are significantly influenced by the interface bonding between lamellae. In this study, the erosion behavior of plasma-sprayed TiO 2 , Al 2 O 3 and YSZ coatings deposited at different deposition temperatures was investigated. The cross section of plasma sprayed coatings was characterized by the scanning electron microscope. It was revealed that the coatings deposited at room temperature exhibit a typical lamellar structure with numerous unbonded interfaces, whereas the coatings prepared at the temperature above the critical bonding temperature present a dense structure with well bonded lamellae. The erosion rate significantly decreases with the improvement of interface bonding. In addition, the erosion mechanisms of the conventional coatings and the lamellae well bonded coatings were further discussed. The unbonded interfaces act as precracks accelerating the erosion of plasma-sprayed coatings. Thus, controlling inter-lamellae bonding based on the critical bonding temperature is conducive to the improvement of erosion resistance of plasma-sprayed ceramic coatings.

In the present study, a novel and practical method, white light interference, was proposed to characterize the lamellar pores covered by thermally sprayed YSZ and LZ splats. In this method, only an ordinary optical microscopy (OM) was employed. Colorful Newton rings and parabolic shapes of the lamellar pores were widely observed by OM. The crack spacing and the shapes of the lamellar pores captured by OM were well consistent with those by scanning electron microscopy (SEM) and focus ion beam (FIB). Besides, mechanical analyses were carried out and the results were well consistent with those by OM. Most importantly, the essential fact that the lamellar pores resulted from transverse cracking/delamination in thermal sprayings was highly elaborated.

Thermally-sprayed LZ/YSZ double-layer coatings are promising candidate for the next generation thermal barrier coatings (TBCs) due to exceedingly low thermal conductivity and superior high-temperature phase stability. However, a delamination failure at LZ and YSZ interface were widely observed during TBCs service. Till today, the interfacial microstructure between LZ and YSZ remains unclear. In the present study, LZ splats were deposited on YSZ substrate to serve as a LZ/YSZ interface. The interfacial microstructure was explored by focused ion beam (FIB) and high-resolution transmission electron microscope (HR-TEM). The interfacial defects at splat interface were clearly observed and thoroughly discussed. These results would shed light on deeply understanding the interfacial failure of double-layer LZ/YSZ coatings.

This study investigates the correlation between thermal barrier coating (TBC) lifetime and thermally grown oxide (TGO) layer thickness. YSZ TBCs were deposited by atmospheric plasma spraying on Ni-base substrates and subjected to burner cycling tests with a thermal gradient and isothermal furnace testing. Both tests revealed that thermal cycling lifetime decreases with increasing TGO thickness, following a power law function, and for a critical TGO thickness of 5-6 μm, the failure mode changes from cracking within the YSZ layer to interface cracking around the TGO. Although either test can be used to evaluate TBC performance, burner cycling tests are better suited for evaluating ceramic topcoats, while furnace cycling test results integrate the effects of bond coat properties, especially oxidation resistance, as well as ceramic topcoat cracking resistance. The two tests can thus be used together to assess the factors that control TBC failure.

One way to reduce plasma jet velocity and prolong the dwell time of spray particles in the jet is to enlarge the orifice of the torch nozzle. In this study, normal and modified nozzles are used to deposit YSZ particles on ceramic and superalloy substrates by plasma spray-physical vapor deposition (PS-PVD). The modified nozzle is shown to increase the evaporation of YSZ particles and thus the quantity of Zr atoms and Zr 1+ ions in the plasma jet, which allows columnar structured coatings to be realized at higher deposition rates using a conventional 80 kW plasma spray system. The columnar ceramic coatings are also shown to have good conformity on cold-sprayed MCrAlY bond coats with high surface roughness.

In this work, liquid plasma spraying is used to deposit composite coatings for potential use as cathodes in intermediate-temperature solid oxide fuel cells. A suspension containing well-distributed Gd-doped ceria (GDC) nanoparticles in a lanthanum strontium cobalt ferrite precursor solution was used as the feedstock, and GDC concentration was varied to study its effect on phase composition, microstructure, surface morphology, and electrochemical performance. The results are presented and discussed.

Transparent hydrophobic materials have broad application prospects in industry and daily life due to its transparent and super-hydrophobic characteristics, while its performance will be significantly influenced by the materials and its surface morphology. Hydrophobicity and transparency are competitive properties from the viewpoint of surface roughness. However, there are still a lot of problems to be solved to satisfy hydrophobicity and transparency simultaneously in the prepared surface. A novel and simple method to fabricate transparent hydrophobic surface is introduced in the present paper. Micron and sub-micron mixed Al 2 O 3 powders were deposited on the glass and plastic substrate by vacuum cold spray. The coatings were modified by 1,1,2,2- Tetrahydroperfluorodecyltrimethoxysilane (FAS) to reduce the surface energy. The surface morphology was characterized by scanning electron microscopy (SEM) and 3D laser microscopy (LSM). The transmittance of the coating was also investigated. The results show that the as-sprayed surfaces exhibit submicron structure. After FAS absorbing treatment, the contact angle of water droplets on the coatings ranged from 108° to 136° depending on the local surface morphology, and the transmittance of Al 2 O 3 coatings are all above 80% in the range of visible light.

A new method to fabricate micro-nano scaled surface with super-hydrophobicity is introduced in the present paper. Micro-nano hierarchical structure coatings based on silica (SiO 2 ) micron particles were successfully deposited on the stainless steel substrates by high-velocity oxygen fuel (HVOF) spraying which were modified by 1,1,2,2- Tetrahydroperfluorodecyltrimethoxysilane (FAS) to reduce surface energy. The influences of the HVOF process parameters on the wettability of the coatings were investigated. The coatings were characterized by scanning electron microscopy (SEM), 3D laser microscopy (LSM), and Fourier transform infrared spectrometer (FTIR). The results show that the as-sprayed surfaces exhibit micro-nano hierarchical structure. The water droplets are strongly adhesive to the as-sprayed surface, while by FAS absorbing treatment, the surfaces exhibit super-hydrophobicity, whose contact angle with water droplets are as high as 150°, and the water droplets tend to roll on the surface with extremely low adhesion with a sliding angle of 3°

In this study, YSZ coatings are deposited by plasma spray-physical vapor deposition using a shroud to limit expansion of the plasma jet and increase its heating ability. Optical emission spectroscopy shows that the shroud significantly increases the evaporation of YSZ particles in the jet, resulting in coatings with a hybrid columnar structure. SEM examination of coating surfaces and cross-sections reveal micro and nanoscale features and, in each case, the mechanisms of formation are discussed.

The aim of this study is to better understand the formation of nonbonded splat-to-splat interfaces in thermally sprayed ceramic coatings. To that end, the surfaces between splats in plasma-sprayed La 0.5 Sr 0.5 CoO 3 (LSCO) coatings were examined and compared to free splat surfaces. The results show that free splat surfaces are relatively smooth, while adjacent surfaces at intersplat interfaces are quite rough. The observation implies that nonbonded splat-to-splat interfaces were never bonded, having fractured due to interface shear stress generated during splat cooling.

In this study, MCrAlY-Al 2 O 3 composite powders were produced by ball milling and deposited by plasma, HVOF, and cold spraying. The results show that Al 2 O 3 fractions can be well controlled using composite powder due to non-preferential impact debonding of the matrix and Al 2 O 3 . The microstructure of spray powders is well retained in HVOF and cold-sprayed coatings due to the unmelted or partially molten condition of the spray particles. In the case of plasma-sprayed coatings, however, most Al 2 O 3 particles segregate at lamellar interfaces, forming a continuous oxide scale on the splat. The cold-spray coatings exhibit the highest hardness due to the work hardening effect of kinetic deposition.

In earlier experiments, plasma-sprayed Al 2 O 3 coatings were deposited on preheated Al 2 O 3 substrates to study the effect of substrate temperature on splat formation and phase transformations. The aim of the present work is to develop a model to better understand the factors that affect phase selection during the solidification of Al 2 O 3 splats. A model based on one-dimensional heat transfer and classic nucleation theory is presented and used to simulate the rapid solidification process and the influence of process parameters on phase selection. The model accounts for under-cooling phenomena, heterogeneous nucleation, and nucleation kinetics. The findings indicate that the relationship between initial substrate temperature and phase selection is primarily based on the catalytic effect of the alumina substrate on the nucleation of Al 2 O 3 phases as a function of contact angle.

In this study, WC-CoWC coatings were produced by HVOF spraying using bimodal-structured WC-Co powder with both micro- and nano-sized WC particles. Due to the melting characteristics of the powder during spraying, the microsized particles are retained in the deposit, but the nanosized particles dissolve into the Co matrix, forming a Co-W-C ternary phase. Compared to coatings sprayed from conventional WC-CoWC powder, the bimodal coatings are more resistant to corrosion and wear and are comparable in microhardness.

Plasma-sprayed thermal barrier coatings are highly insulating due to intersplat pores. High-temperature exposure, however, leads to pore healing and a loss of thermal insulating capability. Previous sintering models based on the healing of iso-thick pores cannot explain the ultrafast sintering kinetics observed during the initial sintering stage. To better understand the sintering process and the kinetics of pore healing, a study was conducted on the changes that occur at the interfaces between lamellae at high temperatures. It was found that splat surfaces develop multiscale convexes, forming bridges between adjacent splats that significantly accelerate pore healing. This model explains the fast sintering kinetics of the initial sintering stage and presents new opportunities to further tailor the sintering kinetics of YSZ coatings and thereby improve TBC performance.

Thermally grown oxide (TGO) that naturally forms on bond coat surfaces plays an important role in determining the lifetime of thermal barrier coatings (TBCs). Splashed particles on thermally sprayed MCrAlY bond coat surfaces are weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides that contribute to TBC failure. In this study, various heat treatments are used to modify the weakly bonded splashed particles on LPPS CoNiCrAlY bond coats in order to restrain the formation of mixed oxides and prevent associated failures.

In this study, La 2 Zr 2 O 7 coatings produced by atmospheric plasma spraying were heated to 1300°C for up to 100 h. Residual pores in the coatings were characterized by SEM. Porosity was found to decrease with increasing sintering duration, although large pores still existed after 100 h. The relationship between the opening of residual pores and sintering time was closely analyzed and is described in detail. The results presented in the paper can inform the design of pores that endow TBCs with higher sintering resistance.

Thermal cycling tests were performed on thermal barrier coatings (TBCs) to evaluate the influence of temperature gradients in the ceramic topcoat on overall lifetime and performance. The coating system tested consists of an Inconel 738 substrate, a cold-sprayed NiCoCrAlTaY bond coat, and an atmospheric plasma sprayed YSZ topcoat. YSZ surface temperatures were 1150, 1200, and 1250 °C, corresponding to temperature gradients of 150, 200, and 250 °C across the 250 µm thick layer. Heating and cooling times were set at 120 sec for each thermal cycle. The results of the study show that lifetime decreases with increasing temperature gradient, although the gradient has little effect on the failure mode.