Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

Simultaneous synthesis by spark plasma sintering of a thermal barrier coating system with a NiCrAlY bond coat

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO₂ (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al₂O₃ TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO₂ in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.     

Thermal conductivity and thermal cycle life of La2O3 and HfO2 doped ZrO2-Y2O3 coatings produced by EB-PVD

The effects of La₂O₃ and HfO₂ addition on thermal conductivity and thermal cycle life of EB-PVD YSZ coatings were investigated. La₂O₃ and HfO₂ were selected as additives, because they significantly suppress the sintering of YSZ. The developed coating showed low thermal conductivity as well as high resistance to sintering. Burner rig tests confirmed that the developed coating have a superior thermal insulating effect and have a longer life than that of a coating with conventional composition.     

An approach to predict the solid film thickness possibly yielded from an alumina sol-gel liquid film

The Landau-Levich-Derjaguin (LLD) equation is widely used to predict the thickness of wet layer deposited on substrate by dip-coating. But it cannot effectively predict the solid film thickness yielded from the sol-gel liquid layer. Considering the solid content, the amount of solution sticking to the surface of substrate and the density of the sol-gel derived solids materials are the main factors determining the solid film thickness, a new approach capable of directly predicting what an oxide film thickness of a liquid layer on the substrate could yield without really sintering at high temperature was developed. It was found that the predicted and measured thicknesses for both compact and porous Al₂O₃ films were in good accordance. The approach uses very common testing techniques and does not concern the aspects such as solution composition, Newtonian or non-Newtonian fluid, withdrawal speed, viscosity, and liquid-vapor surface tension, etc. So the method is much timesaving and economical, and will be a good supplement for thickness determination techniques, especially under some circumstances where use of SEM, XRR, ellipsometer analyses are limited.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

Degradation of SPS-Fabricated YSZ and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>-YSZ Ceramics in Supercritical Water

Zirconia (ZrO₂) ceramics are being considered as a candidate material for thermal insulating barriers in pressure tubes used in the supercritical water (SCW) nuclear reactors. However, the literature suggests that zirconia may undergo a detrimental phase transformation which is accelerated in aqueous environments. In this research, 8 mol% Yttria-Stabilized Zirconia (YSZ) ceramics with the addition of 5 and 10 mol% Nd₂O₃ were manufactured via spark plasma sintering (SPS) process and subsequently subjected to a SCW environment. The weight losses and microstructural evolutions of these materials during SCW exposure were studied. The results suggest that doping YSZ with Nd₂O₃ significantly decreased the degradation rate of the YSZ ceramic and improved its structural stability. X-ray diffraction studies revealed that after degradation testing, the Nd₂O₃ helped to retain the desirable cubic phase of YSZ matrix. In the case of pure YSZ ceramic, a phase change of the matrix toward the monoclinic lattice was observed and likely contributed to the ceramic’s disintegration in SCW environment.     

Impedance Analysis of 7YSZ Thermal Barrier Coatings During High-Temperature Oxidation

ZrO₂-7 wt.%Y₂O₃ (7YSZ) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying. High-temperature oxidation of 7YSZ TBCs was accomplished at 950 °C and characterized by impedance spectroscopy and scanning electron microscopy with energy-dispersive spectrometry. The results indicated that the thermally grown oxide (TGO) mainly contained alumina. The increase of the thickness of the TGO layer appeared to follow a parabolic law. Impedance analysis demonstrated that the resistance of the TGO increased with increasing oxidation time, also following a parabolic law, and that characterization of the TGO thickness based on fitting an equivalent circuit to its measured resistance is feasible. The YSZ grain-boundary resistance increased due to increasing cracks within the coating for oxidation time less than 50 h. However, beyond 150 h, the YSZ grain-boundary resistance slightly decreased, mainly due to sintering of the coating during the oxidation process.     

Microstructure characterization and electrical conductivity of electroless nano Ni coated 8YSZ cermets

Using a common electroless bath, Ni-8YSZ (Ni-8 mol% yttria stabilized zirconia) composite nano powder have been synthesized without use of any expensive sensitizing agent. HRTEM micrographs indicated that the coating morphology of Ni nano particles on the 8YSZ showed a spotty, discontinuous distribution and the Ni nano particles appeared as a crystalline phase. The amount of Ni in the composite powders was varied from 36-51 wt.% by changing the substrate powder loading in the electroless bath. Bar type samples were prepared by uniaxial pressing and sintering at 1300 °C for 2 h with these coated powders. The cubic (c)-zirconia was found to partially dissociate into monoclinic (m)-zirconia on sintering with Ni content 41% or higher and also increases with the increase of Ni content. The microstructure of each Ni-YSZ cermet after reduction in a H₂ + Ar gas atmosphere showed dual scale porosity (micro and submicron porosity). The Ni-8YSZ cermet samples showed metallic electrical conduction behavior, proving the percolation capability of the synthesized nano composite.     

Densification Kinetics of CeO<Subscript>2</Subscript> Reinforced 8 Mol.% Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Stabilized ZrO<Subscript>2</Subscript> Ceramics

The grain growth kinetics of 8YSZ ceramics processed using spark plasma sintering (SPS) has been investigated in the temperature ranging from 1100°C to 1500°C. The activation energy during SPS densification was obtained as 332 kJ/mol with grain boundary diffusion as a dominant mechanism. Further, the effect of CeO₂ on the densification kinetics of 8YSZ ceramic processed via SPS and conventional sintering (CS) has been delineated. The lower grain boundary mobility of CS-processed composites (an order of magnitude lower than SPS) is attributed to the solute drag and lattice distortion mechanism. However, no significant change in the grain boundary mobility was observed with CeO₂ addition (~?14.7–43.9?×?10^?18 m³/N/s for CS and 107.2–116.7?×?10^?18 m³/N/s for SPS) revealing that the defect concentration is nearly constant in 8YSZ. The study highlights the effect of sintering techniques (SPS and CS) and reinforcement (CeO₂) on engineering the desired microstructure of 8YSZ ceramic.     

Phase Transformation and Lattice Parameter Changes of Trivalent Rare Earth Doped YSZ as a Function of Temperature

Yttria-stabilized zirconia (YSZ) co-doped with trivalent oxide Sc₂O₃ and Yb₂O₃ is prepared using mechanical alloying and high-temperature sintering. High-temperature XRD analysis was performed to study the phase transformation and lattice parameter changes of various phases in the baseline YSZ and co-doped samples. The results show that the structure for the co-doped samples tends to be more thermally stable at test temperature above critical value. The lattice parameters for all samples increase with temperature at which XRD is carried out, and the lattice parameters for the two trivalent rare earth oxides co-doped YSZ are smaller than that for 7YSZ under the same temperature.     

Study on gas permeation behaviour through atmospheric plasma-sprayed yttria stabilized zirconia coating

Gas permeation behaviour through atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia (YSZ) electrolyte coating was studied experimentally. YSZ coatings were fabricated using different powder feedstock. The temperature and velocity of in-flight particles during spraying were measured with a diagnostic system. The results showed that particle temperature and velocity were significantly influenced by the size of powders. The gas permeability of these coatings was estimated by a specific instrument with pure O₂, N₂ and H₂. It was found that the gas permeability was reduced by decreasing the size of powder. Gas permeation behaviour through plasma-sprayed YSZ coating was studied. Transition flow was compatible to gas permeation behaviour for all three plasma-sprayed YSZ coatings. The relationship between gas permeation behaviour and coating microstructure is discussed in this article.     

Reaction behavior and pore formation mechanism of TiAl–Nb porous alloys prepared by elemental powder metallurgy

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α₂-TiAl₃/γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.     

Monoclinic zirconia distributions in plasma-sprayed thermal barrier coatings

Phase composition in an air plasma-sprayed Y₂O₃-stabilized ZrO₂ (YSZ) top coating of a thermal barrier coating (TBC) system was characterized. Both the bulk phase content and localized pockets of monoclinic zirconia were measured with Raman spectroscopy. The starting powder consisted of ∼15 vol.% monoclinic zirconia, which decreased to ∼2 vol.% in the as-sprayed coating. Monoclinic zirconia was concentrated in porous pockets that were evenly distributed throughout the TBC. The pockets resulted from the presence of unmelted granules in the starting powder. The potential effect of the distributed monoclinic pockets on TBC performance is discussed.     

Synthesis and Properties of La2O3-Doped 8 mol% Yttria-Stabilized Cubic Zirconia

In this study, 8 mol% yttria-stabilized cubic zirconia (8YSZ) powder as a matrix material and 0-15 wt.% La₂O₃ powder as an additive were used to determine the effect of La₂O₃ addition and its amount on the phase stability, microstructure, sintering, and mechanical properties of 8YSZ. Colloidal processing was used to mix the powders uniformly and to obtain a homogenous microstructure. XRD results showed the existence of only a cubic crystal structure for 1 and 5 wt.% La₂O₃ addition amounts. However, La₂Zr₂O₇ with a hexagonal and cubic crystal structure was observed in 8YSZ specimens doped with 10 and 15 wt.% La₂O₃. Further, up to 5 wt.% La₂O₃ was completely dissolved in the crystal structure of the specimens; however, above 5 wt.%, La₂O₃ reacted with 8YSZ at high temperatures and formed pyrochloric La₂Zr₂O₇. Grain size measurements revealed that the grain size of 8YSZ increased up to 1 wt.% La₂O₃ addition, and then decreased beyond this amount. The hardness and fracture toughness of 8YSZ decreased and increased, respectively, with the increasing La₂O₃ amount.     

Comparison of corrosion resistance between Al2O3 and YSZ coatings against high temperature LiCl-Li2O molten salt

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li₂O molten salt under an oxidizing environment. Al₂O₃ and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al₂O₃ and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li₂O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al₂O₃ coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li₂O molten salt to form a porous reaction layer of LiAl, Li₅AlO₄ and LiAl₅O₈ for the Al₂O₃ coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.     

Fabrication and Characteristics of YSZ�CYSZ/Al2O3 Double-Layer TBC

A novel YSZ?CYSZ/Al₂O₃ (YSZ means 6 wt% yttria partially stabilized zirconia) double-layer thermal barrier coating was fabricated using composite sol?Cgel and pressure filtration microwave sintering (PFMS) technologies. In this double-layer coating, the top layer was YSZ ceramics with a thickness of about 150 ??m and the bottom layer was composed of micro-sized YSZ particles packed by nano-sized ??-Al₂O₃ films and had a thickness of about 10 ??m. Cyclic oxidation tests indicated that this coating possessed superior properties to resist oxidation of alloy and spallation of coating. These beneficial effects could be mainly attributed to that, the alloy substrate could be sealed completely by ??-Al₂O₃ phase and the thermal stress could be decreased by means of better thermal matching and nano/micron structure in YSZ/Al₂O₃ layer. Moreover, thermal insulation capability tests indicated that the thermal barrier effect was improved due to the application of YSZ/Al₂O₃ layer. YSZ/Al₂O₃ layer could be considered as a promising bond coat in TBCs.     

Fabrication and thermophysical properties of porous TiB2 ceramics fabricated by reactive spark plasma sintering

Titanium oxide (TiO₂) and boron carbide (B₄C) were added to TiB₂ raw powders to prepare porous TiB₂ ceramics by reactive spark plasma sintering, and the gas escape (such as CO and B₂O₃) resulted in higher porosity. X-ray Diffraction results indicated that the reduction reaction was completed after the reactive spark plasma sintering process. The porosity could be controlled by changing the ratio of synthesized TiB₂ to raw TiB₂ powders. The porosity of porous TiB₂ ceramics with 20 wt.% and 40 wt.% synthsized TiB₂ ceramics are 18.5% and 22.2%, respectively. The thermal diffusivity of the porous TiB₂ ceramics decreased with the porosity due to the low diffusivity behavior of gas and vacuum in pores, and the thermal conductivity for porous TiB₂ ceramics decreased as the temperature increased throughout the measured temperature range. The results here pointed to a potential method for fabricating porous TiB₂ ceramics with controllable thermophysical properties.     

Preparation and properties of porous ceramics from spodumene flotation tailings by low-temperature sintering

Porous ceramics were prepared with spodumene flotation talings (SFT), kaolin and low-melting point glass (LPG) powder, whose pores were formed by the chemical reaction of hydrogen peroxide (H₂O₂). LPG was used to reduce the sintering temperature of porous ceramics and kaolin was used to realize the adsorption to methylene blue (MB) of porous ceramics. The average flexural strength, compressive strength, apparent porosity, water absorption and maximum MB adsorption capacity were 5.60 MPa, 4.66 MPa, 52.27%, 44.32% and 0.7 mg/g, respectively. Moreover, the results of orthogonal experiments present that the sintering temperature and the dosage of H₂O₂ had great influence on the mechanical properties and apparent porosity of porous ceramics, respectively. The main reason for the improvement of mechanical properties of porous ceramics was that LPG gradually became soft with increasing the sintering temperature, which made the mineral particles adhere to each other closely. Kaolinite was not completely converted into metakaolin at 550 °C, which might be the main reason why porous ceramics had adsorption properties.     

Microstructure and mechanical properties of porous Ti3SiC2

A ≈43 vol.% porous Ti₃SiC₂ ternary compound was fabricated by the reactive sintering of elemental powders. Under cyclic compression, the first loading cycle resulted in a measurable deformation, but subsequent cycles to the same load resulted in fully reversible, closed hysteresis loops that were smaller in area than the first loop. Cyclic nanoindentation loadings on solid bridges separating pores displayed behavior that was qualitatively similar to the uniaxial compression testing results. The results are interpreted in light of our kink band formation model. The significantly larger areas associated with the porous material than the fully dense sample with comparable grain size are ascribed to the formation of more incipient and regular kink bands as a consequence of the lack of constraint due to the presence of pores. The technological implications of having a porous material that dissipates more energy – on an absolute scale – than its fully dense counterpart are discussed.     

Manufacturing of Composite Coatings by Atmospheric Plasma Spraying Using Different Feed-Stock Materials as YSZ and MoSi<Subscript>2</Subscript>

Yttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi₂ as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi₂. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi₂ at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi₂. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi₂, a mixed coating was obtained without decomposition of the MoSi₂, which has been analyzed by means of XRD and SEM.