Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness

Continuous alumina fiber-reinforced yttria-stabilized zirconia (YSZ) composites with a LaPO₄ fiber coating were fabricated by slurry infiltration and spark plasma sintering (SPS). The LaPO₄ coating was deposited on the reinforcement alumina fabrics by a modified sol-gel method. The YSZ slurry with good dispersion and stability was prepared by optimizing the pH value, dispersant addition and ball milling time. The fabricated composite with a high density of ∼ 92 % has a good flexural strength of 277 ± 43 MPa, and a superior fracture toughness of 15.93 ± 0.75 MPa·m^1/2 exhibiting a non-brittle failure behavior. It was found that the LaPO₄ coating reduced the residual stress near the fiber/matrix interface to 131 ± 41 MPa, which was 369 ± 63 MPa in the composite without the fiber coating. The LaPO₄ coating renders a weak interphase to improve the composite toughness by activating several toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior.     

Fabrication of ceramic composite coatings using electrophoretic deposition,reaction bonding and low temperature sintering

We have developed a novel combination of electrophoretic deposition (EPD), reaction bonding and low temperature sintering techniques for the fabrication of yttria stablised zirconia (YSZ)/alumina composite coatings on Fecralloys. A mixture of ethanol and acetylacetone solvent was found to be an effective medium for YSZ and aluminium particle suspension. With the particle size of YSZ and aluminium being significantly reduced during ball milling. By using the EPD process, uniform green form coatings containing YSZ and aluminium particles were produced on Fecralloys. After oxidation of aluminium at 500°C and sintering at 1200°C, a dense and adherent YSZ/Al₂O₃ coating was produced. The presence of aluminium in the green form coatings not only contribute to the bonding between the coating and the metal substrate, but also compensate for the volume shrinkage of the coatings during sintering by the volume expansion arising from oxidation of aluminium to alumina.     

Evaluation of a Single Cell and Candidate Materials with High Water Content Hydrogen in a Generic Solid Oxide Fuel Cell Stack Test Fixture,Part II: Materials and Interface Characterization

A generic solid oxide fuel cell (SOFC) test fixture was developed to evaluate candidate materials under realistic operating conditions. A commercial 50 mm × 50 mm NiO‐YSZ anode‐supported thin YSZ electrolyte cell with lanthanum strontium manganite (LSM)/YSZ cathode was tested to evaluate the stability of candidate materials. The cell was tested in two stages at 800°C: stage I with low (~3% H₂O) humidity and stage II with high (~30% H₂O) humidity hydrogen fuel in constant voltage or constant current mode. Part I of the work, published previously, provided information regarding the generic test fixture design, materials, cell performance, and optical post‐mortem analysis. In part II, detailed microstructure and interfacial characterizations are reported regarding the SOFC candidate materials: (Mn,Co)‐spinel conductive coating, alumina coating for sealing area, ferritic stainless steel interconnect, refractory sealing glass, and their interactions with each other. Overall, the (Mn,Co)‐spinel coating was very effective in minimizing Cr migration. No Cr was identified in the cathode after 1720 h at 800°C. Aluminization of metallic interconnects also proved to be chemically compatible with alkaline‐earth silicate sealing glass. The details of interfacial reaction and microstructure development are discussed.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Different microstructure BaO–B2O3–SiO2 glass/ceramic composites depending on high-temperature wetting affinity

In this work, we propose a simple but effective method to fabricate BaO–B₂O₃–SiO₂ glass/ceramic composites with different microstructures that depend on the high-temperature wetting affinity. The experimental results showed that the wetting affinity between oxide ceramic and the BaO–B₂O₃–SiO₂ glass matrix could strongly affect the driving force of densification and crystallization and finally the microstructure of the glass/ceramic composites. It was found that suitable amounts of alumina powders could obviously increase the driving force for sintering of glass by increasing the capillary pressure. In this case, the contact angle between alumina and glass matrix is about 24° at high temperature and a densified and homogeneous microstructure of glass/alumina composite was obtained. On the contrary, rutile powders additive was found to result in higher phase separation and crystallization during the sintering of glass/rutile composites. In this case, the contact angle between rutile and glass matrix is about 124° at high temperature, and the sintered body has a lower dielectric loss than the sample with alumina additive. Therefore, the microstructure and dielectric property of glass/ceramic composites could be controlled by adjusting the ceramic composition and wetting affinity between ceramic additive and glass matrix in our study.     

Static and dynamic corrosion behavior of SnO2-doped AZS slip cast refractory in SLS glass

The corrosion behavior of a tin IV oxide-doped AZS-refractory, subject to static and dynamic corrosion testing at 1370˚C in soda-lime-silica glass, was studied considering the effect of the microstructural features on corrosion. The refractory was synthesized by slip cast methods through reaction sintering of alumina and zircon raw materials using SnO₂ as a sintering agent. SnO₂ had a considerable influence in the enhanced alumina/zircon reaction sintering and the subsequently evolved microstructures of an interlocked Zr_(1-x)Sn_(x)O₂ solid solution reinforced alumina-mullite composite. The process kinetics of the refractory corrosion followed reasonably well the predicted dependence on the square root of angular velocity under forced convection corrosion. Glass chemical corrosion and erosion of the refractory, under static and dynamic glass conditions, respectively, revealed the Zr_(1-x)Sn_(x)O₂ solid solution-rich mullite matrix as providing the most corrosion resistance and glass compatibility.     

Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia

Gadolinium zirconate (Gd₂Zr₂O₇, GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3 mol% yttria partially-stabilized zirconia (YSZ) incorporation. The fracture toughness of the composite of 90 mol% GZ-10 mol% YSZ (GZ–YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ–YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock lifetime of GZ–YSZ composite coating was improved, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ–YSZ composite coatings were discussed.     

Thermal stability of plasma-sprayed La2Ce2O7/YSZ composite coating

A composite coating composed of La₂Ce₂O₂ (LCO) and yttria-stabilized zirconia (YSZ) in a weight ratio of 1:1 was deposited by the plasma spraying using a blended YSZ and LCO powders, and the stability of the LCO/YSZ interface exposed to a high temperature was investigated. The LCO/YSZ deposits were exposed at 1300 °C for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ scanning electron microscopy assisted by X-ray energy-dispersive spectrum analyses and X-ray diffraction measurements. At an exposure temperature of 1300 °C, the grain morphology of LCO splats in contact with YSZ splats changed from columnar grains to quasi-axial grains with interface healing, and some grains tended to disappear during the thermal exposure. The results indicate that the phases in LCO–YSZ composite coating are not stable at 1300 °C. The element La in the LCO splat diffused towards the adjacent YSZ splat during the exposure, generating the reaction product layers composed of La₂Zr₂O₇ between the LCO and YSZ splats. After exposed for 200 h, the composite coating consisted of a mixture of mainly La₂Zr₂O₇ and CeO₂ and a minor amount of YSZ, accounting for the unusual decrease in the thermal conductivity at the late stage of exposure.     

Durability of YSZ coated Ti2AlC in 1300°C high velocity burner rig tests

A thermal barrier coating (TBC) system survived 500 hours in aggressive, 1300°C burner rig testing. The yttria-stabilized zirconia (7YSZ) TBC was plasma sprayed on an oxidation-resistant Ti₂AlC-type MAX phase and tested in a jet fuel burner at 100 m/s, using 5 hours cycles. No coating spallation or surface recession was observed; Al₂O₃-scale growth produced a slight 2.4 mg/cm² mass gain. The coating surface exhibited craze-cracked colonies of [111]_flourite textured columns, with no visible moisture attack. The 20 μm alumina scale remained intact under the YSZ face, about twice that producing failure for TBC/superalloy systems. TiO₂ nodules, initially formed on the uncoated backside, were removed, and Al₂O₃ was etched through volatile hydroxides formed in water vapor (~10%). Overall, the test indicated exceptional stability of the YSZ/Al₂O₃/Ti₂AlC system under turbine conditions due in large part to close thermal expansion matching.     

YSZ/MoS2 self-lubricating coating fabricated by thermal spraying and hydrothermal reaction

Thermal sprayed ceramic coatings have extensively been used in components to protect them against friction and wear. However, the poor lubricating ability severely limits their application. Herein, yttria-stabilized zirconia (YSZ)/MoS₂ composite coatings were successfully fabricated on steel substrate with the combination of thermal spraying technology and hydrothermal reaction. Results show that the synthetic MoS₂ powders are composed of numbers of ultra-thin sheets (about 7 ~ 8?nm), and the sheet has obvious lamellar structure. After vacuum impregnation and hydrothermal reaction, numbers of MoS₂ powders, look like flowers, generate inside the plasma sprayed YSZ coating. Moreover, the growing point of the MoS₂ flower is the intrinsic micro-pores of YSZ coating. The friction and wear tests under high vacuum environment indicate that the composite coating has an extremely long lifetime (>?100,000 cycles) and possesses a low friction coefficient less than 0.1, which is lower by about 0.15 times than that of YSZ coating. Meanwhile, the composite shows an extremely low wear rate (2.30?×?10^?7 mm³ N^?1 m^?1) and causes slight wear damage to the counterpart. The excellent lubricant and wear-resistant ability are attributed to the formation of MoS₂ transfer films and the ultra-smooth of the worn surfaces of hybrid coatings.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Transparent,thermal insulation and UV-shielding coating for energy efficient glass window

Size-controllable double-shell hollow TiO₂@SiO₂ spheres (DHTSs) were fabricated using a simple sol-gel reaction. The size of DHTSs was controlled by using PS spheres with different size as templates. Moreover, DHTSs was also incorporated into waterborne polyurethane (WPU) matrix to prepare WPU/DHTSs composite film as glass coating. The effect of the diameter of DHTSs on the transparency, thermal insulation and UV-shielding properties of WPU/DHTSs composite film were investigated in detail. And the results revealed that the composite film with 230 nm-sized DHTSs exhibited the optimum performance. In addition, the temperature rise rate in the chamber fitted with the glass coated by WPU/DHTSs composite film (26%) was lower than that fitted with the common glass without coated by anything (48%). Therefore, the transparent coating with excellent thermal insulation and good UV-shielding properties will become a potential energy saving building glass coating.     

Interfacial microstructure evolution of glass‐based coating on IC10 superalloy with a Ni3Al bond‐coat at 1050°C

A glass‐based composite coating incorporating YSZ, corundum, and Ni₃Al was prepared on IC10 superalloy with a sputtering Hf‐modified Ni₃Al bond‐coat. Sealing effect of the glass‐based top‐coat and reactive elemental effect of Hf facilitated the formation of columnar α‐Al₂O₃ at 1050°C. Furthermore, a discontinuous gahnite interlayer formed at the α‐Al₂O₃/top‐coat interface due to interfacial reactions. However, because of the discontinuous microstructure of gahnite, the α‐Al₂O₃ suffered durative attack by the active glass matrix, which caused severe internal oxidation of Hf and Al within the bond‐coat. In addition, the oxygen barrier ability of the top‐coat changed with its microstructure evolution.     

Ceramic matrix composites in the alumina/5-30 vol.% YAG system

The preparation technique of the particulate composite materials in the alumina/YAG system was elaborated. Within alumina particles suspension yttria precursor was precipitated with ammonium carbonate. Drying and calcination at 600 °C resulted in the mixture of alumina and yttria particles, the latter being much finer than alumina particles. This mixture was additionally homogenized by short attrition milling in an aqueous suspension. Sintering of such powders results in the materials composed of YAG inclusions of sizes smaller than shown by alumina grains and evenly distributed within the matrix. YAG particles result from the reaction of Y₂O₃ with Al₂O₃ during heat treatment. YAG inclusions limit effectively grain growth of the alumina matrix. Hardness, fracture toughness, strength, Young modulus and wear susceptibility of composites and pure alumina were measured. Composites show higher hardness and in some cases higher fracture toughness and wear resistance than pure alumina polycrystals.     

Effect of microstructure and mechanical properties on wear behavior of plasma-sprayed Cr2O3-YSZ-SiC coatings

In this study, the microstructure and mechanical properties of the atmospheric plasma-sprayed Cr₂O₃ (C), Cr₂O₃-20YSZ (CZ), and Cr₂O₃-20YSZ-10SiC (CZS) coatings were evaluated and also compared with each other, so as to explain the coatings wear behavior. Microstructural evaluations included X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and porosity measurements. Mechanical tests including bonding strength, fracture toughness, and micro-hardness tests were used to advance our understanding of the correlation between the coatings properties and their wear behavior. The sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of multimodal YSZ and subsequent SiC reinforcements to the Cr₂O₃ matrix resulted in an increase in the fracture toughness and Vickers micro-hardness, respectively. It was found that the composite coatings had comparable coefficients of friction with pure Cr₂O₃ coatings. When compared with the C coating, the CZ and CZS composite coatings with higher fracture toughness exhibited superior wear resistance. Observation of the wear tracks of the coatings indicated that the lower wear rates of the CZ and CZS coatings were due to the higher plastic deformation of the detached materials. In fact, improvement in the wear resistance of the composite coatings was attributed to a phase transformation toughening mechanism associated with tetragonal zirconia which created more ductile tribofilms during the wear test participated in filling the pores of coatings.     

Wear properties and microstructures of alumina matrix composite ceramics used for drawing dies

The alumina matrix ceramics used for drawing dies were prepared by hot-press sintering method. The ceramics materials were made of Al₂O₃/TiC, Al₂O₃/(W,Ti)C and Al₂O₃/Ti(C,N). Mechanical and friction properties of these materials were tested and measured. The experiments for testing friction properties were carried on wear and tear machine. Mechanisms of frictions were analyzed with scanning electron microscope. Results showed that the alumina matrix composite ceramics have good physical and mechanical properties for used as drawing dies. Measured friction coefficients of alumina matrix composite ceramics showed a trend of decline and kept the value of 0.4–0.5 with the rotating speed of 550 rpm. Alumina matrix composite ceramics have smaller wear rate, while the wear rates of Al₂O₃/TiC and Al₂O₃/(W,Ti)C decrease gradually with a rising rotation speed. The wear of alumina matrix ceramics was severe at deformation zone. The primary wear behaviors of alumina matrix ceramics are scraping and furrowing. Even though the mechanisms for wear different, abrasive and adhesive wear were found to be the predominant wear mechanisms for the ceramic drawing die.     

Assessment of the performance of Y2SiO5-YSZ/YSZ double-layered thermal barrier coatings

Y₂SiO₅ is a promising material for the thermal barrier coatings due to its low thermal conductivity, high temperature stability and exceptional resistance for molten silicate attack. However, it suffers low fracture toughness and low coefficient of thermal expansion compared with yttria-stabilized zirconia (YSZ). In this study, a composite coating approach, i.e., incorporating YSZ into Y₂SiO₅ coating, was employed to overcome those limitations. The double-layered Y₂SiO₅-YSZ/YSZ coatings were fabricated using atomospheric plasma spraying and tested under thermal cycling at 1150 °C. The phase compositions, microstructure, mechanical properties and the failure behavior were evaluated. It was found that the amorphous phase during spraying would crystallize at high temperature accompanied by volume shrinkage, leading to cracks and spallation in the coating. With YSZ addition, the composite coatings exhibited a much longer lifetime than the single phase Y₂SiO₅ coating due to a lower volume shrinkage and enhanced toughness.     

Comparison between barium and strontium-glass composites for sealing SOFCs

The present study demonstrates the feasibility of adding micron-scale Y₂O₃-stabilized ZrO₂ (YSZ) powders to modify the properties of two borate glasses used for sealing electrolyte supported SOFCs. The crystallization of the composite made with a Ba-containing glass was found to be independent of the volume fraction of YSZ, as opposed to the situation for Sr-glass composites where the crystallization temperature decreased with the volume fraction of YSZ. The variation of the flow properties of both glass composites was measured using a wettability test, and an increase of the contact angle was measured when the volume fraction of additives was increased. Examining the microstructure showed that initially the Ba-containing glass reacted with YSZ to form a BaZrO₃ compound. Long time exposure at 800 °C caused a large reduction of the coefficient of thermal expansion (CTE), which is explained by increased formation of BaZrO₃ and further change in glass composition. On the other hand, the reaction involving the Sr-containing glass with the YSZ additive shows the initial formation of calcium zirconate (Ca is an ingredient in both glasses) followed by appearance of strontium zirconate with further heating. For this Sr glass composition, the observed reduction of CTE was associated with the change in composition of the remaining glassy phase since the CTEs of the reaction products are close to the CTE of the YSZ additives.     

Aerosol-deposited Al2O3/PTFE hydrophobic coatings with adjustable transparency

With the wide range of requirements for architectural glass, such as transparency, opacity, and hydrophobicity, there is a need to address the issues in the complexity of convention methods. Thus, considering functionality and applicability in various architectural windows, hydrophobic alumina/polytetrafluoroethylene (Al₂O₃/PTFE) composite layers with transparency or opacity were transferred to commercial architectural glass using a facile aerosol deposition (AD) process. We successfully fabricated hydrophobic coating layers with high transmittance (only a 0.03% difference from sheet glass) by optimizing the PTFE content in Al₂O₃ using solution-based synthesized powders to enable a uniform surface topology. The opaque hydrophobic Al₂O₃/PTFE coating layers exhibit a transmittance of approximately 0% with excellent hydrophobicity of 130°. Remarkably, this opaque film was successfully employed onto a large deposition area, curved substrate, and micro-patterned regions. It is believed that our AD-prepared composite layers have great potential for architectural glass in terms of economic feasibility and versatility.