Processing and characterisation of fine crystalline ceria gadolinia–yttria stabilized zirconia powders

For low temperature SOFCs the yttria stabilized zirconia (YSZ)-coated ceria is a promising candidate for replacing YSZ-electrolyte. An important requirement for the co-firing feasibility of such a configuration is the densification of ceria at low temperatures (<1400°C). Fine crystalline gadolinia doped ceria (CGO)-powder readily sinterable at 1250°C was synthesized by co-precipitation with oxalic acid of 0·05 M and crystallization in methanol at 200°C for 6 h. The fabrication and characterisation of solid solution phases with a graded composition (CGO)_x(YSZ)_1−x, to be used as an interlayer between YSZ and CGO, in order to avoid delamination, were also studied and discussed. CGO_xYSZ_1−x powders, prepared by the glycine combustion method, required higher sintering temperatures (1500°C) to densify, while they showed significantly lower ionic conductivity than YSZ and CGO, attributed to the large lattice deformation and scattering of oxygen ions.     

Adhesion and residual stress of plasma sprayed Alumina–titania coatings

Plasma-sprayed coatings are formed by the impacting of particles onto a fixed substrate layer-by-layer. Residual stresses inside the coatings are essential for their influencing on the coatings’ performance and durability during service life. In the present work, heat transfer and elastic–plastic residual stresses generation during plasma spraying in Al₂O₃–13wt.%TiO₂/NiCrAl (AT13) coating system were analyzed by finite element analysis (FEA). The sophisticated spraying process was simulated and the laminated structure of the coating was modeled under three-dimension. In this simulation, radial and axial compressive stresses were concentrated at the interfaces and inside the bond layer. Besides, at the specimen corner of the free edge, there were high tensile radial and axial stress concentrations. Such remarkable stresses, no matter tensile or compressive, may lead to the delamination and failure of coatings. Comparing with the numerical results, X-ray diffraction measurement was conducted on the AT13 coatings. As a result, the tested values matched well with the FEA simulated results.     

Microstructures and tribological properties of vacuum plasma sprayed B4C–Ni composite coatings

A promising wear resistant coating has been fabricated via vacuum plasma spray (VPS) technique by using electroless plating composite powders comprised of B₄C and different amounts of Ni (10 and 20 vol.%). Tribological evaluation from the ball-on-disk test showed that the wear resistance of the composite coatings was superior to that of the pure B₄C coating, and the composite deposit containing 10 vol.% Ni demonstrated the optimum tribological properties. This mainly attributed to the more uniform microstructures of the composite coatings, and the higher thermal conductivity of the composite coating also contributed to its distinguished wear behaviors. For the coatings investigated, the dominant wear mechanism was determined to be oxidation and the formation of a transfer layer on the worn surface.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

Effects of washing and calcination–milling on ionic release and surface properties of yttria stabilized zirconia

Crystallographic features, physical properties and ionic release from yttria stabilized zirconia (YSZ) in suspension were studied by means of XRD, TEM, light-scattering particle size, BET, ICP and zeta potential analysis. It was found that Zr, Y, Na, and to a lesser extent Ca, Hf and Pd leach from 8 mol% YSZ powder. The impurities present increase the zeta potential of suspensions made from as-received YSZ. A trace amount of tetragonal phase observed in 8 mol% YSZ persists following washing and calcination–milling. Dislocations and crystallographic defects together with fractured crystals which form during milling of the calcined powder should lead to the formation of more broken bonds; as a result the surface of the particles can support higher surface charge density. Washing and calcination–milling lead to a shift of the isoelectric point of 8 mol% YSZ from pH 8.4 to pH 6.3 and 6.8, respectively. Due to higher chemical stability and previously shown positive impacts on microstructure and performance of fuel cells, use of calcined YSZ can be more advantageous than as received powder.     

Structure and properties of electrodeposited Ni–Co–YZA composite coatings

The aim is to develop an economical composite coating with high thermal stability. Ni–Co alloys are found to possess better thermal, physical and mechanical properties compared to Ni. Also, oxide particles as distributed phase can impart better thermal stability. Hence, particulates of composite Yttria stabilised zirconia, a commonly used high temperature material and alumina (YZA) were reinforced in various Ni–Co alloy matrices through electrodeposition. The influence of YZA on the microhardness, tribology and corrosion behaviour of Ni–Co alloys with Co contents of 0 wt.%, 17 wt.%, 38 wt.% and 85 wt.% was evaluated. Optical and Scanning Electron Microscopy (SEM) confirmed the presence of YZA particles and Energy Dispersive X-ray Analysis (EDX) revealed the composition. Tribology testing showed that composite containing 38 wt.% Co displayed better wear resistance. It was found from the immersion corrosion studies that Ni–17Co–YZA coating displayed improved corrosion resistance. Thermal stability studies showed that Ni–85Co–YZA coating retained its microhardness at temperatures of 600 °C. Thus, these coatings can be tailored for various applications by varying the cobalt content.     

Physical characteristics and sintering behavior of ultrafine zirconia–ceria powders

Ultrafine zirconia–12 mol% ceria powders have been prepared by the coprecipitation technique. The azeotropic distillation with n-butanol has been carried out to ensure complete elimination of the residual water in the precipitate. This procedure has proved to be quite effective in preventing the formation of agglomerates, which are responsible for inhomogeneities in the sintered microstructure, and for non-densification at low temperatures. The crystallization of the solid solution occurs at 430 °C as determined by thermal analyses. The specific surface of the calcined powder is 127.9 m² g⁻¹ and the pore size distribution exhibits only a maximum at approximately 9 nm. Total shrinkage of the compacted powder reached 30% at 1200 °C. Sintered specimens show six bands characteristics of the tetragonal phase in the Raman spectrum. Specimens with apparent densities >95% of the theoretical density and average grain size of 230–400 nm were obtained after sintering at 1200 °C.     

Atmospheric plasma sprayed silica–hydroxyapatite coatings on magnesium alloy substrates

Silica-doped hydroxyapatite as a bioactive coating presents some advantages compared to the pure one, such as: increased in vivo bioactivity and early bone ingrowth. The aim of this study is to obtain a deposition of silica-doped hydroxyapatite on magnesium alloy plates by atmospheric plasma spraying. The coating material was prepared by a precipitation method with sodium silicate addition as a source of silica, and various methods were used to characterize it. Spraying conditions including powder feed rate and current values were varied. The coating properties were defined by determining the purity, phase composition, morphology and corrosion protection of the HAP–Si deposits on the magnesium plates.     

Failure of plasma sprayed nano-zirconia-based thermal barrier coatings exposed to molten CaO–MgO–Al2O3–SiO2 deposits

Recently, nanostructured thermal barrier coatings have received considerable attention because of some superior properties in comparison with their conventional counterpart. In this study, nanostructured 8 wt% yttria-stabilized zirconia (n-YSZ) coatings were deposited by atmospheric plasma spraying, and the degradation behavior caused by molten calcium-magnesium-aluminon-silicate (CMAS) attack was investigated. Results showed that the thermo-chemical reaction product between CMAS and YSZ (both powders and coatings) is different with the change of CMAS content. At low CMAS concentration, a cubic phase is generated by the diffusion of Ca into YSZ grains. As compared to the conventional YSZ, less C-ZrO₂ is detected for n-YSZ. When CMAS reaches a certain concentration (eg 15 mg/cm²), disruptive phase transformation from tetragonal to monoclinic will occur and the reaction is more readily for n-YSZ. Two different chemical reaction mechanisms governing the CMAS content effect were proposed. It should be noted that the nanozone in the coatings plays an important role in the CMAS degradation process, which enhances CMAS infiltration rate and accelerates the chemical reaction, leading to a poor CMAS resistance of the nanostructured coating than that of the conventional counterpart.     

A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria–zirconia catalysts

Heats of adsorption were measured calorimetrically for O₂ adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria–zirconia catalysts, all with 1 wt% Pd. Significantly more O₂ adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria–zirconia were also 200 kJ/mol, much lower than the heat of reaction for Ce₂O₃ oxidation to CeO₂, but in reasonable agreement with estimates from O₂ desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.     

Evaluation of plasma sprayed alumina–40 wt% titania and partially stabilized zirconia coatings on high density graphite for uranium melting application

Ceramic coatings have been proposed on high density graphite crucibles for the application of uranium consolidation and distillation of molten salt in pyrochemical reprocessing of metallic fuels. Towards this, uranium melting experiments were carried out on plasma sprayed partially stabilized zirconia (PSZ) and Al₂O₃–40 wt% TiO₂ (A40T) coated high density graphite samples at 1350 °C using induction heating system for evaluating the compatibility of these coatings with molten uranium. The coated high density graphite samples were characterized before and after uranium melting test by scanning electron microscopy attached with energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. Microstructural observations revealed that no significant reaction layer or product was formed between uranium and PSZ coating, while uranium significantly adhered to A40T coating. PSZ coating offers better stability and protection to high density graphite crucibles from the chemical attack by molten uranium.     

The effect of the surface nanostructure and composition on the antiwear properties of zirconia–titania coatings

This study describes the preparation, surface imaging and tribological properties of titania coatings modified by zirconia nanoparticles agglomerated in the form of island-like structures on the titania surface. Titania coatings and titania coatings with embedded zirconia nanoparticles were prepared by the sol–gel spin coating process on silicon wafers. After deposition the coatings were heat-treated at 500 °C or 1000 °C. The natural tendency of nanoparticles to form agglomerates was used to build separated island-like structures unevenly distributed over the titania surface having the size of 1.0–1.2 μm. Surface characterization of coatings before and after frictional tests was performed by atomic force microscopy (AFM) and optical microscopy. Zirconia nanoparticles were imaged with the use of transmission electron microscopy (TEM). The tribological properties were evaluated with the use of microtribometer operating in ambient air at technical dry friction conditions under normal load of 80 mN. It was found that nanocomposite coatings exhibit lower coefficient of friction (CoF) and considerably lower wear compared to titania coating without nanoparticles. The lowering of CoF is about 40% for coatings heated at 500 °C and 33% for the coatings heated at 1000 °C. For nanocomposites the wear stability was enhanced by a factor of 100 as compared to pure titania coatings. We claim that enhanced tribological properties are closely related to the reduction of the real contact area, lowering of the adhesive forces in frictional contacts and increasing of the composite hardness. The changes in materials composition in frictional contact has secondary effect.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

The effect of SiC addition on the crystallization kinetics of atmospheric plasma–sprayed basalt-based coatings

The crystallisation kinetics of the conversion of a glass coating layer made from a mixture of natural basalt volcanic rock and SiC into glass-ceramic have been investigated. The process depends on the crystallisation temperature, time and amount of the SiC added. Coating powders were prepared from pure basalt and from basalt containing 10–50 wt% SiC. The powders were coated by an atmospheric plasma spray technique on the pre-coated AISI 1040 steel substrate with Ni–Al. The coating layer was vitrified by sudden cooling. The amorphous structure of the coatings was verified by X-ray diffraction (XRD) analysis. To obtain glass-ceramic, coatings were subjected to crystallisation heat treatment in an argon atmosphere. Crystallisation heat treatment temperatures of 800 °C, 900 °C and 1000 °C were chosen by using DTA. After the heat treatment process, augite, ferrian-diopsite, diopside, albite, andesine, and moissonite phases formed in the coating layer and were verified by XRD analysis. The crystallisation activation energies were determined to be between 323.4 kJ/mol and 253.2 kJ/mol, depending on SiC addition. The crystallisation activation energies decreased with increasing amounts of SiC addition. The Avrami parameters of the crystallisation process varied between 1.60 and 3.33, which indicates that internal crystallisation dominated for all of the compositions.     

Raman and X-ray photoelectron spectroscopic characterizations of thermal stability of 3 mol% yttria stabilized zirconia ceramics

Structural variations and phase stability in 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) subjected to long-term aging at room temperature and thermal treatments in air were investigated by Raman and X-ray photoelectron spectroscopies, and compared with previous cases obtained by hydrothermal treatments. The results revealed a significant difference in the kinetics of zirconia polymorphic transformation in different environments due to the critical roles played by surface and lattice hydroxyl defects. Thermal treatments in air of long-term stored 3Y-TZP revealed a gradual recovery to tetragonal phase due to the ocurrence of dehydroxylation, while thermal treatments of pristine 3Y-TZP caused tetragonal-to-monoclinic polymorphic transformation at low temperature, but then an inverse transformation at higher tempertures due to a competition of hydroxylation and dehydroxylation. Treatments of 3Y-TZP in different environments were found to lead to distinct paths of off-stoichiometric chemistry, including hydroxylation, hydroxyl migration, surface reconstruction, and dehydroxylation during the treatments.     

Phase stability of plasma sprayed YAG–YSZ composite beads/coatings at high temperature

Beads and coatings of YAG–YSZ composite ceramic were prepared by plasma spray. YAG was applied as an additive in the hope of improving the phase stability and oxygen impermeability of YSZ. To achieve this aim, some basic research about crystallization behavior and chemical compatibility of the composite were carried out. Plasma sprayed YAG tended to form amorphous state. With the different content of YAG in the composite, crystallization of YAG and YSZ was delayed by each other to different extent due to the barrier effect of heavy atoms. The relative low melting point of YAG led to dense coatings without obvious splat structure and further distinctive vertical cracks during crystallization within the coatings. The cracks became less severe when YAG content was lower. With the addition of YAG, the development of monoclinic ZrO₂ was suppressed while Y-rich phases were promoted at higher temperature.     

Adhesion properties and fracture mechanism of plasma sprayed NiCrAl/Al2O3–13wt.%TiO2 coatings by post annealing

Plasma-sprayed NiCrAl/Al₂O₃–13wt.%TiO₂ coatings (AT13) deposited on mild steel substrate were annealed with varying temperatures in air. The adhesion of the coating was evaluated by tensile adhesive strength test. The microstructure and the fracture mechanism were studied using optical microscopy, X-ray diffraction, and scanning electron spectroscopy/energy dispersive spectroscopy. It was found that the tensile bond strength of the coatings increased with increasing of annealing temperature at first and then decreased with increasing of annealing temperature further. The as-sprayed coating fractured at the interfaces of substrate/bond layer and bond layer/ceramic coating with a brittle–ductile mixed fracture. The measured strength expressed the adhesive strength and internal adhesive strength of the coating. The failure of the coating annealed at 300, 400, and 500 °C took place at the interface of substrate/bond layer and had a mixed fracture surface of transgranular cleavage fracture and localized ductile fracture. The strength obtained is the adhesive strength between the coating and the steel substrate. The coating annealed at 400 °C had a maximum strength of 42.9 MPa. When the temperature is above 600 °C, the bonding strength would be damaged. Therefore, there is a proper annealing temperature which can significantly improve the bond strength of the coating.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.