Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying,Part I: Microstructure

In this work, two types of yttria-stabilised zirconia (YSZ) powders, a microsized powder and a reconstituted nanostructured powder, were used as the original feedstock for depositing thermal barrier coatings (TBCs) using a high-efficiency supersonic atmospheric plasma spraying (SAPS) system. The effect of the original powder on the coating microstructure was studied by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicated that the microsized powder was fully melted in the plasma jet and that the as-sprayed conventional coating (named MC) was composed of regular-shaped tetragonal ZrO₂ with grain size of 200–500 nm. However, the cross-section morphology of the water-quenched powders revealed that the reconstituted nanostructured powder was partially melted during plasma spraying and that the as-sprayed nanostructured coating (named NC) exhibited a multi-modal microstructure that mainly consisted of unmelted nanoparticles (30–50 nm) and nanograins (60–110 nm), with the latter being the main microstructure of the coating. One visible polycrystalline region consisting of 10 nm grains was also found in NC. In addition, due to the full melting of the microsized powder in the plasma jet, MC exhibited a lower porosity and higher microhardness and Weibull modulus compared with those of NC. In the following paper (Part II), the thermo-mechanical properties, such as thermal shock resistance, oxidation resistance and thermal insulation performance, of the above two coating types will be further studied.     

Thermal conductivity of plasma sprayed Sm2Zr2O7 coatings

Rare-earth zirconates with a pyrochlore structure have been developed for potential application in thermal barrier coating systems to further improve the performance and durability of gas turbines. The Sm₂Zr₂O₇ (abbreviated as SZ) powder was synthesized by solid state reaction and then deposited by air plasma spraying. The phase stability, microstructure and thermal conductivity of SZ and 8 wt% Y₂O₃ stabilized zirconia (8YSZ) coatings were investigated. The X-ray diffraction results indicated that the crystal structure of the as-sprayed SZ coatings was defect-fluorite, and after heat treating at 1200 °C for 50 h, it started to transform to pyrochlore, and the content of pyrochlore increased with increase in temperature of the heat treatment. The thermal conductivities of SZ coatings were significantly lower than those of 8YSZ coatings before and after heat treatments, which increased considerably after heat treatments compared to the as-sprayed states for both coatings due to sintering effects.     

Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells

In this study, 3 mol% yttria stabilized zirconia (3YSZ) is investigated as a SOFC electrolyte alternative to 8 mol% yttria stabilized zirconia (8YSZ). The mechanical and electrochemical properties of both materials are compared. The mechanical tests indicate that the thickness of 3YSZ can be reduced to half without sacrificing the strength compared to 8YSZ. By reducing the thickness of 3YSZ from 150 µm to 75 µm, the peak power density is shown to increase by around 80%. The performance is further enhanced by around 22% by designing of novel electrode structure with regular cut-off patterns previously optimized. However, the cell with novel designed 3YSZ electrolyte exhibits 30% lower maximum power density than that of the cell with 150 µm-thick standard 8YSZ electrolyte. Nevertheless, the loss in the performance may be tolerated by decreasing the fabrication cost revealing that 3YSZ electrolyte with cut-off patterns can be employed as SOFC electrolyte alternative to 8YSZ.     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Thermal cycling behavior of nanostructured 8YSZ，SZ/8YSZ and 8CSZ/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The nanostructured single-ceramic-layer (SCL) 8YSZ thermal barrier coatings (TBCs), double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZ)/8YSZ and SZ doped with 8 wt% CeO₂ nanoscale particles (8CSZ)/8YSZ TBCs were fabricated by atmospheric plasma spraying (APS) on nickel-based superalloy substrates with NiCoCrAlY as the bond coating. The thermal cycling behavior of the three as-sprayed TBCs was investigated systematically at 1000 ℃ and 1200 ℃. The results reveal that the thermal cycling lifetime of the nanostructured DCL 8CSZ/8YSZ TBCs is the longest among them, which is largely due to the fact that the intermediate layer buffer effect of the DCL structure, more porosity and improvement of thermal expansion coefficient from doping CeO₂ nanoparticles can relieve thermal stress to a great extent at elevated temperature. The failure mechanism of the nanostructured TBCs has been discussed in detail.     

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.     

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.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Studies on sintering kinetics and correlation with the sinterability of 8Y zirconia ceramics based on the dilatometric shrinkage curves

Prediction of the dominant sintering mechanism and corresponding temperature regimes become important in arriving at the optimum sintering schedule of ceramics. The sintering kinetics of 8Y zirconia (8YSZ) submicron powders, with particle sizes ～205 nm, has been evaluated through, (i) constant rates of heating and (ii) master sintering curve (MSC) technique. Slip cast green 8YSZ compacts, close to 50% of the theoretical density, have been subjected to dilatometry, for recording the shrinkage behavior, at constant heating rates of 5, 10 and 20 °C/min. The kinetics of the corresponding sintering process has been evaluated by estimating and comparing the apparent activation energies (Q) from the constant heating rate technique, and MSC approach. The observed apparent activation energy (～350 kJ/mol) suggests grain-boundary diffusion to be the dominant mechanism in sintering of 8YSZ ceramics.     

Kinetic and thermodynamic effects of manganese as a densification aid in yttria-stabilized zirconia

This work demonstrates the role of Mn as an effective sintering aid in Yttria-Stabilized-Zirconia (YSZ) is a result of the concomitant reduction of activation energies and change of interfacial energies caused by Mn segregation. Kissinger analyses of the heat of sintering showed a decrease in activation energy from 219.9 kJ/mol for YSZ to 103.4 kJ/mol for YSZ containing 3-mol% Mn. Direct microcalorimetry analyses showed that the average surface and grain boundary energies of YSZ decreased from 0.94 and 0.71 J/m², respectively, to 0.70 and 0.17 J/m² for 3-mol% Mn doped YSZ. The decrease in the ratio between surface and grain boundary energies indicates an increase in dihedral angle from 137.5° to 166.6°, meaning an increase in sintering stress. Segregation of manganese to grain boundaries was experimentally observed and is discussed to be responsible for both kinetic and thermodynamic changes in the system while suggesting interconnection by the thermodynamic extremal principle.     

Thermal stability and sintering behavior of plasma sprayed nanostructured 7YSZ, 15YSZ and 5.5SYSZ coatings at elevated temperatures

Nanostructured scandia, yttria doped zirconia (5.5SYSZ), 7 wt% yttria stabilized zirconia (7YSZ) and 15YSZ thermal barrier coatings (TBCs) were produced by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal stability and sintering behavior of the three as-sprayed TBCs at 1480 °C were investigated. The results indicated that the thermal stability of SYSZ and TBCs was longer than the 7YSZ TBCs due to higher amount of tetragonal phase. Furthermore, the results demonstrated that the nanostructured 7YSZ coating exhibits higher sintering resistance than 5.5SYSZ TBC.     

Reducing sintering temperature of yttria stabilized zirconia through addition of lithium nitrate and alumina

Reducing sintering temperature of yttria stabilized zirconia (YSZ) has been achieved through doping with alumina and lithium nitrate at levels below 1 mol%. Sintering experiments of pure and doped samples have been conducted with the same profile using an optical dilatometer. All samples exhibited anisotropic sintering over a wide range of temperatures but final shrinkage values were comparable in axial and radial directions. Sintering temperature has been reduced by as much as 110 °C. We believe that the reduction in sintering temperatures is due to viscous flow in the first stage sintering. Bimetallic doping (mixture of alumina and lithium nitrate doping) is more effective in reducing sintering temperature than single doping possibly due to better distribution of doping material throughout the matrix material. Separate sintering experiments for 5 h have been conducted at 1250 °C and 1170 °C on doped 8 mol% and 3 mol% YSZ, respectively, and have shown that near full density (～96%) is reachable.     

Enhanced electrochemical activity and long-term stability of Ni–YSZ anode derived from NiO–YSZ interdispersed composite particles

Nickel oxide–yttira stabilized zirconia (NiO–YSZ) interdispersed composite (IC) particles were prepared by a mechanochemical processing using NiO and YSZ nanoparticles. Transmission electron microscopy (TEM) revealed that primally particles of YSZ (75 nm) and NiO (160 nm) were presented alternatively in the composite particles. Specific surface area (SSA) decreased from 8.6 to 7.1 m²/g during the mechanochemical processing. The SSA reduction suggested that the chemically bound NiO/YSZ hetero-interfaces were formed during the processing. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) visualized that the anode made from the IC particles consisted of three-dimensional textured structure of fine Ni and YSZ networks (grain size of them was approximately 500 nm) with 34 vol% of porosity. The anode demonstrated not only low polarization of 152 mV at 1 A/cm² even under the operation at 700 °C but also long-term stability for 920 h.     

Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition

A 500 nm thick thin film YSZ (yttria-stabilized zirconia) electrolyte was successfully fabricated on a conventionally processed anode substrate by spin coating of chemical solution containing slow-sintering YSZ nanoparticles with the particle size of 20 nm and subsequent sintering at 1100 °C. Incorporation of YSZ nanoparticles was effective for suppressing the differential densification of ultrafine precursor powder by mitigating the prevailing bi-axial constraining stress of the rigid substrate with numerous local multi-axial stress fields around them. In particular, adding 5 vol% YSZ nanoparticles resulted in a dense and uniform thin film electrolyte with narrow grain size distribution, and fine residual pores in isolated state. The thin film YSZ electrolyte placed on a rigid anode substrate with the GDC (gadolinia-doped ceria) and LSC (La_0.6Sr_0.4CoO_3?δ) layers deposited by PLD (pulsed laser deposition) processes revealed that it had fairly good gas tightness relevant to a SOFC (solid oxide fuel cell) electrolyte and maintained its structural integrity during fabrication and operation processes. In fact, the open circuit voltage was 1.07 V and maximum power density was 425 mW/cm² at 600 °C, which demonstrates that the chemical solution route can be a viable means for reducing electrolyte thickness for low- to intermediate-temperature SOFCs.     

Effect of calcination temperature on morphology of mesoporous YSZ

A nano-structured mesoporous yttria-stabilized zirconia (YSZ) powders were prepared for the first time using cetyltrimethylammonium bromide (CTAB) as the surfactant and urea as the hydrolyzing agent and using ZrO(NO₃)·6H₂O and Y(NO₃)₃·6H₂O as inorganic precursors. The Brunauer–Emmett–Teller (BET) surface area, Barrett–Joyner–Halender (BJH) pore size distribution and crystallite/particle size of mesoporous YSZ varied with calcine temperatures were studied. Characterizations revealed that the mesoporous YSZ powder calcined at 600 °C was weakly agglomerated and had a high surface area of 137 m²/g with an average grain size of ∼5.8 nm. It was demonstrated that the mesoporous structure remained up to 900 °C. The low-densified YSZ sample with porosity as high as 33% was prepared from mesoporous YSZ powder sintered at 1500 °C for 6 h.     

Rheology,structure, and sintering of zirconia suspensions with pyrogallol-poly(ethylene glycol) as polymeric surfactant

Yttria-stabilized zirconia (YSZ) nanoparticles have been dispersed in water with pyrogallol-poly(ethylene glycol) polymer (i.e., Gallol-PEG) as surfactant. Fluid YSZ suspensions with an apparent viscosity  < 10² Pa s at a shear rate of 10² s^?1 have been prepared with a solids concentration of 45 vol.% for the 90-nm YSZ nanoparticles. Theoretical calculation of interparticle potentials indicates that the adsorbed polymer renders steric hindrance critical to the suspension stabilization. The concentrated YSZ suspensions exhibit shear-thinning flow over a broad shear-rate range, resulted from the weak attractive minimum (～2.7 k_BT) found between the neighbouring particles in close proximity. The Gallol-PEG concentration influences not only the suspension rheology but also the particle-packing structure and sintered density of the slip-casted YSZ compacts. The YSZ suspensions with an optimal Gallol-PEG concentration of 2 wt.% exhibit a low pore volume in the green state, facilitating hence densification (> 99% theoretical) upon subjected to sintering.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.