Nanostructured YSZ Research Articles

Defect chemistry to trigger zirconia densification at low temperatures by Spark Plasma Sintering

In-depth control of reactivity and defects chemistry is shown to be mandatory to reach high density YSZ ceramics at sintering temperatures lower than 900°C. The combination of Cold Sintering Process (CSP) and Spark Plasma Sintering (SPS) has highlighted the role of transient chemistry on the densification of this material. More, optimised conditions were found for achieving densification of YSZ at 850°C in a one-step SPS process. In particular, the vacuum level in the SPS chamber is clearly shown to be a decisive parameter to obtain dense nanostructured YSZ tetragonal ceramics up to 95% below 900°C. The occurrence of oxygen vacancies has thus been addressed associating thermal analyses, X-ray photoelectron spectroscopy, Electron Paramagnetic Resonance, photoluminescence spectroscopy and impedance spectroscopy. Understanding defect chemistry mechanisms and their dependence on the process or combination of processes, is a lever towards high-quality YSZ nanostructured ceramics below 900°C.

Sc3+:Ce4+:Y3+ doped zirconia nanopowders (ScCeYSZ): Synthesis, thermal phase stability and hot corrosion behavior of spark plasma sintered body

This study aims to investigate the high-temperature phase stability and hot corrosion behavior of zirconia with triple doping of Sc3+, Ce4+ and Y3+ ions. FESEM result shows that the mean particle size of ScCeYSZ nanoparticles was 80–90 nm. Next, optimal samples regarding the highest high-temperature phase stability (1.9%Sc8.3%Ce1.8%YSZ, 1.1%Sc9.0%Ce1.8%YSZ and 0.5%Sc9.6%Ce1.8%YSZ) were consolidated at 1550 °C for 15 min via the spark plasma sintering (SPS) method. A hot corrosion test was done in the presence of 45%Na2SO4+%55V2O5 salts on three samples at 900 °C for 2 h. Also, the results of hot corrosion were compared with ceramic resulting from the nanostructured YSZ bulk sample (nanoYSZ).Based on the X-ray diffraction results, the formation of the non-transformable tetragonal phase is confirmed in the synthesized nanopowders. Also, the phase and microstructural results after hot corrosion of sintered samples show the formation of a monoclinic phase and destructive YVO4 crystals and quasi-cubic CeO2 crystals on the surface of sintered samples. The results indicated that the sample containing 1.8% scandia, 8.3% ceria, and 1.9% yttria had the highest phase stability and hot corrosion resistance (low monoclinic percentage and low leaching of stabilizer elements of Sc3+:Ce4+:Y3+ from zirconia lattice and low depth of molten salt).

Design and realization of interface strengthening to GNPs/YSZ nanocomposite coating

Interfacial strengthening of ceramic coatings on metal surfaces is a goal that has been pursued by researchers in this field, but remains unsolved. In this work, the interface strengthening between ceramic coating and metal substrate was designed and successfully realized. GNPs were modified with silica-sol at the molecular scale to disperse in YSZ uniformly and the GNPs/YSZ nanocomposite coatings with high bonding strength were produced due to in-situ generation of interfacial carbides. Results show that the bonding strength of 9 wt% GNPs/YSZ nanocomposite coating increased by up to ∼54% compared to nanostructured YSZ coating without NiCrAlY bond-coat, and even 17% higher than that of nanostructured conventional type YSZ coating with NiCrAlY bond-coat. This technique could be applied to reinforce the interface between various materials, showing great application potential.

Nanomechanical properties of plasma-sprayed nanostructured and conventional 8YSZ thermal barrier coatings

Nanostructured thermal barrier coatings (TBCs) have become a noteble research field for the development of high-performance thermal protection coatings. The mechanical properties are essential for TBCs, which determine the functionality, reliablity and durability of coatings. In this article, the nanostructured 8YSZ (n-8YSZ) and conventional 8YSZ (c-8YSZ) coatings were fabricated by atmospheric plasma spraying (APS). The goal of the work is to do an evaluation and comparison of the nanomechanical properties of n-8YSZ and c-8YSZ coatings. The microstructure of coatings was observed by scanning electron microscopy and the nanomechanical properties was characterized using nano-indenter. Results indicate that the microstructure of the n-8YSZ coating exhibits bi-modal distribution while the microstructure of the c-8YSZ coating shows mono-modal distribution. The elastic indentation work ( W e ) is 11.952 ± 1.243 nJ and the plastic indentation work ( W p ) is 20.266 ± 5.594 nJ for the n-8YSZ coating. Meanwhile, the W e is 11.753 ± 0.634 nJ and the W p is 16.508 ± 3.000 nJ for the c-8YSZ coating. The microstructure of the coating has little effect on W e , but much on W p . The dissipation ratio of plastic deformation work ( η ) is 0.621 ± 0.046 for the n-8YSZ coating and 0.579 ± 0.042 for the c-8YSZ coating. Therefore, it can be predicted that the n-8YSZ coating is superior to the c-8YSZ coating in terms of erosion performance.

Depth-Sensing Indentation Creep Behavior of Nanostructured Thermal Barrier Coatings from As-Synthesized t'-8YSZ Feedstocks.

Nano-indentation is a popular method to characterize the micromechanical properties of nanostructured 8YSZ coatings. However, little research has focused on the creep behavior of nano-indentation and only the elastic modulus and nanohardness have been analyzed. Herein, for the first time, the nano-indentation creep behavior of plasma-sprayed nanostructured 8YSZ coatings using as-prepared nanostructured non-transformable tetragonal (t’) feedstocks was investigated. The indentation creep behavior can be well characterized by the power-law equation and the strain rate sensitivity has been calculated in light of the equation. The strain rate sensitivity was sensitive to the load and the reasons were analyzed in detail. The current results can further guide and design thermal barrier coatings from the point of indentation creep property.

Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings

The nanostructured 8YSZ thermal barrier coatings were deposited by atmospheric plasma spraying onto K417 G nickel-based superalloy with high velocity oxygen fuel sprayed NiCoCrAlYCe bond-coat using as-prepared nanostructured t´-Zr0.9Y0.1O1.95 feedstocks for the first time. The microstructure and mechanical properties of nanostructured and conventional 8YSZ coatings were comparatively investigated systematically. The results revealed that both coatings were composed of t´-Zr0.9Y0.1O1.95 phase and the formation mechanism of t´ phase was elucidated. The nanostructured 8YSZ coatings demonstrated typical bi-modal microstructure, whereas the conventional 8YSZ coatings exhibited mono-modal microstructure. Furthermore, the bi-modal microstructure of nanostructured 8YSZ coatings was analysed by elastic modulus and nanohardness Weibull distribution plots. The high and low slopes in Weibull distribution plots corresponded to unmelted and melted regions of nanostructured 8YSZ coatings, respectively. The fracture toughness and bonding strength of nanostructured coatings were higher than that of conventional 8YSZ coatings. Finally the reasons were explained in detail.

Build-up of the splat with columnar nanocrystalline for thermal barrier coatings derived from nanostructured t'-8YSZ feedstocks

In the present work, nanostructured 8YSZ coatings were produced by atmospheric plasma spraying using synthesized nanostructured t'-8YSZ feedstocks. The splats characteristics were systematically analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicate that there are columnar nano-grains in splats and the preferential growth orientation of columnar nano-grains is [101] direction. Finally, the growth mechanism of columnar nano-grains in splats for plasma-sprayed nanostructured YSZ coatings was proposed in detail.

Thermo-physical and thermal insulation properties of multi-scale nanostructured thermal barrier coatings using as-prepared t′-8YSZ feedstocks

The thermal conductivity and thermal expansion coefficient are important thermo-physical properties for thermal barrier coatings (TBCs). In this work, multi-scale nanostructured 8YSZ coatings were fabricated using as-prepared nanostructured t′-8YSZ feedstocks by atmospheric plasma spraying (APS) onto NiCoCrAlYCe coated K417G superalloys. Meanwhile, conventional t-8YSZ feedstocks were also selected for comparison. The thermo-physical properties of both coatings have been studied. Results indicate that nanostructured 8YSZ coatings have a lower thermal conductivity than conventional counterparts especially when the temperature is above 1000 °C. Nanostructured 8YSZ coatings have better thermal insulation capability compared with conventional 8YSZ coatings. Therefore, the nanostructured coatings using t′-8YSZ feedstocks have a broad prospect for service at elevated temperature.

Bonding strength and thermal conductivity of novel nanostructured La2(Zr0.75Ce0.25)2O7/8YSZ coatings

The thermal protection coatings possessing functionality, reliability, and durability require appropriate bonding strength and low thermal conductivity. In this study, the novel nanostructured La2(Zr0.75Ce0.25)2O7/8YSZ (LCZ/8YSZ) coatings were deposited onto NiCoCrAlYCe coated nickel-based superalloy by atmospheric plasma spraying (APS) using the as-prepared nanostructured feedstocks. The microstructure, bonding strength, and thermal conductivity of nanostructured LCZ/8YSZ coatings were investigated. The results reveal that the microstructure of as-sprayed nanostructured LCZ/8YSZ coatings presents bi-modal distribution with some pores and microcracks. Furthermore, the microstructure of coatings is dense and the interface bonding of 8YSZ/bond coat and LCZ/8YSZ coatings is perfect. The bonding strength of nanostructured LCZ/8YSZ coatings is 25.1 MPa, which is lower than that of nanostructured 8YSZ coatings with the same thickness. The tensile failure mechanism is also analysed. The thermal conductivity of nanostructured LCZ/8YSZ coatings from 0.86 W·m−1·K−1 to 0.33 W·m−1·K−1 is obviously lower than that of nanostructured 8YSZ coatings from 1.24 W·m−1·K−1 to 0.40 W·m−1·K−1 in the temperature range from 30 °C to 1200 °C.

Synthesis and Application of Nano structured Bi layer YSZ LZ Thermal Barrier Coating

Present work is on synthesis of high purity Nano-structured TBC materials, Lanthanum Zirconate and YSZ. They were prepared via wet chemical routes, starting from the indigenous source minerals such as zircon and monazite available in the beach sand. This is first time that the results of TBC materials synthesis from these base minerals, their purification and a high end application being presented comprehensively. Their characterisation and thermal barrier application on aeroengine components have been presented. The total oxide impurities being critical to the life of the coating, could be controlled within 0.03 per cent by weight. On comparison with other powders it was found that the indigenously synthesised YSZ powder had practically 100 per cent tetragonal prime phase and no monoclinic phase; whereas others had significant amounts of monoclinic phases present in them. Both YSZ and LZ powders were sinter agglomerated at 850 °C to preclude the possibility of any contamination and sieved. APS process was used to realise nano-structured bi-layer coating on the exhaust nozzle parts of an aeroengine. The components were subjected to rapid thermal transients during long accelerated endurance testing, equivalent to 1000 h of engine operations. The coatings also withstood the gas erosion of supersonic combustion products, vibratory loads of 4 g and more than 30000 nozzle actuations similar to aircraft maneuver. The paper also presents a brief review of implications of a nano-structured thermal barrier coating and certain nuances of chemical synthesis which forms the backbone of the strategies for durable coatings.

Effect of heat treatment and synergistic rare-earth modified NiCrAlY on bonding strength of nanostructured 8YSZ coatings

In this paper, the nanostructured 8YSZ coatings and two kinds of bond coatings including NiCrAlY and NiCrAlYCe coatings were deposited by atmospheric plasma spraying onto nickel-based superalloys. The microstructure of as-sprayed coatings and the effect of heat treatment on bonding strength of NiCrAlY/8YSZ and NiCrAlYCe/8YSZ coatings were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), together with energy dispersive spectrum (EDS) and X-ray diffraction (XRD) analysis. The results indicate that the coatings exhibit typical bi-modal microstructure and the grain size of 8YSZ coatings keeps nanostructure. As for 8YSZ coatings, the phase transformation occurs from t phase to t′ phase during plasma spraying and the heat treatment does not change the phase structure. The heat treatment and rare-earth modified NiCrAlY coatings play an important role in enhancing the bonding strength of as-sprayed coatings significantly compared with NiCrAlY/8YSZ coatings. The bonding strength of NiCrAlYCe/8YSZ coatings whose temperature of the heat treatment is 800 °C @ 6 h can reach 48.9 MPa, which is 1.5 times than that of as-sprayed NiCrAlYCe/8YSZ coatings.

Effects of Al2O3 diffusion barrier layer (including Y-containing small oxide precipitates) and nanostructured YSZ top coat on the oxidation behavior of HVOF NiCoCrAlTaY/APS YSZ coatings at 1100 °C

In this research, the effects of thin Al2O3 diffusion barrier layer (including Y-containing small oxide precipitates) and nanostructured YSZ top coat on the high temperature oxidation behavior of HVOF NiCoCrAlTaY/APS YSZ coatings were examined by using isothermal oxidation tests at 1100 °C. Nanostructured YSZ top coat could slightly reduce the double layered TGO growth rate and weight gain of thermal barrier coating systems. However, Al2O3 diffusion barrier layer (consisting of Y-containing small oxide precipitates) could substantially decline the oxidation rate and Ta movement (Ta-enriched oxides formation) in the NiCoCrAlTaY/Al2O3/nanostructured YSZ coating at 1100 °C.

Effect of nano-structuration and compounding of YSZ APS TBCs with different thickness on coating performance in thermal shock conditions

Effect of nano-structuration and compounding of YSZ APS TBCs investigated on coating behavior in thermal shock conditions. The coatings were applied on Inconel 738 discs with three different thickness per powder. In order to harmonize the results from the samples, performance factor is defined as a criterion that in the starting of the activity has an amount of about 100 and is reduced after the damage begins. The results revealed that the growth of damage in the YSZ class is almost linear, and this behavior is observed in all samples. The thick TGO in this class shows its high oxygen permeability, and the type of damage indicates that its location is near the TGO region. The nano-structured YSZ class has a very good performance and through an interesting phenomenon, the slope of the damage growth diagrams is decreasing with time. The obvious thing about the CSZ class microstructure is the presence of horizontal and vertical cracks and its dense structure. In this class, the main location of damage is through the coating and after the beginning of damage, its curve has grown with a high rate. The best performance among all samples belongs to the nano-structured YSZ, which due to the presence of nano-zones, has a higher toughness and ability to endure more cycles.

Synthesis and characterization of nanostructured t′-YSZ spherical feedstocks for atmospheric plasma spraying

The properties of feedstocks have a significant effect on the quality of as-sprayed coatings. The 8YSZ feedstocks have been widely used in atmospheric plasma spraying (APS) for the preparation of thermal barrier coatings (TBCs). In this paper, the nanostructured APS 8YSZ spherical feedstocks were successfully synthesized by the nanopowder granulation method. The microstructures and physical properties of as-prepared 8YSZ feedstocks were characterized. By contrast, the phase composition of conventional 8YSZ feedstocks was also analysed. The results show that the nanostructured spherical 8YSZ feedstocks with the median particle size (d50) of 33.22 μm whose grain size is about 20 nm consist of the non-transformable tetragonal (t′) phase and no other phases. Furthermore, the density of the as-prepared feedstocks is high enough and the nanostructured 8YSZ feedstocks are free-flowing. The nanostructured t′-YSZ feedstocks have excellent combination properties, which makes them have a broader application prospect in APS TBCs compared with the conventional 8YSZ feedstocks in future.

Oxidation behaviour of nanostructured YSZ plasma sprayed coated Inconel alloy

ABSTRACTThis paper investigates the effect of nanostructured and micrometre size YSZ powder coated plasma spray coating on high temperature oxidation resistance of Inconel (IN 718) substrate. The oxidation rate and reaction kinetics are studied, and the post-corroded scales are characterised in SEM, field emission SEM and X-ray diffraction. The results clearly indicate that there is an improvement of oxidation resistance in the case of nanostructured YSZ coating as compared to the micrometre size YSZ coated specimen. The detail mechanism of the significant improvement of the oxidation resistance of nanostructured coating is discussed in this paper.

Effect of spraying power on microstructure and property of nanostructured YSZ thermal barrier coatings

Nanostructured yttria stabilized zirconia (n-YSZ) thermal barrier coatings were fabricated by atmospheric plasma spraying (APS) at different spraying powers. The microstructures, phase stability and mechanical properties of the n-YSZ coatings were examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers indentation, respectively. The adhesion strength of coatings was evaluated according to ASTM C633-01 standard. Thermal cyclic oxidation method was carried out to study the effect of spraying power on thermal shock resistance of the coatings. Results showed that the n-YSZ coatings had a bimodal microstructure consisting of well melted splats and partially melted nanostructured areas. Both the porosity and the content of nanostructure in n-YSZ coatings decreased with increasing of the spraying power. After heat treatment at 1573 K, the porosity of the coating decreased and the hardness increased noticeably compared with that of the as-sprayed coating, indicating the sintering effect of the nanostructured coating. However, porosity of the coating deposited by the lowest spraying power (22 kW) still retain above 10% after annealing at 1573 K for 24 h, showing higher sintering resistant ability. Although phase structures certainly changed during heat treatment, phase compositions were more stable for the coatings deposited by the relatively low spraying powers. The average bond strength and thermal cycling life of the coating prepared by the lowest spraying power (22 kW) were about 29 MPa and 292 cycles, respectively, which were approximately twice as much as those of the coating deposited by the highest spraying power (42 kW). The best thermal shock resistance obtained for the n-YSZ coatings prepared at 22 kW can be partially attributed to its high content of nanozones, which play an important role in reducing the oxygen diffusion, releasing the thermal stress and interrupting crack propagation.

Effect of layer thickness on thermal shock behavior in double-layer micro- and nano-structured ceramic top coat APS TBCs

Abstract The performance of thermal barrier coatings with the double-layer top coat in thermal shock condition has been investigated. Used powders to produce coatings are as follows: Yttria Stabilized Zirconia (YSZ, Y), Ceria-Yttria Stabilized Zirconium Oxide (CSZ, C) and nano-structured YSZ (YSZ-N, YN). The samples were classified into four double-layer families, including Y-C, YN-C, Y-YN and YN-Y. At the end of each cycle, samples were photographed, and the surface and edge damage were determined. Furthermore, scanning electron microscope (SEM) Images and energy-dispersive spectrometer (EDS) analysis of sample's cross-section were taken before and after the test. After collecting experiment's data, the effects of various factors on the outputs were checked. The results showed that Y-YN, YN-Y and YN-C families, have the best performance, respectively. Moreover, it was found that using YSZ-N as the top layer, reduces the thickness of TGO, and it has a great effect on performance and the amount of damage.

Comparison of the effect of sintering on the microstructure, micro hardness and phase composition of conventional and nanostructured YSZ TBCs

Abstract This paper reports on a study that investigated the effect of sintering on the microstructure, thermal properties and phase composition of nanostructured and conventional 8YSZ thermal barrier coatings. For this purpose, conventional and nanostructured YSZ coatings were heated at 1100 °C in an atmospheric box furnace for 10, 20, 50, 100, and 150 h. Optical microscope and FE-SEM were used for microstructural studies and XRD was used for phase analysis. The percent of pores and micro hardness were used as measures of sintering. According to the findings, the percent of pores and cracks decreased because of the inter-diffusion of splats. Changes in properties, such as thermal conductivity and micro hardness, during the sintering process were less significant for nanostructured coatings than they were for the conventional ones, but phase changes were more noticeable. XRD investigation showed that picks shift for both samples were the same. The findings showed that using nanostructured coatings could be a way of reducing sintering effects.

Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO2 + 10 mol % Y2O3) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.

Nanostructured SOFC Electrode Scaffolds Prepared via High Temperature in situ Carbon Templating of Hybrid Materials

High surface area solid oxide fuel cell (SOFC) electrode scaffold materials were prepared at traditional sintering temperatures by sintering hybrid inorganic-organic gels in argon. The prepared materials were yttria-stabilized zirconia (YSZ, 8 mol% Y2O3), gadolinia doped ceria (GDC, Ce0.8Gd0.2O3-δ) and strontium titanate (STO, SrTiO3). Gels were prepared via the citric acid (Pechini) method and the propylene oxide method. The gels were sintered between 1050oC and 1350oC in argon, creating an amorphous carbon template in situ, which preserved a ceramic scaffold nanostructure. The carbon template was removed upon heating in air to 700oC. Specific surface areas up to 83, 95, and 99 m2/g were achieved for YSZ, GDC, and STO, respectively. The carbon template concentration and resulting surface areas are tunable by modifying the gel formulations. Symmetric cathode cells were prepared with traditional and nanostructured YSZ. As expected, the gel-derived, nanostructured YSZ improved electrode performance.