Yttria-stabilized Zirconia Powders Research Articles

Fabrication, microstructure, and properties of 8 mol% yttria-stabilized zirconia (8YSZ) transparent ceramics

Fine grained 8 mol% yttria-stabilized zirconia (8YSZ) transparent ceramics with high optical and mechanical properties were fabricated by air pre-sintering and hot isostatic pressing (HIP) using commercial 8YSZ powders as the raw material. The pre-sintered ceramics with fine grains and appropriate relative density play a key role to achieve high transparency and suppressed grain size after HIP post-treatment at relatively low temperatures. With the increase of HIP temperature from 1350 to 1550 °C, the in-line transmittance of 8YSZ ceramics at 600 nm increases from 56.9% to 71.5% (2.5 mm in thickness), and the average grain size increases from 2.4 to 16.3 µm. The corresponding bending strength of 8YSZ transparent ceramics decreases from 328±20 to 289±19 MPa, the hardness (H) decreases from 12.9±0.1 to 12.5±0.2 GPa, and the fracture toughness (KIC) decreases from 1.30±0.02 to 1.26±0.03 MPa·m1/2. Systematical investigations were carried out to study the combination of high optical transparency and excellent mechanical properties in 8YSZ ceramics.

Core shell coatings for nuclear thermal propulsion cermets

Magnetron sputtering has been employed to provide conformal tungsten coatings from approximately 70 to 540 nm thicknesses over yttria stabilized zirconia (YSZ) powders. Through a rotating vessel, the powder was continually cascaded exposing its surface enabling the line-of-sight cathode tungsten target to deposit over the entire surface. With increasing coating thickness, a particulate surface morphology developed. These tungsten particulates were plate-like in morphology and faceted in some cases. The formation of these features is contributed to the continual low energy impacts between the hard YSZ powders during deposition as well as the adhesion and de-adhesion of the coatings connecting particle clusters. The results of this work demonstrate the ability of sputter deposition to create refractory coatings for cermet fuels utilized in nuclear thermal propulsion, as well as broader applications where core-shell powder fabrication is needed.

Novel controllable solid/hollow T'-YSZ nanostructured powders for additive manufacturing

Here, we report novel spherical non-transformable yttria-stabilized zirconia (T'-YSZ) nanostructured powders with solid/hollow and controllable microstructure fabricated via a nanoparticle regranulation method for additive manufacturing (AM). The as-obtained T'-YSZ feedstocks are superior to commercial YSZ powders for thermal barrier coatings when thermal spraying is used as the AM method. Furthermore, we confirm for the first time that the state-of-the-art T'-YSZ feedstocks can be successfully used for 3D printing. Therefore, the as-prepared powders are promising for AM, and they will promote the development of AM techniques.

Initial sintering mechanism and additive effect in zirconia ceramics

AbstractY2O3‐stabilized ZrO2 (YSZ) ceramics have been used for various engineering applications because of their excellent mechanical properties or oxide‐ion conductivity applicable to solid‐electrolyte. The performance of YSZ depends on the sintered texture that is directly determined by sinterability of raw powders. A new kinetic analysis method of diffusion mechanism based on an initial sintering theory (grain‐boundary or volume diffusion) is theoretically derived, the initial sintering mechanism of hydrolytic YSZ powder is experimentally determined, and the effects of powder characteristics on sinterability are discussed. Furthermore, the additive‐enhanced sintering is proposed. A small amount of Al2O3 significantly enhances the densification. Using the additive effect, the low‐temperature degradation that is the fault of zirconia ceramics can be improved by decreasing the sintering temperature.

Synthesis of γ‐Bi 2 O 3 /YSZ composite powders using a facile precipitation method

Low melting point and high ionic conductivity of γ-Bi2O3 make it a promising additive to decrease the sintering and operation temperatures of yttria stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. Herein, γ-Bi2O3/YSZ composite powders with good uniformity and precise control of morphology and phase were successfully synthesized via a low cost chemical precipitation method. Both the concentration of NaOH solution and the reactant adding sequence affect the morphology and synthesis of γ-Bi2O3/YSZ composite powders. When the concentration of NaOH was in the range of 1.25–1.875 M, tetrahedron γ-Bi2O3/YSZ powders were synthesized. While, cubic structural γ-Bi2O3/YSZ powders were obtained when adding Bi3+ and YSZ suspension into 1.5 M NaOH solution. The addition of YSZ facilitates the fabrication of γ-Bi2O3 and widens its process window to a higher NaOH concentration. Thus synthesized γ-Bi2O3/YSZ composite powders effectively decrease the sintering temperature of YSZ to 1050°C due to the uniform distribution of γ-Bi2O3 inside YSZ powders. This work provides a facile method to fabricate γ-Bi2O3/YSZ composite powder with controlled morphology and phase, which will promote the mass production of low cost YSZ-based electrolyte for SOFC applications.

Impact of Preparation Method and Y2O3 Content on the Properties of the YSZ Electrolyte

This study is an effort to cover and interconnect multiple aspects of the fabrication of the yttria-stabilized zirconia (YSZ) from powder preparation to a solid electrolyte suitable for utilization in solid oxide cells. Thus, a series of YSZ electrolytes was prepared, differing in the content of the Y2O3 dopant and in the method of preparation. Combustion synthesis along with the thermal decomposition of precursors was used for YSZ powder synthesis with a dopant content of 8 to 18 mol.%. Post-synthesis treatment of the powder was necessary for achieving satisfactory quality of the subsequent sintering step. The morphology analyses of the YSZ powders and sintered electrolytes produced proved that small particles with a uniform size distribution are essential for obtaining a dense electrolyte. Furthermore, the conductivity of YSZ electrolytes with different Y2O3 contents was examined in the temperature range of 400 to 800 °C. The lowest conductivity was found for the sample with the highest Y2O3 content. The obtained results enable the preparation methods, YSZ powder morphology, and composition to be connected to the mechanical and electrochemical properties of the YSZ electrolyte. Thus, this study links every step of YSZ electrolyte fabrication, which has not been sufficiently clearly described until now.

Effects of Aluminum Oxide Addition on Electrical and Mechanical Properties of 3 mol% Yttria-Stabilized Tetragonal Zirconia Electrolyte for IT-SOFCs.

Composite tetragonal zirconia (3Y-TZP) sinters with Al2O3 contents of 0, 1, 5, 10 and 15 mol% were obtained from a 3-YSZ powder prepared using the gelatin method, and the influence of alumina addition on the mechanical and electrical properties of the obtained sinters was investigated. Al2O3 was added via two different methods, namely during the preparation of the 3-YSZ powder and via impregnation using an alcohol solution of aluminum nitrate. The obtained green bodies were sintered for 2 h in air at 1773 K. The structure and morphology of the two series of sinters were investigated using XRD and SEM-EDS, their electrical properties were determined using impedance spectroscopy, and their hardness and critical stress intensity factor were measured using the Vickers indentation test. We established that both the amount of alumina and the method used to introduce it into the 3Y-TZP matrix significantly affect the physicochemical properties of the obtained polycrystalline material. The 3-YSZ/10 mol% Al2O3 sinter that had Al2O3 introduced during the preparation of the 3-YSZ powder was found to exhibit the most advantageous mechanical and electrical properties while still having sufficiently low porosity.

Effect of spontaneous polarization of tourmaline on the grain growth behavior of 3YSZ powder

AbstractThis paper introduces a new method for adjusting the particle size of ZrO2 powder through the electric field effect of tourmaline. The 3 mol% yttria‐stabilized ZrO2 (3YSZ) powder was prepared by using ZrOCl2∙8H2O, YCl3∙6H2O, NaOH, and nano‐tourmaline powder as raw materials through direct precipitation. The powder was characterized with thermogravimetric‐differential scanning calorimeter, X‐ray diffraction, Brunauere‐Emmette‐Teller, Fourier transform infrared, scanning electron microscope, and transmission electron microscope, and the mechanism of tourmaline in the process of powder preparation was also discussed. Findings demonstrate that the electric field generated by spontaneous polarization of tourmaline can control the size of ZrO2 crystal grains, improve the dispersibility of the powder, and promote the stability of ZrO2 in the tetragonal phase. When the electric intensity of tourmaline is 9.02 × 106 V/m, and the addition amount is 2.5 vol%, tourmaline/3YSZ powders are mainly tetragonal phase, and the grain size is the smallest, with an average crystallite size of 19 nm. Then the mechanical properties of tourmaline/3YSZ ceramics were tested. Compared with 3YSZ ceramics without tourmaline, the flexural strength and fracture toughness were improved by 39.5% and 35.8%.

Achieving high anti-sintering performance of plasma-sprayed YSZ thermal barrier coatings through pore structure design

Sintering is one of the critical factors leading to thermal and mechanical degradation of plasma-sprayed thermal barrier coatings (TBCs) in service under high temperature. The sintering resistance of the coating is expected to be improved through pore structure design. In this study, through a strategically improved powder feeding process, 8YSZ powders with fine-sized and coarse-sized pores were injected from the tail of plasma flame during plasma spraying to design porous coatings containing two differently flattened particles with different pore structures. The microstructural and property evolutions during sintering of the coatings were systematically studied compared with traditionally structured coatings. The effect of pore morphology on its sintering behavior was analyzed by quasi-in-situ observation of microstructure evolution. Results show that the anti-sintering performance of plasma-sprayed coating can be improved by adjusting the pore structure of coating with porous unmelted particles. In particular, the introduction of unmelted particles with coarse-sized pores can ensure that the coating has excellent thermal and mechanical properties for a long time. The results of this study provide guidance for structural design of anti-sintering plasma-sprayed coatings.

Synthesis of YSZ powders with controlled properties by the CDJP method

In this work, YSZ powders were synthesized by the controlled double-jet precipitation at constant pH values in the range from 3 to 8. It has been shown that weakly bound floccule-like particles with diffuse boundaries, obtained at pH 3, 7 and 8, strongly agglomerate during drying and form large particles of irregular shape. The dense spherical aggregates of zirconium hydroxide are formed only at CDJP at pH 4 and 5 due to the layer-by-layer buildup of primary particles on the surface of the aggregates. These dense aggregates capture less salt, do not undergo additional agglomeration during drying and retain a low size dispersion. YSZ powder obtained at pH 5 has a high bulk density and fluidity, as well as a narrower particle size distribution than conventional thermal spray powders. The use of this powder resulted in the formation of a uniform coating with high adhesion, uniform porosity and reduced roughness.

Mechanical Properties of Multi-Sized Porous Thermal Barrier Coatings at Micro and Nano Scales after Long-Term Service at High Temperature

Atmospheric plasma-sprayed multi-sized porous structures in thermal barrier coatings (TBCs) were constructed with hollow spherical (HOSP) 8YSZ powders and polypropylene pore formers. The mechanical properties of the multi-sized TBCs were investigated through the nanoindentation method as comparations of the as-sprayed coating and those serving at 1100 °C for a long-term. The results showed that the introduction of the multi-sized pores into TBCs could lead to the decrease of the hardness and modulus as well as the increase of fracture toughness. The hardness and modulus of the coating increased, and the fracture toughness of the coating decreased with the prolongation of the serving times at high temperatures. The mechanical properties of the coating became stable after 60 days’ serving at 1100 °C. The hardness, elastic modulus and fracture toughness of the TBCs increased to 6.51, 7.79, 10.04 GPa and 126.36, 135.13, 145.22 GPa as well as 1.049, 1.157, 1.255 MPa·m0.5, respectively, corresponding to the multi-sized porous TBCs with 0%, 10% and 15% PP powders. The nano-size pores disappeared gradually in the multi-sized TBCs during serving at 1100 °C. The macropores deposited between the ceramic particles penetrated with little diffusion and were not easy to disappear. The introduction of a multi-scale pore structure into the TBCs could improve the stress-strain tolerance of the TBCs.

Tribological property and thermal shock resistance of NiCoCrAlY coated YSZ composite coatings prepared by different laser additive manufacturing scanning speeds

The operating efficiency of aeroengine is significantly affected by the airtightness of its gas path. In this work, the NiCoCrAlY-coated YSZ powder was used and the NiCoCrAlY/Yttria-stabilized zirconia (YSZ) composite coatings were deposited on IN718 substrate with different laser scanning speeds by laser melting deposition to fabricate the seal coating to improve the airtightness. The microstructure, tribological property and thermal shock performance of composite coatings were studied. Due to the extremely high laser energy density, the composite feeding materials are melted thoroughly as well as the IN718 substrate, forming a metallurgical bonding between the composite coating and the substrate. Compared with the pure NiCoCrAlY coating, the reinforced phases γ-NiAl, γ-Ni, ZrO2 and Al2O3 were generated in the composite coating because of the decomposition of YSZ ceramic particles. Due to the uniform nucleation and refined grains, the hardness improves in composite coating hence the wear resistance compared with the pure NiCoCrAlY coating. With increasing laser scanning speeds, the length and width of the microcracks in the composite coating increased correspondingly, thus degrade the thermal shock resistance. The sample prepared by the laser scanning speed of 8 mm/s shows the best wear and thermal shock resistance in this work since its less microcracks while moderate grain size.

Few-layered-graphene/zirconia composites: Single-step powder synthesis, spark plasma sintering, microstructure and properties

The chemical vapor deposition of carbon is performed onto a commercial yttria-stabilized zirconia (3YSZ) powder bed. This produces few-layered-graphene (FLG) film uniformly covering the 3YSZ grains, without the manipulation of any pre-existing nanocarbon in the form of graphene platelets. The powders are then consolidated by spark plasma sintering, producing specimens where FLG is located along the grain boundaries of the 3YSZ matrix, which is below 0.3 μm in grain size. The samples are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy. The pure 3YSZ exhibits higher toughness and fracture strength compared to composites, but the trend is that their toughness increases upon the increase in carbon content. Crack-deflection and crack-bridging are observed. The composites are electrically conducting with a percolation threshold between 1.48 and 1.98 vol.% of carbon, reflecting the continuous nature of the FLG film over very long distances.

Nano-Dispersed 8 Mol% Yttria Stabilized Zirconia Powder: Phase, Morphology and Conductivity Investigation with Different Chelating Agents Via Modified Sol-Gel Method

Nano-Dispersed 8 Mol% Yttria Stabilized Zirconia Powder: Phase, Morphology and Conductivity Investigation with Different Chelating Agents Via Modified Sol-Gel Method

Effect of spraying ceramic powder pore structure on thermophysical properties of plasma-sprayed thermal barrier coatings

Pore structure is one of the major factors influencing the failure mechanisms of thermal barrier coatings (TBCs) during high-temperature service. Powders exhibiting different pore structures have been produced using the plasma spheroidization technique. To improve the strain tolerance of an aircraft engine's hot sections, the preparation method of the TBCs material—8-wt% yttria-stabilized zirconia powders—has been continuously optimized. In this paper, the constrained effects of pores on the ceramic top coating of plasma-sprayed lamellar TBCs were investigated. Furthermore, the crystal phase structures, thermophysical properties, porosity, microstructures, and synthesis mechanism of powders and coatings were analyzed. Results revealed that the top ceramic coating, which was made from powders cooled by air and distilled water, can influence the coating porosity. Moreover, thermal insulation was substantiated by the finite element analysis method. Consequently, dense powders cooled by water can be used on the top surface in direct contact with high temperature flame flow to resist the diffusion of oxides, and the porous powder cooled by air can be applied in the middle coating to decrease the thermal conductivity. Hence, TBCs with various layers of different porosities can prolong the service life of thermal barrier coatings.

Tribological performance of Yttrium stabilized zirconia coatings on Ti6Al7Nb alloy in Hanks Balanced Salt

Titanium alloy (Ti-6Al-7Nb) is the new trend among the bio-compatible alloys due to its non-cytotoxicity. In this work.Ti-6Al-7Nb is coated with 8YSZ powder coating using plasma spraying with NiCrAlY as the bond coat. The surface morphology, elemental composition, phase structure and micro hardness were investigated using FESEM, SEM, EDS, XRD respectively. To study the tribological wear behavior Taguchi methodology is employed. Pin-on-disc equipment is used conduct the experiment in Hanks' Balanced Salt Solution (HBSS) used as the lubricant in accord with ASTM G99 at room temperature. L16 Orthogonal array is chosen with the process parameters such as distance travelled (500, 1000, 1500 and 2000 m), velocity (0.3, 0.6, 0.9 and 1.2 m/sec), load (5, 10, 15 and 20 N) and coating thickness (100, 200, 300 and 400 µm). It is to be noted that there is no significant change in the hardness among the coated specimens but hardness increased from uncoated to the coated specimens. From the experiment it is concluded that distance is the most influential factor in assessing the coefficient of friction and coating thickness is the most influential factor influencing factor in assessing the weight loss.

Effect of Gd2O3 Addition on the Microstructure and Properties of Gd2O3-Yb2O3-Y2O3-ZrO2 (GYYZO) Ceramics.

Gd and Yb elements have high chemical stability, which can stabilize the solid solution in ZrO2. Gd2O3 and Yb2O3 have high melting points, and good oxidation resistance in extreme environments, stable chemical properties. Therefore, Gd2O3 and Yb2O3 were added to ZrO2 to stabilize oxides, improve the high temperature stability, and effectively decrease the thermal conductivity at high temperature. In this work, 5 wt% Yb2O3 and 5 wt%, 10 wt%, 15 wt% Gd2O3 were doped into 8 wt% Y2O3 stabilized ZrO2 (8YSZ) powders as thermal barrier coating materials, and sintered at 1650 °C for 6 h, 12 h, 24 h. The effects of Gd2O3 addition on the microstructure, density, thermal conductivity, hardness, and fracture toughness of Gd2O3-Yb2O3-Y2O3-ZrO2 (GYYZO) bulk composite ceramics were investigated. It was found that the densification of the 8YSZ bulk and GYYZO bulk with 15 wt% Gd2O3 reached 96.89% and 96.22% sintered at 1650 °C for 24 h. With the increase of Gd2O3 addition, the hardness, elastic modulus and fracture toughness of the GYYZO bulk increased and the thermal conductivity and thermal expansion coefficient of the GYYZO bulk decreased. GYYZO bulk with 15 wt% Gd2O3 sintered at 1650 °C for 24h had the highest hardness, elastic modulus and fracture toughness of 15.61 GPa, 306.88 GPa, 7.822 MPa·m0.5, and the lowest thermal conductivity and thermal expansion coefficient of 1.04 W/(m·k) and 7.89 × 10−6/°C at 1100 °C, respectively. The addition of Gd2O3 into YSZ could not only effectively reduce the thermal conductivity but also improve the mechanical properties, which would improve the thermal barrier coatings’ performances further.

Microstructural and Physical Properties of Yttria Stabilized Zirconia (YSZ) Prepared by Ceramic Injection Moulding (CIM)

Ceramic injection moulding (CIM) is a near net shape process to produce smaller and intricate parts at a competitive cost. However, fine particle size (nano scale) used for such injection moulding process generally leads to agglomeration, higher binder content and critical dimensional shrinkage which results in defects on the sintered components. This study extensively investigates the characteristics of Yttria Stabilized Zirconia (YSZ) for CIM process. YSZ parts were moulded by CIM process that utilized a multi-component binder system using palm stearin (PS) and polyethylene (PE) in 60:40 (vol %) ratio. The powders were characterized using particle size analyzer, pycnometer density and scanning electron microscopic (SEM). The binders were characterized by the pycnometer density, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Powders loading were choosen by the critical powder volume percentage (CPVP) through oil absorption test. YSZ powder was mixed with the binders at different powder loadings ranging from 58 to 60 vol% based on CPVP. The parts were injected moulded, thermal debound, pre-sintered and sintered. The microstructure, green strength and hardness of the sintered part at different powder loadings were investigated. A large porous region was clearly observed at 58 vol% compared to 60 vol%. All samples were sintered at 1350°C, with the highest green strength and hardness were 13MPa and 357.42 HV respectively was given by the sintered part at powder loading of 60 vol.

Effect of the core-shell structure powders on the microstructure and thermal conduction property of YSZ/Cu composite coatings

The microstructure of thermal spray powders plays a significant role in the application performance of thermal protection coatings deposited by air plasma spraying. Based on the design of the spray powder structure, the [email protected] core-shell structure powders have the potential to be a thermal conduction anisotropy material for thermal protection coating, owing to the special alternate-layered structure formed during plasma spraying. To study the mechanism of the thermal conduction anisotropy of YSZ/Cu composite coatings, the YSZ coatings, [email protected] coatings and YSZ-Cu coatings were fabricated by YSZ powders, [email protected] core-shell structure powders and YSZ-Cu mechanical mixture powders, respectively. There were YSZ vertical thermal insulation layer and Cu horizontal thermal condition layer in the [email protected] coatings, which provides a structural basis for the realization of thermal conduction anisotropy. Compared with the YSZ-Cu coatings, the temperature difference between the back-surface center area and the edge area of the [email protected] coatings is significantly reduced by approximately 37%, which is attributed to the synergistic effect of alternate-layered comprising the YSZ layer and the high thermal conductivity Cu layer. [email protected] coatings can effectively prevent the rapid rise in local temperature by the analysis of finite-element simulation, which is beneficial for prolonging the service life of the thermal protection coating in a non-uniform temperature field.

Thermal Conductivity of Multi-Sized Porous Thermal Barrier Coatings at Micro and Nano Scales after Long-Term Service at High Temperatures

Thermal barrier coatings with multi-sized porous structure at micro and nano scales were prepared with hollow spherical YSZ powders and polypropylene powders through atmospheric plasma spraying. The thermal conductivities of the multi-sized thermal barrier coatings after a long-term serving at high temperature were tested through laser flash heating method. Meanwhile, the physical models of thermal barrier coatings with multi-sized porous structure at micro and nano scales were constructed through Ansys Workbench. The evolutions of thermal conductivity of thermal barrier coatings with multi-sized pores after long-term service at 1100 °C were investigated through computation. It was found that the thermal conductivity of the coating increased with the extension of the serving time. When the serving time reached 60 days, the thermal conductivity of the coating tended to be stable and close to the compacted bulk. The computational results were consistent with the tested ones, which could be helpful to explain the thermal conducting evolution in thermal barrier coatings with multi-sized porous structure at nano and micro scales after long-term serving at high temperature.