ZrO2 Grains Research Articles

Selective oxidation/crystallization and their patterning on metallic glass by laser irradiation

Periodic lines, approximately 15-μm-wide and separated by several hundred micrometers, were prepared on the surface of Zr55Cu30Al10Ni5 metallic glass by scanning a focused laser beam across the samples in air. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) suggested that the lines produced by the focused laser beam consisted of localized metal oxides (tetragonal ZrO2, monoclinic ZrO2) and crystallites (Cu10Zr7), which was in contrast to the amorphous substrate. The electron backscatter diffraction (EBSD) also confirmed the successful patterning of the lines which was mainly composed of the fine ZrO2 grains. The energy dispersive X-ray spectroscopy (EDS) mapping clearly exhibits that high amounts of zirconium (161% of the nominal composition) as well as oxygen (58.9%) and a certain amount of aluminum (80% of the nominal composition) were presented in the beam-scanned area. The Auger electron spectroscopy (AES) was also conducted to see the atomic percentage from the top surface to the depth direction and showed that the created oxides were thermally stable and therefore oxidization cannot proceed with increasing the number of laser scan. We confirmed that the laser irradiation method has a large degree of freedom for designing patterns, such as interdigital, dots, and letters because of the precise control over the scanning stage.

Fabrication and characterization of electrospun ZrO2/Al2O3 nanofibers

Abstract Novel ZrO2/Al2O3 nanofibers with the average diameter of around 110 nm are fabricated by heat treatment of electrospun zirconium oxychloride (ZrOCl2)/aluminium nitrate (Al(NO3)3)/polyvinylpyrrolidone (PVP) composite fibers at temperature of 1200 °C. Intermediate and the final ceramic filaments are characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). The ZrO2/Al2O3 nanofibers contain mainly tetragonal zirconium dioxide in contrast to pure ZrO2 nanofibers consisting of monoclinic zirconia. It is revealed that Al2O3 impedes the ZrO2 grain growth and stabilizes the tetragonal phase of zirconia forming Al2O3-ZrO2 solid solution. Fabricated zirconia nanoscale filaments with varying phase composition can be considered, for instance, as promising one-dimensional high-temperature supports for heterogeneous catalysts with controlled activity and selectivity.

Consolidation of undoped, monoclinic zirconia polycrystals by flash sintering

AbstractConsolidated, monoclinic ZrO2 polycrystal was produced from undoped ZrO2 powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO2 was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal‐to‐monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO2 body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO2 without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain‐boundary second phase in the flash‐sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash‐sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO2 grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash‐sintered specimen for 5 minutes. The alignment of ZrO2 grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.

Effect of intra-oral aging on t→m phase transformation, microstructure, and mechanical properties of Y-TZP dental ceramics

The aim of the present study was to evaluate the influence of intra-oral aging on the tetragonal-to-monoclinic (t→m) phase transformation of two Y-TZP dental ceramics – Lava Frame (Frame) and Lava Plus (Plus) – and determine the impact of this response on their microstructures and mechanical properties: flexural strength, Young's modulus, microhardness and fracture toughness. Standardized ceramic specimens were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). After the baseline analysis, the specimens were attached to personalized intra-oral resin appliances and exposed to the oral cavity of 20 subjects for 60 days and then analyzed again. Specimens produced for mechanical properties evaluation were also analyzed before and after the 60-day intra-oral aging. The data were analyzed using two-way ANOVA and Tukey HSD's post hoc test (α=0.05). Weibull analysis was used to evaluate the strength reliability. Both Y-TZP ceramics suffered t→m phase transformation after 60-day intra-oral aging (Plus=4.7%/Frame=7.7%). SEM and AFM analyses showed dislodgement of ZrO2 grains and a significant increase in roughness after intra-oral aging for both ceramics. Both Y-TZP ceramics suffered a decrease on flexural strength, Young's modulus and fracture toughness after intra-oral aging (p<0.05). Only Plus presented a decrease of microhardness after intra-oral aging (p<0.05). Intra-oral aging for 60 days produced t→m phase transformation, changed the microstructure and decreased the mechanical properties of two Y-TZP dental ceramics.

In-situ oxidation and short-time corrosion investigation on strain and dislocation during the generation and growth of ZrO2

An In-situ oxidation test and a short time exposure corrosion test of Zircaloy-4 were carried out. A lot of dislocations generated in the nucleation and growth process of ZrO2 are observed. Dislocations will be firstly distributed at the interface of ZrO2. With the growth of m-ZrO2 grains, the number of dislocations at the interface increases. When the grain size of ZrO2 reaches a critical value, the dislocations in the grains of ZrO2 begin to appear. A compressive strain between semi-coherent planes of oxides was detected in a range from −0.3% to −6%.

Microstructure evolution of a W‐doped ZrB2 ceramic upon high‐temperature oxidation

AbstractThis basic research deals with the microstructure evolution of a W‐doped ZrB2 ceramic, as‐sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B2 solid solutions surround the original ZrB2 nuclei, whereas refractory W‐compounds at triple junctions and clean grain boundaries are distinctive of this ceramic. After oxidation, the microstructure is typified by intragranular nanostructures, in which nanosized W inclusions remained trapped within ZrO2 grains, or decorate their surfaces. The understanding of the oxidation reactions occurring in the system as a function of the oxygen partial pressure was fundamental to conclude that W‐based compounds do not notably suppress or retard the oxidation of ZrB2 ceramics.

Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500–1650 °C temperature range

The microstructure evolution of ZrB2 hot pressed with 15 vol% TaSi2 was studied in the as-sintered state and after oxidation for 15min at 1500 and 1650°C in stagnant air. In the pristine material, the original ZrB2 nuclei are surrounded by a mixed (Zr,Ta)B2 solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain boundaries are distinctive of this ceramic. After oxidation, the solid solution evolves into ZrO2 grains encasing intragranular nano-structured TaB2 particles. Here we show that the operating limit temperature of this composite is related to the critical oxidation of TaB2 to Ta2O5 above 1600°C, accompanied by large volume expansion and local liquid formation, which ruptures the ZrO2 grains and structure.

Single grain analysis on a nanoscale in ZrO2:Al2O3 nano-composites by means of high-resolution scanning transmission electron Microscopy

Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by microwave plasma process synthesis and studied by high resolution scanning transmission electron microscopy. The ZrO2:Al2O3 nanocomposite samples showed a structure of randomly arranged, crystalline grains of both ZrO2 and Al2O3 with a size in the range of 3–5 nm. For each constituent, both cubic and monoclinic grains were observed. Similarly prepared, single phase Al2O3 nanoparticles were found to be completely amorphous. In the case of ZrO2, the crystallite size and the fraction of the monoclinic ZrO2 phase is noticeably reduced in the ZrO2:Al2O3 nanocomposite in comparison to single phase samples. The observation of ultra-small crystalline Al2O3 in the composite samples, is tentatively ascribed to a modification of the interface energy by the contact of Al2O3 and ZrO2 as well as a possible uptake of ZrO2 into the Al2O3 grains from the gas phase during synthesis.

Segregation as a driving force in the formation of nanocomposite ZrO2-Al2O3 coatings

ZrO2-Al2O3 coatings were deposited on a floating substrate at a temperature of 370°C. Various coating compositions, ranging from pure zirconia to 50at.% alumina content, were deposited by reactive pulsed-DC magnetron sputtering using 51mm diameter Zr and Al targets. The coating had a Zr-Al-O solid solution structure that was composed of nano-size cubic-ZrO2 grains (20±5nm) with aluminum cations distributed inside them. The deposited coatings were annealed up to 1350°C and their structural changes were studied using Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM) and Vickers hardness measurement. Two exothermic annealing events were observed. The first event appeared at 700–750°C and was contributed to aluminum segregation to the grain boundaries, and the formation of a nanocomposite-ZrO2/amorphous-Al2O3 structure. The second event appeared at 1000–1200°C and was related to coarsening (“Ostwald ripening”) of the ZrO2 grains. The coating hardness after the first event was stabilized to 19.5±1.1GPa because of the segregation, which hindered grain growth. However, after a second DSC cycle of the coating, only one sharp endothermic transformation peak was observed at onset temperature of 1060±5°C. This was associated with grain growth accompanied with zirconia transformation from the cubic to the monoclinic phase.

Effect of Particle Shape on Dispersion Formation of Harmonic Microstructure of Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;-ZrO&lt;sub&gt;2&lt;/sub&gt;

Silicon nitride with 50 mass zirconia ceramic matrix composites were processed by mechanical milling (MM) followed by spark plasma sintering (SPS). Two different of Si3N4 particle shapes for create harmonic microstructure were investigated. The microstructure of Si3N4-ZrO2 with initial Si3N4 shape is like coin/flakes after MM for 144 ks was failed to create the harmonic microstructure after SPS. With another sphere shape of Si3N4 after MM for 144 ks, the harmonic microstructure could be formed after SPS. Thus, the initial powders shape of Si3N4 have an effect in the formation of harmonic microstructure could be fully achieved. The highest mechanical properties of Si3N4-ZrO2 are on the powders with mechanical milling time for 144 ks. The Si3N4 ceramic with homogeneous fine grains of ZrO2 dispersed on its surface was obtained, and the mechanical properties were improved. The Vickers hardness obtained on 144 ks is 1031 MPa and the bending strength is 262 MPa. The main factors explaining the improvement in the mechanical properties of Si3N4-ZrO2 are considered to be the porosity decrease caused by the particles shape and appropriate condition of powder processing i.e. MM and SPS.

Microstructure and properties of (1 − x)La0.9Ca0.1CrO3/x ZrO2 (x = 0–30 wt-%) composites

The microstructure and properties of Ca doped lanthanum chromate [La0.9Ca0.1CrO3 (LCCO)] and zirconia (ZrO2) composites [abbreviated as (1 − x)LCCO/xZrO2, x = 0–30 wt-%] prepared by solid state reaction method are investigated in this paper whose potential application in heat resistant ceramics are also discussed. The growth and fusion of the LCCO grains are hindered by the ZrO2 grains, which make the matrix grains refined. In the stable system, it is discovered that the density of the composites decreases, the porosity increases and the decrease amplitude of the bend strength after thermal shock at 600°C reduces with the increasing ZrO2 content, which results in diminishing the bend strength gradually and enhancing the thermal shock resistance. When x = 10–15 wt-%, the samples show the best thermal shock resistance realising a maximum thermal cycle times of 8. The material with x = 20 wt-% exhibits excellent slag resistance.

Effect of ZrO2 content on ageing resistance and osteogenic cell differentiation of ZrO2–Al2O3 composite

Two commercial 3mol% yttria-partially stabilized zirconia powders, with 0.3wt% Al2O3 (YZA) and without Al2O3 (YZ), were used to produce alumina (Al2O3)–zirconia (ZrO2) slip cast composites. The influence of the ZrO2 content and the ZrO2 grain size on the ageing behavior of the different Al2O3–ZrO2 composites was investigated. In addition, the in vitro biocompatibility and osteogenic cell differentiation of Al2O3–ZrO2 surfaces were evaluated before ageing (ba) and after ageing (aa) using the rat bone marrow-derived osteoblast cell culture model; the osteogenic potential of preosteoblast MC3T3-E1 cells on the same surfaces was also assessed. The ageing susceptibility of the composites significantly increased with increasing the ZrO2 content over 22vol%. For ZrO2 contents ≤22vol%, the grain size did not influence the transformability of tetragonal ZrO2 under ageing conditions; however, in the composites with 50vol%, the greater grain size of 50vol% YZ with respect to 50vol% YZA enhanced the ageing degradation. Overall, the cell culture experiments revealed no significant differences among the composites before ageing in terms of osteogenic cell differentiation, except for the higher mineralization of bone marrow-derived osteoblast cells grown on 50vol% YZ ba compared with the ones grown on 50vol% YZA ba. The ageing process tended to rescue the osteogenic potential of these cells grown on 50vol% YZA while inhibiting the one on 50vol% YZ. In conclusion, the low ageing sensitive of the composites with ZrO2 contents ≤22vol% did not change the osteoblast biocompatibility, whereas the greater ageing degradation of the composites with 50vol% ZrO2 seemed to alter the osteogenic potential of bone marrow-derived osteoblast cells.

The influence of zirconia precursor/binding polymer mass ratio in the intermediate electrospun composite fibers on the phase transformation of final zirconia nanofibers

Nanofibrous zirconia was fabricated by calcination of electrospun zirconium oxychloride/polyethylene oxide (PEO) composite fibers with different mass fraction of the components. ZrO2 nanofibers were characterized by scanning electron microscopy (SEM), nitrogen adsorption at 77 K, and X‐ray diffractometry (XRD). It was revealed that increase in ZrOCl2/PEO mass ratio above the threshold value significantly decreases tetragonal phase (t‐ZrO2) content and increases monoclinic phase (m‐ZrO2) content in final ceramic nanofibers. Distinct t‐ZrO2 → m‐ZrO2 transformation takes place when average ZrO2 grain size approaches to 30 nm.

Study of diffusion processes in the oxide layer of zirconium alloys

Abstract In the active zone of a nuclear reactor where zirconium alloys are used as a coating material, this material is subject to various harmful impacts. During water decomposition reactions, hydrogen and oxygen are evolved that may diffuse through the oxidic layer either through zirconium dioxide (ZrO2) crystals or along ZrO2 grains. The diffusion mechanism can be studied using the Ion Beam Analysis (IBA) method where nuclear reaction 18O(p,α)15N is used. A tube made of zirconium alloy E110 (with 1 wt. % of Nb) was used for making samples that were pre-exposed in UJP PRAHA a.s. and subsequently exposed to isotopically cleansed environment of H2 18O medium in an autoclave. The samples were analysed with gravimetric methods and IBA methods performed at the electrostatic particle accelerator Tandetron 4130 MC in the Nucler Physics Institute of the CAS, Řež. With IBA methods, the overall thicknesses of corrosion layers on the samples, element composition of the alloy and distribution of oxygen isotope 18O in the corrosion layer and its penetration in the alloy were identified. The retrieved data shows at the oxygen diffusion along ZrO2 grains because there are two peaks of 18O isotope concentrations in the corrosion layer. These peaks occur at the environment-oxide and oxide-metal interface. The element analysis identified the presence of undesirable hafnium.

Fine grained Al2O3–ZrO2 (Y2O3) ceramics by controlled crystallization of amorphous phase

It is classic to prepare nanoceramics by crystallization of amorphous solids; however, this method was seldom reported in the classic system of Al2O3–ZrO2. In present work, fine grained Al2O3–ZrO2 (Y2O3) ceramics were obtained through controlled crystallization of dense amorphous bulks by heat treatment. Phase composition, microstructure and mechanical property were investigated during heat treatment. Crystallization and densification were almost completed after heat treatment at 1400°C for 2h with relative density of 98.5%, and as-prepared sample exhibited homogenous mixture of Al2O3 and ZrO2 grains at about 440nm. The fine grained ceramic exhibited high hardness of 15.5GPa and fracture toughness of 5.2MPam1/2. This work may provide new guidance for preparing AZY nanoceramics through crystallization of amorphous phase in both experiment and theory.

Calcia-magnesia-alumino-silicate (CMAS)-induced degradation and failure of air plasma sprayed yttria-stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs) used in gas-turbine engines experience severe degradation by calcia-magnesia-alumino-silicate (CMAS) deposits during high-temperature operation. The present study identified and evaluated the chemical and microstructural changes in air plasma-sprayed (APS) 7 wt.% Y2O3 stabilized ZrO2 (7YSZ) TBCs caused by CMAS attack under isothermal conditions at 1340 °C. Additionally, a ‘model’ experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300 °C for different exposure times. The combined results from both studies highlight the importance of local CMAS glass composition on the 7YSZ/CMAS interaction. Specifically, low Y-content in the glass, caused by a relatively large glass 'sink,' produces Y-depleted ZrO2 grains that undergo tetragonal (t) → monoclinic (m) phase transformation upon cooling. Alternatively, small pockets of Y-enriched glass induce the formation of t″-ZrO2, a phase characterized by its high stabilizer content. After prolonged high-temperature exposure, solution-reprecipitation induces the formation of both m-ZrO2 and t″-ZrO2 throughout the APS 7YSZ TBC in accordance with the phase diagram. Using a thermomechanical model it is shown that the strain associated with the martensitic t→m phase transformation plays an important role in the delamination failure of TBCs attacked by CMAS.

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO2/Y2O3 ceramics from mixture of Al2O3, SiO2, ZrO2 and Y2O3 by gradually adding of α–β Si3N4 nanopowder from 1 to 5wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400°C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si3N4 additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33g/cm3, while for traditionally sintered samples, the value is 2.55g/cm3, and the compressive strength for SPS grows with Si3N4 additives, reaching 600N/mm2. In the case of traditional sintering, it decreases to approximately 100N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO2 grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si3N4 additive, are filled with mullite crystalline formations.

Surface Structure and High-Voltage Charging/Discharging Performance of Low-Content Zr-Oxide-Coated LiNi1/3Co1/3Mn1/3O2

The surface structures, high-voltage charging/discharging performances, and Li-ion transfer at the electrode/electrolyte interface of LiNi1/3Co1/3Mn1/3O2 coated with small amounts of Zr oxide by the sol-gel method were investigated. Island-like ZrO2 grains as well as grain-free (sea-like) zones were observed on the surfaces of the coated LiNi1/3Co1/3Mn1/3O2 samples. The ratio of the area of these sea-like zones to the total surface area of the LiNi1/3Co1/3Mn1/3O2 sample increased from 44 to 85% when the Zr oxide content was decreased from 1.0 to 0.125 wt%. A compound oxide phase containing Zr, Ni, Co, and Mn was formed uniformly on the sea-like zones; the depth of this phase was approximately 2 nm. The initial discharge capacities of the coated LiNi1/3Co1/3Mn1/3O2 samples were equal to that of bare LiNi1/3Co1/3Mn1/3O2. Further, the cycling performances of the coated LiNi1/3Co1/3Mn1/3O2 samples were significantly higher, as were their discharge capacities and discharge voltages, while their charge-transfer resistances (Rct) were lower. It was also found that the coated LiNi1/3Co1/3Mn1/3O2 samples had lower activation energies (Ea) for charge transfer than the bare LiNi1/3Co1/3Mn1/3O2. Thus, it can be surmised that the area and structure of the sea-like zones determined the Rct and Ea values and cycling performances of the coated LiNi1/3Co1/3Mn1/3O2 samples.

Three different alumina–zirconia composites: Sintering, microstructure and mechanical properties

Two commercial 3mol% yttria–partially stabilized zirconia powders, with 0.3wt% Al2O3 (Y–PSZA) and without Al2O3 (Y–PSZ), and a Zr (IV) precursor were used to produce alumina (Al2O3)–zirconia (ZrO2) slip cast composites. The influence of both the zirconia content and the reduction of zirconia particle size on the sintering behavior, microstructure development and mechanical properties were investigated. The increase in the zirconia content from 10.5 to 22vol% increased the hardness; whereas, above 22vol% ZrO2 the hardness decreased. A significant increase in the fracture toughness with increasing the ZrO2 content over 22vol% was obtained by the stress-induced phase transformation. The flaw size limited the strength below 22vol%; whereas, above 22vol% ZrO2 the strength was controlled by the stress-activated phase transformation. For 10.5vol% ZrO2, the smaller ZrO2 grains produced by using the Zr (IV) precursor were more effective in preventing the Al2O3 grain growth resulting in higher hardness. However, the tetragonal–monoclinic (t–m) transformation of some unstabilized ZrO2 grains during cooling reduced Young's modulus and fracture toughness.

Al₂O₃/ZrO₂/Y₃Al₅O12 Composites: A High-Temperature Mechanical Characterization.

An Al2O3/5 vol%·ZrO2/5 vol%·Y3Al5O12 (YAG) tri-phase composite was manufactured by surface modification of an alumina powder with inorganic precursors of the second phases. The bulk materials were produced by die-pressing and pressureless sintering at 1500 °C, obtaining fully dense, homogenous samples, with ultra-fine ZrO2 and YAG grains dispersed in a sub-micronic alumina matrix. The high temperature mechanical properties were investigated by four-point bending tests up to 1500 °C, and the grain size stability was assessed by observing the microstructural evolution of the samples heat treated up to 1700 °C. Dynamic indentation measures were performed on as-sintered and heat-treated Al2O3/ZrO2/YAG samples in order to evaluate the micro-hardness and elastic modulus as a function of re-heating temperature. The high temperature bending tests highlighted a transition from brittle to plastic behavior comprised between 1350 and 1400 °C and a considerable flexural strength reduction at temperatures higher than 1400 °C; moreover, the microstructural investigations carried out on the re-heated samples showed a very limited grain growth up to 1650 °C.