High Yttria–Zirconia and Yttria Ceramics for Petrochemical Reverse‐Flow Reactor Applications

Reverse‐flow reactors achieve the desired hydropyrolysis reaction of natural gas and other hydrocarbon feeds at very high temperatures of up to 2000°C, which enables the production of many high‐value chemicals. To identify refractory ceramic materials suitable for constructing key components of the reactor, the full range of solid solutions between zirconia and yttria having 18 to 100 mol% yttria have been tested in a laboratory reactor. Conventional yttria‐stabilized zirconia (YSZ) materials having 8 mol% Y₂O₃ appear to accommodate reactor thermal severity, but are prone to a new form of corrosion termed ceramic dusting that is observed when pyrolysis and oxidation cycles are alternated under reverse‐flow conditions. Yttria and high yttria–zirconia ceramics having ~80 mol% or more yttria suppress ceramic dusting corrosion in steam‐free pyrolysis environments. The addition of low levels of steam of ~5% to the pyrolysis gas stream increases the stability of YSZ materials substantially, so that the stability threshold is closer to 40 mol% Y₂O₃ in the yttria–zirconia system. The two approaches can be combined to optimize reactor performance. Key experimental results are presented and discussed taking into account the thermodynamic phase stability of the different phases.     

Fracture resistance of yttria stabilized zirconia manufactured from stabilizer-coated nanopowder by micro cantilever bending tests

Yttria stabilized tetragonal zirconia polycrystal (Y-TZP) owes its high toughness to transformation toughening, a mechanism that requires the development of a process zone. It is important to measure if and to what extent the size of components can be reduced. In this study, tests were carried out using focused ion beam or picosecond milled pre-notched, 3 mol percent Y₂O₃ (3Y-) TZP micro-cantilever (10–250 μm) beams. The tests show clearly that the maximum fracture resistance is size dependent and the plateau toughness was not reached in any of the small-scale samples. A correlation between transformability of the tetragonal phase and the measured fracture resistance became visible only for the largest micro cantilever but did not reach the values measured in macroscopic samples. Based on these results, it is not advantageous to use very tough zirconia materials in components with dimensions smaller than ～0.25 mm, as the high toughness is not fully realized.     

Thermal cycling performance of La2Ce2O7/50 vol.% YSZ composite thermal barrier coating with CMAS corrosion

La₂Ce₂O₇ (LC) is receiving increasing attention due to its lower thermal conductivity, better phase stability and higher sintering resistance than yttria partially stabilized zirconia (YSZ). However, the low fracture toughness and the sudden drop of CTE at approximately 350?°C greatly limit its application. In this study, the LC/50?vol.% YSZ composite TBC was deposited by supersonic atmospheric plasma spraying (SAPS). Compared to YSZ or double layered LC/YSZ coating, the thermal cycling life of LC/50?vol.% YSZ coating with CMAS attack increased by 93% or 91%. The latter possessed higher fracture toughness (1.48?±?0.26?MPa?m^1/2) than LC (0.72?±?0.15?MPa?m^1/2) and better CMAS corrosion resistance than YSZ owing to the formation of Ca₂(La_xCe_1-x)₈(SiO₄)₆O_6–4x with <001> orientation perpendicular to the coating surface. The sudden CTE decrease of LC was fully suppressed in LC/50?vol.% YSZ coating due to the change of temperature dependent residual stresses induced by YSZ.     

The Influence of Platelet‐Like LaMgAl11O19 on the Toughness of 3 mol% Yttria Partially Stabilized Zirconia Ceramic

LaMgAl₁₁O₁₉‐3 mol% yttria partially stabilized zirconia ceramics were successfully prepared by pressureless sintering at 1550°C for 3 h. The ceramic's mechanical properties were measured, and the phase composition and microstructure observed using X‐ray diffraction and scanning electron microscopy. The results show that the mechanical properties of the ceramic were initially improved on addition of LaMgAl₁₁O_19, but further additions were detrimental. When the amount of LMA added was equal to 2 wt.%, the bending strength and fracture toughness reached 812 ± 37 MPa and 14.0 ± 0.3 MPa·m^1/2. This equates to an increase of 8.0% and 6.9% compared with untreated 3 mol% yttria partially stabilized zirconia (3YSZ) ceramic, respectively. However, the bending strength and fracture toughness both decreased when the amount of LaMgAl₁₁O₁₉ added was 4 and 6 wt.%. A crack propagation and force analysis of the crack tips in LaMgAl₁₁O₁₉‐platelet‐reinforced 3YSZ ceramic were also carried out. The results indicate that phase transformation and crack deflection were the dominant toughening mechanisms in the LaMgAl₁₁O₁₉‐3YSZ ceramic. At the same time, energy dissipation by the LaMgAl₁₁O₁₉ platelets also helps to restrain crack propagation in the matrix, which improves toughness more effectively.     

Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing,Microstructure, and Thermal Aging

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd₂Zr₂O₇ which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

Development of Ce‐PSZ‐/Y‐PSZ‐Toughened Alumina Inserts for High‐Speed Machining Steel

The nanocomposite CeO₂/Y₂O₃ partially stabilized zirconia (Ce‐PSZ/Y‐PSZ)‐toughened alumina was prepared by wet chemical simultaneous coprecipitation process. The thermal stability of phases and morphology of powders were characterized by TG‐DTA, FTIR, and FESEM. The microstructure, stabilization of phases and compositional analysis with different mol% CeO₂/Y₂O₃‐doped zirconia in alumina are characterized by FESEM, XRD, and EDAX spectra. Significant improvement in fracture toughness and flexural strength has been observed in 10 vol% of partially stabilized zirconia (2.5 mol% Y₂O₃ in ZrO₂/9 mol% CeO₂ in ZrO₂)‐toughened alumina, which is suitable for high‐speed machining applications.     

Ceramic matrix composites in the alumina/5-30 vol.% YAG system

The preparation technique of the particulate composite materials in the alumina/YAG system was elaborated. Within alumina particles suspension yttria precursor was precipitated with ammonium carbonate. Drying and calcination at 600 °C resulted in the mixture of alumina and yttria particles, the latter being much finer than alumina particles. This mixture was additionally homogenized by short attrition milling in an aqueous suspension. Sintering of such powders results in the materials composed of YAG inclusions of sizes smaller than shown by alumina grains and evenly distributed within the matrix. YAG particles result from the reaction of Y₂O₃ with Al₂O₃ during heat treatment. YAG inclusions limit effectively grain growth of the alumina matrix. Hardness, fracture toughness, strength, Young modulus and wear susceptibility of composites and pure alumina were measured. Composites show higher hardness and in some cases higher fracture toughness and wear resistance than pure alumina polycrystals.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

Influence of chemical composition on sintering ability of ZTA ceramics consolidated from freeze dried granules

Dense zirconia-toughened alumina (ZTA) ceramic composites with ZrO₂ = 0, 5, 10, 15, 20, 30, 60 and 100 wt.% have been prepared by sintering green compacts obtained by dry powder pressing of freeze dried granules consisting of α-alumina and a yttria partially stabilized zirconia (YPSZ) at various temperatures ranging from 1450 to 1650 °C for 1-2 h. The characteristics of sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), Archimedes principle, Vickers indentation method and by 3-point bend test. Characterization results revealed that adding YPSZ increased the 3-point bend (flexural) strength, fracture toughness and homogeneity of the microstructure, but slightly decreased the hardness and the sintering ability of alumina. A 20 wt.% YPSZ was sufficient to increase the fracture toughness and flexural strength of specimens sintered for 2 h at 1600 °C from 2.5 to 4.6 MPa m^1/2 and 150 to 400 MPa, respectively. The XRD results revealed that there is no solid-solution formation between zirconia and alumina constituents of ZTA ceramic composites upon sintering.     

Effect of yttria doping on the microstructure and mechanical properties of Al2O3–FeAl2O4 nanocomposites developed via solid state precipitation

We have recently reported the production of Al₂O₃-matrix nanocomposites via solid state precipitation of nanosized FeAl₂O₄ particles within the matrix grains during aging of Al₂O₃–10 wt.% Fe₂O₃ solid solutions in a reducing atmosphere (N₂ + 4% H₂). In addition to these nanoparticles, however, coarse micron-sized FeAl₂O₄ particles were present along the matrix grain boundaries. In the present work, we show that the addition of ～250 ppm yttria to the solid solutions suppressed the development of these intergranular particles, reducing their size by a factor of ～2 with optimum aging. A fracture toughness improvement by 45% and flexural strength improvement by 50% with respect to monolithic Al₂O₃ were recorded with the yttria-containing nanocomposite developed by aging for 20 h at 1450 °C. Aging also improved the hardness with respect to the solid solution. The change in fracture mode in the presence of the nanosized intragranular particles was believed to be the major contributing factor towards the improvement in toughness and therefore the strength. The higher strengths obtainable in the presence of yttria were attributed to the reduction of intergranular precipitate size relative to yttria-free nanocomposites.     

Integrated high strength and high toughness induced by elongated TiB2 grains in reactive sintered TiB2–TiC–SiC ceramics

Although the addition of other phases into TiB₂ matrix to form ceramic composites has been widely used to improve the mechanical properties of monolithic TiB₂ ceramics, it is still difficult to greatly enhance the flexural strength and fracture toughness simultaneously. In this work, TiB₂–TiC–SiC composites were successfully prepared by reactive spark plasma sintering of Ti₃SiC₂–B₄C–Ti powder mixtures. During the sintering process, TiB₂ grains grew into an elongated morphology, endowing the composites with integrated high strength and high toughness. The growth mechanism of TiB₂ grains was attributed to the evaporation–condensation kinetics induced by the presence of B₂O₃. These findings can accelerate the exploration of ceramic composites with excellent comprehensive properties.     

Corrosion resistance and thermal-mechanical properties of ceramic pellets to molten calcium-magnesium-alumina-silicate (CMAS)

Because gas turbine engines must operate under increasingly harsh conditions, the degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) is becoming an urgent issue. Mullite (3Al₂O₃·2SiO₂) is considered a potential material for CMAS resistance; however, the performance of mullite in the presence of CMAS is still unclear. In this study, mullite and Al₂O₃–SiO₂ were premixed with yttria stabilized zirconia (YSZ) in different proportions, respectively. Porous ceramic pellets were used to conduct CMAS hot corrosion tests, and the penetration of molten CMAS and its mechanism were investigated. The thermal and mechanical properties of the samples were also characterized. It was found that the introduction of mullite and Al₂O₃–SiO₂ mitigated the penetration of molten CMAS into the pellets owing to the formation of anorthite, especially at 45 wt% mullite/55 wt% YSZ. Compared with Al₂O₃–SiO₂, mullite possesses a higher chemical activity and undergoes a faster reaction with CMAS, thus forming a sealing layer in a short time. Additionally, the thermal expansion coefficient, thermal conductivity, and fracture toughness of different samples were considered to guide the architectural design. Considering the CMAS corrosion resistance, thermal and mechanical performance of TBCs systematically, a TBC system with a multilayer architecture is proposed to provide a theoretical and practical basis for the design and optimization of the TBC microstructure.     

Effect of molten V2O5 salt on the corrosion behavior of micro- and nano-structured thermal sprayed SYSZ and YSZ coatings

The corrosion resistance of micro-and nano-structured scandia and yttria codoped zirconia (nano-4 mol%SYSZ and micro-8.6SYSZ) and yttria doped zirconia (4YSZ) in the presence of molten vanadium oxide were investigated. To this end, duplex TBCs (thermal barrier coatings), composed of a bond coat (NiCrAlY) and a top coat (4SYSZ or 4YSZ), were deposited on the IN738LC Ni-based supper-alloy by atmospheric plasma spraying (APS). The corrosion studies of plasma sprayed TBCs were conducted in 25 mg V₂O₅ molten salt at 910 °C for different times. The nanostructured coating, as compared to its micro-structured counterpart, in spite of a further reaction with the V₂O₅ salt, showed a higher degradation resistance during the corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones. Finally, the corrosion resistance and degradation mechanism of SYSZ and YSZ coatings were compared with the presence of molten NaVO₃ and V₂O₅ salt, respectively.     

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (～4.0?mol% Y₂O₃–ZrO₂, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200?°C. Here we report an excellent zirconia system co–doped with 16?mol% CeO₂ and 4?mol% Gd₂O₃ (16Ce–4Gd) presenting nontransformable feature up to 1500?°C, in which no detrimental monoclinic (m) ZrO₂ phase formed on partitioning. It also exhibits a high fracture toughness of ～46?J m^?2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500?°C.     

SiC and ZrN nano-particulate reinforced AlON composites: Preparation,mechanical properties and toughening mechanisms

Due to excellent chemical stability, high rigidity, superior corrosion and wear resistance, aluminum oxynitride (AlON) has been considered as one of most promising candidate ceramic materials in high-performance structural, advanced abrasives and refractory fields. However, it usually exhibited relatively low flexural strength and poor fracture toughness. The study is aimed to develop silicon carbide (SiC) and zirconium nitride (ZrN) nano-particulate reinforced AlON composites with improved mechanical properties and fracture resistance via a hot-press sintering process. It was found that the addition of ZrO₂ nanoparticles would be transformed into ZrN during sintering. Due to the pinning effect of SiC and ZrN nano-particles positioned at grain boundaries of micro-sized AlON particles, the presence of SiC and ZrN nano-particles resulted in the reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in AlON to intragranular cracking in composites. With presence of 8 wt% SiC and 5.2 wt% ZrN nano-particles, the relative density, microhardness, Young’s modulus, flexural strength and fracture toughness increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in flexural strength and fracture toughness.     

Relationships between fracture toughness,Y2O3 fraction and phases content in modern dental Yttria-doped zirconias

The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y₂O₃-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y₂O₃ was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4−5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y₂O₃ concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (K_Ic) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between K_Ic and bulk mol% Y₂O₃ content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between K_Ic and the fraction of the transformable t-phase was established within the range of 30–70 vol%.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.