Failure mechanisms of calcium magnesium aluminum silicate affected thermal barrier coatings

Depth profiles of the phase composition of two examples of calcium magnesium aluminum silicate (CMAS) affected thermal barrier coatings (TBCs) from an aero gas turbine engine were obtained using a monochromatic and collimated beam of synchrotron radiation. One TBC was deposited by plasma spray and the other by electron beam physical vapor deposition. These examples were complemented with an X‐ray diffraction (XRD) study of mixtures of TBC zirconia powder and sand heated in a furnace. The XRD results were compared with electron backscatter images and energy dispersive spectroscopy studies of the cross sections and mixtures. It was found that when liquid, the CMAS enhances mass transport leading to the densification of the zirconia, which then leads to spalling because of the increased residual stresses generated on cooling. Even without spalling densification will reduce a TBC's ability to thermally insulate. The enhanced mass transport can also lead to destabilization of the zirconia if yttrium ions preferentially transfer to the liquid or greater stabilization if calcium or magnesium ions transfer from the liquid to the zirconia. Zircon also precipitates when the zirconium from the TBC reacts with the silicon in the liquid CMAS.     

Element leachability from a coal stockpile and associated coastal sand deposits

The mobilities of major and trace elements in stockpiled sub-bituminous coal and the coastal sand on which the stockpile has been built have been evaluated by batch leaching tests using demineralised water and controlled-pH (acidic) solutions. The results were compared with those from column leaching tests on the coal with similar liquids, and with a column test in which the coal was placed above a bed of the coastal sand material.Batch testing showed that most trace elements, especially As, Cr, Cu, Mo, Pb, Sb, Sr, V and Zn, were released in higher concentrations from the sand than from the coal. Some elements were released in greater concentrations from the coal (although values were still low), while a few were released in low but approximately equal concentrations from both materials. Comparison of the column and batch test results suggests that most of the elements in the leachate from the column in which coal was underlain by sand were derived from the sand component. The column tests further indicate that concentrations of many elements mobilised from the coal are likely to be reduced by interactions associated with percolation through the sand bed.     

耐碱玻璃纤维原料用锆英砂中二氧化锆（铪）含量测定方法的探讨

探讨了EDTA快速测定锆英砂中二氧化锆（铪）含量的方法,对试验酸度、滴定温度和Fe3＋对测定的影响进行了研究,着重选用简便的标定物Zr标准溶液对操作步骤进行简化,通过4种锆英砂样品进行方法验证以及加标回收试验,结果表明, Zr标准物质标定结果与基准试剂标定结果吻合,二氧化锆（铪）测定结果与重量法测的值比对绝对差均不大于0．18%,加标回收率达到98%以上。     

Effect of dopants on the properties of metal-doped zirconia prepared by the glycothermal method

The effects of dopants on zirconia prepared via the glycothermal method were investigated by XRD to determine the crystal structure and crystallite size. Morphologies of products were observed by SEM. The basic sites of zirconia were studied by CO₂-temperature programmed desorption (CO₂-TPD). The functional group in the samples was determined using IR. The intensity of Zr³⁺, characterized by ESR, could be described as the oxygen coordinatively unsaturated Zr sites. The results suggest that doping elements can modify the surface chemistry of ZrO₂ to form hydroxyl groups and surface energies depending on the structure (cubic, tetragonal) in different dense phase. ESR peaks of Pb- and Bi-doped zirconia are different from the others, which showed high intensity of Zr³⁺.     

玻璃窑炉池底电熔锆刚玉铺面砖的质量改善

通过砂型在保温箱内的摆放形式,详细介绍改善前后浇铸成型及保温退火等过程,使制品的体积密度、成品率、生产成本等大大改善。同时对玻璃生产运行的安全性及延长窑炉的使用寿命都有了可靠保证。     

Making heating elements on the basis of zirconium dioxide for high-temperature furnaces

Conclusions A technology was developed and resistor elements designed on the basis of zirconium dioxide for operation in air at up to 2100°C.We also developed a design for test rigs which include a furnace for individual testing of the heating elements and a furnace with three zirconia elements, and also an electric operating circuit with automatic control with respect to the working current of the heating elements.The results of tests show that in the working region of the furnace with three zirconia elements it is possible to carry out heating in an air atmosphere up to 1950°C, with a surface temperature on the hightemperature part of the heating elements of 2050–2100°C. One of the essential conditions for operating such furnaces is individual current regulation of each element.The furnace of the construction described can be used for laboratory testing with the single use of the heating elements, maintaining them at the maximum temperature for a period of several tenths of an hour.Translated from Ogneupory, No. 1, pp. 36–41, January, 1971.     

Synthesis of high-purity zircon,zirconia, and silica nanopowders from local zircon sand

High-purity zircon (ZrSiO₄) nanopowder was successfully produced from Indonesian natural zircon sand using a low-cost purification approach via magnetic separation, immersion in HCl, and reaction with NaOH, followed by a top-down nanosizing process using wet ball-milling for 10?h and annealing at 200?°C for 2?h. Furthermore, polymorph zirconia (ZrO₂ – amorphous, tetragonal, and monoclinic) and silica (SiO₂ – amorphous and cristobalite) nanopowders were also successfully derived from the purified zircon powder using a bottom-up method via alkali fusion and co-precipitation processes followed by calcination. The crystallite size of the powders was estimated from X-ray diffraction (XRD) data analysis to give 40, 31, 61, and 149?nm, respectively, for the zircon, tetragonal- and monoclinic-zirconia, and cristobalite. Microstructural characteristics of the zircon, silica, and zirconia nanopowders were revealed in transmission electron microscopy (TEM) images which confirmed that the average sizes of the particles were in a good agreement with the XRD estimated values.     

Annealing temperature influence on sol-gel processed zirconium oxide thin films for electronic applications

A study of zirconium oxide (ZrO₂) thin films obtained by the non-alkoxide sol-gel method at different annealing temperatures, up to 450?°C, is presented. Morphological, compositional, and optical characterizations of zirconia thin films show high transparency and high bandgap, besides homogeneous and non-porous surface. Metal-insulating-metal (MIM) devices were assembled from this zirconia material for electrical characterizations and have shown high electric resistivity and high specific capacitance. A study of the thin film composition shows residues of S and Cl elements from the precursor solution that contributes for reduction of the dielectric constant of the zirconia thin films, even though they still present higher values when compared to SiO₂, which is a positive alternative to replace this oxide in electronic devices. A parallel study of MIM assembled on polymeric substrate and annealed at 100?°C also leads to positive results concerning high electrical insulating and capacitance. This study aims the understanding of the relations between annealing temperature and impurities found in sol-gel based thin films, as well as their relations to dielectric characteristics of zirconia thin films that impact the final properties of electronic devices, such as in field effect transistors.     

Modelling of elastic modulus of a biphasic ceramic microstructure using 3D representative volume elements

In this work a methodology to reconstruct three-dimensional microstructures, representative of real biphasic ceramics using Neper free software is proposed. Finite element analysis in Ansys was implemented in order to calculate the effective elastic modulus of the simulated microstructures.Fine grained and dense zirconia toughened alumina (ZTA) materials with 5 and 40 vol.% of Yttria Stabilized Zirconia (YTZP) have been chosen to validate the proposed methodology. First, the effects of the size of the representative volume elements (RVEs) and the characteristics of the grain shapes are analysed. Second, the compliance with the isotropic condition is also verified.Agreement between the numerical and experimental values of the elastic modulus of the considered ZTA materials has been found. For these materials, zirconia fractions higher than 10 vol.% lead to bi-continuous microstructures which make the elastic properties deviate from the Voigt limit due to the increased number of contacts between zirconia grains.     

氧化锆、不锈钢复合粉体浆料流变性的研究

初步探索金属-陶瓷复合粉体浆料的粘度与浆料的pH值、分散剂含量、固体含量之间的关系,制备出高固体含量(体积分数为40%)、低粘度(0.32Pa·s)的氧化锆、不锈钢复合粉体浆料.     

Reactive wetting and interfacial characterization of ZrO2 by SnAgCu-Ti alloy

The reactive wetting behavior of zirconia with SnAgCu-x%Ti (SAC-x%Ti, wt%, x?=?1, 4) alloy was investigated via the sessile drop method in isothermal experiments. As temperatures elevated, the final contact angle decreased and the minimum contact angle of 21° and 7° were obtained at 1000?°C for SAC-1%Ti and SAC-4%Ti droplets, respectively. Kinetic calculations indicated that the spreading of SAC-Ti droplets on zirconia was controlled by interfacial reaction and the wetting activation energy was 108.8?kJ/mol. The reaction products distribution and morphology in droplets were influenced vastly by the addition of Ti. Along with the increase of Ti content from 1% to 4%, a great deal of Ti-Sn intermetallic compounds (IMCs) were generated in droplets, thereby the outline of droplets were transformed from hemispherical into similar trapezoidal due to the limited spreading and fluidity of droplets. Owing to the interfacial reaction between active elements Ti and zirconia and the subsequent formation of the Ti-O layer, the wettability of SAC-Ti/zirconia was greatly promoted. According to transmission electron microscopy (TEM) analysis, the thin Ti-O reaction layer consisted of the Ti₂O, Ti₄O₇, Ti₇O₁₃ and TiO₂ phase.     

Thermal Shock Resistance of Zirconia with 15 Mole % Titanium

The thermal shock resistance of zirconia with 15 mole % titanium prepared either by cold-pressing and vacuum-sintering or by vacuum hot-pressing was determined by radially quenching disks with thermally insulated faces under various conditions of heat transfer. The thermal shock resistance of calcia-stabilized zirconia disks was determined for comparison. For quenches from below the transformation temperature range of zirconia, the thermal shock resistance of zirconia with 15 mole % titanium was much better than that of calcia-stabilized zirconia, but for quenches from above the transformation range it was slightly inferior. The thermal shock resistance of zirconia with 15 mole % titanium is fairly insensitive to the methods of manufacture used in this investigation. In an attempt to identify the physical properties responsible for the improved thermal shock resistance of zirconia with 15 mole % titanium, the heat capacity, thermal expansion, modulus of rupture, modulus of elasticity, and thermal conductivity as functions of temperature were determined.     

The Effect of Zirconia Reinforcing Agents on the Microstructure and Mechanical Properties of Hydroxyapatite-Based Nanocomposites

A hydrothermally treated zirconia colloid was introduced during the precipitation of hydroxyapatite (HAP) to achieve an HAP–zirconia nanocomposite. High dispersion of zirconia was achieved at zirconia loadings below 8 wt%. Vickers hardness was optimized with a 1.5 wt% loading of zirconia. A low loading (1.5 wt%) of tetragonal zirconia was able to increase the bending strength of nanocrystalline HAP from 183 to 243 MPa. This HAP–zirconia nanocomposite was sintered to full density by 1000°C under an applied load of 50 MPa. The average grain sizes of HAP and zirconia were maintained at ≤100 nm. With the high strength and low loading of secondary phase, these HAP-based nanocomposites should be attractive for orthopedic and dental implant applications.     

Laboratory studies of manufacture of zirconia parts

Conclusions When a number of zirconia mixtures for the manufacture of parts were tested, the best results were shown by those made from mixtures containing 45 – 50% fraction 1.5 – 0. 5 mm and 55 – 60% finer than 0. 088 mm.The addition of 5 to 10% clay as a binder to the zirconia mixture considerably improves the pressing operation, improves the density of the parts after drying and firing and makes it possible to reduce the firing temperature.The fine fractions of fired zirconia can be partially replaced by crude zirconia.Small size parts can be made from the crude zirconia.     

Effects of sandblasting distance and angles on resin cement bonding to zirconia and titanium

ObjectivesThe aim of this study was to evaluate effects of sandblasting distance and angles resin to zirconia and titanium bonding.MethodsDensely sintered zirconia and cp2 titanium specimens were prepared and randomly divided into groups, and then sandblasted with various distance (5 mm, 10 mm and 15 mm) and angles (45°, 60°, 75° and 90°). After surface treatment, each specimen surface underwent a silane primer application (RelyX, 3M ESPE), followed by bonding of a resin cement (RelyX Unicem Aplicap, 3M ESPE). Then, each cylindrical resin stub (diameter 3.6 mm×2 mm) underwent a shear adhesive (bond) strength test and surface roughness evaluation. SEM evaluation and EDX analysis were used to observe surface properties of both zirconia and titanium samples. Results were statistically analyzed using analysis of variance (ANOVA) and Turkey test (α=0.05).ResultsSurface roughness showed a significant difference amongst the different distances and angles for both the zirconia and titanium materials and these changes in surface roughness were evident in the SEM imaging photos. As for the adhesive strength, there was a significant difference in the adhesive strength for the titanium and zirconia with different angles. In general, 75° gives the best results although this is not significantly different from 90°. However, no significant difference was observed in changes of sandblasting distance for both materials. EDX analysis at the surface revealed elements carbon, oxygen, silicon, aluminum, and zirconia on the surface.ConclusionsSandblasting at various distance and angles contributes differences in surface roughness when it comes to both zirconia and titanium materials. Despite both 75° or 90° sandblasting angle could yield a sufficiently high adhesive strength for resin to titanium or zirconia bonding, sandblasting at 75° seems to be optimal to increase the adhesive strength.     

Electrospinning Zirconia Fiber From a Suspension

A zirconia suspension containing 5–10 nm size zirconia particles was modified by adding different amounts of polymer solution to enable electrospinning of zirconia fibers from a range of compositions. The electrospun fibers were heat treated at 600° and 1200°C, and analysis of size distribution reveals that zirconia fibers down to about 200 nm in diameter can be prepared in this way, in contrast to other spinning processes, which are able to produce zirconia fibers having diameters ≥3000 nm.     

Destabilization of Zirconia Thermal Barriers in the Presence of V2O5

Impurities, such as vanadium, degrade the operating performance of yttria-stabilized zirconia thermal-barrier coatings. V₂O₅ reacts preferentially with Y₂O₃, forms YVO₄, and leads to the destabilization of zirconia thermal-barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria-stabilized zirconia to V₂O₅ vapor. The growth of YVO₄ from yttria-stabilized zirconia and the destabilization of cubic yttria-stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected-area diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO₄ and cubic zirconia is discussed.     

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.     

Testing zirconia dinas bricks in arc furnaces

Conclusions Throughout the campaign of 5 ton electric furnaces dinas zirconia brick in the arches stands up better than ordinary dinas. The residual length of dinas zirconia bricks ranges from 140 to 220 mm and that of dinas from 70 to 150 mm. Dinas zirconia wears by an average 1.5 times less than dinas bricks set in less exposed areas of the arch.The production of a large batch of dinas zirconia should be undertaken for large-scale testing in electric arc furnaces instead of dinas.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.