Search

This paper presents the micro-shot peening of zirconia (ZrO)-advanced ceramics applicable for bio-medical, dental, automotive and aerospace sectors. A ZrO-advanced ceramic was micro-shot blasted with selected parameters as a first-step investigation focused on the topography, microstructure, surface hardness and the surface fracture toughness ( K) characteristics. A new technique of micro-blasting was conducted using a portable shot blaster. A white-light interferometer, scanning electron microscopy and Vickers indentation technique were employed for the analysis. This was followed by determining the K using an empirical equation. Surface roughness was improved by 34 % after micro-shot blasting treatment. No surface cracking was present which generally exists due to the brittle nature of the ceramic. The hardness, however, reduced by 5.6 % with a reduction in the Vickers crack length of 9 %. This improved the K by 3 % when comparing the micro-shot peened surface to the original, as-received surface. It is difficult to conclude if the ZrO-advanced ceramic has undergone plastic deformation and the movement of dislocations increased to strengthen the ZrO ceramic at this stage. However, based on the results, it can be predicted that a level of surface compression was induced beneath the micro-shot peened layer as indicated from the result of the surface topography and integrity. This would justify the hardness modification and the enhancement in K. [ABSTRACT FROM AUTHOR]

In this paper, the densification of a ceramic powder matrix that contained large macropores of the size much larger than the average grain size of the matrix was studied. The grain-boundary diffusion controlled sintering models of ceramic matrix contained large macropores were first established. A sintering potential associated with the macropores was also verified and introduced into the models. The experimental data were collected for the sintering of porous 8 mol% yttria-stabilized zirconia system with 20 vol% large pores. The predictions of the model were found in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

The processing of yttria-stabilised zirconia (Y-ZrO2)-based ceramic nanocomposites by means of pulsed electric current sintering (PECS) is described. A nanometer-sized electrically conductive secondary TiCN phase was added to the insulating zirconia matrix in order to make the composite electrically conductive. The paper focuses on the importance of processing conditions and highlights the benefits of the PECS method as compared to more traditional hot pressing. The mechanical and microstructural properties of the ZrO2–TiCN composites have been determined, and the benefits of using an electrical current to densify these composites were explained in terms of the evolution of the electrical properties of the densifying powder compact. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cells are currently constructed using a yttria stabilised zirconia electrolyte membrane. However, zirconia has a number of disadvantages associated with its use, such as the high operational temperatures required for it to exhibit acceptable levels of ionic conductivity. Alternative ceramics such as doped cerium oxide show promise as electrolytes capable of operating at reduced temperatures, but introduce additional problems such as electronic conduction and inferior mechanical properties. This paper describes the manufacture and characterisation of a number of prototype electrolytes consisting of a mixture of yttria stabilised zirconia and gadolinium doped ceria. Traditional ceramic processing techniques were used to produce the samples, which were then examined using dilatometry, impedance spectroscopy and X-ray diffraction. Results show a lowering of the ionic conductivity of zirconia with the addition of doped ceria. X-ray diffraction patterns obtained from the samples suggested that this effect could be attributed to the formation of a solid solution of ceria in zirconia. [ABSTRACT FROM AUTHOR]

Advanced ceramic materials with complex design have become inseparable from the current engineering applications. Due to the limitation of traditional ceramic processing, ceramic additive manufacturing (AM) which allows high degree of fabrication freedom has gained significant research interest. Among these AM techniques, low-cost robocasting technique is often considered to fabricate complex ceramic components. In this work, aqueous ceramic suspension comprising of commercial nano-sized yttria-stabilized zirconia (YSZ) powder has been developed for robocasting purpose. Both fully and partially stabilized YSZ green bodies with complex morphologies were successfully printed in ambient conditions using relatively low-solid-content ceramic suspensions (<38 vol%). The sintered structures were able to retain the original morphologies with >94% of the theoretical density despite its high linear shrinkage (up to 33%). The microstructure analysis indicated that dense fully and partially stabilized YSZ with grain size as small as 1.40 ± 0.53 and 0.38 ± 0.10 μm can be obtained, respectively. The sintered partially stabilized YSZ solid and porous mesh samples (porosity of macro-pores >45%) exhibited hardness up to 13.29 GPa and flexural strengths up to 242.8 ± 11.4 and 57.3 ± 5.2 MPa, respectively. The aqueous-based ceramic suspension was also demonstrated to be suitable for the fabrication of large YSZ parts with good repeatability. [ABSTRACT FROM AUTHOR]

Porous yttria-stabilized zirconia (ZrO2-8 mol% Y2O3, YSZ) ceramics with ultra-low thermal conductivity (as low as 0.06 W/mK) could be fabricated by tert-butyl alcohol (TBA)-based gel-casting process with low solid loadings of 10 and 15 vol%. High porosity (52–76%) and fine pores with average pore size of 0.7–1.8 μm formed after sintering at 1350–1550 °C. These air-containing pores were believed to affect the through-thickness heat transfer propagation, resulting in low thermal conductivity. The thermal conductivity of porous YSZ ceramics with different porosities fits well with computed values derived from Effective Medium Theory (EMT). [ABSTRACT FROM AUTHOR]

Capability of the recycling of high strength and high fracture toughness yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) sintered body utilizing “low-temperature annealing degradation” phenomenon was investigated. Hydrothermal treatment was employed to induce the phase transformation from tetragonal to monoclinic zirconia and to disintegrate the Y-TZP sintered body. 3 mol% Y2O3–ZrO2 specimens sintered at 1,550 °C and more were disintegrated without leaving the original appearances when the treatment temperature was between 200 °C and 400 °C. The size of the disintegrated fragments of Y-TZP sintered body was much affected by hydrothermal treatment conditions. Only with hydrothermal treatment and simple ball milling, the sintered body was pulverized into the primary particle level. This technique is expected to apply to a sustainable recycling system for the zirconia ceramics, which restrains an energy consumption compared to crushing zirconia using mechanical procedures. [ABSTRACT FROM AUTHOR]

High temperature creep of a metal–ceramic composite consisted of yttria tetragonal zirconia polycrystals (YTZP) with a certain amount of nickel grains (Ni–YTZP, 2.5% vol.) has been studied. The average grain size of YTZP grains was 0.20 μm, whereas that of the Ni grains was 50–70 nm. This work reports the mechanical response under creep of such composite materials. It is found that the activation energy for creep is significantly higher than that reported in similar systems with much larger nickel grains. This fact has been explained in terms of the chemical nature of the metal–ceramic interfaces. [ABSTRACT FROM AUTHOR]

Ceramic monolithic and multilayered composites were produced by stacking water based green ceramic cast tapes. Two different mixtures of composites within the Al2O3/YTZP, system were investigated. The difference in the coefficient of thermal expansion (CTE) between the composites fabricated, resulted in the development of residual stresses when the same compositions were combined in a multilayered structure. The magnitude and the distribution of these residual stresses were assessed by the piezo-spectroscopic technique. The stress within the layers varied with a parabolic trend, with the highest values at the interfaces and a reduction toward the centre of the layers. The influence of symmetry of the multilayered laminate on the magnitude and distribution of the residual stresses within the structure are discussed. [ABSTRACT FROM AUTHOR]

Dense nanocrystalline ZrO2-3 wt% Y2O3 ceramics with grain sizes ranging between 23 to 130 nm were tested by ultrasonic pulse echo and Vickers hardness. The elastic modulus and hardness results were corrected for the residual porosity and the phase content. The corrected elastic moduli exhibited continuous decrease with decrease in the grain size. In contrast no correlation was found between the corrected hardness and grain size. The percolative composite model was used to describe the changes in the elastic moduli in terms of percolation of the elastic wave through the intercrystalline phase at the percolation threshold. The absence of correlation with the hardness results was explained due to the other energy absorbing mechanisms such as microcracking beneath the indenter. [ABSTRACT FROM AUTHOR]

A high performance 500 nozzle in-line piezoelectric drop-on-demand print-head mounted on a linear table with z-axis control was used to prepare arrays of fine ceramic pillars. Such structures may find applications in miniature heat exchangers, catalyst supports, for cell up-regulation in prosthetics or in polymer-ceramic piezoelectric 1–3 composites. The ink conveyed 14 vol% fine zirconia and deposited a wax-based suspension in an octane-alcohol mixture. The dried ink contained 63 vol% zirconia. [ABSTRACT FROM AUTHOR]

ZrO2 and SiC ceramic thin films and their bilayer have been successfully prepared by a newly developed electrostatic atomization technique. This technique can generate fine spray of ceramic suspensions in a micrometer sized range with a narrow size distribution which is crucial for preparation of uniform thin films of these ceramic materials. Compared to some other thin film deposition techniques, such as Chemical vapour deposition (CVD), physical vapour deposition (PVD) and plasma spray (PS) etc. the thin film deposition process using electrostatic atomization is not only cheap but also controllable. The prepared ZrO2 and SiC thin films were investigated using scanning electron microscopy (SEM) and energy dispersion analysis (EDA) techniques. These thin films were observed to be homogenous with a particle size less than 10 μm. The ZrO2-SiC bilayer was found to have an abrupt interface, implying that the deposition process is controllable and also that functionally graded ceramic/ceramic materials can be prepared in this way if the thickness of each layer is accurately controlled. [ABSTRACT FROM AUTHOR]

Effect of YNbO4 addition on the fracture toughness and stress-strain characteristics of ZrO2(3Y) ceramics was investigated using micro-indentation, X-ray diffractometry and in-situ compression-diffraction techniques in the present work. X-ray diffraction patterns show that sintering of YNbO4 and ZrO2(3Y) mixture results in formation of a solid solution, and the tetragonality of ZrO2(3Y) ceramics increases with the content of YNbO4. The ratio of 002t to 200t peak intensity in X-ray diffraction spectra of the specimens changes significantly after a grinding process at low fractions of YNbO4 (less than 5 mol%) and approaches to a constant value as the content of YNbO4 is higher than 5 mol%, implying that the addition of YNbO4 reduces the domain switchability of the ZrO2(3Y) ceramics. Fracture toughness of modified-ZrO2(3Y) specimens with the same heat treatment conditions could be greatly enhanced by an appropriate addition of YNbO4. In addition, stress-strain curves of YNbO4-modified ZrO2(3Y) specimens exhibit an extraordinary elastic behavior. Data of in-situ loading-diffraction experiment show that an unidentified stress-induced transformation occurs and the peak intensity varies with the stress conditions. Analysis of the stress-strain characteristics and X-ray diffraction results suggest that the fracture toughness of the specimens cannot be simply attributed to the t-to-m phase transformation. [ABSTRACT FROM AUTHOR]

In this study, a slip casting method was used for the production of tensile specimens for superplastic deformation from zirconia powders containing 3 and 8 mol% yttria. It was seen that the slip casting process allowed the production of complex, net-shape and economical production of a relatively small number of individual shapes. The factors affecting the casting characteristic of the slip system, namely, solid content, slip viscosity, dispersing agent concentration and moulding conditions were investigated and optimum values for the above were determined. The effect of shaping process on superplastic deformation was also investigated for slip-cast and die-pressed specimens. It was seen that superplastic ductility obtained from slip-cast specimen was higher than that of die-pressed one. The reason for ductility enhancement was due to the homogeneous dispersion of the powder and elimination of agglomerates in slip-cast specimen, compared to die-pressed specimen in which agglomeration caused non-homogeneous sintering leaving crack-like voids responsible for early fracture. [ABSTRACT FROM AUTHOR]

The crystal structure of compositionally homogeneous, nanocrystalline ZrO2 - X mol% Y2O3 ( X = 2.8, 4, 5, 6, 7, 8, 9, 10, 11, and 12) powders synthesized by a nitrate-citrate gel-combustion process has been studied by X-ray diffraction. By applying the Rietveld method, it was found that all the powders presented the tetragonal phase ( P42/ nmc space group). The axial ratio c/ a f decreased with increasing Y2O3 content and became almost unity at 9 mol% Y2O3. However, powders in the compositional range of ZrO2 - 9 to 12 mol% Y2O3 exhibited the oxygen atoms displaced from their ideal sites of the cubic phase along the c axis, which is known as the t′′-form of the tetragonal phase. A conjunct analysis of crystallite size and microstrain of all the powders is also presented. [ABSTRACT FROM AUTHOR]

Indentation fatigue testing is carried out for four kinds of zirconia ceramics; 6Y-FSZ, 3Y-TZP, Mg-PSZ and Ce-TZP; which have different strengthening-toughening mechanisms one another. Each material shows different indentation fatigue behavior. Large sized surface chipping is produced easily in early cycles for non-toughened 6Y-FSZ, while 3Y-TZP toughened by anelasticity has high resistance against chipping. There is a possibility that anelastic characteristic works as resistance against indentation fatigue damage. Stress induced phase transformation also improves fatigue damage, but this effect depends on the extent of transformation and/or occurrence of microcracking. [ABSTRACT FROM AUTHOR]

The zirconia particles in zirconia-containing ceramics have a size distribution similar to Gaussian distribution. The spontaneously martensitic start temperature (Ms) of different particles are not the same. The larger the particle, the higher its Ms. On cooling from high temperature to a value lower than the macro Ms of the materials, the stress-induced and spontaneous transformation will happen, so that the toughness of the materials increases at first, then gradually drops to a value slightly higher than that of the matrix. The size distribution plays an important role in affecting the toughness of the materials. When the average particle size increases, the maximum toughening (ΔKc)max and the temperature (Tmax) at which (ΔKc)max happens will all increase. The toughness at given temperature will increase at first and then drop also to a value slightly higher than that of matrix with increasing of average particle size. The stronger concentration of the size distribution, the higher (ΔKc)max will be. The weaker concentration for size distribution (either the range of zirconia particle sizes becomes wider or the scale parameter of the distribution increases), the lower (ΔKc)max, but the range of temperature in which the toughness is larger than certain value will become wider. Some suggestions of designing ceramics with high toughness at different temperatures are given. [ABSTRACT FROM AUTHOR]

The mechanical strength of ceramic multilayers for use in solid oxide fuel cell stacks was examined. The multilayers consist of a substrate, made of yttria stabilised zirconia (YSZ), with a NiO/YSZ composite coating on both faces. Under some processing conditions the mechanical strength of the multilayers was similar to that of pure YSZ, while a much lower strength was found for other processing conditions. Inspections of the fracture surfaces revealed markedly different appearances, suggesting that a transition in failure mode had occurred. Fracture mechanics models for thin films on substrates were used to interpret the observed fracture modes, providing insight to the underlying failure mechanisms. [ABSTRACT FROM AUTHOR]

Alumina and zirconia ceramics joints were made by diffusion bonding through platinum and palladium gaskets in air at temperatures between 1300 and 1550°C. The instrumentation and the procedure for these joints manufacturing are described. The effects of temperature, pressure and exposition time on 3-pt bending strength of resulting joints were established. The maximum bending strength of the samples reached 500 MPa. During thermal resistance testing the samples withstood several hundreds of thermal cycles at temperatures up to 800°C in air. [ABSTRACT FROM AUTHOR]