Effects of porosity and pore distribution on static strength properties of porous zirconia

Bulk porosity, along with size and spatial distribution of pores, play key roles in strength of porous ceramics, as reported in a study on porous alumina. Hence, a fracture mechanics procedure was proposed to evaluate their strength by presuming that behavior of pore distribution is equivalent to that of crack distribution, and each pore is surrounded by virtual crack. In contrast to alumina, zirconia has distinct spherical‐shaped pores. Moreover, its strength properties vary with stabilizing additives. In this research, strength properties of yttria‐stabilized zirconia ceramics were studied to verify applicability of the procedure proposed for simulating strength of porous ceramics. The effect of pore characteristics on static strength properties was determined experimentally and confirmed by Monte‐Carlo simulations. It was revealed that simulated strength coincided with experimental results within a narrow scatter band, ie, factor of 2^1/2. Therefore, the proposed procedure was found to be appropriate for estimating strength of porous zirconia.     

熔融沉积法3D打印制备氧化锆陶瓷及其力学性能研究

在传统熔融沉积方法的基础上,采用颗粒混合料和螺杆挤出机构3D打印制备了致密和多孔氧化锆陶瓷,系统研究了颗粒原料的打印性能、坯体显微结构特征和陶瓷材料的力学性能。研究结果表明,该方法可以实现倾角达165°和跨度为5.5mm的无支撑结构的打印成型。研究了两种打印路径对致密氧化锆陶瓷抗弯强度及抗弯强度Weibull模数的影响,结果表明与传统单线填充模式相比,"单线+矩形"复合填充模式可以得到更高致密度和更优力学性能的陶瓷(抗弯强度达到637.8 MPa, Weibull模数达到9.10)。研究了不同气孔率多孔氧化锆陶瓷的压缩力学行为,结果表明陶瓷的抗压强度和气孔率之间存在复合指数规律,低气孔率时异面压缩的应力–应变曲线只呈现弹性阶段,高气孔率时出现弹性阶段和坍塌阶段,均未出现密实阶段。     

Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods

Porous alumina–zirconia ceramics were obtained by infiltrating porous alumina ceramics, which were prepared by tert-butyl alcohol (TBA)-based gel-casting method. Back scattering images of the fracture surface and energy dispersive spectroscopy were performed to obtain composition profiles on the fracture surface and across sections of the sintered composites. The porosity, pore size distribution and compressive strength were also investigated. The results show that the content of zirconia can be adjusted effectively by infiltration times and it decreases with increasing distance from the surface of the samples. The porosity and compressive strength can also be controlled by the infiltration times. With increases of the infiltration times from 1 to 3 cycles, the open porosity decreases slightly from 62.43% to 56.62%, while the compressive strength of the porous alumina–zirconia ceramics increases from 13.57 ± 1.21 to 26.87 ± 2.01 MPa, indicating that the porous ceramics with high porosity and high strength can be prepared by TBA-based gel-casting method combined with the infiltration process.     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al₂O₃ and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al₂O₃ content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al₂O₃, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.     

The porosity dependence of flexural modulus and strength for capsule-free hot isostatically pressed porous alumina

Structural properties such as flexural moduli and strength have been measured for a range of porous alumina specimens of different initial powder sizes and final porosities, sintered using the capsule-free hot isostatic pressing method. This processing method produces a porous body in which the closed porosity is negligible. The relationship of these structural properties to total porosity has been investigated. The results indicate that both a power and an exponential function could adequately describe the porosity dependence of flexural strength. The strength values obtained were test method dependent, and were significantly lower for specimens with sintering aids. A power law model based on a critical porosity, as proposed by Phani, gave the best fit for the modulus measurement data. No dependence of mechanical properties on particle size was observed. The strength measurement results did not appear to support suggestions that better strength could be obtained by the capsule-free hot isostatic pressing method than conventional sintering, as reported elsewhere.     

Effect of YSZ fiber addition on microstructure and properties of porous YSZ ceramics

Yttria-stabilized zirconia (YSZ) fiber was introduced as the reinforcement for porous YSZ ceramics fabricated by tert-butyl alcohol-based gel-casting process and pressureless sintering. Effect of YSZ fiber addition on the microstructure and properties of porous YSZ ceramics was studied systematically. Results showed that YSZ fiber obviously obstructed densification during the sintering process and therefore higher porosity could be achieved with the same solid loading of the initial slurry. Mean pore size regularly increased with increasing fiber addition. The reinforcing effect reached its optimum with 10 wt% YSZ fiber addition, yet decreased with increasing porosity. Fiber addition significantly changed the fracture mode of the porous ceramics from brittle to quasi-ductile with increasing fiber additions; fiber pull-out and crack deflection play major roles in the process. Compared with the porous ceramics without fibers, the thermal conductivity decreased a little. With improved mechanical and thermal properties, YSZ fiber-reinforced porous YSZ ceramics are more applicable in thermal insulation materials.     

Preparation of zirconium pyrophosphate bonded silicon nitride porous ceramics

Abstract

A new method for preparing high bending strength porous silicon nitride ceramics with controlled porosity was developed using a pressureless sintering technique, using zirconium pyrophosphate as a binder. The fabrication process was described in detail and the sintering mechanism of porous ceramics was analysed by an X-ray diffraction method. The microstructure and mechanical properties of the porous Si₃N₄ ceramics were investigated, as a function of the content of ZrP₂O₇. The resultant porous silicon nitride ceramics sintered at low temperature (1000 and 1100°C) showed fine micropore structure and a high bending strength. Porous silicon nitride ceramics with porosity of 34–47%, a bending strength of 40–114 MPa and a Young's modulus of 20–50 GPa were obtained.     

Effect of grain size distribution on the strength of porous Si3N4 ceramics composed of elongated β-Si3N4 grains

The grain size distributions (diameter and aspect ratio) of porous Si₃N₄ ceramics composed of elongated -Si₃N₄ grains were evaluated statistically, and their effect on the pore size distribution and the flexural strength of the porous Si₃N₄ was investigated. Porous Si₃N₄ ceramics having porosities of 27 to 43% and median pore diameters of 0.56 to 0.96 m were used as specimens. The grain diameter distribution was well correlated to the pore size distribution of the porous Si₃N₄ ceramics. We concluded that the strength of the porous Si₃N₄ ceramics increased with increasing grain length of -Si₃N₄ as well as with decreasing porosity.     

RETRACTED ARTICLE: Characteristics of porous zirconia coated with hydroxyapatite as human bones

Since hydroxyapatite has excellent biocompatibility and bone bonding ability, porous hydroxyapatite ceramics have been intensively studied. However, porous hydroxyapatite bodies are mechanically weak and brittle, which makes shaping and implantation difficult. One way to solve this problem is to introduce a strong porous network onto which hydroxyapatite coating is applied. In this study, porous zirconia and alumina-added zirconia ceramics were prepared by ceramic slurry infiltration of expanded polystyrene bead compacts, followed by firing at 1500°C. Then slurry of hydroxyapatite-borosilicate glass mixed powder was used to coat the porous ceramics, followed by firing at 1200°C. The porous structures without the coating had high porosities of 51–69%, high pore interconnectivity, and sufficiently large pore window sizes (300–500 μm). The porous ceramics had compressive strengths of 5·3∼36·8 MPa, favourably comparable to the mechanical properties of cancellous bones. In addition, porous hydroxyapatite surface was formed on the top of the composite coating, whereas a borosilicate glass layer was found on the interface. Thus, porous zirconia-based ceramics were modified with a bioactive composite coating for biomedical applications.     

Effect of particle size on microstructure and strength of porous spinel ceramics prepared by pore-forming <Emphasis Type="Italic">in situ</Emphasis> technique

The porous spinel ceramics were prepared from magnesite and bauxite by the pore-forming in situ technique. The characterization of porous spinel ceramics was determined by X-ray diffractometer (XRD), scanning electron microscopy(SEM), mercury porosimetry measurement etc and the effects of particle size on microstructure and strength were investigated. It was found that particle size affects strongly on the microstructure and strength. With decreasing particle size, the pore size distribution occurs from multi-peak mode to bi-peak mode, and lastly to mono-peak mode; the porosity decreases but strength increases. The most apposite mode is the specimens from the grinded powder with a particle size of 6·53 μm, which has a high apparent porosity (40%), a high compressive strength (75·6 MPa), a small average pore size (2·53 μm) and a homogeneous pore size distribution.     

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity

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).     

Properties of porous Si3N4 ceramic electromagnetic wave transparent materials prepared by technique combining freeze drying and oxidation sintering

A simple and low-cost technique combining freeze drying and oxidation sintering is explored to prepare Si₃N₄ ceramics with high porosity and complex shape. The effects of sintering temperature and time on the phase composition, microstructure, porosity, pore size and dielectric constant of the porous Si₃N₄ ceramics are studied. Due to the variations of phase composition and microstructure, the porous Si₃N₄ ceramics sintered at different temperature possess characteristic in flexural strength. The porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h have the highest flexural strength of 71–74 MPa. The changes of porosity and composition have much effect on the dielectric constant of porous Si₃N₄ ceramics. Because of the high porosity and SiO₂ volume fraction, the porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h possess low dielectric constant of 3.4–3.6 and small pore size of 0.9 μm. The porous Si₃N₄ ceramics are good structural/functional and promising electromagnetic wave transparent material.     

Hydroxyapatite coating on porous zirconia

Since hydroxyapatite has excellent biocompatibility and bone bonding ability, porous hydroxyapatite ceramics have been intensively studied. However, porous hydroxyapatite bodies are mechanically weak and brittle, which makes shaping and implantation difficult. One way to solve this problem is to introduce a strong porous network onto which hydroxyapatite coating is applied. In this study, porous zirconia and alumina-added zirconia ceramics were prepared by ceramic slurry infiltration of expanded polystyrene bead compacts, followed by firing at 1500 °C. Then a slurry of hydroxyapatite–borosilicate glass mixed powder was used to coat the porous ceramics, followed by firing at 1200 °C. The porous structures without the coating had high porosities of 51% to 69%, a high pore interconnectivity, and sufficiently large pore window sizes (300 μm–500 μm). The porous ceramics had compressive strengths of 5.3˜36.8 MPa and Young's moduli of 0.30˜2.25 GPa, favorably comparable to the mechanical properties of cancellous bones. In addition, porous hydroxyapatite surface was formed on the top of the composite coating, whereas a borosilicate glass layer was found on the interface. Thus, porous zirconia-based ceramics were modified with a bioactive composite coating for biomedical applications.     

Effects of rheological properties on ice-templated porous hydroxyapatite ceramics

Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.     

Mechanical behavior of alumina and alumina-feldspar based ceramics in an acetic acid (4%) environment

This study investigates the mechanical properties of alumina-feldspar based ceramics when exposed to an aggressive environment (acetic acid 4%). Alumina ceramics containing different concentrations of feldspar (0%, 1%, 5%, 10%, or 40%) were sintered at either 1300, 1600, or 1700 °C. Flaws (of width 0%, 30%, or 50%) were introduced into the specimens using a saw. Half of these ceramic bodies were exposed to acetic acid. Their flexural strength, K_IC, and porosity were measured and the fractured samples were evaluated using scanning electronic- and optical microscopy. It was found that in the ceramic bodies sintered at 1600 °C, feldspar content up to 10% improved flexural strength and K_IC, and reduced porosities. Generally, it was found that acetic acid had a weakening effect on the flexural strength of samples sintered at 1700 °C but a beneficial effect on K_IC of ceramics sintered at 1600 °C. It was concluded that alumina-based ceramics with feldspar content up to 10% and sintered at higher temperatures would perform better in an aggressive environment similar to oral cavity.     

Experimental Investigation on Steel Foams Fabricated by Sintering-Dissolution Process

This article describes a new process to manufacture open-cell steel foams. Calcium chloride anhydrous is used as a space holder. By changing the values of the main manufacturing parameters such as volume percentage, and the size and shape of the space holder, we produce different steel foam samples which cover a wide range of solid fraction, pore size, and shape. The effects of space-holder content and sintering condition such as temperature and time on the porosity of steel foam samples are discussed. The microstructure and composition of steel foam samples are observed and analyzed by scanning electron microscope and X-ray diffraction. The compressive curves of steel foams are measured by a universal testing machine. The experiment results show the compressive strength of steel foam samples with porosities between 65% and 85% is in the range of 66.4 ～ 12.9 MPa. The compressive strength depends mainly on the porosity and pore shape. The absorbed energy per unit volume (W) of steel foams with porosities between 85% and 65% is in range of 6.8 ～ 31.2 MJ/m³. Under the condition of identical porosity, the absorbed energy per unit volume (W) of steel foam is about three times of aluminum foam. In compression, steel foam specimens show heterogeneous macroscopic deformation.     

Effect of porosity on the crack pattern and residual strength of ceramics after quenching

The effect of porosity on the crack characteristics of ceramics after water-quenching is studied by measuring the cracks in ceramic sheets. The result reveals that the pore volume fraction has a slight effect on the enhancement of thermal shock resistance of ceramics when the porosity ranges from 0 to 20 %, because the length and density of the long crack in porous alumina are always slightly less than that in dense alumina. This result is in agreement with the prediction based on the minimum potential energy principle using experimentally measured data. Moreover, the proportion of the strength reduction in the third regime decreases significantly with increasing porosity, because the strength of unquenched specimens decreases more rapidly than that of quenched specimens with increasing porosity. The results of this study may help to further understand the thermal shock behavior of ceramics.     

Dielectric properties in GHz range of porous Si3N4–BN–SiO2 ceramics with considerable flexural strength prepared by low temperature sintering in air

Abstract

Porous Si₃N₄–BN–SiO₂ ceramics with ultimate apparent porosities between 0·140 and 0·799 were fabricated in air at 1100°C by partial sintering using core starch as both consolidator and pore former in the green bodies. The pores were derived from burning off the starch, the partial oxidation of silicon nitride and the stack of particles of the start materials. Effect of retaining time on the microstructure of sintering bodies was analysed by SEM analysis. Reference intensity ratio (RIR) technique based on the X-ray diffractometry results demonstrated the phase components content of sintered bodies. Influence of porosity on the flexural strength of porous Si₃N₄–BN–SiO₂ ceramics was investigated. The ceramic with a porosity of 0·140 attained a maximal flexural strength of 60±4·11 MPa. In addition, the dielectric constants and loss tangents were presented for porous Si₃N₄–BN–SiO₂ triphase ceramics in the frequency range of 18–40 GHz, and the real part of dielectric constant of the materials reached as low as 2·67 at the porosity of 0·732 at a frequency of 20 GHz.     

Mechanical and microstructural properties of cordierite-bonded porous SiC ceramics processed by infiltration technique using various pore formers

Cordierite-bonded porous SiC ceramics were prepared by air sintering of cordierite sol infiltrated porous powder compacts of SiC with graphite and polymer microbeads as pore-forming agents. The effect of sintering temperature, type of pore former and its morphology on microstructure, mechanical strength, phase composition, porosity and pore size distribution pattern of porous SiC ceramics were investigated. Depending on type and size of pore former, the average pore diameter, porosities and flexural strength of the final ceramics sintered at 1400 °C varied in the range of ~ 7.6 to 10.1 µm, 34–49 vol% and 34–15 MPa, respectively. The strength–porosity relationship was explained by the minimum solid area (MSA) model. After mechanical stress was applied to the porous SiC ceramics, microstructures of fracture surface appeared without affecting dense struts of thickness ~ 2 to 10 µm showing restriction in crack propagation through interfacial zone of SiC particles. The effect of corrosion on oxide bond phases was investigated in strong acid and basic salt medium at 90 °C. The residual mechanical strength, SEM micrographs and EDX analyses were conducted on the corroded samples and explained the corrosion mechanisms.