Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics

The structural and morphological evolutions of cordierite-based ceramics produced from stevensite/andalusite mixture sintered from 1150 to 1350 °C were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior was investigated by three-point bending and Brazilian tests. The elastic properties were evaluated using ultrasonic non-destructive testing (NDT). XRD results revealed that the main crystalline phase formed at 1300 and 1350 °C was cordierite with traces of mullite. A linear-elastic behavior followed by brittle fracture was observed in three-point bending test with the presence of multiple discontinuities. Flexural and diametral compression strength values of cordierite sintered at 1300 °C were 39.4±4 and 21.8±2 MPa, respectively. The elastic properties such as Young's modulus (38.7–45.1 GPa), shear modulus (17.90–19.42 GPa) and Poisson ratio (0.08–1.6) of cordierite-based ceramics produced at 1350 and 1300 °C were also determined.     

Mechanical properties of cellular ceramics obtained by gel casting: Characterization and modeling

Dense and cellular ceramics were produced from yttria partially stabilized zirconia powders by gel-casting, using agar as a gelling agent and polyethylene spheres (125–300 μm diameter) as volatile pore forming agent to create 50–65 vol.% spherical macropores, uniformly distributed in a microporous matrix.The mechanical properties of both dense and porous samples were investigated at the microscale by nanoindentation testing. The influence of micro-porosity on the mechanical properties of samples was evaluated by the analysis of hardness and modulus depth profiles, coupled with FIB-SEM section observations of selected indentation marks. The intrinsic elastic modulus of the zirconia phase resulted to be of the order of 220 GPa. Mechanical characterization at the macroscale consisted of uniaxial compression tests and four point bending tests. Elastic moduli of about 170 GPa were measured for about 93% dense ceramics, lowering down to 44 and 13 GPa with the addition 50 and 65 vol.% macropores, respectively. Digital image based finite element analysis (DIB-FEA) procedures were implemented in order to verify their applicability for the prediction of mechanical behavior of this type of cellular materials: results confirmed that a very good match between measured and calculated values of elastic modulus can be achieved, provided that the effects of micro-porosity are considered by the proper choice of the elastic properties to be assigned to each individual phase identified by Image Analysis.     

On the nonlinear mechanical behavior of macroporous cellular ceramics under bending

This work presents the results of an experimental work on the mechanical behavior under bending of Si–SiC reticulated foams. Particularly it is here studied a transient regime after their linear elastic behavior and before their catastrophic failure. During this regime part of the mechanical work is dissipated by local struts failure. This behavior is interesting because, if well understood, could give important information on the prediction of reticulated ceramics’ life time. Besides load cell and deflection readings, acoustical emission and electrical resistance were also recorded during the tests. Data were further processed using Weibull statistics.Resultsevidence that the duration and effects of this regime strongly depend on ligaments’ microstructure and thus on the manufacturing technique. A correlation with the average acoustical events per sample and the Weibull modulus is also evidenced.     

多孔氧化铝陶瓷的制备、表征及性能研究

以蔗糖溶液为低温介质,采用冷冻干燥法和退火工艺制备多孔Al2O3陶瓷。研究了烧结温度和退火时间对多孔陶瓷孔隙结构、开孔率和力学性能的影响。结果表明,随着烧结温度的升高,试样的线性烧成收缩率有明显的升高,开孔率和维氏硬度先缓慢降低,当烧结温度为1600℃时迅速下降。平均晶粒尺寸是影响甚至决定多孔氧化铝陶瓷维氏硬度的主要原因。随着退火时间的延长,多孔陶瓷的孔径显著增大,多孔陶瓷开孔率范围为40.35%～64.58%,退火处理后的孔隙率比未退火处理提高了60.05%。多孔陶瓷的抗压强度随退火时间的延长而降低,而在最长的24h退火时间后,多孔陶瓷的抗压强度仍能达到25.9MPa,可以满足许多应用领域的强度要求。可以通过调节退火时间来控制多孔陶瓷的孔隙结构、开孔率和抗压强度。     

多孔陶瓷材料的制备及性能研究

从分析网状结构多孔陶瓷材料的孔隙成形机理着手，描述了高孔隙网状结构陶瓷材料的制备工艺，包括高孔隙纤维网状结构陶瓷材料的制各，并分析了多孔陶瓷的力学性能及渗透性能。     

Effects of sintering behavior on microstructure and piezoelectric properties of porous PZT ceramics

Porous lead zirconate titanate (PZT) ceramics with interconnected pores were fabricated by using a novel tert-butyl alcohol (TBA)-based gel-casting method. The resultant samples were sintered at different temperatures and subsequently characterized in terms of both microstructure and piezoelectric properties to study effects of sintering behavior. Both microstructure and piezoelectric properties exhibited obvious dependence on sintering temperature. It was noted that porosity and grain size played dominant roles in determining the magnitudes of dielectric constant (?) and piezoelectric constant d₃₃, respectively. With the increase of sintering temperature, the porosity declined and the grain size increased which exerted opposite influences on piezoelectric properties mainly via ? and d₃₃. Since anti-interference ability of the ceramic correlates linearly with ?, it is possible to adjust sintering temperature to tailor porosity and pore morphology in order to achieve optimum piezoelectric properties as well as relatively high anti-interference ability.     

The modified Mori-Tanaka scheme for the prediction of the effective elastic properties of highly porous ceramics

In this paper, two modifications are proposed to be applied to the well-known Mori-Tanaka (MT) scheme to improve its performance in the estimation of the mechanical properties of highly porous ceramic structures containing complicated agglomerates of merged and open-cell spherical pores of different radii. In the first modification, the effect of the merged pores is considered by estimating their number with the theory of geometrical probabilities and treating them as corresponding ellipsoids of the same volume. In the second modification, porous structures containing open pores are treated as a damaged material with reduced load-carrying capacity and the formulations are modified to consider the effect of the open pores. In order to investigate the reliability of the analytical estimations, different groups of artificial porous structures with porosity values ranging from 10% to 90% are constructed by random positioning of the spherical voids of different radii in a representative volume element (RVE) and their effective elastic properties are obtained by means of the finite element method (FEM). For each level of porosity, a total of 30 random structures are examined to assess the variations caused by the statistical nature of the microstructure. Comparison between the findings of the statistical FEM and the analytical results show that the proposed modifications considerably increase the precision of the MT scheme in the estimation of the effective elastic moduli of highly porous materials. Furthermore, unlike the classical MT method, the modified formulations are capable of demonstrating of the probable anisotropy in the effective elastic properties of the porous structures. Good agreement is also observed between the results obtained from the developed formulations and published numerical and experimental observations for ceramic structures.     

碳化硅-堇青石多孔陶瓷的制备及其性能

以碳化硅粉末、高岭土和滑石等为原料,按堇青石的化学计量比设计原料配比,制备了堇青石理论生成量分别为0、10%、15%、20%、100%的碳化硅-堇青石多孔陶瓷,测定了试样的抗折强度、显气孔率和热膨胀系数,并分别用XRD和SEM分析了试样的晶相组成和断面形貌。结果表明:与碳化硅多孔陶瓷相比,碳化硅-堇青石多孔陶瓷的抗折强度显著提高,热膨胀系数明显降低,但显气孔率有所降低。SEM分析结果表明:碳化硅-堇青石多孔陶瓷中碳化硅颗粒排列较紧密,断面呈“网格状”结构;而在多孔碳化硅陶瓷中,晶粒形貌清晰且排列较疏松,气孔平均孔径较大。     

Relationship between elastic and mechanical properties of dental ceramics and their index of brittleness

The purpose of the study was to verify the effects of a number of materials’ parameters (crystalline content; Young's modulus, E; biaxial flexure strength, σ_i; Vickers hardness, VH; fracture toughness, K_Ic; fracture surface energy, γ_f; and index of brittleness, B) on the brittleness of dental ceramics. Five commercial dental ceramics with different contents of glass phase and crystalline particles were studied: a vitreous porcelain (VM7/V), a porcelain with 16 vol% leucite particles (d.Sign/D), a glass-ceramic with 29 vol% leucite particles (Empress/E1), a glass-ceramic with 58 vol% lithium-disilicate needle-like particles (Empress 2/E2), and a glass-infiltrated alumina composite with 65 vol% crystals (In-Ceram Alumina/IC). Discs were constructed according to manufacturers’ instructions, ground and polished to final dimensions (12 mm × 1.1 mm). Elastic constants were determined by ultrasonic pulse-echo method. σ_i was determined by piston-on-3-balls method in inert condition. VH was determined using 19.6 N load and K_Ic was determined by indentation strength method. γ_f was calculated from the Griffith–Irwin relation and B by the ratio of HV to K_Ic. IC and E2 showed higher values of σ_i, E, K_Ic and γ_f, and lower values of B compared to leucite-based glass-ceramic and porcelains. Positive correlations were observed for σ_i versus K_Ic, and K_Ic versus E^1/2, however, E did not show relationship with HV and B. The increase of crystalline phase content is beneficial to decrease the brittleness of dental ceramics by means of both an increase in fracture surface energy and a lowering in index of brittleness.     

Processing and piezoelectric properties of porous PZT ceramics

5–45% porous lead zirconate titanate (PZT) ceramics were fabricated by adding pore formers such as polymethyl methacrylate (PMMA) and dextrin and sintering at 1200 °C for 2 h. The optimum heating procedure was decided according to the thermogravimetric analysis of pore formers. The effects of different pore formers and their content on the microstructure and piezoelectric properties were investigated. With an increase in the content of pore formers, the porosity of sintered ceramics increased, which led to reduced dielectric constant (ɛ₃₃) and longitudinal piezoelectric coefficient (d₃₃) as well as enhanced hydrostatic piezoelectric voltage coefficient (g_h) and hydrostatic figures of merit (d_h g_h). The hydrostatic figures of merit (d_h g_h) of 41% porous PZT were 10 times more than that of 95% dense PZT.     

Production and properties of porous unfired zircon ceramics

Conclusions We examined the conditions for obtaining porous, uniformed ceramics from zircon. We studied the influence of the concentration and size of the macropores on certain properties of the resulting ceramic. An assessment of these properties suggests that it is prospective to use porous unfired zircon ceramics as heat insulation for high-temperature equipment.Translated from Ogneupory, No. 1, pp. 34–37, January, 1984.     

Mechanical properties and fracture behavior of fibrous Al2O3/SiC ceramics

Fibrous Al₂O₃ ceramics with a mixture of SiC and Al₂O₃ as the cell boundaries were fabricated by extrusion-molding and hot-pressing techniques. The effects of the cell boundary composition on the mechanical properties and fracture behavior are investigated. It is shown that a 65:35 mixture of SiC:Al₂O₃ can act as a suitable cell boundary for Al₂O₃ cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and apparent toughness are up to 1349 J/m² and 6.0 MPa m^1/2, respectively.     

Porous microcracked ceramics under compression: Micromechanical model of non-linear behavior

Stress–strain curves for porous microcracked ceramics (such as aluminum titanate) under compression exhibit non-linearity, hysteresis, and a sharp increase in stiffness when changing from loading to unloading. A micromechanical model is developed that expresses these features in terms of porosity and crack density. The mentioned features are linked, in a quantitative way, to closures and frictional sliding of microcracks. The model also allows one to extract information on crack densities from stress–strain curves, which provides insight into the evolution of open, closed and sliding cracks. The model is approximate due to uncertainty factors, but its predictions are generally in agreement with the data and the information conveyed by SEM images.     

Mechanical properties of porous carbon material: Woodceramics

The mechanical properties of Woodceramics which were made from medium-density fiberboard have been investigated with special reference to the effect of burning temperature on their bending Young's modulus and bending strength. Woodceramics made from beech wood have also been tested to clarify the compressive strength anisotropy, and the role of phenol resin impregnation in strengthening the beech based charcoal.The bending Young's modulus hardly varies for burning temperatures between 300 and 500°C, but it improves remarkably for burning temperatures between 500 and 800°C. The bending strength degrades with temperature for burning temperatures between 300 and 500°C, but it improves remarkably with increasing temperature of burning between 500 and 800°C. The bending Young's modulus and bending strength gradually degrade with temperature for burning temperatures at and above 2000°C.The compressive strength of beech wood burned at 800°C in the longitudinal direction is greater than that in the radial direction, which in turn is greater than that in the tangential direction; this reflects the anisotropy of wood. Woodceramics made from beech wood have a compressive strength superior to beech charcoal in any of the following three directions: 4.5 times in the longitudinal direction, 3.4 times in the radial direction, and 2.0 times in the tangential direction. Both for beech charcoal and beech Woodceramics, brittle fracture is brought about by the buckling of cell wall in compression along the longitudinal direction but by the bending of cell wall in the compression along radial and tangential direction.     

Structural phase transition and electrical properties of Sr2+ substituted porous PMN‐PZT ceramics

In this study, Sr²⁺ modified porous PMN‐PZT ceramics with one‐dimensional pore channels were produced by the ionotropic gelation process of alginate/PMN‐PZT suspensions. The ion‐exchange method was employed during the fabrication procedure to adjust the content of SrO addition. With the SrO addition increasing from 1.653 wt.% to 2.957 wt.%, the structural phase of porous PMN‐PZT ceramics transformed from rhombohedral (R) to tetragonal (T) perovskite phase. Accordingly, the electromechanical coupling coefficient (k_t) value of porous PMN‐PZT ceramics decreased from 64.7% to 58.7%, while the hydrostatic figure of merit (HFOM) value increased from 1510 to 5547 × 10^?15 m²/N, and acoustic impedance Z value ranged from 4.59 to 7.55 MRayls, which helped for applications in underwater transducers or hydrophones.     

Processing and properties of cordierite-silica bonded porous SiC ceramics

Cordierite-silica bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC with cordierite sol followed by sintering at 1300–1400 °C in air. The porosity, average pore diameter and flexural strength of the ceramics varied 30–36 vol%, ~ 4–22 µm and ~ 13–38 MPa respectively with variation of sintering temperature and SiC particle sizes. In the final ceramics SiC particles were bonded by the oxidation-derived SiO₂ and sol-gel derived cordierite. The corrosion behaviour of sintered SiC ceramics was studied in acidic and alkaline medium. The porous SiC ceramics were observed to exhibit better corrosion resistance in acid solution.     

Corrosion behavior of porous silicon nitride ceramics in different atmospheres

The corrosion behavior of silicon nitride (Si₃N₄) ceramics with a porosity of 46% at 1200–1500 °C under different conditions including dry O₂, O₂ containing 20 vol% H₂O and Ar containing 20 vol% H₂O is compared. The results show that porous Si₃N₄ ceramics exhibit good oxidation resistance up to 1200 °C. Their corrosion behavior varies depending on the temperature and atmosphere. Water vapor can obviously affect the morphology of the reaction product and thus accelerate the corrosion rate due to its specific inward diffusion mechanism and devitrified effect at high temperature. In view of the reaction kinetics, it proceeds in a diffusion-controlled manner in dry O₂ while follows the parabolic-linear law at water-containing atmosphere. Furthermore, a new model considering both oxidation and volatilization reactions is established. These provide a baseline for expanding the application fields of non-oxide porous ceramics such as Si₃N₄ and silicon carbide (SiC) etc.     

Phase transition behavior and temperature-stable piezoelectric properties of new quaternary (K,Na)NbO3 based ceramics

(K, Na)NbO₃-based lead free materials have been found to exhibit good piezoelectric properties due to the orthorhombic–tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, this transition correspondingly results in a strong temperature dependence of the dielectric and piezoelectric properties. In this work, new quaternary (1?x) (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST)–xSrTiO₃ (ST) lead-free piezoelectric ceramics were fabricated by a conventional ceramic technique and their structure and piezoelectric properties were also studied. The results of X-ray diffraction reveal that SrTiO₃ diffuses into the KNLNST lattices to form a new solid solution with a perovskite structure. After the addition of SrTiO₃, tetragonal–orthorhombic phase transition shifts to lower temperatures. The good piezoelectric properties of 0.995 KNLNST–0.005 ST material were found to be d₃₃～295 pC/N, k_p～42%, and ε_r～1902, with greatly improved temperature stability over the temperature range of 0–100 °C, demonstrating practical potential for actuator and ultrasonic transducer applications.     

Mechanical and dielectric properties of 3D printed highly porous ceramics fabricated via stable and durable gel ink

A novel, efficient, versatile strategy was carried out to fabricate highly porous ceramic parts based on the combination of strong colloidal gel ink fabricated with high boiling point organic solvents and DIW technique. The preparation and optimization of inks and the effect of heating temperature on the phase composition, microstructure, mechanical properties and dielectric properties of ceramic parts were systematically investigated. The strong colloidal ink exhibits excellent ambient stability and printability. The sintering temperatures bring about the evolution of phases, structural mechanical properties and dielectric properties of ceramic parts. Ultimately, Si₂N₂O single wall ceramic parts with a frame density of 1.07?1.14 g/cm³ and an apparent porosity of 53.13 ± 1.29% were successful fabricated. The dielectric constant and dielectric loss of Si₂N₂O sample (1650℃) are only 4.24 and 0.0049, respectively. This strategy provides a reference for in-situ synthesis of high-performance porous ceramic components based on the DIW.     

Mechanical and thermal behavior of cellular mullite materials

In this paper, the mechanical behavior and thermal properties of cellular mullite bodies obtained by thermal direct-consolidation of foamed aqueous suspensions of mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose (MC) were studied. The mechanical behavior of cellular mullite materials sintered at 1600 °C was evaluated by diametral compression at room temperature, 1000 °C and 1300 °C. The variation in the thermal diffusivity and thermal conductivity at temperatures up to 900 °C was determined using the laser-flash method. The results of the mechanical and thermal evaluation were analyzed based on the porosity features of the sintered materials, which was determined in turn by the starting system used for shaping the bodies.