Strength-Processing Defects Relationship Based on Micrographic Analysis and Fracture Mechanics in Alumina Ceramics

Characterization of processing defects in structural ceramics is very important for the investigation of mechanical property, because processing defects are origin of fracture and govern the strength of products. The relationship between defects and strength is represented by simple equation which contains factor of defect size. In this study, size of defects in alumina ceramics was measured by optical microscopy with sintered body thinned to about 150 μm. The size distribution was used for strength calculation based on linear fracture mechanics. Average and Weibull modulus of calculated strength were very agreed with experimentally measured them. The result shows that defects in sintered body govern the strength and are quantitatively related to strength by fracture mechanics.     

Effect of CIP on the strength of ceramics produced from ZrO2 + 3 mol.% Y2O3 powder

It is shown that the strength of ceramics from zirconia partially stabilized with yttria (PSZ) is determined substantially by the nature of the surface or subsurface defects. The kind of defects is connected with the fullness of formation of grain boundaries. It is shown that growth of the CIP pressure causes a change in the nature of the defect responsible for fracture, which in turn causes substantial hardening of the ceramics. When PSZ powder is pressed at 0.4 GPa the resulting ceramics has an ultimate bending strength of 1700 MPa. As a rule ceramics with such a strength are obtained only with the use of hot isostatic pressure.     

Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities

The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics.     

Effectiveness of the acoustic-emission method for evaluating the strength properties of ceramics and refractories depending on the specific features of their deformation

Conclusions The effectiveness of the method of AE for controlling the defectiveness of the structure and nondestructive evaluation of the strength of ceramics and refractories depends on the specific features of their deformation which can be taken into account on the basis of the characteristic structure-sensitive brittleness index of the materials. In this case, the method of AE can be most effectively used for relatively brittle ceramic materials. When using this method for testing brittle ceramics, in most cases, it is possible to distinguish only the specimens containing large structural defects that control their strength.Translated from Ogneupory, No. 6, pp. 13–18, June, 1988.     

Effect of the loading rate of the strength of GB-7 ceramics

The effect of the loading rate on the strength of an alumina ceramics of grade GB-7 in air is studied. It is established that the strength of the ceramics decreases substantially with increase in the loading rate due to the interaction between the calcium borosilicate vitreous phase and steam, which causes substantial growth of the microstructural defects existing in the ceramics.     

多元稳定剂复合掺杂对增韧ZrO_2陶瓷性能的影响

Y_2O_3、CeO_2,MgO分别单独作为稳定剂制备增韧ZrO_2陶瓷材料,各有优点和不足。为了发挥不同稳定剂的优点而抑制其不足,本文研究了多元稳定剂复合添加对增韧ZrO_2陶瓷性能的影响。试验发现Y、Ce、Mg三种稳定剂复合掺杂的(Y、Ce、Mg)—PTZ陶瓷保持了Y稳定剂使材料强度、硬度较高的优点,发挥了Ce稳定剂使材料高韧性的优点,而Mg的加入实现了液相促进下的低温烧结,克服了Ce稳定剂使材料晶粒粗大,穿晶断裂等缺点,可获得高韧性、高强度ZrO_2陶瓷材料。     

Finite element analysis of fracture behavior in ceramics: Competition between artificial notch and internal defects under three-point bending

Evaluation of the nonlinear relationship between the surface defect size and fracture strength of ceramics is important for engineering applications. In this study, we aim to predict the apparent nonlinear relationship between the defect size and fracture strength of single-edge notched beams (SENBs) using the finite element method. Specifically, we applied the methodology for predicting fracture strength from microstructure distribution data (relative density, pore size, aspect ratio, and grain size) to a finite element analysis (FEA) model in which the shape and size of the initial defects are defined at notch locations. By reproducing the apparent nonlinearity caused by the competition between the surface and internal defects within the framework of linear elastic fracture mechanics, the effectiveness of the FEA methodology for the evaluation of strength scatter and allowable crack size in ceramics was demonstrated.     

Investigation of Partial Discharge and Fracture Strength in Piezoelectric Ceramics

Although piezoelectric ceramics are widely used, the structural reliability of these materials in long‐term service is still a concern. In this study, a nondestructive method for assessing material strength reliability using partial discharge (PD) tests is proposed. The PD inception electric field and the flexure strength are both controlled by the defects in the ceramics which suggests that the PD test can be an alternative nondestructive method for evaluating the structural reliability of piezoelectric ceramics. PD and four‐point flexure tests were carried out on piezoelectric ceramics. The influence of the PD test on samples was investigated by comparing the microstructure and electrical properties before and after the PD test. Samples which were not PD tested were also fractured and compared. It was found that no strength degradation was caused by the PD test. Weibull analysis revealed a correlation between the distribution in values for PD inception electric field and flexure strength.     

Influence of corrosion and mechanical loads on advanced ceramic components

Advanced oxide ceramics are prospective materials for severe application conditions, including corrosion, particularly, in oxygen-rich environments, combined with the action of mechanical loads. The corrosion behavior and mechanical strength decrease of oxide ceramics, such as high alumina, alumina–mullite and zirconia-based ceramics, were studied in water steam supercritical conditions (elevated temperatures and pressures). The strength decrease under the action of the studied aggressive environment is mostly dealt with the glassy phase dissolution and intergranular corrosion for alumina–mullite and high alumina ceramics, while degradation of zirconia-based ceramics is also dealt with the phase transformation. The influence of structure defects related to processing of the ceramics on corrosion is considered.     

Dopant effects on high temperature mechanical properties of zirconium carbide ceramics

Combining theoretical and experimental methods, the effects of 5?vol.-% WC dopant on the microstructure evolution, sintering behaviour and mechanical properties of ZrC ceramics were investigated. WC dopant was found to improve the high temperature elastic modulus and bending strength of ZrC ceramics. Both calculations and experimental results showed that the formation of (Zr, W)C solid solution promote dissolution of the impurity oxygen from the starting powder into the lattice sites, resulting in less oxygen defects in grain boundaries. Internal friction curve can also conform that the ZrC ceramics doped with WC have cleaner grain boundaries, which improved higher elastic modulus and bending strength in WC doped ZrC ceramics at elevated temperature.     

Flash joining of alumina ceramics under a small current density

The ceramic joining using electric field (E-field) has garnered significant research attention due to the decreased joining barrier and enhanced reliability. However, the underlying mechanism of E-field assisted joining remains unclear. Herein, we report the rapid joining of alumina ceramics using a small current. Moreover, the E-field is applied in both perpendicular and parallel directions to the faying surfaces, demonstrating a significant difference in terms of joint strength. These results indicate that the E-field generates defects and promotes the joining process by facilitating ionic diffusion.     

Strength Retention in Hot-Pressed Si3N4 Ceramics with Lu2O3 Additives after Oxidation Exposure in Air at 1500°C

The effect of oxidation exposure on room-temperature flexural strength was examined in 3.33- and 12.51-wt%-Lu₂O₃-containing hot-pressed Si₃N₄ ceramics exposed to air at 1500°C for up to 1000 h. After oxidation exposure, the room-temperature strength of the ceramics was degraded, and strength retention decreased with time at temperature, dependent on the amount of additive. The retention in room-temperature strength displayed by the two compositions after 1000 h of oxidation exposure was 75%–80%. The degradation in strength was attributed to the formation of new defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk during oxidation exposure.     

DRYING OF FINE CERAMICS

The drying mechanism of shaped ceramics is reviewed and some methods for eliminating defects produced by drying are discussed in this report. The types of defects depend upon the shaping method, shape, properties of the raw materials, drying process and other items. Most defects of dried ceramics are produced during the initial or middle drying stage when large shrinkage of the ceramic body occurs. These defects may be successfully eliminated by heating the body from the inside. Self-deformation caused by weight is also a serious defect. The near net shaping of fine ceramics to reduce machining requires uniform body shrinkage and hence, highly controlled drying. Examples of electric current drying, dielectric drying, and dewaxing are also shown in this report.     

喷雾造粒粉制备ZTM陶瓷的结构缺陷与强度研究

结合喷雾料浆,造粒颗粒的相关工艺控制,对喷雾造粒粉制备的ＺＴＭ陶瓷的结构缺陷及强度进行了研究。结果发现：较低固相含量的料浆易形成具有坚硬外壳的空心颗粒,这种颗粒在成型过程中难以破坏,以坯体中形成结构缺陷;颗粒的残余水分对颗粒的变形性具有重要作用,随残余水分的增加,坯体的断裂从沿颗粒向穿过颗粒的形式过渡。研究还表明：较大颗粒间的气孔,如在成型过程未被完全破坏和变形,则会遗留在烧结体中,导致材料强度的     

ZrO2表面改性及凝胶注模研究

凝胶注模成型工艺能够制备出高强度,高致密性,且形状复杂的氧化锆陶瓷.粉体团聚对凝胶注模成型工艺存在不良影响,粉体团聚导致陶瓷内部产生缺陷,导致致密性、均匀性变差,力学性能降低.本文通过对氧化锆粉体表面改性,改善粉体团聚,将改性粉体用于凝胶注模成型,制备出低粘度浆料.通过SEM、粘度仪、显微镜等仪器,对强度、粘度及粉体分...     

Ultrastrong tough zirconia ceramics by defects-engineering

We report the preparation of a category of ultrastrong tough zirconia ceramics by engineering defects using an oscillatory pressure during pressure assisted sintering. The introduced oscillatory pressure enhances the dynamic grain rearrangement, plastic deformation, mass transportation, and pore removal, leading to the formation of pore-free ceramics characterized by the rich coherent grain boundaries among individual mesocrystalline grains with intragranular quasi-interfaces. As a proof of concept, the pressure required by oscillatory pressure sintering (OPS) for preparing fully dense 3Y-TZP ceramics is significantly reduced, which is just ∼1/5 of that required by hot isostatic pressing. The OPS-prepared 3Y-TZP ceramics reached a record-breaking high bending strength and fracture toughness, being up to 1.8 GPa and 16 MPa·m^1/2, respectively. This success illustrates a universal principle of engineering defects for making breakthrough in exploring other ultrastrong tough ceramics.     

Effects of granule compaction procedures on defect structure, fracture strength and thermal conductivity of AlN ceramics

The effect of granule compaction procedures on defect structure, fracture strength and thermal conductivity was examined for AlN ceramics using the same starting granules. For this study, changing the order of the cold isostatic pressing (CIP) and dewaxing procedure was performed, which changed the compaction behavior of the granule bed. The structural examination of green and sintered bodies showed that the change of granule strength strongly influenced the size and concentration of the granule related defects in sintered bodies. A large difference of the fracture strength associated with the change of granule strength was noted, and it was ascribed to the difference between the defect structures in the sintered bodies. On the other hand, the thermal conductivity was kept almost constant (> 150 W/m K) against the process alteration.     

Near-net-size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

Porous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade-off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near-net-size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α-Al₂O₃ powders as the starting materials and using MoO₃ to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid-state reaction during reaction sintering. The in situ formed three-dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near-net-shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.     

CONDUCTIVE DRIlNG CHARACTERISTICS OF GELATINIZED RICE STARCH

ABSTRACT

The drying mechanism of shaped ceramics is reviewed and some methods for eliminating defects produced by drying are discussed in this report. The types of defects depend upon the shaping method, shape, properties of the raw materials, drying process and other items. Most defects of dried ceramics are produced during the initial or middle drying stage when large shrinkage of the ceramic body occurs. These defects may be successfully eliminated by heating the body from the inside. Self-deformation caused by weight is also a serious defect. The near net shaping of fine ceramics to reduce machining requires uniform body shrinkage and hence, highly controlled drying. Examples of electric current drying, dielectric drying, and dewaxing are also shown in this report.