Creep Deformation of Ceramics in Four-Point Bending

Flexural testing was investigated as a method of studying the creep of ceramic materials at elevated temperatures. Three techniques were used to evaluate the steady-state stress exponent: two of them were based on the measurement of the surface curvature of a flexure specimen after testing; the third was based on the more conventional technique of measuring displacement rate as a function of applied load and exposure time. Applied to a grade of commercial vitreous-bonded alumina, the techniques yielded disparate results for the steady-state stress exponent. This discrepancy in results is believed to be a consequence of the fact that ceramics tend to creep more readily in tension than in compression, leading to a shift in the neutral plane for stress and strain in flexural specimens, which results in extended primary creep. A local enhancement of creep under the loading points of the test specimens was observed in the materials tested; this creep enhancement was attributed to contact stresses at the loading points.     

Anisotropy of Mechanical Properties of Zirconia-Based Ceramics Tested for Bending

The effect of bending stress s of different magnitudes and signs on the fracture toughness K _1c of polycrystalline specimens of partially stabilized zirconia (PSZ) is considered. A method for testing pre-stressed PSZ specimens by Vickers indentation using a four-point bending scheme is proposed. The dimensions of the impression from a diamond pyramid and the length of the radial cracks generated thereby are determined. An anisotropy of strength properties is revealed in the specimens tested, which is explained by the involvement of two mechanisms: forcing action of an external stress on the crack opening and activation of the tetragonal- monoclinic phase transition in the tensile stress field.     

Indentation Thermal Shock Test for Ceramics

This paper presents an indentation-quench method to determine the resistance of ceramics to thermal cycling. The method defines a critical quench temperature difference, Δ T _c, based on a criterion which includes a minimum amount of crack growth and a minimum fraction of growing cracks. The method is applied to three different materials, which are ranked according to the individual values of Δ T _c. The variation of ΔT_c with different indentation loads and indentation positions is investigated. This technique uses a small number of specimens, avoids subsequent mechanical testing, and provides information about the ΔT C of the materials. The effect of repeated cycling on crack extension is also discussed.     

Computer-Aided Design of Chemical Compounds with Controlled Properties

A problem of designing chemical compounds with controlled properties is considered. An algorithm for computer solution of the problem is presented, and its complexity is evaluated. Software implementation of the proposed approach allows one to perform the automated design of virtual chemical compounds with controlled properties and to plan their synthesis and use.     

Measurement of Residual Stres in Injection-Molded Ceramics

This paper discribes the adaptation of the layer removal method for the measurement of residual stresses in the plane of a plate for a ceramic injection molding suspension. The latter contained 56 vol% alumina in a polypropylenebased organic vehicle. A strain gauge method was used to measure curvatures. The experimental precautions are discussed and the errors evaluated. The method is applied to conventional moldings made at high and low pressures, to annealed moldings, and to moldings made with a heated sprue.     

Adhesive Contact of Nanowire in Three-Point Bending Test

A new adhesive receding contact model is presented in this paper for a nanowire in a three-point bending test. Because of its flexability, the nanowire in such a test, may lift-off or separate from its supporting elastic medium; this can dramatically change the nanowire boundary conditions and deformations. The changes of the nanowire boundary conditions and deformations, have a significant impact on the interpretation of the experimental data of the nanowire material properties. Through the model developed here, some explanations are offered for the different and contradicting observations of the nanowire material properties and the nanowire boundary conditions, found in recent experiments.     

Controlled Crystallization of the Amorphous Phase in Silicon Nitride Ceramics

The controlled crystallization of amorphous second phases in SiAlON is demonstrated in two systems. In a magnesia-containing SiAlON, cordierite crystallized on annealing after hot-pressing. Similarly, garnet crystallized in yttria-contain-ing SiAlON. The resulting microstructures are characterized by TEM and qualitatively related to changes on room-temperature toughness and high-temperature deformation.     

磨加工及残余应力对氮化硅陶瓷强度的影响

利用X-射线衍射方法测量了经不同磨削的氮化硅陶瓷表面残余应力及其对抗弯强度的影响。结果表明,磨削工艺所引入的残余应力是拉应力,对陶瓷抗弯强度有显著降低。     

Crack Healing and Bending Strength of Aluminum Titanate Ceramics at High Temperature

Bending strength and Young's modulus of aluminum titanate ceramics at room temperature to 1300°C were examined. Bending strength increased from 62 MPa at room temperature to 280 MPa at 1100°C. Young's modulus also increased, to 99 GPa at 1100°C. These increments were caused by crack healing. In particular, crack cylinderization occurring at 1000° to 1100°C markedly increased the mechanical strength. The thermal-hysteresis curves also showed healing of grain-boundary cracks.     

复合膜的透气性测试

介绍了提高材料阻隔性的常用方法,并针对复合膜的透气性特点,指出在测试时应注意的事项以及相应的改善方法。     

Controlled Flaws in Ceramics: A Comparison of Knoop and Vickers Indentation

Application of indentation fracture analysis to Knoop and Vickers indentation is examined, with particular emphasis on determining the limitations of the point force representation for the residual stress field. Deviation from the point force approximation is insignificant for crack-size/plastic-zone-size ratios 1.3. The Vickers deformation/fracture configuration in brittle materials invariably conforms to this requirement, whereas the Knoop configuration does not (except at very high indentation loads). However, stable crack growth during a failure test extends the crack sufficiently that the strength degradation for both types of indentation is well described by the point force approximation.     

An Experimental Measurement of Effective Diffusion Distance for the Sintering of Ceramics

The traditional models of sintering predict a pronounced dependence of densification rate on the scale of the microstructure as measured by the grain size. This study evaluates the grain size exponent for densification during isothermal sintering of an aggregated nanocrystalline zirconia powder, and for a submicrometer alumina powder. The results gave grain size exponents that are much higher than those anticipated for the expected sintering mechanisms. Furthermore, microstructural analysis showed that this overestimate of the exponent could be due to the spatial heterogeneity in the microstructure on the scale of the diffusion distance. To assess this issue, pore boundary tessellation was used to determine a new measurement of effective diffusion distance that takes into account the local spatial arrangement of pores. This measurement gives exponents much closer to those expected for the sintering of tetragonal zirconia by volume diffusion, and for the sintering of the alumina by grain-boundary diffusion.     

Use of the Notched-Beam Test for Evaluation of Fracture Energies of Ceramics

The mathematical analysis of the notched-beam specimen for fracture-energy determination considers a beam containing a zero-volume crack, i.e. a crack with zero width. Such a configuration is difficult to reproduce in practice with ceramics, and artificially cut notches with widths large relative to the scale of the ceramic microstructure are usually used. Evidence is presented which suggests that incorrect results can be obtained using such notches even when real cracks exist at the notch root. The effects of notch width and depth are examined for Al₂O₃, graphite, and SiC. A tentative specification of a root-crack-notch-radius relation is proposed for the application of this technique to ceramics.     

Design Data for Engineering Ceramics: A Review of the Flexure Test

The uniaxial strength of engineering ceramics is often measured by the well-known flexure strength test method there is a risk that flexure data are not representative of the properties of fabricated components. Reliability estimates for components based upon statistical extrapolation techniques from flexure data may not be valid. This paper reviews the problem and judges the usefulness of flexure data for design purposes. It is shown that some of the limitations of flexure data apply; to other modes of testing, including direct tension testing     

某测量系统直流变换器测试台的设计与实现

针对某测量系统直流变换器的性能参数要求,研究并设计相应的测试设备。介绍了手动和自动两种方法测试8台单机共56路信号的具体实现方案,给出了自动测试时数据的采集与处理算法。     

Chemically Bonded Phosphate Ceramics: II, Warm-Temperature Process for Alumina Ceramics

This is the second of three papers on a dissolution model that describes the formation of chemically bonded phosphate ceramics. In this paper, we discuss the kinetics of formation of aluminum phosphate ceramics between 100° and 150°C. Using basic thermodynamic formulations, we calculated the temperatures of maximum solubility of alumina and its hydrated phases and predicted the temperatures of formation of ceramics. Differential thermal and X-ray diffraction analyses on samples made in the laboratory confirm these temperatures. The resulting ceramics of alumina bonded with aluminum phosphate (berlinite) show a high compressive strength of 16 000 psi. We have concluded that rapid evaporation of excess water in the slurry generates porosity in the ceramics, and that better processing methods are needed. A consolidation model is presented that describes the microstructure of the ceramic. It predicts that a very small amount of alumina must be converted to form the bonding phase; hence, the product is mostly alumina with a thin coating of berlinite on the surface of alumina particles.     

Controlled Firing of Reaction-Bonded Aluminum Oxide (RBAO) Ceramics: Part II, Experimental Results

The reaction-bonded aluminum oxide (RBAO) process is an attractive alternative to conventional processing of ceramics, because of advantages such as lower costs, enhanced green machinability, near-net-shape forming, and broad microstructural variability. However, various problems are still encountered in the production of RBAO ceramics. Part I of the paper presented model predictions that may allow for the controlled firing of RBAO ceramics. The current work investigates the reaction behavior of RBAO ceramics under the model-predicted conditions (i.e., for varying oxygen content, heat loss, heating cycles, and scale) via thermogravimetry, differential thermal analysis, and analysis of samples that have been fired in a box furnace. By controlling the reaction, one can produce large, crack-free RBAO ceramics.     

Controlled Firing of Reaction-Bonded Aluminum Oxide (RBAO) Ceramics: Part I, Continuum-Model Predictions

The reaction-bonded aluminum oxide (RBAO) process is a novel, reaction-forming technique for producing monolithic, alumina-based ceramics. Although there has been extensive work on the RBAO process, it is often difficult to reproduce the process and avoid sample cracking. To solve the problems that are associated with the RBAO process, it is necessary to have a fundamental understanding of the reaction-bonding process and the effects of various processing parameters on the reaction behavior. To gain some insight into the process, a continuum model has been developed. The model, which considers the interaction between the macroscopic material and energy balances, is used to predict conditions under which RBAO bodies may be fired in a controlled manner, i.e., avoiding the runaway reaction. In particular, the effects of the oxygen content of the atmosphere, the heat loss by convection and radiation, the heating cycle, and scale (sample size) have been investigated. For small sample sizes, model predictions indicate that the reaction may be controlled by reducing the oxygen content of the atmosphere, increasing the heat loss, and/or incorporating an isothermal hold into the heating cycle at a temperature just below the ignition temperature. For larger sample sizes, model predictions indicate the need for multiple low-temperature holds at increasing temperatures. It is believed that firing RBAO bodies in a controlled manner will allow one to avoid sample cracking. Part II of this work presents a complementary experimental study that investigates the reaction behavior and structural integrity of samples that have been fired under the predicted conditions.