Key Engineering Materials Vols. 353-358

Abstract: The effect of electromechanical loadings including their sequence, which was motivated by the property of crack energy density for piezoelectric material, was experimentally investigated. Three-point bending fracture test was performed for two piezoelectric ceramics under different electromechanical loading conditions. It was found that the fracture loads under closed circuit condition are greater than those under open circuit condition. Effect of applied electric field on fracture load in the test varied with materials. Applied electric field always enhanced crack extension in C-21 ceramics regardless of their directions, while it produced very little effect on crack extension under negative electric fields in C-2 ceramics. It was also found that electromechanical loading sequence clearly affects fracture strength, although its effect varies also with materials.

Abstract: The dependence of strength on notch radii of ceramic components is presented. The strength is exponential decay as the notch radius decrease, the smooth specimen that corresponds to the infinity notch radius topped the fracture strength, but the lowest value relys on the micro feature of the materials. Plotted the strengths on Weibull and Normal probability paper, the fitness coefficients reveal that the strength of smooth specimen, as the past literatures indicated, follows Weibull distribution, but the strength of notched specimen follows Normal distribution better. Such result is due to the fact that the decreasing notch radius reduces and limits the effective original fracture volume, when the notch becomes a slim crack, the weakest link place becomes deterministic.

Abstract: Effects of the body centered cubic distribution of the dispersed SiC and (W,Ti)C particles on the residual thermal stresses of (W,Ti)C/SiC/Al2O3 ceramic composite were analyzed with the finite element method, especially with the comparison with that of the face centered cubic distribution. Results show that the residual thermal stress is affected not only by each material compositions but also by the distribution forms of the dispersed SiC and (W,Ti)C particulates. Both distribution and magnitude of residual thermal stress differs with the difference in the particle distribution forms. The calculation results of different particle distribution forms, however, are just the reflection of different cases of the residual thermal stresses inside the ceramic composite from different points of view. Both reflections can research agreement in trends.

Abstract: The purpose of this study is to determine the bi-axial flexural strength, weibull modulus and fracture mode of bilayered alumina glass-infiltrated core and the veneering porcelain. Forty disk specimens were fabricated from alumina glass-infiltrated core (HSDC-A) and veneer porcelain (Vintage AL). The specimens were equally divided into four groups as: MV, monolithic specimens of veneer material; MC, monolithic specimens of core material; BV, bilayered specimens with the veneer in tension; BC, bilayered specimens with core material in tension. Mean flexure strength, standard deviation and associated Weibull modulus were determined using bi-axial flexure (ball-on-ring) for each group. Both optical and scanning electron microscopy were employed for identification of the fracture mode and origin. The surface loaded in tension influenced the bi-axial flexural strength and reliability of the composites. The frequency of specimen delamination, Hertzian cone formation and sub-critical radial cracking in the bilayered discs are also dependent on the surface loaded in tension.

Abstract: In this research, SrBi2+xTa2O9 (SBT) powders with different Bi content were prepared using conventional solid-state reaction method. The SrBi2+xTa2O9 ceramics (x=0 ,0.2 ,0.4 ,0.6) were fabricated by the cold isostatic compaction and sintering in the air. Microstructure and dielectric, ferroelectric properties of SBT ceramics were investigated by XRD, SEM, EDAX and dielectric tester. The effects of Bi content on dielectric, ferroelectrc properties in SBT ceramics were discussed.

Abstract: In this paper, ZrB2-based ceramics containing up to 15 vol% nano-SiC whiskers were prepared by hot pressing at 1950°C under 20MPa pressure in flow argon. SEM and XRD techniques were used to characterize the sintered compacts. A fine and homogeneous microstructure was observed. The relative density of ZrB2-based ceramic containing 10vol% SiC whiskers reached to 97.7%. The bending strength and fracture toughness of the composite were 550 MPa and 8.08 MPa·m1/2 respectively, while those of the monolithic ZrB2 ceramic (0 vol% SiC whiskers added) were 424 MPa and 4.52 MPa·m1/2 respectively. The grain size of the ZrB2-based ceramics was reduced greatly by the addition of nano-SiC whiskers during the sintering process.

Abstract: A thin electrode layer at the interface between two dissimilar linear piezoelectric materials under electromechanical loading is investigated. The complex function theory is employed to obtain the exact solution to a finite thin conductive layer. Special consideration is devoted to the structure of singular stress and electric fields near the tip of the thin electrode between two dissimilar piezoelectric materials. The stress and electric field are found to have an inverse square root singularity. The electric field intensity factor characterizes uniquely the singular fields close to the edge of the conductive line sheet.

Abstract: Porous SiC/Si3N4 composites were fabricated by reaction between Si3N4 and C powder in 0.5 MPa nitrogen atmosphere at 1700oC, 1800oC and 1900oC for 0.5h, 1h and 2h, respectively. The characteristics of in situ porous Si3N4/SiC composites were further investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Results show that the SiC particles are very fine. And the transformation from α-Si3N4 to β-Si3N4 is obvious at 1800oC for 2h and at 1900oC. Otherwise, at 1700oC, the formation of rod-like β-Si3N4 is inhibited. The compressive test was conducted at a cross-head speed of 0.5 mm/min to get the basic mechanical performance of the porous ceramics. The highest value of strength of these composites was obtained when sintered at 1800oC for 1h.

Abstract: Effects of mullite on the structure, microhardness and machinability of fluorphlogopitemullite glass-ceramics were investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and microhardness tester. Results show that the microhardness and machinability are related closely to its microstructure. The microhardness decreases remarkably with the increase of crystallization and the formation of interlocking structure. The addition of mullite can decrease the size of mica crystals, which results in the spoil of interlocking structure. The microhardness of the glass-ceramics is slightly increased and then decreased with the addition of mullite. When the content of mullite is 15wt%, the glass-ceramics shows good machinability after reheated at 1200°C for 2h.

Abstract: Friction and wear tests of TiB2 sliding against SiC were conducted without lubricant from room temperature to 1200°C in air and in vacuum. The friction coefficient of the couple of TiB2/SiC is affected obviously by the oxidation of TiB2. It increases with the increase of temperature and reaches a maximum at some temperature in air, then decreases remarkably. The friction coefficient of TiB2/SiC in vacuum exhibites almost a constant and keeps smaller value than that in air. Transition of TiB2 onto the sliding surface of SiC was observed, which improved the wear resistance of SiC at high temperatures.