Key Engineering Materials Vols. 353-358

Abstract: Amorphous Si-B-C-N ceramics obtained by high energy ball milling and hot pressing using hexagonal boron nitride (h-BN), graphite (C) and amorphous Si as starting materials have been studied. The mechanical milling with high energy resulted in the generation of large amounts of amorphous composites only milled for 5 h. Si-B-C-N powders were consolidation by hot pressing at 1850 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that small amount of BN and SiC crystal lies in the amorphous matrix. The flexural strength reached the maximal value of 137.2 MPa at a mole ratio of BN/(Si+C) being 0.6.

Abstract: By means of forced non-resonance under 3-point bending method, the internal frictions of 2D-C/SiC composites are carried out from 25°C to 400°C in air. The effects of temperature, amplitude and frequency on internal frictions have been analyzed. Results show that interface layer plays a dominant role to internal friction. When temperature increases from 25°C to 400°C, internal friction of 2D-C/SiC composite increases slightly, and storage modulus decreases monotonically. With frequency increases from 1Hz to 100Hz, internal friction reduces to a minimum around 10Hz, whereas above 50Hz it arises sharply. In amplitude scale from 0.5μm to 10μm, internal friction increases linearly with the increase of amplitude.

Abstract: An electrical nonlinear constitutive model for piezoelectric material is studied in this paper. Electric yielding criterion and yielding effect on CED are investigated. Using the mechanical part of CED as a fracture parameter, it is shown that the predicted fracture loads through FEA for compact tension specimen is in accordance with corresponding experimental results.

Abstract: AlN-SiC-TiB2 ceramics were synthesized by self-propagating high temperature synthesis (SHS) and hot isostatic pressing (HIP) methods. The powder mixtures of Al, 6H-SiC and TiB2 were shaped by isostatic cool pressing method, and then combustion reaction was carried at the pressure of 100-200 MPa N2 by an ignitor. The solid solution of AlN and 2H-SiC in AlN-SiC-TiB2 ceramics was formed. The phase composition and microstructure were investigated by XRD and SEM. The mechanical properties of composite were measured as functions of composition. The maximum value of flexural strength and fracture toughness of composites were 430 MPa and 3.9 MPa⋅m1/2 respectively.

Abstract: The large-sized sapphire (Ø225×205 mm, 27.5 kg) was grown successfully by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at cooled center). The surface quality of the specimens was characterized by micro-Raman spectroscopy, and double crystal X-ray diffractometry (DCD) was utilized to investigate its crystalline perfection. The measurement of rocking curves was performed on various specimens from different region of large sapphire boule. The experimental results showed that CMP (chemo-mechanical polishing) with subsequent suitable chemically etching can develop the best-quality sapphire crystal surface and the values of FWHM obtained by conventional DCD were in the range from 27” to 58”. The infrared spectral transmission (2.0-4.5 5m) of sapphire crystal exceeded 82%. It is confirmed of SAPMAC growth method characteristics with in-situ annealing, small temperature gradient and low residual stress level by numerical simulation analysis.

Abstract: Ytterbia and Lutecia were adopted to stabilize α-sialon ceramics during a two-step hot press sintering, respectively. Although Yb3+ and Lu3+ possess the similar small ion size (0.86 Å and 0.85 Å) as two adjacent rare-earth elements, Yb- and Lu-α-sialon ceramics present very different microstructure and properties. Yb-α-sialon ceramic is fully composed of α-sialon phase without any secondary phase. The ceramic incorporated with Lu2O3 also consists of α-sialon phase, but a trace amount of intergranular phase J′ (Lu4Si2-xAlxO7+xN2-x) still remains at the boundary. Microstructure observation indicates the fully equiaxial grain morphology of Yb-α-sialon ceramic. However, elongated grains appear in the Lu-α-sialon ceramic, coexisting with small equiaxed grains. Owing to the almost full α-sialon phase assemblage, both ceramics present high hardness with the value of over 21 GPa. Lu-α-sialon ceramic possesses good toughness, which is attributed to the elongated grains assisting the facilitation of toughening mechanism. The equiaxied grain morphology results in a comparatively lower toughness for Yb-α-sialon ceramic.

Abstract: In this paper, nano-silica particles were incorporated to a fused silica-boron nitride dual phase ceramics originally using micrometer sized particles as starting powders. Effects of nano-silica content on density, sintering behavior, mechanical properties and microstructure of the dual phase ceramics were investigated. It was found that with the addition of nano-silica particles, the density and mechanical properties of the dual phase ceramics increased. Flexural strength and fracture toughness of them attained to 51.4MPa and 0.8MPa·m1/2, respectively. The increased density and spatial network structure of fused silica attribute to the improvement of the mechanical properties of the dual phase ceramics.

Abstract: ZrO2(3Y) ceramics were sintered by vacuum hot-pressing and impacted by split Hopkinson method. The microstructure of the materials was studied by X-ray diffraction and scanning electron microscopy. The results show that the phase transformation from tetragonal phase to monocline phase occurred in the ceramics during dynamic loading process, and the transformation rate is about 30%. From the results of scanning electron microscopy, it can be found that the microcracks formed in the materials under the dynamic loading, and there is a kind of liked-amorphous layer appeared on the fragment surface.

Abstract: Al2O3 composites with different volume fractions of 3Y-TZP (3 mol% yttria-doped tetragonal zirconia polycrystalline) were prepared by using a colloidal process and pressure-casting into green specimens, then were pressureless sintered to full density at 1550oC. The resulted two-phase microstructure contained uniformly distributed and submicro- meter-sized grains. The addition of 15 vol% ZrO2 resulted in an increase in bending strength (average 656 MPa by 4 pt. test). Appreciate amount of m-ZrO2 phase was detected after fracture. The enhanced transformability results in an increase in flexural strength and toughness. The addition of 30 vol% ZrO2 resulted in best toughness (5.4 MPam0.5) and outstanding wear resistance (7.2×10-8 cm3/Nm).