Key Engineering Materials Vol. 655

Abstract: The transparent β-Si₃N₄ ceramic with a whisker-like microstructure was prepared by hot-pressing at 2000 °C for 26 h, with MgSiN₂ as an additive. The resultant material achieves the maximum transmittance of 70 % at the wavelength of about 2.5 μm and the transmittance value keeps higher than 60 % in the range of 700-4500 nm wavelength, which is attributed to the very small amount of the intergranular amorphous phase along with high density. The present transparent β-Si₃N₄ ceramic exhibits an indentation fracture toughness of 7.2±0.3 MPa m^1/2.

Abstract: Wave-transparent porous Si₃N₄ ceramics were fabricated using 9wt% multiple sintering additives of Y₂O₃ and Yb₂O₃. The influence of sintering additive composition on the porosity, mechanical properties and microstructure of porous Si₃N₄ ceramics was investigated. The results showed that the porous Si₃N₄ ceramics with porosity ranging from 32% to 43% were prepared. For the porous Si₃N₄ ceramics sintered at 1690°C, flexural strength, fracture toughness and the β-Si₃N₄ phase content increased as the Yb₂O₃ content of multiple sintering additives increased from 0 to 9 wt%. In most specimens, microstructures with interlocked fibrous β-Si₃N₄ grains and uniformly distributed fine pores were obtained with employing scanning electron microscopic examination and X ray diffraction analysis. Sinterability and phase transformation of Si₃N₄ with more Yb₂O₃ content were more pronounced than others.

Abstract: To investigate the influence of β-Si₃N₄ powder on thermal conductivity of silicon nitride, coarse, fine β-Si₃N₄ powder and various β-Si₃N₄/α-Si₃N₄ ratios of starting powders were adopted to fabricate ceramics by spark plasma sintering at 1600°Cand subsequent high-temperature heat treatment at 1900°C with the sintering additives of Y₂O₃ and MgO. It is found that with more fine β-Si₃N₄ powder in the starting powder, β-Si₃N₄ grains exhibit high thermal conductivity, which is partly resulted from the compaction of β-Si₃N₄ grains.

Abstract: In the presented work the effect of graphene platelets (GPLs) grade and mixture homogenization parameters on the microstructure and selected properties of Si₃N₄ – graphene composites have been investigated. Three different commercial grades of the multilayer graphene platelets were used as a filler for ceramic matrix. The mixtures of silicon nitride powder with 2wt% of GPLs were prepared using planetary mill at 200 – 400 rpm for 1 – 16 h. The water, acetone and isopropanol have been tested as a wetting medium. The composites were sintered using Spark Plasma Sintering – SPS at 1650 °C under 35 MPa of uniaxial pressure during the whole cycle. Microstructure, density, Young modulus, hardness and indentation fracture toughness of the sintered samples have been investigated. The microstructure homogeneity of the composites depends mainly on the milling duration. Milling process should last for about 4 – 8 h at 200 rpm for optimum results. Mechanical properties of the composites are slightly varied depending on the mixture preparation parameters. The homogenization time has a little stronger influence on the Young's modulus and hardness in the case of composites containing larger graphene sheets, in connection with the fragmentation of its microstructure. The best properties have composites reinforced by the GPLs characterised both by the smallest thickness and lateral size of nanosheets.

Abstract: cBN/SiAlON composites were prepared by spark plasma sintering (SPS) method using Si₃N₄, AlN, Al₂O₃, cBN and Y₂O₃ powders as raw materials. The sintering process is at the temperature of 1500°C holding for 5 min. Effect of the Y₂O₃ content on phase composition, microstructure, bulk density, hardness and fracture toughness of the cBN/SiAlON composite was investigated. The experimental results showed that when the Y₂O₃ content was 0.2 wt. % the bulk density and fracture toughness of the composite had the maximum values of 3.0 g/cm³ and K_IC = 5.7 MPa∙m^1/2, respectively. The cBN/SiAlON composite with 0.8 wt. % Y₂O₃ addition got the maximum hardness of 16.4 GPa.

Abstract: In the present work, SiC was pressureless solid state sintered with 3 wt% C and 0.6 wt% B₄C as sintering additives. The friction and wear behavior of the PSSS SiC ceramics was investigated by using a block-on-ring tribometer. The wear volume and friction coefficient was measured. It is as expected that the friction coefficient increased with the elevation of the normal load and sliding speed. The microstructure of the worn surface was observed, based on which the wear mechanism was analyzed. Different degrees of oxidation during the friction process was found and the degree of oxidation was related to the severity of wear. The normal load was found to exert great influence on the wear of the SSiC ceramics.

Abstract: The average particle size of SiC used as raw materials is 60 μm, the filter membrane in a mass ratio of each component are: SiC : binder: sodium carboxymethyl cellulose (CMC): deionized water =9:1:1:20. Method of producing membrane by means of spin coating, the sintering temperature was set in different ranges. The Archimedes drainage method was used to measure its porosity, and the filter pressure of membrane was charactered by the filter pressure testing system, SEM was performed to characterize its surface morphology and microstructure. The results show that: when the temperature rises from 1225 °C to 1325 °C, the filter pressure of SiC porous ceramic filter membrane decrease firstly and then increase; the atlas trend of the porosity is roughly opposite with the filter pressure, the maximum of the porosity value is about 42% when the temperature reaches 1300°C, at this temperature, the filter pressure reaches a minimum value and the surface of the filtration membrane is the most flat. When the temperature reaches 1350 °C, the surface of the film started showing a plaque.

Abstract: Spin coating method was adopted for preparation of two types of SiC filter membrane with different particles size. Sintering temperature was set at 1300°C. SEM was performed in order to characterize its micro-structure. For measuring the porosity of the membrane, we used Archimedes drainage method. The filter pressure testing system was used to character the membrane filter pressure drop. The experiment results indicated that, when the filter membrane was composed of average diameters 45μm and 23μm SiC particles, with their thickness reached about 300μm and 150μm respectively and the amount of ceramic binder was 10%, the porosity of the membrane achieved 43%, the surface of filtration membrane structure was uniform, and its filter pressure was smaller. On the premise of satisfying specific filtering accuracy and efficiency, larger size of silicon carbide particle could offer better performance of the membrane.

Abstract: Hot pressing sintering process for cemented carbide preparation was investigated with cBN-WC/Co composite powders with different Co content from 4wt% to 10wt% at temperatures between 1300°C and 1400°C. The results showed that the Co content had remarkable influence on the densification and mechanical properties of cBN-WC/Co composites. Near-full densification can be obtained when cBN-WC/8wt%Co powders were sintered at temperatures at 1350°C, and pressures at 20MPa, combining an excellent Vickers hardness of 17.57GPa with an acceptable flexure strength of 601.58MPa.