Key Engineering Materials Vols. 280-283

Abstract: ANSYS software was used to simulate the superposition rule of multi-thermal-source temperature field. The SiC productions synthesized by multi-thermal-source furnace were examined through X-ray diffractometer (XRD) and energy depressive spectroscopy (EDS). The crystalline and density were determined by scanning electron microscopy (SEM). The superposition rule and influence of multithermal- source temperature field on SiC synthesizing were clarified.

Abstract: Porous ceramics green bodies of silicon carbide were formed by freeze-casting of silicon carbide/carbon black aqueous slurries in a mold where temperature distribution was controlled. The influence of solid loading on the pore structure of green bodies was investigated. Pore morphology of green bodies formed from slurries with various solid loading ranging from 25 to 50vol% experienced a gradual change. Appropriate solid loading of silicon carbide/carbon black slurry was concluded for fabrication of SiC ceramics with oriented pore structure. Two means were employed for changing ice crystallizing velocity, so that the influence of ice crystallizing velocity on pore size of green bodies was explored. It was concluded that pore size became larger under slow freezing condition, whereas smaller with the pre-cooling of silicon carbide/carbon black slurry.

Abstract: This paper describes the investigation of gelcasting of silicon carbide/carbon black aqueous slurry. Aqueous slurries are prepared by using polyvinylpyrrolloidone and tetramethyl ammonium hydroxide as dispersants. Rheological behaviors of the slurries have been studied. Two initiator systems are used to initiate free-radical polymerization of acrylamide in silicon carbide/carbon black aqueous slurry. In order to reduce strong effect of carbon black on inhibiting polymerization of acrylamide, acetylacetone is added into the slurry. The experimental result shows that controllable free-radical polymerization of acrylamide in silicon carbide/carbon black aqueous slurry can be realized by adding small quantity of acetylacetone.

Abstract: Microflaws were detected on the cross-section of polymer-derived silicon carbide fibers and their formation mechanism was studied by varying the curing degree and the firing rate. The results show that microflaws decrease in size with increase of the curing degree due to an increased ceramic residue. The results also show that microflaws decrease in size with decrease of the firing rate. No microflaws are detectable when the firing rate is as low as 10K/h. This indicates that the microflaws are the main channels of evolution gases and the pressure of these gases leads to their formation and propagation. So a high curing degree and a low firing rate are both preferred in the preparation of dense silicon carbide fibers.

Abstract: Mullite powders of stoichiometric composition were synthesized by sol-gel process with TEOS and Al(NO3)3·9H2O as silicon and aluminum source. Homogeneous mullite powders were used as the starting materials to prepare the porous mullite coating on the porous silicon carbide support by repeated dip-coating method. XRD analysis revealed that the mullite precursor gels almost completely transformed to orthorhombic crystalline mullite at 1200°C. The average pore diameter and porosity of the porous mullite coating were about 4.5µm and 52.3%, respectively.

Abstract: CF/SiC and Hi-Nicalon/SiC composites were prepared by precursor pyrolysis-hot pressing, and the microstructure and fracture behavior of the composites were investigated. Because of a strongly bonded fiber/matrix interface primarily resulting from the direct reactions between the fibers and matrix, Hi-Nicalon/SiC composite exhibited a typical brittle fracture behavior. However, CF/SiC composite displayed a tough fracture behavior with extensive fiber pullout, which was primarily attributed to a weakly bonded fiber/matrix interface as well as higher strength retention of the fibers. As a result, CF/SiC composite achieved better mechanical properties of 691.6 MPa in strength and 20.7 MPa•m1/2 in toughness, which were much higher than those of Hi-Nicalon/SiC composite.

Abstract: Amorphous SiC coatings were deposited by RF magnetron sputtering from a sintered SiC target onto Si(100) substrate at room temperature. The influence of RF power on the surface morphology and the RMS surface roughness of the resulting SiC coatings was studied by using atomic force microscopy. Two types of surface morphologies were obtained. The corresponding forming mechanisms were also discussed.

Abstract: The ultra-fine treatment of industrial SiC powder was carried out by fluidized bed opposed jet milling (QLM-100K), and the influences of grinding parameters on the ultra-fine treatment were studied. A better grinding result is achieved through a higher crushing pressure and a higher sorter frequency and the ultra-fine treated SiC powder possesses a smaller particle size, a narrower particle-size distribution, a higher specific surface area and reduced agglomeration. The effects of ultra-fine treatment on the compaction behaviors, sintering behaviors, properties and microstructure of SiC ceramics were investigated. The compact with ultra-fine treated powder has a higher density and a lower sintering temperature. The densification and mechanical properties of the ceramics can be ameliorated obviously and the microstructure defects of the ceramics decrease.

Abstract: Si3N4-SiCp composites were prepared using fine Si powders as the starting materials for Si3N4, and SiCp as the aggregates. The erosive wear behavior of reaction sintered Si3N4-SiCp composite ceramic was investigated in liquid-solid flow. The results display that the composite ceramic hold a better capability of erosive wear resistance than 92 Al2O3 ceramics. The SEM pictures of the worn surface indicate that the main erosive wear mechanism of the composite ceramic is coexistence of micro-cutting and chisel-cutting.

Abstract: The friction and wear of the silicon carbide (SiC) and hot pressed silicon nitride (Si3N4) against zirconia (Y–TZP) sliding under dry friction and room temperature conditions were investigated with pin-on-disk tribometer at sliding speed of 0.56 m·s-1 and normal load of 50 N, 80 N and 120 N, respectively. It was found that, the coefficient of friction and wear rate are dependent on the test duration as well as the normal load. Through analyzing and comparing, the wear rates of the two frictional couples both are in the 10-6 mm3 (N·m)-1. Based on the variety regulation of the wear maps, the wear mechanisms of the two couples were analyzed. Between the two couples, the friction and wear characteristics of the SiC/ZrO2 couple are better than the Si3N4/ZrO2 couple’s.