Papers by Author: Yi Bing Cheng

Abstract: ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) using exothermic reaction of ZrO2-C-Mg system. By theoretical calculating, the adiabatic temperature (Tad) for the system is about 2235K enough to react as SHS process. The Tad observed during experiment is 1850K. The results show that high pure ZrC powder is obtained with appropriate Mg contents. The scanning electron micrograph shows that the average size of ZrC particles is about 2μm.

Abstract: Mesoporous silica-carbon nanocomposites (C-SiO2) were synthesized and used as a host carrier in carbothermal reduction to fabricate highly crystalline silicon carbide nanoparticles and nanofibers. SiC nuclei were introduced into the mesopores as seeds by infiltration of preceramic precursor polycarbosilane (PCS) prior to the heat-treatment of carbothermal reduction. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption analysis were used to characterize C-SiO2 nanocomposites and SiC products. Crystalline SiC was not formed in the mesoporous C-SiO2 nanocomposites with a low carbon content (e.g. C/SiO2 ratio = 1.01) at 1450 °C. However, when a given amount of PCS was infiltrated into the mesoporous C-SiO2, SiC nanofibers and nanoparticles were produced at 1450 °C even in the low carbon content sample. The major morphology formed from the mesoporous C-SiO2 nanocomposites without PCS infiltration was nanoparticles, while nanofibers dominated in the products of the PCS infiltrated compositions. The results indicate that the conversion of PCS into SiC nuclei in mesopores prior to carbothermal reduction has facilitated the formation of SiC nanofibers. Therefore infiltration of seeds into mesopores of C-SiO2 precursors appears to be an effective means in accelerating the reaction and controlling of nanostructures of silicon carbide.

Abstract: Reaction sintering of boron carbide represents an attractive densification process. In this work, sintering mechanisms of silicon carbide and boron carbide composites were studied. Mixed boron carbide/graphite mixtures were sintered in a vacuumed graphite furnace between 1380 and 1450oC. The samples were in contact with bulk silicon metal which melts at 1410oC. Reaction sequence of the composition was investigated by X-ray diffraction, SEM and TEM. It was found that a reaction between molten silicon and B4C occurred and the reaction produced silicon carbide and silicon-containing boron carbide. Dense composites can be achieved by pressureless sintering at 1450oC and the final microstructure consists of silicon carbide, boron carbide, silicon-containing boron carbide and residual silicon at grain boundaries.

Abstract: The wear behaviour of Ca a-sialon ceramics of two distinct microstructures, fine equiaxed grains (EQ) and large elongated grains (EL), with the same chemical composition was investigated as a function of apparent contact pressure and sliding speed, using ball-on-disc type tribometers at room temperature and at 600°C. For room temperature tests, the EL microstructure exhibited a lower wear rate than EQ in the severe wear regime due to a greater resistance to large crack-induced material removal. As the apparent contact pressure decreased, mild wear appeared for both microstructures. The mechanism that dominated the material removal in EQ was grain pullout. In contrast, the controlling mechanism for EL was transgranular fracture. Therefore, EL had a lower wear rate than EQ in the mild wear regime. For wear tests at 600°C, crack-induced severe wear occurs in both EQ and EL samples for all contact pressures. EL had a slightly lower wear rate than EQ. Wear particles were generated on the wear track, but no tribofilm was observed and no oxidation products were detected. Wear models revealed that the grain aspect ratio plays a more important role than grain diameter in influencing the crack propagation during severe wear and grain pull-out during mild wear.