High-Performance Ceramics III

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Authors: Ren Li Fu, Ke Xin Chen, José Maria F. Ferreira
Abstract: In this paper, b-SiAlON whiskers were synthesized by combustion of silicon and aluminium powders with SiO2 and Y2O3 as additives and a-Si3N4 or b-SiAlON powders as diluents. SEM observation of the products showed that the crystals with an aspect ratio higher that 5, which could be assigned to almost pure and well crystallized b-SiAlON. The influence of the amount of NH4F additive, the type of diluent on the phase formation and on the morphology of products was investigated.
Authors: Qian Lin Chen, Jian Qing Wu, Xian Ping He, Jian Xin Cao
Abstract: In this article, the β–Sialon with average diameter size less than 20µm is prepared and researched by use of aluminum, silicon, and corundum at 1300°C holding temperature for 8h under nitrogen atmosphere. The preparation temperature of β–Sialon powder was optimized according to the results from the experiments that different aluminum and silicon reacted with pure nitrogen at different temperature. The reaction at the preparation of β–Sialon powder was discussed by thermodynamic calculation. The reaction mechanism was investigated by x–ray diffraction (XRD) and scanning electron microscope (SEM) was discussed in detail.
Authors: Jun Ting Luo, Kai Feng Zhang, Guo Feng Wang, Guo Qing Chen
Abstract: Si3N4 ceramic bodies were prepared by liquid phase sintering (LPS) with the amorphous nano-sized Si3N4 powders. Nano-sized Al2O3 and Y2O3 powders were introduced as additives. XRD analysis showed that the sintered body consists of β-Si3N4 and Si2N2O which confirms that phase change temperature of β-Si3N4 is lower than traditional Si3N4. SEM examination showed that the grain size of sintered body is smaller than 300 nm. Superplastic forming can be undertaken at the low temperature of 1550°C in a nitrogen atmosphere when the forming velocity is less than 0.5 mm/min. The formed parts rupture when the forming velocity is 1 mm/min or the forming temperature is 1500°C. Only a few defects are observed in the blank before forming, but many cavity groups are present in the formed workpiece.
Authors: Zong Han Xie, Mark Hoffman, Robert J. Moon, P. R. Munroe, Yi Bing Cheng
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.
Authors: Xin Lu, Xiao Shan Ning, Wei Xu, He Ping Zhou, Ke Xin Chen
Abstract: In the study, the spark plasma sintering (SPS) method was adopted to fabricate Si3N4 ceramics with Y2O3-MgO additives. Specimens with different grain dimensions, grain shapes, α/β phase ratios, densities were obtained by changing the heating rate and dwell time of SPS. The relationship between the microstructure and the thermal conductivity was studied. Results show that the heating rate and the dwell time have great influence on the microstructure and properties of Si3N4 ceramics. Both equiaxed and columnar β- Si3N4 grains are formed during sintering, but the thermal conductivity of Si3N4 ceramics is affected only by columnar grains. The thermal conductivity of the ceramics increases together with the formation and the growth of the β- Si3N4 columnar grains.
Authors: Jian Qing Wu, Fang Li Yuan, Xin Jun Zeng, Ping Gen Rao
Abstract: A new method called cation extraction process has been applied to improve the oxidation resistance of silicon nitride ceramics with sintering additives. It is found that the high temperature oxidation rate of the Si3N4 ceramic treated by the process decreased remarkably in contrast to the oxidation rate of the Si3N4 without the process. The oxidation rate of Si3N4 ceramics with the cation extraction process is reduced more than 50% comparing with that of the Si3N4 ceramics without the process. The morphological characteristic and phase composition of the oxides were analyzed by means of SEM and XRD. The oxidation rate of silicon nitride after cation extraction treatment is determined by the quality of the “purified layer” formed below the pre-oxide scale. Thicker oxide layer formed in pre-oxidation, higher extraction temperature and longer extraction time are helpful to form a good “purified layer”.
Authors: Su Min Zhu, Ruo Ding Wang, Na Li, Hong'an Xi, Qin Li, Shu Qiang Ding
Abstract: Porous silicon carbide (SiC) ceramics were fabricated by a polycarbosilane (PCS) conversion bonding technique, in which PCS was used as a binder to bond SiC particles with each other. In the preparing process, SiC particles were first coated with PCS, and then the powder compacts were heat-treated in an inert atmosphere. During the heat-treatment, the PCS decomposed and gradually converted to inorganic covalent solids composed mainly of Si-C networks. The pyrolysis process of PCS, the pore structures and flexural strength of the as-prepared specimens were analyzed and discussed. Preparing temperature as low as 1100°C was adopted in this process and the porous SiC ceramics with a flexural strength of 20 MPa at an open porosity of 43% was obtained. Since PCS was used as a binder, the critical feature of this technique was that the preparation of porous SiC body was achieved at a low temperature.
Authors: Yan Xiang Wang, Shou Hong Tan, Dong Liang Jiang
Abstract: The effects of microstructure on strength, toughness and corrosion resistance of reactionformed silicon carbide and heat treatment on the properties of reaction-formed silicon carbide have been investigated in this paper. The results show strength and corrosion resistance of reaction-formed silicon carbide decrease with increasing Si content. Fracture toughness shows no rule with free Si content. The average SiC grain size and bonding area of SiC grain increase during heat treatment at 1850°C. The strengths of the samples after heat treatment decrease compared with that of the samples before heat treatment due to the pores in the heat treatment bodies.
Authors: Rui Zhang, Hai Long Wang, Lian Gao, Shao Kang Guan, Jing Kun Guo
Abstract: Copper coated silicon carbide clusters were used to fabricate nanocomposite. Compacts were isostatically pressed and heated in nitrogen atmosphere. Microstructure observations were carried out to show the spheroid growth of the coated clusters. Spheroid growth was found to proceed through coalescence of smaller spheroids of the coated Cu/SiC composite particles. The densification process contains steps of agglomeration – kernel-shell formation – slumping movement. Grain growth of the adherent Cu particles is suppressed due to the constraint of rigid SiC particles. This is the deterministic characteristic of the coated composite particles.
Authors: Xu Guang Tong, Xiao Zhan Yang, Jian Bao Li, Geliang Sun
Abstract: In this paper, the models of multilayer absorbing coating and hollow ball are studied. For the former model, a computer program is made to simulate absorbing results with the electromagnetic transmission line theory. The results show that there is a relevant absorbing peak at different frequencies for different dielectric constant (ε) and the apex’s height and position change with ε and thickness of the multilayer coatings. This program can predict the coating’s absorbing property efficiently. With the latter model, it is found that SiC hollow micro-balls whose porosity is about 0.89 make the best absorbing effect.

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