Papers by Author: Zheng Ren Huang

Abstract: Silicon nitride (Si₃N₄) ceramics with Y₂O₃-Al₂O₃ as sintering additives were prepared by pressureless sintering technology using two kinds of Si₃N₄ powders as raw materials. The particle size of Si₃N₄ powders ball-milled with various times were characterized and the effects of particle size on the densification behavior of Si₃N₄ ceramics were investigated. The relative density and microstructure of the sintered bodies at different sintering temperatures were determined. The experimental results show that the powders with longer milling time have finer particle size and the samples with fine particle display higher shrinkage-rate at lower sintering temperature than the one with coarse particle. By extending milling time, the density gap between the sintered ceramics using different Si₃N₄ powder become narrower. The main factor resulting in the difference of sintering densification behavior is the particle size of Si₃N₄ powders rather than metallic oxide impurity of raw powders.

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: Abstract. The reaction layer microstructure of SiC/SiC joints brazed by Ag-Cu-Ti filler metal, including composition, morphology, grain size were investigated by X-ray diffraction, electronic probe microanalysis, transmission electron microscope. An obvious reaction layer composed of TiC and Ti5Si3 was observed at the interface of SiC substrate and filler metal. There is a representative structure of SiC substrate/continuous fine TiC layer /discontinuous coarse Ti5Si3 layer/filler metal in the reaction layer. The continuous TiC layer, composed of about 10 nm roundish grains, is 350 ~ 400 nm thick. Ti5Si3 layer is composed of only one row of Ti5Si3 grains, which disperse with diverse size from 100 ~ 500 nm. Different growth behavior of TiC and Ti5Si3 is the main reason to form this microstructure.

Abstract: Ultra-high temperature ceramic composites of ZrB2-SiC were densified by pressureless sintering. Ultra-fine ZrB2-SiC composite powders synthesized by sol-gel method were mixed with commercial ZrB2 and SiC powders. The sintered body of hybrid powders (combined commercial and synthesized composite powders) showed excellent properties not only in the relative density but also the flexure strength. Dry-pressed compacts using 4wt% Mo as a sintering aid were sintered to nearly full density at 2200°C/2h. The average strength was ~560MPa and the maximum was ~632MPa. SEM and TEM showed that SiC particles were distributed homogenously in the ZrB2 matrix and the average particle size was ~5μm. From HRTEM observations, the grain boundaries were apparently free of glassy phases and no intermediate phases existed.

Abstract: Solid state sintered silicon carbide (S-SiC) ceramic is one of the top optical materials for high space reliability and other excellent properties. Two microstructures were produced by sintering under different conditions. The effects of microstructure on removal rates of SiC ceramics during polishing processes were studied. The material removal mechanisms during polishing were analysed and modeled. With the increase of the aspect ratio and grain diameter size during polishing, grain pull-out is more difficult in elongated grains than in exquiaxed grains. The SiC ceramic with high hardness has high removal resistance leading to get bad surface quality under the same mechanical procedure. The samples with elongated microstructure have low hardness and surface toughness.

Abstract: Stable ZrB2-based ultra-high temperature ceramic slurries were prepared and characterized with a solid content 40 vol% by aqueous gelcasting which was suitable to form high quality and complexshaped ceramic parts. In the present work, Ammonium Citrate Tribasic (ACT) was used as the dispersant. The properties of ZrB2 slurry, and the influencing factors were investigated by conventional techniques such as sedimentation tests, particle size distribution measurements, electrokinetic measurements and rheological analysis. High solids loading and low viscosity slurry was obtained by controlling the optimal conditions for the ZrB2-based ceramic powders.

Abstract: The effects of high-energy ball milling on SiC powders were studied using a planetary apparatus. Conditions to obtain nanostructured SiC powders with an average crystallite size of 4 nm were determined and powders were characterized by XRD, SEM and TEM analyses. This process was applied to prepare fine powders leading to dense SiC ceramics by sintering at 1900oC for 30 minutes under 30 MPa in argon.

Abstract: To obtain high performance ceramic matrix composites (CMCs), fiber coatings are often fabricated as the interphase between fiber and matrix. The SiC coating was synthesized at low temperature and reduced pressure in the present experiment. SiC was derived from a gaseous methyltrichlorosilane (MTS)/H2 precursor by chemical vapor infiltration (CVI). The thickness of the coating was inspected by SEM. The correlation between the coating thickness and the depositing conditions, i.e. the deposition temperature, the pressure, the deposition time per pulse and the pulse number were investigated. Based on these work, the C/SiC double-layer coating was fabricated.

Abstract: 3-D braided C fiber preform was used to reinforce SiC matrix by polymer infiltration and pyrolysis (PIP). The effect of PCS pyrolysis process on the uncoated carbon fiber was studied. During the pyrolysis, amorphous SiCxOy and some free silicon yielded. The Si element diffused into the C fiber from the matrix because of the concentration gradient at high temperature and destroyed the intrinsic structure of the uncoated C fiber. At the same time, the free Si reacted with the uncoated C fiber. Thus, strong bonding between the fiber and matrix was formed. As a result, bending strength of the composite was decreased.