Polyamide 6 composites reinforced with silicon nitride whiskers: Synthesis,interface interaction,and mechanical properties

In this study, polyamide 6 (PA6)/silicon nitride whisker (SNW) composites were fabricated via in situ hydrolytic ring‐opening polymerization of ε‐caprolactam. By this novel method, SNWs can be dispersed uniformly in PA6 matrix. The interface interaction between SNW and PA6 was investigated, and the reinforcing efficiency of SNW on PA6 was evaluated. It was revealed by Fourier transform infrared spectroscopy that a large amount of polar groups, such as ? SiOH, ? NH, and ? NH₂, were present on the SNWs, by which hydrogen bonding and covalent bonding can be formed at interface. Mechanical test showed that at a loading of 5.0 wt % SNW, the tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength of PA6/SNW composite were 37.9, 80.5, 60.3, 73.9, and 64% higher than those of neat PA6, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Formation of SiC whiskers/leucite-based ceramic composites from low temperature hardening geopolymer

In this study, SiC whiskers (SCWS) reinforced geopolymer composites (SCWS/KGP) and their ceramic products (SCWS/leucite) were prepared, and effects of SiC whiskers contents on the microstructure and flexural strength of the SCWS/KGP and SCWS/leucite composites were investigated. The results show that the whisker addition has little influence on both phase composition and thermal shrinkage of the KGP composites, but a suitable content of whisker will result in the improved flexural strength, and when the SCWS content is 2 wt%, flexural strength of the SCWS/KGP composite is enhanced by 95% compared with the neat geopolymer. The flexural strength of the composites can be further enhanced significantly after the composites being treated at 1100 °C and 1200 °C and flexural strength of the composite with SCWS content of 2 wt% was 107% and 125% higher than the untreated counterpart, respectively. The increase in flexural strength of the composites should be attributed to the strong leucite formation, whisker debonding and pulling out from matrix during the fracturing process based on the good interfacial bonding state between whisker and leucite matrix.     

半水硫酸钙晶须水化质量分数对其水化能力的影响研究

半水硫酸钙晶须的水化能力对其应用有较大影响,为掌握半水硫酸钙晶须水化质量分数对其水化能力的影响规律,对半水硫酸钙晶须在不同水化质量分数下的水化过程进行了研究.在此基础上,采用XRD、SEM等对晶须水化产物进行了表征,结果表明:随着半水硫酸钙晶须水化质量分数的增大,其水化速度减慢;当半水硫酸钙晶须水化质量分数为1.0％时...     

Synergism of Toughening Mechanisms in Whisker-Reinforced Ceramic-Matrix Composites

The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al₂O₃/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.     

Synthesis of Mullite Whiskers and Their Application in Composites

Mullite whiskers were synthesized by a vapor-solid reaction. Mullite composition xerogels were fired at 900° to 1600°C with AlF₃ in an airtight container. An average whisker increased in length from 7 μm at 1100°C to 10 μm at 1600°C, whereas an average whisker decreased in aspect ratio from 25 to 10 with increased firing temperature. The whiskers elongated to the c -axis and the side planes were the {110}. A clear lattice image corresponding to (110) lattice spacing up to the edges of the whiskers was observed with high-resolution electron microscopy, and no droplet was observed on the tips of the whiskers. The chemical composition of the whiskers synthesized below 1100°C showed an apparent Al₂O₃-rich composition of about 72 mol%. Composites reinforced by 15 vol% of mullite whiskers in the matrix of 75 vol% mullite/25 vol% Y-TZP enhanced the fracture toughness compared with those materials without mullite whiskers.     

Colloidal Processing and Mechanical Properties of Whisker-Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD-processed ultrafine mullite powder (<0.1 μm), -Si₃N₄ whisker and -mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze-dried porous green compacts were hot-pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N₂ atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si₃N₄ whisker increased the density, flexural strength, and fracture toughness of the hot-pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.     

Direct Pyrolysis of Raw Rice Husks for Maximization of Silicon Carbide Whisker Formation

A new alternate route for the production of SiC from rice husks has been described. The proposed single-stage process has the advantage of reduced time of production and a much higher yield of SiC whiskers than the conventional two-stage process. Direct pyrolysis of raw rice husks was carried out in a graphite furnace under vacuum and the reaction products were analyzed through macroscopic examination, XRD, SEM, and chemical analysis. Effects of time and temperature on the relative yields of the various reaction products have been determined over a wide range. The pyrolysis of raw rice husks has been found to be accompanied by four different competitive processes, viz., formation of SiC whiskers, formation of SiC polycrystals, crystallization of amorphous silica, and graphitization of amorphous carbon. Crystallization of silica and formation of SiC( w ) are dominant processes at 1150° to 1310°C. At higher temperatures formation of SiC polycrystals and graphitization of carbon are more favored. A multistep pyrolysis to 1310°C was found to yield a still higher amount of SiC( w ) than direct pyrolysis.     

Effect of Processing Temperature on the Morphology of Silicon Carbide Whiskers

Two kinds of SiC whiskers were annealed at temperatures similar to those for the processing of ceramic matrix composites. The morphology and structure of the as-received and postannealed whiskers were investigated by SEM, TEM, and XRD, and the influence of processing temperature on the mechanical properties of ceramic matrix composites was discussed.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.