Advanced Materials Research Vols. 105-106

Abstract: SiC reinforced MoSi2 composites have been successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. The sintering temperatures and holding time were 1500°C~1650°C at a heating rate of 10K/min and 1 hour, respectively. The microstructure and mechanical properties of the as-sintered composites were investigated. SEM micrographs of SiC/MoSi2 composites showed that SiC particles were homogeneously distributed in MoSi2 matrix. The Vickers hardness, flexural strength and fracture toughness of the SiC/MoSi2 composites were up to 15.50GPa, 468.7MPa and 9.35MPa•m1/2, respectively. The morphologies of fractured surface of the composites revealed the mechanism to improve mechanical properties of MoSi2 matrix. At last, the cyclic oxidation behavior of the composites was discussed. The results of this work showed that in situ SiC/MoSi2 composite powder prepared by MASHS technique could be successfully sintered via pressureless sintering process and significant improvement of room temperature mechanical and anti-oxidation properties could be achieved.

Abstract: Carbon nanotubes (CNTs) are good reinforcement of composite materials, through add appropriate amount of carbon nanotubes to MoSi2 can be improve the strength and toughness of MoSi2. In this paper, the material of CNTs/MoSi2 was made through vacuum hot pressing technology. And the split hopkison press bar (SHPB) technology was used for testing the dynamic mechanical properties of CNTs/MoSi2 composite material which has different proportion of CNTs. In the SHPB experiment, in order to get better waveforms, the waveform shaping technology was used for improving the waveform quality, and also the strain gauge technology was used for testing the real strain of the specimen in the dynamic loading process. Through calculating, the dynamic stress-strain curves which under different high strain rate are given. At the same time, the strength, deformation and other test results are analyzed, and also compared them with the static compression experiment results of the CNTs/MoSi2 specimen which is tested by the electronic universal testing machine, and then obtained the dynamic performance of CNTs/MoSi2 composite material.

Abstract: Several Ti3C2-Cu(Al) cermets were prepared by pressureless sintering or by in-situ hot-extruding a mixture of Ti3AlC2 and Cu powders, their microstructure and properties were investigated. The Al of Ti3AlC2 was dissolved and diffused into liquid Cu, forming Ti3C2-Cu(Al) cermet. Due to strong combination between Ti3C2 and Cu(Al), and special network microstructure formed by Cu(Al) phase surrounding sub-micro-sheet layered Ti3C2 phase, the cermets have quite high fracture strength and electric conductivity. Properties of cermets fabricated by in-situ hot-extruding were further increased.

Abstract: Highly pure and dense bulk Ti2AlC was prepared by hot-pressing a mixture of the “312” phase Ti3AlC2 powders, and the element Ti and Al powders. Different ratios of the starting materials and different sintering temperature were attempted in order to obtain a highly pure and dense bulk Ti2AlC sample. Phase analysis and microstructures observing were performed by using by XRD, SEM as well as an X-ray fluorescence spectrometer. The results show that a nearly full dense bulk Ti2AlC sample can be prepared at 1300°C and 30MPa for 30 minutes in argon atmosphere. A dominant mechanism to form the “211” phase Ti2AlC can be attributed to the directly connecting between Al and Ti6C octahedron, which is as an intermediate phase in the Al-rich liquid mediator during the hot pressing.

Abstract: A cold-extruding and temper means was developed in order to densify the Ti3AlC2 toughened Cu matrix composites with a lower content of Ti3AlC2 ceramic. The Ti3AlC2/Cu samples, with 10%, 15% and 20% of Ti3AlC2 in volume ratio, ware prepared by pressless sintering a mixture of copper and Ti3AlC2 powders, after which were formed by the cold isostatic compaction. The sintered samples ware extruded at room temperature, and then tempered at 950oC. Such treatment was performed twice for obtaining a more remarkable densifying effect. Changes in microstructures were observed by SEM after every extrusion and temper. The results show that particle size of Ti3AlC2 was reduced more than 50% after secondary extrusion and temper, and the particles distribution became more uniform in compared with the untreated samples. As a result of the microstructural change, the densities of the Ti3AlC2/Cu samples were increased about 5 %, and defects such as pores and microcracks were almost entirely slaked.

Abstract: The microstructure evolution during sintering and preparation of nanoceramic materials is studied by molecular dynamics (MD) simulation. A 3D model for nanoceramic crystal body including mainly crystal planes of (100) and (110) is developed. This model is used to simulate the hot pressing of nanoceramic, especially for SiC, by rapid heating-up and cooling-down under certain pressure. In this model, the Tersoff potential function is used to simulate the interatomic force between atoms. The microstructure during melting process and crystallizing process are investigated by analyzing energy evolution, pair correlation function and the graph of instantaneous place of the atoms. The results show that the microstructure is amorphous after melting, and crystal planes of (100) and (110) are different in density, melting point and microstructure, showing anisotropy.

Abstract: Aerogel composites were prepared by immersing ceramic fiber into silica aerogel precursor via supercritical fluid drying. The mechanical behavior of silica aerogel composites was investigated with shrinkage, tension, bending and compression. The influences of fiber volume fraction, aerogel density and heat treatment were examined. After reinforced by ceramic fiber, the shrinkage of aerogel and aerogel-fiber composites was retarded obviously. The mechanical property of aerogel-fiber composites increased first and then decreased with increasing fiber volume fraction from 5.8% to 11.5%. For a given fiber volume fraction, the mechanical strength enhanced with increasing aerogel density. In addition, the mechanical property of aerogel composites did not decrease but increased after heat treatment at 600°C.

Abstract: Apparent activation energy in low temperature aging of two kinds of Y-TZP ceramic was studied in this paper. The ceramics were processed into small pieces, and aging in 100°C, 110°C, 120°C, 130°C and 140°C respectively. The content of monoclinic phase was calculated by X-ray diffraction patterns and the microstructure was scanned by SEM. According to MAJ law and Arrhenius formation, straight lines were fitted in coordinate systems of ln (ln (1/(1 − f )))-ln t and ln b-1/T, and the kinetic parameters of low temperature aging of Y-TZP ceramic were calculated according to these lines. The results revealed that grain size significantly affected apparent activation energy of Y-TZP ceramics.

Abstract: This paper focuses on the influence on the mechanical properties of hydroxyapatite / poly-hydroxybutyrate (HA / PHB) composites by different HA contents, and the effect of Silane coupling agent on improving the interface of HA/PHB composite materials. The HA/PHB composites were prepared by ball milling, hot-pressing, the HA content were 0, 5, 10, 15, 20 and 30wt%, respectively. Silane coupling agent was used for HA surface treatment in HA/PHB composites to compare with not having HA surface treatment in HA/PHB composite materials. The impact strength and flexural strength of HA / PHB composites were tested, and the microstructure of the fracture surface was observed by SEM. The results showed that HA could enhance the bending strength of HA/PHB composites. With HA content increased, the bending strength increased first and then decreased, when the HA content was 10wt%, the maximum bending strength reached 32.74MPa, increased about 41% comparing with pure PHB. Silane coupling agent can improve the interface of HA / PHB, then enhance the mechanical strength. When the HA content was 15wt%, the maximum bending strength reached 46.6MPa, Increased about 56% comparing with the same proportion of untreated HA/PHB composite material, and about double comparing with pure PHB.

Abstract: Artificial neural network (ANN) back-propagation model was developed to predict the fracture design parameters in reinforced ceramic matrix composites (CMCS).Training ANN model was introduced at first. And then the Training data for the development of the neural network model was obtained from the experiments. The input parameters, the presetting deflection and tip radius of the notch, and the output, the cycle times of fracture were used during the network training. The neural network architecture is designed. The ANN model was developed using back propagation architecture with three layers jump connections, where every layer was connected or linked to every previous layer. The number of hidden neurons was determined according to special formula. The performance of system is summarized at last. The ANN model exhibited excellent comparison with the experimental results. It was concluded that predicted fracture design parameters by the trained neural network model seem more reasonable compared to approximate methods. It is possible to claim that, ANN is fairly promising prediction technique if properly used.