Key Engineering Materials Vols. 353-358

Abstract: Grafting of maleic anhydride (MAH) onto high density polyethylene (HDPE) was carried out in melt state through the mechanical force initiation caused by high-speed rotation of the twin-screw, investigating its functional reaction and the grafting degree influenced by the rotation of the twin-screw, observing the influence on its mechanical property. The results showed that the increase of grafting degree made the interface phase produce a firm chemical coupling, which improved the tensile-strength and flexural strength of the composites. The produced interfacial stress from the shrinkage of specimen melts in injection molding process can strain-induce the forming of the extended-chain crystals in the matrix and obviously improve the notched impact strength of the composites.

Abstract: An inexpensive fly ash (FA), which is from a waste production, has been employed to fabricate fly ash/epoxy composites in our work. Three kinds of fly ash with the most probable diameters of 74"m, 119"m and 146"m were filled in the modified epoxy resin (EP). The purpose of this study is to characterize the dynamic mechanical properties of such composites, and the dynamic mechanical behaviors of the composites are investigated in the temperature range from -40 to 150oC using a tension-compression mode. The results indicate that the dynamic elastic moduli for the fly ash/epoxy composites are (1.4~2.0) GPa, and the peak values of loss factor (tanδ) for these composites can reach (0.79~0.90) in the test specification. In addition, a scanning electron microscope (SEM) has been used to observe the distribution of fly ash particles in the matrix, as well as the photographs of fracture surface of composites.

Abstract: The graphite/2024Al composites have been fabricated by improved Squeeze Exhaust Casting (SQEC) method. Two kinds of graphite preforms with porosities of 13% and 17% respectively were infiltrated with 2024Al (Al-5Cu-2Mg) alloy under the pressure of 73MPa. The disadvantages of traditional Squeeze Casting (SQC) were avoided and the distribution of aluminum alloy appeared homogenous 3D network in the composites. Flexural strength and Young’s modulus were determined at room temperature. Compared to graphite preform, the composites exhibited a significant enhancement of mechanical properties. The flexural strength and Young’s modulus of X-Y direction of G186/2024Al composites increased from 38.6MPa to 99.7MPa and from 10.1GPa to 19.7GPa, respectively. The fracture mechanism of the composites was discussed on the basis of fracture surfaces.

Abstract: The Si3N4- Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method. The sintering temperatures ranged from 1500°C to 1700°C. Microstructure and component of the composites were performed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results show that sintered body consists of Si2N2O and β-Si3N4, with an average grain size about 1μm. The maximum value of flexural strength of the material is 680MPa when sintered at 1700°C. Transcrystalline cracking is the main fracture mechanism of the composites.

Abstract: Nanoparticles can be made into bulk material by sintering process in order to obtain some excelled properties of nanomaterials. Because ZrO2 has a good thermal-resistance property, it has been widely used as thermal-barrier material, including functionally graded material (FGM) and thermal barrier coating (TBC). In the present paper, ZrO2 nanoparticles with a size of 10, 50, 80 and 100 nm were fabricated into bulk material respectively with the help of cold and isostatic pressing processes. The thermal diffusion coefficient, the hot expand coefficient and the specific heat of these bulk materials were measured. The experimental results showed that after ZrO2 nanoparticles being pressed and sintered, the nanoparticles were found in a state of aggregation in the bulk materials, but there are some nanometer size effects in their thermal physic properties. It was found that the thermal diffusion coefficients between 100-150oC were obviously different, as the samples were made of ZrO2 with different nanometer dimensions. The less the nanometer size of particles, the higher the thermal diffusion coefficient. The hot expand coefficient of the sample from 100 nm ZrO2 particles was 96.9741×10-7 K-1 between 30-300 oC. However, the hot expand coefficient of the sample from 10 nm ZrO2 particles was 100.2345×10-7 K-1. On the other hand, the specific heat of the bulk material from ZrO2 nanoparticles was much higher than that of the bulk material from micron ZrO2 particles. When the temperature was over 350 oC, the size of ZrO2 nanoparticles influenced the specific heat of bulk material even more. With the decreasing of size of ZrO2 nanoparticles the specific heat of bulk material increased continuously.

Abstract: Metal particles were embedded in a silicon carbonitride (SiCN) matrix, derived from the commercially available polysilazane Ceraset®. Metal powders, such as Fe, Mn, Co, Ni, were mixed and milled with pre-cross-linked polysilazane and subsequent pyrolysis. The metals act as active fillers to increase the density of composite. The phases and microstructures of metal/ceramic composites were studied using XRD, SEM and EDS. The magnetic property was measured with Magnetic Property Measurement System at –196oC and 27oC. The results show that there were two main domains in composites, one was metal-rich domain and another was metal-poor domain. The reaction compound between metal and matrix had great effect on the magnetic properties of composites, filled with different metals.

Abstract: SiC/SiC composites were fabricated by hot pressing (HP) via liquid phase sintering (LPS) using carbon coated 2D woven Tyranno SA fabrics as reinforcement. Both nano-SiC and micro-SiC powders with sintering additives were used for matrix. The effects of preparation conditions on the microstructure and mechanical properties of the composites were characterized. Highly densified composite was obtained at 1780°C under 20MPa with nano-SiC particles. The strength and elastic modulus of the composite were enhanced. When micro-SiC powder was used, higher strength revealed for the composite sintered at 1780°C under 15MPa, although it was not densified enough. Higher sintering temperature (1800°C) is beneficial for the densification of the composite, but is not obvious for the improvement of mechanical properties.

Abstract: The emerging cause of arc-microcracks is interpreted and the interaction between ceramic particles and arc-microcracks is accounted for determining the average disturbance strain and equivalent eigen strains by using Mori-Tanaka method. Then the micromechanical stress field of the cermets is estimated. Results show that the micromechanical stress field is not associated with the distribution of arc-microcracks. The stress in matrix and ceramic particles will increase when the volume fractions of arc-microcracks increases.

Abstract: The influences of different sintering aids, including Fe2O3, Fe(NO3)3·9H2O, Y2O3+Al2O3 and MgO+Al2O3+SiO2, on the nitridation of reaction bonded Si3N4/BN ceramics were conducted at 1350°C for 2h. Results indicate that the addition of sintering aids could facilitate the nitridation process resulting in higher nitridation percent due to the reactions between sintering aids and surface silica on silicon powder. When 5wt. % of Y2O3 and 2wt. % Al2O3 were added, the nitridation percent reached to 94.4%. The addition of sintering aids has obvious effect on the ratio of α-Si3N4/β-Si3N4. The increase of β-Si3N4 ratio was attributed to the direct reaction of silicon with nitrogen.

Abstract: The feasibility of fabricating h-BN-SiC high-temperature ceramics by in-situ combustion synthesis was demonstrated by igniting the mixture of boron carbide and silicon powder under 100MPa nitrogen pressure. The reaction thermodynamics and the adiabatic combustion temperature were calculated theoretically. The phase composition, microstructure and mechanical properties of composite were identified by XRD and SEM. The maximum bending strength and fracture toughness of the composite were 65.2 MPa and 1.4 MPa·m1/2 under room temperature, respectively. The effects of h-BN and SiC dilution contents on the mechanical properties of composite were also discussed.