Key Engineering Materials Vols. 317-318

Abstract: Fibrous Si3N4 monolithic ceramics with a hexagonal cell were fabricated in the preferred orientation. The densified Si3N4 fibrous monolithic showed fracture toughness of 17 MPa
m1/2. An application of SiC whisker as reinforcement exhibited higher fracture toughness (24 MPa
m1/2). The influences of fiber diameter and reinforcement on the fracture toughness of these fibrous monolithic ceramics were studied in details.

Abstract: A fracture toughness model of fibrous monolithic ceramics revealed that the major factor that contributed to the fracture toughness in ceramics was the actual energy absorbed by crack propagation rather than the total work of fracture. The load-displacement curve and the crack propagation path were predicted using the derived model mirror image with that of experimental data.

Abstract: A novel transparent polycrystalline silicon nitride was fabricated by hot-press sintering with MgO and AlN as additives. The mixed powder with 3 wt.% MgO and 9 wt.% AlN was sintered at 1900oC for 1 hour under 30 MPa pressure in a nitrogen gas atmosphere. Transparent polycrystalline silicon nitride was successfully fabricated. The mechanical properties such as density, hardness, young’s modulus, fracture strength and fracture toughness were evaluated. The effect of α/β phase on the mechanical properties of transparent polycrystalline silicon nitride was investigated. The properties were changed depending on the amount of α/β phase. The hardness and Young's modulus increased with increasing the volume fraction of α-phase fraction as a reflection of the higher hardness of α-phase Si3N4. The fracture toughness and fracture strength decreased with decreasing the volume fraction of β-phase Si3N4.

Abstract: The conductive composites in which nano RuO2 particles are dispersed throughout a glass matrix have been successfully fabricated by sintering at 850°C. The sensing properties of conductive composites were investigated in real time during tensile testing by measuring the electrical resistance. It is shown that the excellent sensing ability based on electrical resistance changes in the low strain range was due to deformation of conduction paths between nano RuO2 particles by brittle fracture of the glass matrix. The change of electrical resistance depends strongly on the volume percent of Al2O3 fiber reinforcement.

Abstract: The photon emission accompanying fracture of a polycrystalline MgO was investigated at room temperature under N2 gas pressures from 10-4 to 105 Pa. At fracture, the ultraviolet, visible and infrared photon emissions instantaneously increased, and then rapidly decreased in most of the experimental conditions. However, in a N2 gas pressure of around 100 Pa, their peak counts lasted for about 10 milliseconds, and the amount of the UV photon emission was fifteen times larger than those obtained in the other N2 gas pressures. This abrupt increment in the emission was explained by the luminescence due to N2 gas discharge according to the classical Townsend’s theory. In conclusion, the photon emission accompanying fracture of a polycrystalline MgO mainly originated from the excited defects as reported by the authors previously, but the N2 gas discharge had a supplementary effect on the emission around a specific N2 gas pressure.

Abstract: Methylsilsesquioxane films were formed on glass substrates by dropping a sol prepared from methyltriethoxysilane and then heat-treated in an oven. Nanoindentation test was performed to assess the elastoplastic properties of the films, including the relative residual depth ξr, Meyer hardness HM, work-of-indentation WI and the elastic modulus E’. The values of ξr, HM and WI were obtained by a Berkovich indenter and E’ was determined by a spherical indenter on the basis of Hertz elastic theory. ξr decreased with the increase in the heat treatment time, whereas HM , WI and E’ significantly increased with the time. The changes in the mechanical properties with the heat treatment time well reflected the evolution of the Si-O-Si network structure in methylsilsequioxane film.

Abstract: Anodic aluminum oxide (AAO) was prepared in three types of aqueous solutions with various applied voltage. The mechanical property of AAO prepared in different electrolyte was investigated and hardness was increased on account of the increase of the thickness between pores. The mechanical property and microstructure change of AAO prepared in oxalic acid at 40V was investigated by heat treatment. AAO prepared in oxalic acid at 40V was transformed from amorphous to crystalline phase by heat treatment above 800oC and hardness was increased about 2.6 times with increase of heat treatment temperature.

Abstract: Nano-structured aluminum was fabricated by accumulative roll-bonding (ARB) process using different rolling methods. One is the ARB using conventional rolling (CR) in which the speed of two rolls (3.0m/min) was equal to each other. The other is the ARB using differential speed rolling (DSR) in which the speed of two rolls is different to each other. The roll peripheral speed of one roll was 2.0m/min and that of another roll was 3.6m/min. The roll speed ratio was kept at 1.8. The ARB was conducted up to 6 cycles at ambient temperature without lubrication. In both cases, the ultrafine grains were developed in the samples. The grains formed by the DSR-ARB were more equiaxed and finer than those produced by the CR-ARB. Tensile strength of the DSR-ARB processed sample was superior to that of the CR-ARB processed one. The elongation was not affected significantly by the number of ARB cycles in both cases. Texture analysis demonstrated that the shear strain, in the case of DSR-ARB, was introduced into the center of thickness. It was concluded that the DSR-ARB process was more effective for grain refinement and strengthening than the CR-ARB process.

Abstract: Infiltrant aluminum was infiltrated into molybdenum disilicide preforms in N2 atmosphere at different temperatures by liquid reactive infiltration processing. The mechanical properties and phase composition of these materials were analyzed and discussed. The experimental results showed that the samples were corresponding to 10 at.% Al averagely after sintered, and consisted of Mo(Al,Si)2 phase and Al-Si alloy phase. High infiltration temperature would lead to Al deficiency mainly in Al-Si alloy phase. The highest bending strength of 737 MPa was reached at infiltration temperature of 1350 °C because the sample had fine and integrated grains, and the strong combination between particles. When the infiltration temperature was higher than 1350 °C, the bending strength of material prepared would decrease because of the phase fragmentation.

Abstract: 6061 aluminum alloy composite reinforced with α-alumina and aluminum borate is fabricated by casting method. The above mentioned composite has been compared to a similar composite without aluminum borate reinforcement for their machinability. The aluminum borate filler was controlled up to (2.5, 5.0 and 30vol %) on the surface of α-alumina. The reaction zone between alumina and the matrix in both the systems are analyzed with the help of TEM micrographs as well as X-ray diffraction profiles. At the reaction zone between alumina and the aluminum alloys, some spinel-like compounds (MgAl2O4) are identified. Finally, machinability was analyzed with the single point tool machining.