Key Engineering Materials Vols. 368-372

Abstract: The B4C/BN laminated ceramics were prepared by tape casting/coating. The effect of structure on the mechanical behavior of the B4C/BN was studied. The results showed that the fracture toughness and bending strength are optimal when the layer thickness ratio (eB4C/eBN) is 15. The SEM photographs of B4C/BN laminated ceramic indicated that the high fracture energy mainly resulting from crack deflection and crack delamination at the BN weak interface.

Abstract: The fabrication of electrodes is one of the key techniques in constructing thermoelectric elements for the practical applications. In this work, the commercial active brazing alloy “Incusil-ABA” was used for the joining of CoSb3 to the Cu surface of the graded electrode materials (Cu/AlN/Cu) by using spark plasma sintering (SPS). The bonding was performed in vacuum at temperatures 500°C for 10min. The brazing and diffusion bonding process were investigated by analyzing the crystal structure and microstructure of the bonding interface using X-ray diffraction and scanning electron microscopy, and its composition distribution was also analyzed by energy dispersive X-ray.

Abstract: Splat morphologies of YSZ/NiCrCoAlY TBCs by plasma spray were characterized using SEM. Based on some assumptions, plasma jet temperature field was established by numerical simulation and the effects of spray parameters are individually studied to optimize the spray process parameters. Furthermore, the interaction wetting ability was taken into account in order to investigate the interaction effect between ZrO2 and NiCrCoAlY droplets. It shows that T enhances markedly with increasing in I and decreasing in FAr; while in despite of the insignificant effect of FHe on T, the temperature of powder in plasma jet enhances with increasing in FHe. As a result of higher thermal conductivity of powder in He plasma gas. NiCrCoAlY droplet impacting solidified NiCrCoAlY splat wets well and spreads fluently while that impacts solidified YSZ splat spreads restrictedly for the poor wetting on the YSZ splat surface.

Abstract: Multilayered mullite/Mo cermet functionally gradient material (FGM) was fabricated through a powder stacking process and sintering in vacuum up to 1500°C. The composition distribution, microstructure and microhardness of the FGM were analyzed. The results indicated that the mullite/Mo FGM was well densified with composite graded distribution and there was no new phase appeared. The microhardness of the FGM increase from 467HV to 1543HV, the density from 2.919 to 7.106 g/cm3 as the content of mullite increase from 20% to 80%. The relative density exceeded 90% as the content of mullite reached 80%. Thermal shock test showed that the cracks passed through the joint without deflection.

Abstract: A new kind of functionally graded materials (FGM) with density gradient has come to show great potentials as flier-plates for creating quasi-isotropic compression waves. In order to meet the demand of lower density in the front face and wider density range for such flier-plate, Mg with a density of 1.74g/cm3 and W of 19.3g/cm3 are selected to make Mg-W system density graded materials. Mg-W alloys with various mass fractions of Mg and W were sintered by spark plasma sintering (SPS) technique at low temperatures, and the processing of densification is mainly investigated. It is found that, up to 92wt%W, the Mg-W alloys can be fully densified at 873K due to the conglutination of Mg particles. The Mg-W alloys still exist as a mechanical mixture of Mg and W. Finally, the Mg-W density graded materials with a density change from 1.74g/cm3 to 10.55g/cm3 have been successfully prepared.

Abstract: WC-Cr3C2 cermet coating on carbon steel was fabricated by laser controlled reactive synthesis method. Effect of diluents is the key factor influencing the cladding process. When the amount of diluents is 30%, the reaction becomes steadily and the smooth coating can be achieved. Observations of the microstructure and phase composition of the sample showed that the coating was a complex phase system which consisted of WC and Cr3C2. In order to improve bond strength of the coating, Ni was selected as additive. The interface morphology showed that the bonding between coating and substrate was metallurgical bonding.

Abstract: Using the processes of field-activated and pressure-assisted combustion synthesis (FAPACS), FGM materials (FGMs) were prepared under the conditions of field-assisted and the hot-press. The microstructure and the phase composition of the interface of the graded materials were investigated and the results showed that the metallurgical joining layer was formed in the interfaces of the (TiB2)pNi/Ni3Al/405 steel. The mechanical characterization of the gradient materials showed that the composition and the micro-hardness of the gradient material were gradient distributed, and its surface Rockwell hardness and wear resistance are better than that of hardened 20Cr steel.

Abstract: Nb-based systems have been considered to be the most attractive ceramic system in the research field of lead-free piezoceramics, since excellent piezoelectric properties were reported in (Li,Na,K)NbO3-based solid solution (LNKN) systems. This article reviews the characteristic and recent research activity for several kinds of the niobate systems including LiNbO3, NaNbO3, KNbO3, tungsten-bronze niobate. The importance of material design for enhancement of piezoelectric properties and their stability in the wide temperature range is also described for LNKN system.

Abstract: KNbO3 (KN) powder was synthesized by modified Pechini method (polymeric precursor method). KN powder with different crystal structure and grain size distribution was synthesized by different drying temperature. In this paper, the mechanism of preparation of KN powder was discussed in thermal decomposition behavior. The obtained powder was characterized by XRD, TEM, FT-IR and TG-DTA.

Abstract: Powders of the solid solution series [Na0.5K0.5NbO3]1-x - [LiTaO3]x, with x = 0.06, are shown to transform from an orthorhombic to a tetragonal crystal system as calcination temperature is increased in the range 850 °C to 1100 °C. Addition of 3 wt% of each alkali carbonate prior to calcination allows retention of the orthorhombic phase to 1100 oC. The latter changes in phase content are attributed to a partial compensation of alkali oxide volatilisation losses. The approach of adding extra alkali carbonates lead to an increase in d33 coefficient in sintered ceramics.