Key Engineering Materials Vols. 336-338

Abstract: Effects of different sintering methods such as pressureless sintering and hot press sintering on relative density and microstructure of the Al2O3p/LiTaO3 (ALT) composite ceramics were investigated to obtain a preferable sintering process. Relative densities of all ALT composites are below 90% when sintered with the cold isostatical pressing followed by pressureless sintering at temperatures of 1250 to 1350°C. The relative densities and microstructure of ALT composite ceramics with the hot press sintering process in a N2 atmosphere at 1150 and 1300°C were investigated. The relative density of ALT composite hot pressed at 1150°C is only 77%, and almost theoretical density at 1300°C. This indicates that sintering pressure plays an important role in the densification of ALT composite ceramics in temperature range of 1150 to 1350°C. Investigation on morphologies of the composites shows that the Al2O3 particles distributed along grain boundaries of LiTaO3, which leads to a fine-grained microstructure in the ALT composite ceramics

Abstract: The sintering kinetics model of initial stage by spark plasma sintering (SPS) is discussed in this paper. During SPS, there are discharges among the powder particles. And the particles surface will be melted and form viscose flow. These phenomena accelerate the particles rearrangement and reduce the sintering time. The relationship between the shrinkage ratio of particles and the sintering time during the initial stages of sintering by SPS has been obtained. The results show that
L/L0 is linear to the time.

Abstract: The critical times of phosphorus segregation in low alloy steel 12Cr1MoV at different solution temperatures, 1050°C and 1300°C, are studied and the diffusion coefficient of complexes, Dc, and diffusion coefficient of phosphorus, Di, are calculated. Results show that the calculated result with the kinetic equations perfectly fits with the experimental observations. And that the higher the solution temperature, the higher is the segregation level of phosphorus, and also the longer is the critical time. It also shows that in the segregation process, the rate of phosphorus segregation at first is high due to a large diffusivity of the complex, Dc. Then the segregation rate decreases due to decreasing complex concentration gradient. In the de-segregation process, the level of phosphorus segregation decreases with increasing holding time t. But the rate of phosphorus desegregation from grain boundaries to centre is slower compared with that in segregation for the diffusion coefficient of phosphorus atoms, Di, is far lower than that of the complexes, Dc.

Abstract: Highly structure controlled ceramics were required for improving their properties. Here, we demonstrate such ceramics fabricated by novel colloidal processing; (i) dense nano-sized zirconia without pressure sintering, (ii) porous ceramics with controlled pore size by hetero-coagulated suspension of ceramics and polymer, and (iii) textured ceramics with feeble magnetic susceptibility by electrophoretic deposition in a high magnetic field.

Abstract: The colloidal forming is an important process to prepare complex-shaped parts of ceramics, especially for industrialization of structure ceramics. It has been paid widely attention to by ceramic scientists, industries and governments in the past three decades. In 90’s, because the homogeneous green bodies were prepared using colloidal ways, many new colloidal forming processes were developed continuously such as gelcasting, DCC, TIF, colloidal vibration casting, colloidal injection moulding, HAS, and so on.. However, crackings of sintered bodies from colloidal ways have also often been occurred, especially for large size and solid parts of ceramics. In this paper, some important views about shrinkages from colloidal forming were discussed. The internal stresses in green bodies to prepare using colloidal forming resulted from the shrinkage of suspension during from liquid to solid. The shrinkage led to different cracks during sintering and/or debindering. Finally, it is put forward that the shrinkage from liquid to solid in colloidal froming have to be regarded in the future.

Abstract: The dispersion and rheological behavior of aqueous alumina, zirconia, and mixed alumina/ zirconia slurries have been studied in terms of solid loading and the incorporated multiple processing additives. Darvan-C, Dynol 604, Glycerol, and PVA were used as the dispersant, surfactant, cryoprotectant, and binder, respectively. Sedimentation density increased significantly on adding the dispersant; its effectiveness was similar for both the alumina and zirconia suspensions. With further addition of binder, the sedimentation density decreased. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high order structure formation in the suspended particles. Shear rate rheology of suspensions with high dispersant content showed a shear thinning but with low dispersant content (≤2 wt.%), however, it showed a shear thickening at high shear rate (>600 s-1).

Abstract: The binary micrometer YSZ-TZP suspensions were prepared for the gel-casting molding of YSZ electrolyte in order to improve the mechanical properties of YSZ. The potential, sedimentation and viscosity of YSZ, TZP and TZP-YSZ suspensions were investigated. The results indicated that the dispersant (PAA NH4) can greatly affect the surface chemical properties of both YSZ and TZP, and it can also change the isoelectric points (IEP) of the two powders to lower pH value. The potential and sedimentation tests indicate that the stable suspensions are prepared when pH is between 8 and 10. The viscosity measurements showed that there exists an optimum dispersant content for each suspension. The lowest viscosity can be obtained at the optimum dispersant content. The solid content of the stable micrometer TZP-YSZ suspensions can reach as high as 50vol%.

Abstract: Advanced silicon nitride ceramic compact was prepared by colloidal process. The rheological behaviors of the Si3N4 powders were emphatically investigated. At the same time, the effects of the dispersant, pH value and ball milling time on the silicon nitride slurry were discussed. The results showed that the addition of the dispersant could shift the value of Zeta potential and improve the fluidity of silicon nitride slurries. The well fluidity can be achieved when the amount of the dispersant was 1.2wt%. Besides, a ball milling time of 8h was appropriate. Statistic pattern recognition was used to predict the processing parameters. Then the high density of green compact could be attained while the volume fraction of solid powders was up to 40%, and the fracture toughness of the sintered body reached 7.2MPa·m1/2. The microstructure of final sintered ceramic was observed by scanning electron microscope (SEM).

Abstract: Si3N4 specimens were joined in a high vacuum atmosphere under same experimental conditions using 50μm thick foils of three types of stainless steels (AISI 304, 316 and 321). The mechanical strength of these joints was evaluated by shear tests using a lap bond configuration and small but significant differences were found among them. Nanoindentation experiments were done on cross-sections of the bonded assemblies with a diamond Berkovich tip using an imposed penetration depth of 500 nm to reduce either surface effects or indentation size effect.

Abstract: Joining of ceramics is of importance from both technical and economical points of view. Brazing is a widely used process to join ceramics. In order to increase the working temperature and weld strength of joints, a high temperature brazing process using Ni-Cr-SiC powders (consisting of Ni, Cr and SiC powders) as filler to join recrystallized SiC ceramic has been investigated. The obtained optimized technological parameters are joining temperature of 1360°C, holding time of 5min and filler mass of 280mg. Under these conditions the maximum relative bending strength of joints, 70.5%, is achieved. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer, of which the major phase is Ni2Si, exists between the welding base material SiC ceramic and the filler reaction product layer, called as interlayer, of which the major phase is Cr23C6.