Preparation And Characterization Of Si₃N₄ Ceramic With Aligned And Elongated Grains

Silicon nitride (Si₃N₄) ceramics with aligned and elongated grains have many excellent properties, such as, high fracture toughness, flexural strength, thermal conductivity, vickers hardness and good frictional behaviour. So it can be used as engine parts, wire-drawing dies, capacitors and matrix materials in the field of high-performance microelectronics In the paper, silicon nitride ceramic with aligned and elongated grains is prepared by addingα-Si₃N₄ whiskers, tape casting technique and hot-press sintering technique.By detecting the effect of dispersant on Zeta potential of silicon nitride powders, the pH value of the slurry should be controlled at about 11; The optimal contents of additives are determined, containing solvent (absolute ethyl alcohol and butanone), dispersant (triethyl phosphate), binder (polyvinyl butyral), plasticizer (polyethylene glycol and glycerine ). The slurries for tape casting with good dispersity containingα-Si₃N₄ whiskers are made, whose viscosity is less than 900 mPa·s. A baffle plate whose space width is 0.13mm) is added to makeα-Si₃N₄ whiskers align unidirectionally in the tape. The tape with uniform thickness, smooth surface, good flexibility, certain strength, is obtained.The green body are hot pressed sintered at 1350℃-1600℃and 30 MP. When adding the same contents of theα-Si₃N₄ whiskers, the densities of the ceramics sample aligned and elongated grains increase with the increasing of the sintered temperatures. When the samples are sintered at the same temperature, the densities of the samples increase with the increasing of the content ofα-Si₃N₄ whiskers, and the density of the sample sintered at 1600℃is 3.23 g/cm³. The results demonstrate that the densification of the samples is realized by liquid phase sintering with the process of solution- deposition- precipitation of grains,. The rod-likeα-Si₃N₄ grains which transformβ-Si₃N₄ phase is seeded, while the liquid phase sintering process promotes the rod-like grains to become coarse and grow. The results indicate that the rod-like large grains align unidirectionally in the tape casting direction.The analysis of the sintered samples aligned and elongated grains shows that the mechanical properties (fracture toughness, flexural strength, and vickers hardness) are increased with the increasing of the contents ofα-Si₃N₄ whiskers and the sintering temperature. The mechanical properties of different planes of the sintered samples are anisotropic, that of the plane parallel to the tape casting direction is best, that of the side plane is worse, that of the plane normal to the tape casting direction is worst. The highest values of fracture toughness, vickers hardness, and flexural strength of the samples are 10.96 MPa·m^1/2,16.76GPa,925.00MPa, respectively.