Studies On Preparation, Microstructure And Properties Of Silicon Nitride With Ternary Sintering Additives

With the development of the aerospace, automotive and other manufacturing industry, the usage of the hard cutting material (such as cast iron, high temperature alloy) has increased dramatically. With the new requirement of the manufacturing, no cutting fluid was used, but the cutting temperature was more than 800 ℃. The cutting life decreased rapidly. It was urgent to develop the high temperature tool materials. Due to the high mechanical properties, high wear resistance, high-temperature ability and economics, silicon nitride was suitable for the tool materials. This dissertation was based on the application of the silicon nitride cutting tools, studying the influence of powder characteristics and ternary sintering additives on the microstructure, mechanical poperties and wear resistance of silicon nitride by hot pressing sintering. With the addition of ternary sintering additives, the high toughness and strength silicon nitride cutting tools were obtained, and the cutting performance was tested. A novel texturing method was put forward.The influence of the starting powder characteristics on sintering behaviors, microstructures and mechanical properties of hot-pressed Si3N4 ceramics with and without the MgO-Yb2O3-La2O3 sintering additives was investigated. The five Si3N4 powders were synthesized by diimide decomposition, direct nitridation, self-propagating high temperature synthesis and vapour phase synthesis. For the system without sintering additives:the high amount of the impurities in the starting powders could facilitate the densification process and promote β-Si3N4 grain growth. However, the whisker-like particles (a phase) present in the initial powders inhibited the sintering process, which led to a lower density of Si3N4 ceramics. The whisker was dissolved in the high temperature. The influence of other powder characteristics was not obvious on the densification and microstructure development. For the system with sintering additives:the effects of impurities content in the initial powders on the densification and micro structure were not apparent. And there was tendency that finer powder size resulted in finer microstructure. However, the high a-phase ratio in the initial powders could accelerate the abnormal grain growth and thus lead to better mechanical properties.The influence of ternary sintering additives MgO-Lu2O3-Re2O3 and MgO-Yb203-Re2O3 on the densification, microstructures and mechanical properties of hot-pressed S13N4 ceramics was studied. The influence of the rare earth on the grain boundary phase and grain boundary film was studied by using the HR-TEM. The relative densities of all the samples were more than 97% with the ternary sintering additives. With the addition of the MgO-Lu2O3-Re2O3, the Sm2O3 and Er2O3 could facilitate Lu2O3 to crystallization as Lu4Si2N2O7 in the silicon nitride. With the addition of the MgO-Yb2O3-Re2O3, the Gd2O3 could facilitate Yb2O3 to crystallization as Yb4Si2N2O7. By the addition of the MgO-Lu2O3-Re2O3(La2O3, Gd2O3, Er2O3), the flexure strength and toughness of the silicon nitride were more than 900 MPa and 9 MPa-m1/2, respectively. By the addition of the MgO-Yb2O3-La2O3, the flexure strength and toughness of the silicon nitride were 852 MPa and 11.84 MPa·m1/2, respectively. By the addition of the MgO-Yb2O3-Lu2O3, the flexure strength and toughness of the silicon nitride were 905 MPa and 9.44 MPa-m1/2, and Weibull modulus was 8.28. By the addition of the MgO-Yb2O3-Re2O3 sintering additive, the grain boundary film thickness was influenced by the multiple rare earths. The film thickness was in the 0.5-5.5 nm. And there were no amorphous phase in some grain boundary.The influence of ternary additives on wear behavior and cutting performance of Si3N4 ceramics was studied. And the influence of second phase crystallization on the cutting performance was studied. By the addition of MgO-Lu2O3-Re2O3, with the increasing of the radius of Re3+, the frictional coefficient decreased. The Lateral-crack chipping model based on the fracture toughness and hardness explained the specific wear rate of the silicon nitride increased with the increasing of the hardness-toughness factor. The sample SN-LuSm and SN-LuEr with second phase Lu4Si2N2O7 showed higher wear resistance than the liner relationship. The cutting life of the silicon nitride tools decreased with the increasing of the hardness-toughness factor. Due to the crystallization of the Lu4Si2N2O7, the cutting life of the silicon nitride tools after heat treatment extended 1.37 times.It proposed a novel hot pressing flowing sintering for texturing ceramics. With the addition of the MgO-Yb2O3-La2O3, the equiaxed a-Si3N4 particles changed to the rod-like B-Si3N4 nuclei during earlier sintering stage. And the specimen with rod-like β-Si3N4 nuclei flowed along the plane which perpendicular to the hot pressing direction under pressure. Due to the small size of the B-Si3N4 nuclei and the high porosity of the flowing specimen, the perfectly 2-dimensional textured Si3N4 ceramics were fabricated by hot pressing flowing sintering. The Lotgering orientation factor fL for Si3N4 texture was 0.9975. After aligned, the growth of the B-Si3N4 grains was dynamic grain growth with little steric hindrance. So the grains size and aspect ratio were higher than hot pressed specimen.