Effects Of Processing Conditions On Microstructure And Properties Of Silicon Nitride

The in-situ toughening is a new process developed recently, which can efficiently improve the fracture toughness of ceramics. Currently, the worldwide attention has been focused on the research of in-situ toughened Si3N4 because of its simple manufacturing techniques, low cost, and excellent performance. The in-situ toughened Si3N4 with Y2O3-La2O3 as additives is a candidate of thermal-structure components.Effects of hot-pressure sintering processes and gas-pressure pre-sintering processes on the microstructure and densification of silicon nitride are investigated in this thesis. The relations between sintering processes and microstructure, densification are analyzed. Influences of machining on the flexure strength of Si3N4 are discussed. The main results of investigation are as follows:(1) By hot-pressing sintering with equimolar Y2O3 and La2O3 as additives, a dense in-situ toughened Si3N4 is prepared. Its density is achieved 99.8 percent of theory density (To) of Si3N4. While Si3N4 over 99 percent To can be prepared by sintering for l~2h at 1800-1840℃.(2) By adjusting sintering processes, the rules of distribution of size and aspect ratio of silicon nitride crystals are investigated. Lower pre-sintering temperature would benefit forming crystals with bigger size and larger aspect ratio within extent of 1640~1700癈, higher pre-sintering temperature would form regular microstructure. Elevating final sintering temperature could accelerate the growth of crystals, with increasing the size and aspect ratio of crystals; meanwhile prolonging final sintering time is of benefi to increasing aspect ratio of crystals, and enlarging the distribution of size.(3) During gas-pressure pre-sintering, the temperature of gas-pressure sintering oven was too high, which leads to some accompanying phenomena. Components of the accompanying thing (for example: whiskers, sinter) are analyzed, the results are shown that the growth of SiC whiskers follow the mechanism of VS.(4) Conduct gas-pressure pre-sintering experiments in hot-pressure sintering oven, and investigate the relation between gas-pressure sintering processes and microstructure, densification of Si3N4. After gas-pressure pre-sintering at 1800℃, the density of Si3N4 don't attach the value which adapt to next step sintering, and the pre-sintering temperature is needed to elevate with higher nitrogen pressure which to prevent the decomposition of Si3N4 in elevated temperature. The higher the first sintering temperature is, the larger the crystals size and aspect ratio are.(5) After several gas-pressure pre-sintering, Si3N4 in powder bed reacts with graphite, and creates silicon carbide. Powder bed could be used repeatedly.(6) After treatment of cutting, grinding, gritting and polishing, defaults on the surface of Si3N4 sample are created by diamond reamer during cutting. There are a few of surface faults created by reamer in high linear velocity. Surface faults of samples are efficiently reduced after polished. A model of Si3N4 suffering force during cutting is established.