Investigations On The Centrifugal Shaping And Properties Of High Performance Ceramics

Alumina ceramics possess the attractive properties of high strength and excellent heat resistance, corrosion resistance, abrasion resistance and dielectric properties. They are used widely in the industrial areas of electronics, metallurgy, machinery, transportation, energy, aerospace and biotechnology. Attentions of researchers have been specially paid to the improvement of Al2O3 powders and fabrication technologies of alumina based composites, aiming at refining microstructure and improving properties and reliabilities of alumina ceramics. Centrifugal slip casting, as a colloidal compaction method of ceramics, has the advantages of high green density, good green homogeneity, and low compaction defects. It is thus one of the advanced processing methods of high performance ceramics and composites. In the present work, the centrifugal slip casting fabrication of Al2O3 ceramics, Al2O3-ZrO2 ceramic matrix composites, and Al2O3/Ni functionally gradient materials (FGM) have been systematically investigated.High purityα-Al2O3 nanopowders with particle sizes ranging from 60～100nm were prepared from the high temperature decomposition of aluminum ammonium sulfate. Centrifugal compaction of theα-Al2O3 nanopowders was investigated to fabricate high performance alumina ceramics. It was found that the amount of dispersant, pH value, and solid content have a remarkable effect on the properties of alumina ceramic slurry. Using a slurry of good fluidity, comparatively high solid content and good stability, alumina greenbodies of high density and good homogeneity has been produced. Alumina ceramics of fine microstructure and good mechanical properties has been obtained from the greenbodies using vacuum sintering. The alumina slurry made with a polycarboxylic dispersant addition of lwt%, a pH value adjusted to 11, and a solid content of 38 vol% has good fluidity and stability, and the density of the greenbody obtained by centrifugal slip casting is 52.36%. Translucent alumina ceramics were fabricated by a pre-heat-treatment at 1200℃for 120 min, flowed by vacuum sintering at 1550℃for 150 min, with a average grain size less than 5μm. The density, three point bending strength, hardness and fracture toughness of the alumina ceramic are 3.98g/cm3,712MPa,19.4GPa and 3.92MPa·m1/2, respectively.The Al2O3-5vol%ZrO2 ceramic matrix composites were fabricated by the centrifugal slip casting method, and the compact density, distribution of Al2O3 and ZrO2, relative density of the sintered composites, microstructure and mechanical properties of the composites were studies in detail. The results show that slurry with good fluidity and stability can be obtained by using suitable amount of dispersant and adopting optimum solid content, which can avoid the phase separation due to the density difference between Al2O3 and ZrO2 during the centrifugal compaction process. For the sintered samples made with 40 vol% and 45 vol% solid content slurries, the microstructure is homogenous through out the sample, and there is nearly no obvious difference in Al2O3 and ZrO2 distribution and sample density along the centrifugal direction. After sintering at 1450℃, the composite sample is homogenous in both microstructure and density, with a mean particle size less than 2μm. For the Al2O3-5vol%ZrO2 composites, compared with the conventional dry compaction method, the centrifugal slip casting process is advantageous in 5% higher green density,50℃lower in sintering temperature, and remarkable higher bending strength (710 MPa compared to 620 MPa).The Al2O3/Ni functionally gradient composites with continuous gradient distribution of phases were fabricated by the centrifugal slip casting method. Though adjusting the characteristic of the slurry, it is possible to control the degree of phase separation due to the density difference between Al2O3 and Ni during the centrifugal slip casting profess, so that the required gradient distribution of Al2O3 and Ni in the functionally gradient composites can be achieved. On the basis of experiment, the amount of binder and solid content in the slurry and ball milling time were optimized. The influencing factors on porosity, three point bending strength and hardness were investigated. It is show that the slurry made with 2 wt% PEG binder,63 vol% solid contents, and a milling period of 36 hours is suitable for the fabrication of the Al2O3/Ni functionally gradient composites by centrifugal slip casting. An Al2O3/Ni functionally gradient composite with excellent mechanical properties, low porosy (0.4%), and a continuous gradient distribution of microstructure and hardness was produced. The composition gradient of the Al2O3/Ni FGM can be tailored by adjusting the solid content and amount of dispersant, and thus its microstructure can be designed and controlled.Monodispersed SiO2 microspheres were prepared by a modified Stober method. Retaining high concentration of ammonium hydroxide was beneficial to the preparation of monodispersed SiO2 microspheres. Highly ordered two dimensional colloidal crystal can be readily obtained by dropping single droplet of SiO2 microsphere suspension with concentration higher than 0.5 vol%. Large size highly ordering three dimensional SiO2 photonic crystal can be easily prepared by centrifugal settling method. The 3D SiO2 photonic crystal had FCC structure, and the main defects were stacking faults and cracks induced by solvent evaporation. Filling the pore of SiO2 photonic crystal with Sb2S3 and then removing SiO2 by hydrogen fluoride etching, the inverse opal structure Sb2S3 photonic crystal was obtained.