Preparation Of Silicon Carbide Ceramics Reinforced By Particles Or/and Whiskers And Their Application In Mechanical Seals

Silicon carbide ceramic has been widely used in the mechanical sealing of industrial equipments used in all kinds of special conditions, which has become the fourth mechanical seals because of its low weight, high strength, high hardness, resistance to abrasion and corrosion, good chemical stability and so on. The present industrial silicon carbide seals are mainly prepared by reaction-bonding method, which can not be used in some complicated conditions because of their low high-temperature resistance, abrasion resistance and mechanical properties. Seals with excellent mechanical properties and good corrosion resistance are produced by solid state pressureless sintering, and have high hardness and elastic modulus. However, their flexural strength and fracture toughness are still not high enough and they are easy to wear due to high friction coefficient and low self-lubrication, which reduces their reliability and working life. In addition, solid state pressureless sintering needs a temperature as high as 2300℃and a large amount of energy, which raises the cost and restricts the extension of use of silicon carbide seals. A large number of scholars have studied silicon carbide ceramics prepared by liquid phase pressureless sintering, but industrial liquid phase sintered silicon carbide seals are hardly reported.With industrial silicon carbide powders as matrix, liquid phase pressureless sintering was applied in the manufacutre of silicon carbide seals. The paper focused on the preparation of silicon carbide ceramics reinforced by particles or/and whiskers and their application in silicon carbide seals. The secondary phases with high mechanical properties were added to improve the flexural strength and fracture toughness of silicon carbide ceramics. In addition, self-lubricating graphite was also added to improve the friction properties of silicon carbide ceramics. Silicon carbide seals with high performances were produced by industrial producing process with a low cost. The main results are as follows:Nanometer and micrometer SiC whiskers, SiC nanoparticles. SiB6 nanoparticles. TiN nanoparticles were used as the secondary materials, and the effects of pH value, the kind and amount of dispersants on the dispersion of the secondary materials in aqueous media and the rheology and spray-drying of silicon carbide composite slurry were investigated. The results showed:(1) pH value has an effect on the dispersion of SiC nanoparticles, nanometer and micrometer SiC whiskers, but the effects of pH value disappeared when the sedimentation time lasted longer. (2) Sodium polyphosphate could disperse the secondary materials mentioned above, and the relative sedimentation height was all above 94.0% when its content was appropriate. The main mechanism of sodium polyphosphate was improving the static electricity among whiskers. (3) Carboxyl methyl cellulose sodium could disperse micrometer SiC whiskers effectively. When the content of carboxyl methyl cellulose sodium was 4.0wt% and the sedimentation time was 22.0 hours, the relative sedimentation height of micrometer SiC whiskers was 98.0% and the dispersion modes of carboxyl methyl cellulose were increasing the hydrophilicity and zeta potential of micrometer SiC whiskers. (4) Silicon carbide composite slurry showed a shear thinning rheology. which had the characteristics of pseudoplastic fluid. Silicon carbide composite granules were prepared by spray-drying and had a good flow characteristics and forming properties.Six kinds of materials were used as the secondary phases, which were SiB(, nanoparticles, micrometer ZrB2 particles, nanometer or micrometer SiC whiskers, SiC nanoparticles as well as micrometer SiC whiskers, TiN nanoparticles as well as micrometer SiC whiskers. SiC/nmSiB6, SiC/μmZrB2, SiC/μmSiCw, SiC/nmSiCw, SiC/nmSiC/μmSiCw, SiC/nmTiN/μmSiCw composite ceramics were prepared using liquid phase pressureless sintering, and the effects of the secondary phases on silicon carbide ceramics were investigated. The results showed:(1) Effects of the secondary phases on silicon carbide ceramics depended on the composition and properties of the secondary materials and the processing conditions. (2) The addition of SiB6 nanoparticles and micrometer ZrB2 particles can improve Vickers hardness and fracture toughness of silicon carbide composites, while ZrB2 could also improve flexural strength. (3) Effects of nanometer and micrometer SiC whiskers on silicon carbide ceramics varied with their contents and sintering temperature. When the content of micrometer SiC whiskers was 30wt%, the maximum fracture toughness was 8.4MPa·m1/2. When the content of nanometer SiC whiskers was 10wt%, Vickers hardness was 33.6GPa, which was maximum. (4) When both SiC nanoparticles and micrometer SiC whiskers were added, flexural strength and Vickers hardness of SiC/nmSiC/μmSiCw composites increased with the increase of the mass ratio of SiC nanoparticles to micrometer SiC whiskers, while fracture toughness decreased firstly and increased then. (5) When TiN nanoparticles as well as micrometer SiC whiskers were added, flexural strength of SiC/nmTiN/μmSiCw composites increased obviously while Vickers hardness and fracture toughness varied with the increase of temperature. SiC/nmTiN/μmSiCw composite with 2.5wt% TiN nanoparticles and 2.5wt% micrometer SiC whiskers had the highest flexural strength and fracture toughness. which were 1123MPa and 8.9MPa-m1/2 respectively.The strengthening and toughening mechanisms of silicon carbide ceramics by particles or/and whiskers were investigated on the basis of the above experiments. The results showed:(1) The strengthening and toughening mechanisms of SiB6 nanoparticles, TiN nanoparticles, SiC nanoparticles and micrometer ZrB2 particles to silicon carbide ceramics were as follows:1) Inhibiting the normal growth of grains by the pinning effect of these particles to the grain boundaries of the matrix, which was beneficial for homogeneous microstructure with fine grains.2) The residual stress induced by the differences of linear expansion coefficients between these particles and silicon carbide matrix strengthened the ceramics.3) Crack deflection and crack bridging improved fracture toughness.4) Reasonable aggregate gradation of grains improved the microstructure of grain boundary, which lead to the improvement of flexural strength. (2) The strengthening and toughening mechanisms of nanometer and micrometer SiC whiskers were as follows:1) Crack deflection and crack bridging improved fracture toughness.2) The residual stress induced by the differences of elastic modulus between silicon carbide whiskers and silicon carbide matrix improved flexural strength.3) The pores in ceramics could absorb some fracture energy. (3) When TiN or SiC nanoparticles as well as micrometer SiC whiskers were added at the same time, the synergistic effects of the strengthing and toughening mechanisms of nanoparticles and whiskers could be obtained, which improved the mechanical properties of silicon carbide ceramics effectively.With graphite as lubricant, SiC/graphite composite ceramics were prepared and effects of graphite content and sintering temperature on the self-lubrication and mechanical properties of the composites were investigated. Self-lubricating mechanisms were analyzed. The results showed that:(1) Flexural strength and Vickers hardness of SiC/graphite composites decreased with the increase of graphite content, while frictional coefficient also decreased. When the content of graphite was 1 Owt% and the sintering temperature was 1850℃, the frictional coefficient decreased from 0.46 (without graphite) to 0.21, which indicated good self-lubrication. (2) The self-lubricating mechanisms of graphite to silicon carbide ceramics were as follows:1) Some graphite separated from ceramic matrix attached to the frictional surface and the self-lubricating property of seals was improved by the structure of graphite.2) The graphite particles were broken into tiny pieces and they formed a layer on the surface of seal faces, which were separated from each other, and frictional properties were improved.Silicon carbide seals with excellent mechanical or self-lubricating properties were industrially produced using liquid phase pressureless sintering, and some economic and social profits were obtained. The results showed that:1) Silicon carbide seals with 2.5wt% micrometer SiC whiskers as well as 2.5wt% TiN nanoparticles had much higher flexural strength and fracture toughness than the present seals. Friction coefficient of the one with 10wt% graphite was lower than that of the present products. Although their mechanical properties were relatively lower, they could meet the standard of mechanical seals.2) The machining properties and producing efficiency of the two kinds of new seals were improved, and 20%～30% energy was saved.