Ultra-high Pressure High Temperature Sintered Sic

SiC has been used in many area because of its high strength at high temperature,anti-oxidization,anti-wear,low coefficient of thermal expansion,high hardness,anti-heat shock and anti-chemic erosion.SiC is very hard to sinter to high density,because of it's high strength covalent bond and low self diffusion coefficient.Without sinter additives,pure SiC is very difficult to densify with traditional techniques.However,highly densified SiC ceramics with good property can be obtained by using ultra high pressure sinter technique at lower temperature,less time,fewer sinter additives compared with traditional technicals.Thus, this study trys to make clear how the sinter technology parameters effect the SiC's properties and micro-structure of SiC by using ultra high pressure sintering technique.Nano-grain Size SiC had been prepared at ultra high pressure;using nano-SiC and sinter addiatives as major raw materials.Ceramics' phase formation,chemical composition, micro-structure,density and micro hardness had been tested by using XRD,XPS,SEM, Archimedes method and micro indenter.The results indicate that SiC can be sintered at ultra high pressure and lower temperature comparing with the traditional method.The phase formation test shows that there is no phase transfer in SiC after sinteed,β-SiC remains as major phase.Al₂SiO₅ is found in SiC with Al₂O₃ as additive.Al₂SiO₅ and Al₂O₃can help sintering.The ceramic's grain size and crystal lattice constant has calculated by using Sherr's formula and MDI Jade 5.0 program separately. The result shows that the grain size of SiC's growth isn't obvious and the ceramic still remains as nano grain(20-40nm).The crystal lattice shrinks is about 0.3%～0.45%,because the high remain stress after ultra-high pressure sinter.Microstructure analysis indicates that the density of the sintered ceramic is 92-98%of the therotical.There is Al₂O₃ enrichment area in the ceramics with additive added.SiO₂ and Si₃N₄ has been found inside SiC because absorbed N₂,O₂ on roughcast.React with SiC during sintering.The reactions can eliminate hole and make for sintering.The ceramic's density and micro hardness had also been tested.Results show that sinter condition can affect the ceramic's density and micro hardness.The density and micro hardness increases with the pressure,temperature and sinter time.The relative density and micro hardness of the pure SiC ceramics are 92%-99%and 2800-4000Hv1.96 respectively. Results also show that SiC can be sintered to full density at lower temperature when about 2wt%Al₂O₃ is added as sinter additive.Micro-hardness of the SiC increases with the Al₂O₃ content.Sinter condition such as pressure,temperature,sinter time also effect ceramics'(with Al₂O₃ added) density and micro hardness.The influence is just similar to the pure SiC.