Preparation And Properties Of SPS Sintering Of Si₃N₄/BN Composite

Si₃N₄/BN composite is an important aerospace wave-transparent material which has excellent dielectric properties,mechanical properties and good stability in harsh environments.Because of the poor sintering property of Si₃N₄ and BN ceramics,the use of BN to enhance the dielectric properties,thermal stability and machinability of Si3N4 ceramics leads to the rapid deterioration of the mechanical properties of its composites,which limits the application of Si₃N₄/BN composite.This paper is based on the promotion of densification by SPS sintering method and the better dispersion characteristics of cubic boron nitride?c-BN?ceramic powder and the conversion of cubic boron nitride to hexagonal boron nitride?h-BN?at high temperature.The effects of SPS process and c-BN introduction on the sintering densification process and the microstructure of sintered Si3N4/BN composites were compared and studied.The mechanical properties and dielectric properties of Si3N4/BN composites were improved.The main research work is as follows:?1?The effects of different sintering processes?sintering temperature,sintering pressure and holding time?on sintering densification,phase transformation and grain growth behavior of Si₃N₄/BN composites during spark plasma sintering?SPS?were studied.It was found that the sintering time of Si₃N₄/BN composites can be greatly reduced by SPS,which was shortened from the conventional few hours to the ten minutes of SPS.Si₃N₄/BN composites sintered by SPS process can achieve the phase transition of Si3N4 from?to?phase and densification in a short time,and refine the growth of Si3N4 grain.The relative density and phase transition of Si3N4/BN composites were promoted with increased sintering temperature.Increased sintering pressure can make the sample densify at lower temperatures and reduce the grain size of Si3N4.When sintering pressure increased from 20 MPa to 80 MPa,the mean grain size decreased from 1.29?m to 1.10?m.The densification and phase transition of Si3N4 were promoted by extended holding time.?2?Si₃N₄/BN composites were prepared by SPS by directly compounding Si₃N₄with hexagonal boron nitride.The effects of h-BN on Sintering Densification process,grain morphology,distribution and properties of Si3N4/BN composites were investigated.It was found that the introduction of h-BN inhibited the dissolution and precipitation of Si₃N₄,resulted in the sintering of Si₃N₄/BN composites with h-BN at higher temperatures than that of Si₃N₄.At the same time,h-BN can inhibited the growth of Si₃N₄ grains in the c-axis direction and reduced the transformation rate of Si₃N₄.When the content of h-BN increased from 5 wt%to 30 wt%,the transformation rate decreased from 100%to 45.4%.When the content of h-BN was low?5 wt%?,under the action of pressure,the deformation of BN particles can promote particle rearrangement and filling voids,thus increasing the relative density of the composites.However,excessive boron nitride content would lead to the increase of the voids introduced by the BN sheet accumulation structure in the samples,which was not conducive to the increase of densification degree,and the mechanical properties of the composites would change accordingly.Within the range of 0-5 wt%BN content,the properties of the composites increased with the increase of density,but with the further increase of h-BN content?from 5-30 wt%?,the hardness of composites decreased from 144 GPa to 6.9GPa,the Young's modulus decreased from 270 GPa to 165 GPa,and the flexural strength decreased from 731 MPa to 400 MPa.The dielectric constant and tangent of dielectric loss angle of composites decreased continuously with the increase of BN content.?3?Si₃N₄/BN composites were synthesized in-situ through the sintering of Si₃N₄/c-BN composite powders by the phase transformation of cubic boron nitride at high temperature.The effect of c-BN on sintering densification,structure and properties of Si3N4/c-BN composites were studied.It was found that c-BN could be uniformly dispersed among Si₃N₄ particles in the mixing stage of composite powders,which improved the structural uniformity of the materials compared with the agglomeration phenomenon of h-BN.The transformation of c-BN to h-BN in the temperature from1300°C to 1600°C increased the effect of deformation of BN particles on sintering densification.The addition of c-BN also inhibited the crystalline transformation and growth of Si₃N₄,but the inhibition was less than that of h-BN directly introduced into the matrix.With the same BN content,the dielectric constant of the composites with c-BN increased slightly,and the tangent value of dielectric loss angle did not change significantly.Hardness decreased slightly,while flexural strength and fracture toughness increased significantly.This may be related to the characteristic scale formed by dispersion and the interface bonding state formed in-situ,and further research is needed.