Preparation And Properties Of Silicon Nitride Ceramics

The bimodal microstructure of Si₃N₄ ceramics with rod-like grain composition can exceed 1 GPa,but its thermal conductivity is only 50 W/m/K,which limits the development of further applications;using a suitable preparation process to obtain high orientation Si₃N₄ ceramics improve their thermal conductivity.In this paper,?-Si₃N₄powder was used as raw material,a small amount of?-Si₃N₄ was added as seed crystal,and two processes of dry pressing and casting were used to prepare Si₃N₄ ceramics.Hot-pressing sintering was performed at 1900?for 60 min under 30 MPa pressure.The mechanical properties,oxidation resistance and thermal conductivity of the materials were studied.The phase composition and microstructure of the materials were observed by modern analytical methods.Two kinds of ceramic materials were prepared by dry pressing-hot pressing sintering process.The composition was?-Si₃N₄ powder,?-Si₃N₄ powder and sintering aid Y₂O₃,the mass ratios were 95:0:5 and 90:5:5,respectively,and marked as SY-1 and SY-2.The flexural strengths of SY-1 and SY-2 were 397 MPa and 369 MPa,respectively,and the fracture toughness was 4.4 MPa·m¹/2/2 and 7.3 MPa·m^1/2,respectively.The thermal conductivity of SY-1 and SY-2 decreases with increasing temperature and exhibits anisotropy.The thermal conductivity in the parallel direction is higher than the perpendicular direction;the thermal conductivity of SY-1 in the parallel direction is 45.2W/m/K,the thermal conductivity of SY-2 reaches the highest value of 50.3 W/m/K.The XRD results showed that the c-axis of?-Si₃N₄ crystal used to improve the thermal conductivity grows perpendicular to the hot pressing direction,and the presence of?-Si₃N₄crystal is more conducive to the precipitation of the liquid phase on the?-Si₃N₄ crystallites.The crystal form has long rod-like large grains,and the directional arrangement of the?-phase grains makes the material anisotropic.Two kinds of layered ceramic materials were prepared by tape casting-hot pressing sintering process.The composition was?-Si₃N₄ powder,?-Si₃N₄ powder and sintering aid Y₂O₃,the mass ratios were 95:0:5 and 90:5:5,respectively,and labeled SY-3 and SY-4.In order to meet the requirements of the casting process,the binder content and stirring time were studied.The results showed that the binder was 17 wt.%,the plasticizer was 10 wt.%,and the stirring time was 36 h.The thickness of the cast sheet is uniform,smooth and flat,the grain is clear,and has certain strength and toughness,which meets the requirements of the casting molding process.Under the protective atmosphere of N₂,the temperature was raised to 500?at 0.2?/min,and the degreasing process was confirmed by keeping it for1 h.The flexural strengths of SY-3 and SY-4 are 483 MPa and 486 MPa,respectively,and the fracture toughness is 4.8 MPa·m¹/2/2 and 7.5 MPa·m^1/2,respectively.The fracture mode is brittle fracture,but?-Si₃N₄ rod-like grains of SY-4were formed,and the bridging of?-Si₃N₄ rod-like grains consume a large amount of fracture energy,and thus improves the fracture toughness.The XRD results showed that the combination of tape casting and hot pressing sintering promotes the preferential orientation of the?-phase grains,and each property exhibits anisotropy.Antioxidant experiments showed that the oxidative weight gain of SY-3 and SY-4increased with the increase of oxidation temperature,and the flexural strength gradually decreased with the increase of oxidation temperature.SY-4 reached a maximum value of81.6 W/m/K in the parallel direction,and the thermal conductivity after oxidation at1500°C was reduced to 41.0 W/m/K.The surface of the oxidized sample forms a SiO₂glass phase,and the thermal conductivity of the glass phase is low,preventing the diffusion of heat.Similarly,the glass phase covered by the internal?-Si₃N₄ particles also prevents heat conduction;Oxidation-induced?-Si₃N₄ particle cracking destroys the continuous thermal diffusion channel,reduces heat transfer efficiency,and thus reduces thermal diffusivity.