Research On Preparation,Microstructures And Properties Of Dense Si₃N₄ Ceramic

Silicon nitride ceramics are widely used in automotive,aerospace,chemical,machinery and electronic fields due to its high hardness,high strength,thermal shock resistance,corrosion resistance,high temperature oxidation resistance,wear resistance and self-lubricating properties.However,its application is limited due to the difficulty of post processing.The use of a suitable sintering process can achieve near-net shape molding,which is of great significance for the further development of silicon nitride.In this paper,dense silicon nitride ceramic were prepared by using high purityα-Si₃N₄powder（0.7μm）as the main raw material,Y₂O₃ and Al₂O₃ as the composite sintering additives by pressure-less sintering/gas pressure sintering+hot isostatic pressing treatment.The phase,microstructure and mechanical properties of the samples were tested.The relationship between the preparation process,microstructure and mechanical properties of silicon nitride ceramics was investigated.The results show that when the silicon nitride is sintered without pressure,the material is difficult to be sintered and compacted under the condition of low sintering aid（5wt%）.When the temperature is lower than 1700°C,the presence ofα-Si₃N₄ can be detected.Theα-sialon solid solution is formed with the increase of holding time at the sintering temperature of 1730^oC,silicon nitride begins to decompose obviously sintered at 1760^oC.At the same sintering temperature（1730^oC）,the density of silicon nitride increases first and then decreases with the increase of sintering aid.When the addition amount is 8wt%,the relative density of the sample reaches a maximum value value of 97.7%,and the corresponding Vickers hardness,fracture toughness and flexural strength are 15.63±0.24GPa,5.15±0.32MPa·m^1/2,542.5±53.7MPa respectively.When the sintering aid content is 8wt%and 10wt%,there is a formation of an Y₂SiAlO₅N solid solution.After the pressureless sintering sample was treated by hot isostatic pressing（1600^oC+50MPa+90min）,the pores inside the material were further reduced,the grain boundary strength of the material increased,and the fracture mode was dominated by transgranular fracture.The mechanical properties of the sample,such as the density,are increased,and the relative density can reach 98.8%.Corresponding samples have a Vickers hardness of 17.83±0.30GPa,a fracture toughness of6.43±0.51MPa?m^1/2,and a flexural strength of 672.2±33.9MPa.The EDS results show that elements such as O,Al and Y are mainly distributed along the grain boundaries.Bridging and deflection occur during the crack propagation,which improves the fracture toughness of the material.In gas pressure sintering,the main crystalline phase of silicon nitride isβ-Si₃N₄at all sintering temperatures.As the sintering temperature increases,the density of the sample increases first and then decreases.At low temperatures,the grain growth of the sample is incomplete and pores are present inside.When the sintering temperature is increased to 1850^oC,liquid phase volatilization and abnormal growth of crystal grains will occur.There are a large number of pores in the interior,and the fracture mode of the material is mainly along the crystal fracture with the low density.An increase in nitrogen pressure reduces the strength of the grain boundary phase.At the same temperature（1780^oC）and nitrogen pressure（2MPa）,the relative density of silicon nitride ceramics increases first and then decreases with the addition of sintering aids.When the addition amount of the sintering aid is 8wt%,the relative density of the sample reaches the highest value of 99.1%,together with the Vickers hardness of 15.82±0.13GPa,the fracture toughness of 5.25±0.25MPa?m^1/2 and the flexural strength of 638.5±36.2MPa.After hot isostatic pressing,the sample sintered by air pressure has a small increase in density.As in the case of pressureless sintering,elements such as O,Al and Y are mainly distributed along the grain boundaries.The sample after hot isostatic pressing has a density of 99.4%,a Vickers hardness of 16.61±0.23GPa,a fracture toughness of 5.51±0.35MPa?m^1/2,and a flexural strength of 53.2±43.0MPa.The strength of the grain boundary phase of the treated sample is not greatly improved,which is related to the atmospheric pressure and the volatilization of the liquid phase in the gas pressure sintering.The corresponding crack propagation process only has crack deflection,so the toughness is not significantly improved.