Fabrication Of High Performance Silicon Nitride Ceramics Based On The Addition Of Seeds At Low Temperatures

Silicon nitride?Si₃N₄?is one of the important structure ceramic materials due to the properties of high hardness,self-lubrication,and high wear resistance,oxidation resistance at high temperatures.It also can resist thermal shock,and the material would not facture during the rapid cooling process after even heating up to above 1000?.Thus,Si₃N₄ ceramics are often to be used to manufacture bearings,steam turbine blades,mechanical seal,mechanical components,and permanent mold.Due to its superior high temperature mechanical properties and oxidation resistance,the Si₃N₄ceramics offer the advantages for application in the field as high temperature structural components.In this research,the rod-like ?-Si₃N₄ seeds with different aspect ratio were prepared by using different sintering additives,sintering temperature,nitrogen pressure,and holding time.The?-Si₃N₄ seeds were then added into the?-Si₃N₄ powders with selected sintering additives.The powder mixture was then processed with ball-mill,drying and sieving,and hot-press sintering method to obtain high density silicon nitride ceramics and with excellent mechanical properties.In addition,the high-performance silicon nitride ceramic materials were manufactured to exhibit high-strength,high-hardness outer layer and high-toughness inner layer through microstructural engineering.The highly-dispersed ?-Si₃N₄ seeds with different aspect ratios were prepared in this study.Results showed that the aspect ratio was between 2.3and 35.7,and the longest length could be up to 20.17±2.68?m,and the shortest was 1.59±0.44?m.Also,results showed that the mechanical performance of silicon nitride could be significantly improved by the addition of?-Si₃N₄ seeds.The silicon nitride with La₂O₃ sintering additive,added with 2 wt%of?-Si₃N₄ seeds with the aspect ratio of around 15 and sintered at 1700?,could exhibit a high Vickers hardness of 19.4±0.4GPa.The silicon nitride with the maximum bending strength of 1066±54MPa could be obtained by using Y₂O₃ and Al₂O₃ as the sintering additives.On the other hand,the silicon nitride sintered with Y₂O₃ and Al₂O₃ additives at1600?could achieve the maximum fracture toughness of14.7±0.4MPa·m¹/2/2 measured by the method of SENB by addingthe 5 wt%?-Si₃N₄ seeds and with the aspect ratio of about 8.Furthermore,the functionally graded silicon nitride ceramic was prepared to exhibit the surface Vickers'hardness of up to 19.8 GPa,the flexure strength of up to769 MPa,and the fracture toughness of 14MPa·m^1/2.Machining study also showed that thecutting performance and wear properties offunctionally graded Si₃N₄ material are much better than the normal silicon nitride material with bimodal microstrcuture.