| The improvement of flexural strength and fracture toughness is a key problem to the application of Si3N4 ceramic. Nano-powders reinforcing method and the in-site toughening are new process developed recently,which can efficiently improve the flexural strength and fracture toughness of ceramic.The in-site toughening Si3N4 ceramic,synthesis of Si-C-N nano-powders and nano-powders reinforced Si3N4 were the main content in this thesis. Effects of sintering additives on mechanical properties and microstructure of the in-site toughening Si3N4 ceramic were investigated,and effects of ratio of raw materials on compose and characteristic of precursor were studied. Moreover,compose and microstructure of Si-C-N nano-powders pyrolysized in different atmosphere were also researched. The properties,microstructure,strengthening and toughening mechanism as well as oxidation behavior and thermal shock were investigated.Effects of mixing methods and amount of Y2O3 and La2O3 on the mechanical properties and microstructure of the in-site toughened Si3N4 ceramic are studied systematically in this thesis. Results show that the in-site toughened Si3N4 ceramic with 5wt%Y2O3-10wt%La2O3 exhibits better mechanical properties of 911MPa in flexural strength and 10.02MPa.ml/2 in fracture toughness,prepared by solution mixing additives method (SMAM) when the ratio of Y2O3 and La2O3 was 1/2. At the same time,when the ratio of Y2O3 and La2O3 is 2/1,the mechanical properties of Si3N4 ceramic containing 20wt%Y2O3-10wt%La2O3 added by mechanical mixing method (MMM) are better than solution mixing additives method,which flexural strength and fracture toughness were 934 MPa and 11.78MPa.m1/2 respectively. The difference between microstructure of two method result difference mechanical properties. The microstructure of in-site Si3N4 ceramic by SMAM was well-distributed while a bimodal structure was achieved prepared by MMM. SEM observation shows that crack deflection and pulling out of the P -Si3N4 rod-like grains are the maintoughening mechanism in this material.A new low molecule weight polynitrisilicane precursor was synthesized by mixing (CH3)2SiCl2 with CH3SiHCl2 according to different ratio. The effect of the ratio of raw materials on characteristic and compose of the precursor was investigated. The properties and microstructure of Si-C-N nano-powders are also researched under different pyrolysised atmosphere. The result showed that the new precursor exhibited better properties than precursor synthesized by separate (CH3)2SiCl2 or CH3SiHCl2. The oxide amount in the Si-C-N nano-powders decreased and the purity increased with the amount of (CH3)2SiCl2 increased. Three orbicular organic compounds containing Si,C and N element were the main objects of the precursor. With the amount of CH3SiCl2 increased,however,the linear molecule and orbicular organic compounds containing Si,O and N element were formed,which resulted decrease of gasification velocity of precursor and purity of Si-C-N nano-powders. The compose of Si-C-N nano-powder can be adjusted by changing the flow of NH3 in pyrolysis atmosphere. Si-C-N nano-powder begin to form a-Si3N4 and p-SiC at 1500 when being heat treated under N2 atmosphere,which holding time is forty minute. a-SiC,P-SiC and little p-Si3N4 are main grain phase when heat treating temperature is 1700.The effects of compose and amount of Si-C-N nano-powders on relative density,flexural strength and fracture toughness of SiCNnp/Si3N4 composites were understood. The research results showed that the strength and fracture toughness value present peak value variation under the condition of 20wt% Si-C-N powders,and when the powder contain 22wt% carbon element,the SiCNnp/Si3N4 composites exhibit excellent performance of 878.5MPa in bending strength and 11.96MPa.m1/2 in fracture toughness.The high temperature bending strength and fracture toughness of Si3N4 ceramic prepared by in-site toughening and nano-powders strengthening were studied. At 1350,the elevate fracture tough prepared by in-site toughening was 1...
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