| With the development of mechanical manufacturing and the application of high speed machining technology,the traditional tool steel and carbide cutting tools can't meet the needs of modern processing industry,Si3N4 ceramic cutting tools with high strength,high hardness,high wear resistance,good thermal shock and mechanical shock resistance,were widely used in cutting material such as gray cast iron and nickel base superalloy.With the introduction of sintering additives,Si3N4 ceramic material with high performance could be prepared based on liquid phase sintering mechanism.The nature of the liquid?such as the eutectic temperature and viscosity?had an important influence on densification,phase transformation,grain growth,microstructure and mechanical properties for Si3N4 ceramic,and the sintering temperature,species and content of additives were important factors to control liquid properties.Therefore,the suitable sintering temperature and content should be determined for different sintering additives systems.In addition,as a cutting tool,it is necessary to explore the relationship between the cutting performance of Si3N4 ceramic tool and the microstructure or mechanical properties.Based on Al2O3-Y2O3 and MgO-Y2O3 sintering additives,Si3N4 ceramic cutting tool materials were prepared by hot-pressing through controlling the sintering temperature and additives content.Effects of sintering temperature,species and content of additives on densification,phase transformation,grain growth,microstructure and mechanical properties for Si3N4 ceramic were studied.In addition,the relationship between cutting performance and preparation process,microstructure and mechanical properties were studied by high speed machining gray cast iron experiment using Si3N4ceramic cutting tools with different microstructure.The conclusions are as follows:?1?Si3N4 ceramics with 10wt%Al2O3-Y2O3 or 10wt%MgO-Y2O3 additives were prepared respectively by hot-pressing at 1600?,1700?and 1800?.Effects of sintering temperature on density,phase composition,microstructure and mechanical properties for Si3N4 ceramics with the two additives were studied.The results showed that compared with Al2O3-Y2O3 additives,MgO-Y2O3 additives could improve the densification of Si3N4 ceramics better.Si3N4-MgO-Y2O3 ceramics could be completely densified at 1600?.For both of Si3N4-Al2O3-Y2O3 and Si3N4-MgO-Y2O3 ceramics,the increase of sintering temperature could promote the???transformation and the formation of the bimodal distribution.With the addition of Al2O3-Y2O3,Si3N4-Al2O3-Y2O3 ceramics prepared at 1800?had better comprehensive mechanical properties,and the hardness,flexural strength and fracture toughness were 15.60±0.27GPa,1105.99±68.39MPa and 7.13±0.37MPa·m?,respectively.With the addition of MgO-Y2O3,the Si3N4-MgO-Y2O3 ceramics prepared at 1600?had better comprehensive mechanical properties,and the hardness,flexural strength and fracture toughness were16.53±0.21GPa,1166.90±61.73MPa and 6.74±0.17 MPa·m?,respectively.Therefore,based on the comprehensive analysis of two Si3N4 ceramics prepared at different temperature,Si3N4-Al2O3-Y2O3 ceramics is suitable for high temperature sintering,and the best sintering temperature is 1800?,while Si3N4-MgO-Y2O3 ceramics is suitable for low temperature sintering,the best sintering temperature is 1600?.?2?Based on the above comprehensive analysis,Si3N4 ceramic with Al2O3-Y2O3additives were sintered at 1800?,and Si3N4 ceramic with MgO-Y2O3 additives were sintered at 1600?.Effects of the content of additives?6wt%,8wt%and 10wt%?on density,phase composition,microstructure and mechanical properties for Si3N4ceramic were studied.The results showed that Si3N4 ceramics with high density could be obtained by adding 6wt%Al2O3-Y2O3 or MgO-Y2O3 additives.The increase of additives content had little effect on density for the two kinds of Si3N4 ceramics,but promoted???transformation for Si3N4-Al2O3-Y2O3 ceramics prepared at high temperature.With the increase of the addition of Al2O3-Y2O3,the grain diameter and aspect ratio of Si3N4 were increased,and the elongated grain increased significantly.With the addition of MgO-Y2O3,the content of additives had little effect on the grain size of Si3N4 ceramics,but increased the aspect ratio greatly.By analyzing the comprehensive mechanical properties of two Si3N4 ceramics with different content of additives,the hardness would be reduced and the fracture toughness would be improved with the increase of additives content,but it had little effect on the flexural strength.?3?Based on cutting gray cast iron experiment using Si3N4-Al2O3-Y2O3 and Si3N4-MgO-Y2O3 ceramic cutting tools with different microstructure prepared at different sintering temperature and different additives content,the intrinsic relationship between cutting performance and preparation process,microstructure,mechanical properties for Si3N4 ceramic cutting tools was explored.The results showed that the main wear mechanism of the two Si3N4 ceramic cutting tools were abrasive wear and adhesion wear.With the addition of Al2O3-Y2O3,Si3N4 ceramic cutting tool prepared at 1800?had more excellent cutting performance.With the addition of MgO-Y2O3,Si3N4ceramic cutting tools prepared at 1600?were better.Meanwhile,regardless of the species of additives,the cutting performance of two kinds of Si3N4 ceramic tools could be improved significantly by reducing the amount of additives.Based on the relation between cutting performance and microstructure,it was found that the microstructure with fine grain and elongated grain of high aspect ratio was helpful to improve the cutting performance.Furthermore,cutting life of Si3N4 ceramic changed the same with the hardness,and high fracture toughness didn't guarantee excellent cutting performance.Therefore,a reasonable values of hardness and fracture toughness should be controlled in practical application,especially fracture toughness. |
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