Rapid Texturing And Properties Of Si₃N₄ And SiAlON Ceramics Based On High Temperature Plastic Deformation

Silicon nitride(Si₃N₄)and Sialon(SiAlON)ceramics possess excellent properties such as low density,high hardness,high toughness,high heat resistance,and low thermal expansion coefficient etc.They have been widely applied as cutting tools and high-speed bearings etc.Compared with the conventionally manufactured Si₃N₄ and SiAlON ceramics,the textured Si₃N₄ and SiAlON ceramics prepared via the high temperature plastic deformation process could exhibit better properties in specific orientations and are anticipated to be ideal structural materials for application in extreme environments.However,there are still some issues remained to be soloved for the texturing of Si₃N₄ and SiAlON ceramics using high temperature plastic deformation process,such as low preparation efficiency,simple texturing form,and lack of application research.In order to overcome these issues,Si₃N₄ and SiAlON ceramics with randomly two-dimensional texture,centripetal texture,and one-dimensional texture were rapidly prepared by simple compression sinter-forging,centripetal sinter-forging,and extrusion using the spark plasma sintering technology.The anisotropy of microstructure and mechanical properties of the Si₃N₄ and SiAlON ceramics with different texturing modes were systematically investigated in this research.Subsequently,the properties of textured SiAlON ceramics were optimized,and the textured SiAlON ceramic compact cutting tool inserts were prepared.The study on the cutting performance and properties in turning of GH4169 Ni-based superalloy was then carried out.The effects of different texturing forms on the cutting performance,wear morphology,and wear mechanism of SiAlON ceramic cutting tools were studied.The specific research contents and results obtained are summarized as follows:The randomly two-dimensional textured Si₃N₄ and SiAlON ceramics were rapidly prepared by simple compression with flowing sinter-forging process using spark plasma sintering technology.The results showed that the strong texture of Si₃N₄ and SiAlON ceramics can be realized under the rapid plastic deformation under outward flowing plastic deformation,and the texture degree of which was 0.75 and 0.78,respectively.The hardness of the randomly two-dimensional textured Si₃N₄ and SiAlON ceramics on the plane perpendicular to the forging direction was higher,reaching 1744 HV and 1841 HV,respectively;their bending strength with the fracture direction parallel to the forging pressure direction was higher,reaching 1350 MPa and 1393 MPa,respectively;their fracture toughness with the crack direction perpendicular to the long axis of the grain was higher,reaching 7.01 MPa·m^0.5 and6.30 MPa·m^0.5,respectively.The new type of two-dimensionally centripetal texturing of Si₃N₄ and SiAlON ceramics were then prepared by centripetal flowing sinter-forging process using spark plasma sintering for the first time.The results showed that the rod-like grains of Si₃N₄ and SiAlON ceramics grew and rotated towards the radial direction in the high temperature centripetally plastic flow,forming centripetal texture with the grain long axis pointing to the center and parallel to the radial direction;the degree of microstructural texture of Si₃N₄ and SiAlON ceramic was 0.81and 0.79,respectively.According to the three-dimensional strain of centripetal flow,a physical model of grain rotation under centripetal flowing sinter-forging process was proposed,and the mathematical analytical equations of the model were obtained.The experimental results were in a very good agreement with the calculated results.The hardness of the centripetally textured Si₃N₄ and SiAlON ceramics was the highest in the axial section,which was 1701 HV and 1821 HV,respectively;while their hardness was the lowest in the chord section in the cylinder shape sample,which was 1585 HV and 1626 HV,respectively.The strength was the highest when the fracture surface was perpendicular to the radial direction,reaching 1264 MPa and 1351 MPa,respectively.Also,the fracture toughness was the highest when the crack was perpendicular to grain direction,reaching 7.53 MPa·m^0.5 and 6.27MPa·m^0.5,respectively.On the other hand,the highly one-dimensional textured Si₃N₄ and SiAlON ceramics were also prepared by central extrusion process using spark plasma sintering technology for the first time.The results showed that during high temperature extrusion,the rod-like grains of Si₃N₄and SiAlON rotated toward the extrusion direction,and the grain long axis tended to be parallel to the extrusion direction,forming a one-dimensional orientation,with the degree of microstructural texture reaching 0.77 and 0.88,respectively.The hardness of one-dimensionally textured Si₃N₄ and SiAlON ceramics was the highest in the cross section parallel to the extrusion direction,reaching 1671 HV and 1814 HV,respectively;while it was the lowest in the cross-section perpendicular to the extrusion direction,which was 1499 HV and 1559 HV,respectively.Also,the fracture toughness was the highest in the crack direction perpendicular to extrusion,reaching 7.67 MPa·m^0.5 and 6.37 MPa·m^0.5,respectively;while it was the lowest in the crack direction parallel to the extrusion direction,falling in 3.82MPa·m^0.5 and 3.89 MPa·m^0.5,respectively.In order to evaluate the performance and properties of Si₃N₄ and SiAlON ceramics in high speed machining of superalloys,a comparative study was carried out on cutting GH4169Ni-based superalloy using conventionally sintered Si₃N₄ and SiAlON ceramic tools.The results showed that the cutting performance of SiAlON ceramic tool was better than that of Si₃N₄ ceramic tool,which was mainly attributed to its superior high temperature oxidation resistance and higher hardness than that of the Si₃N₄ ceramic,although their strength and toughness were similar.The SiAlON ceramic cutting tool failed by the stably average flank wear and its tool life was about 235 m,and the tip chipping occurred in the Si₃N₄ ceramic tool after cutting 108 m.The wear modes of Si₃N₄ and SiAlON tools were flank wear,boundary notched wear,micro-chipping,tool tip wear,and crater wear.The main wear mechanisms identified were adhesion wear,abrasive wear,diffusion wear,and peeling.The wear mechanism of the Si₃N₄ tool also included chipping.According to the above research results obtained,the SiAlON ceramic tool was more suitable for machining Ni-based superalloys.Therefore,the research on the cutting performance of textured SiAlON ceramic tool was carried out.The textured SiAlON ceramics were firstly cut into small pieces,and then prepared into compact tool inserts based on grain orientation,sample size,and comprehensive stability of the textured tool to ensure that the grain at each cutting edge of the insert was consistent with the relative direction of the cutting edge.After the process of compact tool insert preparation,SiAlON ceramics maintained the same microstructural texturing form,and,thus,the mechanical properties almost remained unchanged.Consequently,the performance and wear mechanism in cutting of GH4169 Ni-based superalloy were studied.The results showed that the grain orientation played an important role on the cutting performance of SiAlON ceramics.The cutting performance was observed to be the best with the rod-like grain parallel to the cutting edge,and the worst with the grains perpendicular to the rake face;cutting performance of the textured tool with the grain perpendicular to the flank face was slightly better than that with the grains perpendicular to the rake face.In the case with grains randomly oriented,the cutting performance was ranked in the middle.Boundary notched wear and tool tip wear were identified to be the donimated wear forms,and the main wear mechanisms were adhesive wear,abrasive wear,diffusion wear,and peeling.In the case that the grains were parallel to the cutting edge,the main cutting force direction was perpendicular to the grain orientation,and the textured tool in this direction exhibited the best performance of anti-notched wear and peeling,failure mode of which was slow and stable flank wear.However,when the grains were perpendicular to the rake or flank face,the workpiece movement or chip flow direction was parallel to the grain direction,resulting in quick peeling of the tool grains along the workpiece or chip friction direction,and thus rapidly catastrophic failure.