Study On Preparation And Cutting Properties Of α-SiAlON Ceramics

Tool inserts of self-toughenedα-SiAlON ceramics are developed recently for high-speed turning nickel alloys,due to good flexural strength,good fracture toughness and high corrosion resistance ofα-SiAlON against nickel.The good mechanical properties of theα-SiAlON ceramics are dependent on formation of bimodal microstructures where growth of a few large grains could be controlled to impart R-curve fracture to theα-SiAlON ceramics.R-curve fracture is featured by steady increase in fracture toughness with length of crack subcritical propagation,enhancing performance of theα-SiAlON tool inserts.However,there is still large room for improving performance ofα-SiAlON tool inserts in view of optimizing the bimodal microstructures for very limited understanding of the of relations between the bimodal microstructures and the cutting performance of theα-SiAlON ceramics.In this work,Dy-and Ca-α-SiAlON ceramics with a formula of M_m/vSi_12-（m+n）Al_m+nO_nN_16-nwhere M^v+=Dy³⁺or Ca²⁺,m=1.5,n=1.2 were prepared by hot pressing at1900℃ under 30 MPa for 1 h.Two techniques,i.e.extra addition of 0-5 wt%Dy₂O₃ to the Dy-α-SiAlON and seeding of Ca-α-SiAlON by 0-16 wt%Y-α-SiAlON seeds,were taken to optimize the bimodal microstructures of theα-SiAlON ceramics via controlling growth of the large grain in the bimodal microstructures.Effects of concentration of the Dy₂O₃and the Y-α-SiAlON seed addition on densification,constituent phases,microstructures and mechanical properties of theα-SiAlON ceramics were investigated.Dy-and Ca-α-SiAlON tool inserts were made and their performance for turning Inconel 718 alloy was tested.（1）For the Dy-α-SiAlON,all the compositions were fully densified after hot pressing,despite the different Dy₂O₃ contents.The constituent phases of the resultant materials were dominated byα-SiAlON phase.The minor phases were Dy₂Si₃O₃N₄（M’）andβ-SiAlON.The compositions with 0.5-5 wt%Dy₂O₃ additions formed distinct bimodal microstructures that were consisted of large elongated grains homogeneously dispersed in matrix of fine-equiaxed grains,whereas formation bimodal microstructure was not so significant in the 0wt%Dy₂O₃ composition.The Vickers hardness slightly decreased with increasing the Dy₂O₃content to give a measurement of 19.6-18.7 GPa at 10-20 kg indentation load.All the compositions showed increasing fracture toughness with crack subcritical propagation length to engender useful R-curve fractures.Performance of the Dy-α-SiAlON tool inserts in high-speed turning Inconel 718 alloy was tested.No nodulation caused as a sign of adhesive wear was presented.The main failure mode was determined as chipping in the nose wear and notch wear regions.Preferential orientation of the elongated grains under the axial pressure of the hot pressing showed a significant effect on life of the cutting tips.（2）For the Ca-α-SiAlON,all the compositions were fully densified after hot pressing,despite the different Y-α-SiAlON seed contents.The constituent phases of the resultant materials were dominated byα-SiAlON phase.Minor phase of Al N polytypes（Al₈O₃N₆）was detectable.All the Ca-α-SiAlON ceramics showed formation of bimodal microstructures,although dependence of bimodal microstructures on Y-α-SiAlON seed concentrations was revealed.The Vickers hardness slightly decreased with increasing the Y-α-SiAlON seed content to give a measurement of 17.9 GPa for the 0 wt%composition and15.9-16.4 GPa for the 2-16 wt%compositions at 10-20 kg indentation load.All the compositions showed increasing fracture toughness with crack subcritical propagation length to engender useful R-curve fractures.The fracture surfaces observed interfacial debonding,crack deflection,bridging and frictional pull-out of the large elongated grains as the dominant reinforcing mechanics.Performance of the Ca-α-SiAlON tool inserts in high-speed turning Inconel 718 alloy was tested.The main failure mode of the 8-16 wt%compositions was chipping at the notch wear regions.The 4 wt%composition showed slight nodulation at the notch wear region.The 0 wt%composition showed very good cutting performance in view 0 notch wear after 390 m of turning length in combination with a steady slow increase in the major flank wear,the minor flank wear and the nose wear with cutting length.The main mass loss of the Ca-α-SiAlON tool inserts was caused by abrasive wear.