Study On Fabrication And Cutting Tool Wear Reliability Of Al₂O₃-based Composite Ceramic Tools Based On Defect Control

In order to control the defects of ceramic tool materials and reduce its brittleness,the advanced Al2O3-based composite ceramic tools with good mechanical properties and cutting performance were developed.In this study,phase composition,microstructure,room-temperature mechanical properties,high-temperature flexural strength,growth mechanisms of the elongated grains,low-defect sintering mechanisms,toughening and strengthening mechanisms,cutting performance and tool wear reliability of the advanced ceramic tool materials were studied systematically.New methods based on defect control technology and microstructure design based on defect tolerance method were proposed during the development of the advanced composite ceramic tools.In this paper,defect formation and evolution laws were revealed during the preparation of composite ceramic tool materials,and the methods of controlling main defect sources such as microcracks and pores were proposed,and the low-defect control theory model was established.Three series of Al2O3-based composite ceramic tool materials(namely AS,AST and ASN)with different microstructure defects and mechanical properties were developed.AS ceramic tool materials had the fewest defects,homogeneous microstructure and the optimum comprehensive mechanical properties.However,for AST and ASN tool materials with more microstructure defects,microstructure uniformity were poor and mechanical properties were reduced in different degrees.Flexural strength,fracture toughness,Vickers hardness and relative density of the low-defect AS ceramic tool materials were 1093 MPa,6.8 MPa·m1/2,19.5 GPa and 99.6%respectively.Flexural strength,fracture toughness,Vickers hardness and relative density of AST ceramic tool materials were 1079 MPa,6.4 MPa·m1/2,19 GPa and 98.5%respectively.Flexural strength,fracture toughness,Vickers hardness and relative density of ASN ceramic tool materials were 977 MPa,6.4 MPa m1/2,18.2 GPa and 98.1%respectively.The vacuum hot-pressing sintering process of the advanced ceramic tool materials including stage-by-stage heating and holding time was proposed and investigated.Holding time and sintering pressure of AS,AST and ASN ceramic tool materials were 20 min and 32 MPa respectively during the sintering process.AS and ASN ceramic tool materials made up of 73.5 wt.%Al2O3-25 wt.%a-Si3N4-1.5 wt.%Y2O3 composite ceramic powders were sintered at 1500? and 1550? respectively.AST ceramic tool materials made up of 58.5 wt.%Al2O3-15 wt.%TiC-25 wt.%a-Si3N4-1.5 wt.%Y2O3 ceramic powders were sintered at 1500?.The flexural strength and fracture behaviour of AS,AST and ASN series ceramic tool materials at the elevated temperatures of 800??1200? were investigated.The results indicated that the stress of AS,AST and ASN ceramic tool materials was almost increased linearly with a progressive increase in term of strain at the high temperature,and it was seen from the fracture surface morphology that the fracture modes of three ceramic tool materials were mainly transgranular fracture,indicating the elastic strain stage before fracture at the high temperature.The elastic modulus,high-temperature flexural strength and the grain boundary viscosity of three ceramic tool materials were gradually decreased with an increase in item of temperature.The high temperature resistance and crystallization phases such as Y2Si2O7 and Y3Al5O12 and the small grains precipitated secondarily at grain boundary inhibited the grain boundary sliding,and thus,creep resistance of ceramic tool material was improved at the high temperature.The flexural strengths of AS,AST and ASN series ceramic tool materials at 800? were 664 MPa?538 MPa and 549 MPa respectively,the flexural strengths of AS,AST and ASN series ceramic tool materials at 1000? were 525 MPa?508 MPa and 510 MPa respectively,and the flexural strengths of AS,AST and ASN series ceramic tool materials at 1200? were 407 MPa?398 MPa and 392 MPa respectively.Heterogeneous nucleation on rough composite substrate surface and anisotropic growth mechanisms of in-situ synthesized and elongated ?-Si3N4 and(3-SiAlON grains were studied,and the dynamic models of nucleation and growth were established.The results indicated that the main growth processes of the elongated ?-Si3N4 and ?-SiAlON grains included the formation of embryos,embryos growing into stable crystal nuclei and crystal nuclei growing into the elongated grains.Nucleation and nuclei growth mechanisms were a dissolution-transport-precipitation process,and ?-SiAlON embryos were formed by interstitial solid solution reaction.The hot-pressing pressure enhanced nucleation of ?-Si3N4 and P-SiAlON embryos,and promoted the directional growth of?-Si3N4 and ?-SiAlON nuclei along the c axis.The low-defect sintering mechanisms of AS,AST and ASN composite ceramic tool materials were studied,and the results indicated that grain-boundary diffusion was the predominant densification mechanism.A low-defect sintering dynamic model of the advanced ceramic tools was established in the liquid-phase and hot-pressing sintering.Strengthening and toughening mechanisms of AS,AST and ASN composite ceramic tool materials were investigated.The results indicated that in-situ growth elongated ?-Si3N4 or ?-SiAlON grains and the uniform dispersion of ?-Si3N4 and TiC grains all played the predominant roles in strengthening and toughening of three kinds of ceramic tools.Interlocking and reticular skeleton structure,intra-type nano-grains and core-shell structure played a certain toughening and reinforcing effect on the advanced ceramic tool materials.Main strengthening and toughening mechanisms were the bridging effect and pullout of the elongated grains/grains,crack deflection,crackbranching and crack pinning,etc.The cutting performance and tool wear mechanisms of the advanced Al2O3-based composite ceramic tools when continuously wet-turning nickel-based superalloy GH3536 and hot die steel H13 were studied.The results indicated that cutting performance and wear resistance of AS,AST and ASN ceramic tools were superior to the contrast tool LT55.When three kinds of ceramic tools cut GH3536,the main failure modes of cutting tools were brittle exfoliation,breakage and groove wear.The wear mechanisms of AS ceramic tool were boundary groove wear and adhesive wear,the wear mechanisms of AST ceramic tool were boundary groove wear,adhesive wear,abrasive wear and oxidation wear,and the wear mechanisms of ASN ceramic tool were boundary groove wear,abrasive wear and adhesive wear.When three kinds of ceramic tools cut H13,the main failure modes of cutting tools were brittle exfoliation and breakage of the rake face,micro breaking of the tool tip and abrasive wear of the tool flank,and the wear mechanisms of AS,AST and ASN ceramic tools were abrasive wear,adhesive wear and boundary wear.Stochastic characteristics of mechanical properties and cutting tool wear life as well as tool wear reliability of the advanced AS,AST and ASN composite ceramic tools were studied.The Weibull distribution models of flexural strength,fracture toughness and Vickers hardness of the three ceramic tools were established,and the results indicated that effects of defect on the stochastic characteristics of mechanical properties were consistent,namely the coefficient of variation and the Weibull modulus in order were Cs(?)>Cs(KIC)>Cs(HV)and m{HV)>m(KIC)>m(?).The Weibull distribution models of cutting tool wear life and tool wear reliability models of AS,AST and ASN ceramic tools were established when continuously wet-turning GH3536 and H13,and the results indicated that the coefficient of variation and the Weibull modulus of tool wear life in order were Cs(ASN)>Cs(AST)>Cs(AS)and m(AS)>m(AST)>m(ASN)· Effect laws of defect on the stochastic characteristics of mechanical properties and tool wear life were revealed,and indicating a good repeatability of the mechanical properties and a high wear reliability for the ceramic tool materials with good density and uniform defect distribution.