Study On Fabrication And Cutting Performance Of TiB₂ Matrix Ceramic Tools Based On Multi-phase And Multi-scale

Aimed at improving the complex mechanical properties of ceramic tool,based on the new thought for designing multi-phase and multi-scale nanocomposites,a new kind of multi-phase and multi-scale TiB₂ matrix nanocomposite ceramic tool materials have been developed according to the chemical and physical compatibility with the sintering and conduction character.The liquid-phase hot-pressing sintering technology was adopted.The micro-scale and nano-scale TiN particles were added into TiB₂ with Ni and Mo as sintering additives.The relationship among sintering process,mechanical properties and microstructure was investigated in this paper.Meanwhile the toughening and strengthening mechanisms were analyzed,and the cutting performance was studied systematically.The influence of different dispersing conditions on the microstructure and mechanical properties of the multi-phase and multi-scale TiB₂ matrix nanocomposite ceramic tool materials was investigate.The results showed that the mechanical properties with absolute ethyl alcohol as dispersing medium were apparently better than that with deionized water as dispersing medium.The latter led to the worsening properties of the materials due to the reaction between TiN and water vapor while the former caused the increase of the mechanical properties of the materials as a result of the clean grain boundary,closely combined grains and no reaction.The new kind of multi-phase and multi-scale TiB₂ matrix nanocomposite ceramic tool materials BN10N5-2 and BN20N5-2 were fabricated successfully with the optimized sintering process of 1550℃under 25MPa with a holding time of 25 minutes.The flexural strength,fracture toughness and Vickers hardness of BN10NS-2 were 1056MPa, 6.52 MPa·m^1/2and 18.6GPa,respectively.The corresponding mechanical properties of BN20N5-2 were 1053MPa,6.92 MPa·m^1/2and 18.8GPa,respectively.The grains were obviously refined by adding the micro-scale and nano-scale TiN particles according to the microstructure research results.At the same time,there were more drawn grains and intragranular fracture but low porosity at the cross section. The influences of multi-phase and multi-scale on strengthening and toughening mechanisms of the new multi-phase and multi-scale TiB₂ matrix nanocomposite ceramic tool materials were studied.Ni and Mo can improve the combination strength of grain boundary and induce crack bridging,crack transgranulation and crack deflection.TiN particles can inhibit the excessive matrix grain growth and generate crack deflection,crack spinning and residual stress.Simultaneously,the particles can also easily cause dislocation to increase mechanical properties.Multi-scale TiN particles can produce toughening mechanisms of both residual stress and crack deflection. Intragranular nano-scale TiN particles can induce frontal process zone broadening and decrease the crack propagation length effectively to produce intragranulare fracture.Compared to the commercial SG4 ceramic tool,the cutting performance and failure mechanism of BN10N5-2 and BN20N5-2 in continuous machining quenched 40Cr alloy steel,quenched 45# steel and quenched T10A were studied.The results showed that in continuous machining quenched 40Cr alloy steel,the wea(?)resistance of BN10N5-2 was a little better than SG4 at a low cutting speed while the difference among BN 10N5-2,BN20N5-2 and SG4 was not significant.The main wear mechanism of BN10N5-2 and BN20N5-2 was abrasive wear on flank surface while abrasive wear and adhesive wear on rake surface at a low cutting speed.Furthermore,there was slight oxidation on rake surface at a high cutting speed.In continuous mach(?)ning quenched 45# steel,the cutting performance of BN10N5-2 and BN20N5-2 was equivalent to that of SG4 at a low speed while the wear resistance of BN10N5-2 and BN20N5-2 was better than SG4 at a high speed.The main wear mechanism of BN10N5-2 and BN20NS-2 was abrasive wear on flank surface while abrasive wear and adhesive wear on rake surface at a low speed.Moreover,there was oxidation wear and adhesive wear on rake surface at a high cutting speed.In continuous machining quenched T10A,both BN10N5-2 and BN20N5-2 appeared bigger wear size at a low cutting speed and caused tipping easily at a high cutting speed.The failure mode was wear and breakage,which was the comprehensive effect of high thermal stress and mechanical stress during machining process.The main wear mechanisms were abrasive on flank surface and bonding wear with oxidation wear on rake surface.