Research On Spark Plasma Reaction Sintered Boron Carbide Superhard Tool And Its Cutting Performance

Boron carbide ceramic has the advantages of high hardness,high wear resistance and high melting point,so it has a wide range of application prospects in cutting tools,armor and high temperature coatings.However,there are outstanding problems in the preparation of boron carbide ceramics such as difficulty in sintering,low fracture toughness and high manufacturing cost.This thesis took the spark plasma reaction sintering technology in-situ synthesis of boron carbide superhard tools as the research object.Comprehensive experiments and theoretical analysis were carried out on the phase composition of the reactants,the composition ratio,the sintering process parameters,the toughening of the material,the friction and wear performance of the material and the cutting performance of the tool.The phase evolution during the reaction sintering of raw materials B,Ti and Graphite was studied.The results showed that Ti and B reacted to form Ti B₂ at 800°C,and B and Graphite reacted to form B₄C at 1250°C.When the raw material ratio was 60.6 wt%B-13.6wt%Graphite-25.8 wt%Ti;the ratio of the reaction product B₄C-Ti B₂ was 75-25 vol%;the sintering temperature was 1900°C;the holding time was 15 min;the reaction time was 6 min,the sintered composite ceramics obtained the best density,Vickers hardness,fracture toughness and flexural strength of 98.6±0.01%,26.6±0.01 GPa,5.9±0.13 MPa·m^1/2 and605 MPa,respectively.The influence of the content of MWNT on the phase composition,mechanical properties and microstructure of B₄C-Ti B₂superhard ceramic tool material was studied,and the toughening mechanism of MWNT were studied.The MWNT and the material matrix have good chemical compatibility and high temperature stability.When 1 vol%MWNT was added to the raw material,the fracture toughness of the material was 7.0±0.21 MPa·m^1/2,which was 18.6%higher than that without MWNT.The main toughening mechanisms of MWNT included large-angle deflection,branching and bridging of cracks,and the fracture and pull-out effects of MWNT and Ti B₂ particles.The friction and wear properties of superhard ceramic tool materials were studied,and the results showed that an appropriate amount of MWNT is beneficial to improve the friction properties.When titanium alloy（TC4）was used as a counter-grinding pair,as the normal load increased,the friction coefficient gradually decreased,and the wear mechanism is adhesive wear.As the friction speed increased,the friction coefficient and wear rate first decreased,then increased and then decreased.The wear mechanism was adhesive wear and oxidative wear.When bearing steel(GCr15 was used as the dual pair,the wear mechanism was abrasive wear at low speed and low load,and abrasive wear and oxidation wear at high speed and high load..The cutting performance of composite ceramics was studied.For dry milling processing TC4,the best milling parameters were 0.04 mm/z per tooth feed,1 mm axial depth of cut,radial depth of cut 40 mm,and milling speed 650 m/min.When the milling speed was less than 550 m/min,the tool wear mechanism was mainly abrasive wear,adhesive wear and lamellar spalling;when the milling speed was greater than 600 m/min,the tool wear mechanism additionally increases the oxidative wear and breaking loss effect.When the cutting depth was less than 0.75 mm,the wear of the tool was mainly the micro chipping of the cutting edge and the abrasive wear of the flank surface,adhesive wear and laminar spalling.When the cutting depth is greater than 1 mm,the tool wear mechanism increased oxidative wear and mechanical/thermal fatigue wear.When milling CFRP,the best milling parameters were axial depth of cut of 2 mm,radial depth of cut of 40 mm,feed per tooth of0.10 mm/z,and milling speed of 600 m/min,and the main form of tool failure was abrasive grinding.