Research On Interfacial Structure And Properties Of Polycrystal-line Cubic Boron Nitride Cutting Tool Materials

Polycrystalline cubic boron nitride（PCBN）material,as a tool for precision cutting of hardened steel and others materials,requires the characteristics of heat resistant damage and wear,chemical abrasion resistance,and excellent processability.Which is prepared generally by sintering under high temperature and high pressure（HPHT）with a ceramic binder and a low content of fine-grained c BN,and the complexity of process leads to many uncertain factors affecting the material performance.Therefore,the material properties with high quality and high stability depend on the higher sintering strength.Focusing on the interface structure based on the preparation and properties of PCBN materials,the thesis studies the effects of raw material selection,formulation,and HPHT sintering process on the interface structure and properties of PCBN materials.A series of beneficial innovations have been achieved:1.The horizontal planetary ball milling method is used to explore the initial raw material dispersion process,which effectively guarantees the acquisition of a uniform and dense sintered structure;The milling mechanism is discussed through the force analysis of the grinding ball,and the appropriate speed[n_sb,n_sc]interval is calculated as[146 r/min,206 r/min].It is concluded that the grinding and impact action of the grinding ball in the tank increases with the increase of the ball mill speed.Further experimentsts show that the particle size of 4-8μm is the inflection point suitable for ball milling and mixing.Ball milling at a speed of 180 r/min for 12hours can obtain a uniform and non-doped sintered structure,with a particle refining rate of 3.07%～4.01%,and the coarser the particles,the more obvious the refinement and doping.2.Black and brown c BN micropowders with different particle size,and ceramic powders of Ti C,Ti N,Ti CN and Ta C,were selected to undergo the ultra-high pressure of5-6 GPa extrusion crushing experiment at ambient temperature.The crushing behavior and law of c BN and ceramic powder particles under ultra-high pressure were revealed by using laser particle size analyzer,SEM,and XRD.Studies have shown that c BN particles are plastically deformed and fracture,and the densification process of particle crushing and void filling is completed within one minute.The black c BN and coarse-grained particles with lower toughness are more easily broken,with the maximum particle crushing rate of 22.81%;However,ceramic particles are broken into brittle fractures or internal cracks,which are easily agglomerated and have poor fluidity.The ceramic powder has a primary densification of ultra-high pressure and a secondary densification during HPHT sintering.The ceramic grains embedded in each other in the microstructure with partial sintering morphology.The active surface and fine particles formed by particle crushing contribute to HPHT sintering,but the"internal damage"such as cracks will also have a negative impact on the characteristics of sintered body.3.The batching scheme design model of Al-Ti N-c BN composite was established by using Horsfield theory and Dinger-Funk equation.Based on the design and calculation of three formulas,the PCBN obtained by HPHT sintering has excellent tool life and workpiece surface finish.The sintering reaction mechanism of PCBN is discussed by thremodynamic Gibbs energy calculation and phase diagram analysis,and also its influence on material properties is studied.The results show that the order of the sintering reaction between Al-N has priority over Al-Ti and Al-B,and is affected by the phase concentration in the contact zone.Aluminum binder in PCBN provides a sintering liquid phase environment due to the influence of phase concentration in the contact area.It has successively reacted with Ti N and c BN to form grain boundaries,which are mainly distributed around c BN and the interface boundaries area,which effectively improves the hardness,density,compressibility and impact strength of the sintered body.4.c BN-c BN grain boundary is formed by sintering c BN of 2-4μm at 2100℃and7.3GPa.The transition width of the interface region is S_B≈15-30 nm,which contains high-density lattice distortion,lattice slip,and dislocation accumulation,forming semi-coherent twin boundaries.The formation mechanism is as follows:c BN is broken under HPHT,resulting in layered cleavage,partial hexagonal transformation to h BN,and the broken fine grains fill the pores to provide a growth source,when sufficient sintering pressure and temperature are available to form solid solution between the grain boundary where c BN grains contact,and then,B and N atoms in the structure form c BN on the grain surface,or h BN is distorted to form c BN under the action of HPHT,with the formation and migration of the interface,c BN-c BN bonding is realized.The study on the sintering of the Al-Ti N-c BN system shows that c BN-Ti N grain boundarys are formed in the sintered body,and the transiton width of the interface ragion is S_B≈5-7nm and has a distribution of nano-crystalline grains.Both c BN and Ti N grains are closely combined along the{111}and{220}crystal planes,dense dislocations and twin grains are accumulated near c BN grains,and a gradual transition smooth structure near Ti N grains.Lattice mismatch calculation shows that c BN-Ti N mostly form a semi-coherent interface,and periodic dislocation accumulation and lattice distortion in the interface area effectively improve the stability of the bonding interface and the strength of the interface.5.The PCBN composites with a diameter of（?）62mm and a flat structure was successfully prepared on the cubic press.PCBN material layer thickness is 0.7～0.9 mm,the structure is uniform and compact without defects,and had good EDM properties.PCBN materials with different binders have a micro-hardness of 28-42 GPa and a flexural strength of 810-1100 MPa.Different degrees of c BN-c BN and c BN-Ti N are formed in the microstructure.Research on the interface between PCBN and cemented carbide layer shows that the interface area presents a partitioned structure of mutual diffusion,which effectively improve the bonding strength of the interface.Theform and degree of diffusion are related to the binder composition contained in PCBN.Mutual diffusion significantly forms an inter-embedded interfacial structure when a metal adhesive is used,and has a higher interfacial bonding strength;However,when the ceramic binder is used and its content incrases,the mutual diffusion weakens,the interface layer presents a regular interface structure of mutual infiltration and bonding,and the interfacial bonding strength is weakened.