| In this paper,in view of the c BN sintering problem of difficult to form under the current industrial production conditions in the ultra-high pressure apparatus,and based on the theoretical knowledge of materials science,high-pressure physics,powder metallurgy and mechanics,etc.,the effect of high-pressure sintering pressure,temperature,time and particle size on the high-pressure sintering behavior of c BN micro-powders were studied by XRD,Raman,HRTEM,SEM and other modern analysis means,focusing on the plastic deformation behavior and the cohesion mechanism of the sintered c BN body caused by the c BN plastic mechanism of c BN in the ultra-high pressure sintering process,and the high-performance pure Pc BN tool materials were successfully sintered under the industrial production conditions of 6.2GPa,2000?and 240s in the 650 type cubic press made in China.In order to reveal the mechanism of the effect of pressure in the c BN high-pressure sintering process,in this paper four single-grain c BN micropowders were used to carry out the experimental research on the c BN plastic deformation behavior and brittle fracture law by Raman spectral stress test and XRD line analysis technique.The test results of Raman spectroscopy show that the internal stress of the sintered coarse grain W15-25 sample is as high as 1.75GPa to 2.20GPa,which is close to the brittle fracture strength of the c BN crystal,while the internal stress in the finer grain W1-2c BN sample is only 0.403GPa to 0.785GPa,its internal stress of the former increase of 332.41%than that of the later.It is believed that the ultra-high-pressure cold crushing of coarse c BN micro-powder is the reason for the obvious expansion of c BN particle size distribution after cold pressure.The micro-strain,total layer dislocation probability f Tand the deformation layer dislocation probability f Dwere further calculated by XRD analysis technology,and the results showed that the micro-strain,total layer dislocation probability f Tand the deformation layer dislocation probability f Din the coarse grain W15-25 sintered sample were-1.50×10-3,7.50×10-3,22.72×10-3respectively,which increased by74.4%,50%and 290%compared to that of the fine grain W1-2 sintered sample,respectively,indicating that coarse c BN particles has experienced much larger cold plastic deformation than fine c BN particles under the same pressure,coarse c BN particles may occur plastic deformation with layer dislocation,and the probability fine particles c BN plastic deformation of layer dislocation is very low.Based on the difference between the plastic deformation and the degree of brittle fracture of different c BN size under ultra-high-pressure cold crushing,the ultra-high-pressure cold crushing model of c BN micro-powder is proposed in this paper for the first time.The effect of sintering process on the compactness and properties of pure sintered c BN samples were studied by SEM and Image J image analysis software.The results showed that the surface porosity of the sintered c BN samples decreases from 4.05%to 0.2%and the porosity decreases below d<1.0?m,the porosity roundness R tends to 0.82-1.00,close to spheroidization,and the relative density?increases from 92.1%to 99%,when sintering temperature increasing from 1550?to 2000?under the conditions of the original c BN particle size of 8-12?m,fixed sintering pressure of 6.2GPa and sintering time of 240s,and the surface porosity of the sintered c BN samples decreases from 4.46%to 0.21%and the porosity decreases from d<2.5?m to d<1.0?m,the porosity roundness R tends to 0.64-1.00,close to spheroidization,and the relative density?increases from 92%to about 99%,when sintering time increasing from 180s to 240s under the conditions of the original c BN particle size of 8-12?m,fixed sintering pressure of 6.2GPa and sintering temperature of 2000?.Continue to raise the temperature and extend the sintering time,the sintering sample dense parameters change little,inferred that dense process has been completed in the sintered pure c BN sample at temperature 2000?,and sintering time 240s.However,under this sintering condition,the grinding ratio and compress strength of the sintered sample increase from 4760 and 1.78GPa to 10200 and 2.37GPa,respectively,with the sintering pressure rising from 5.6GPa to 6.2GPa,and there is the continued to increase trend of grinding ratio and compress strength in the sintered sample when continued increasing the sintering pressure.Because the sintering temperature of this experiment is less than c BN volume diffusion temperature,it is inferred that the sintering mechanism is the plastic flow and surface diffusion,and the main factor of c BN sintering dense process is pressure,the main sintering mechanism is the plastic flow,and the second is the sintering temperature and time that affect the function of the surface diffusion.In order to further reveal the particle size-induced ultra-high-pressure plastic deformation behavior and sintering dense law,the microstructure and performance differences of the ultra-high pressure sintering c BN samples of different grain sizes were investigated under the optimal sintering conditions of 6.2GPa,2000?and 240s using four c BN micro-powders of different particle sizes.The Raman Spectroscopy analysis results showed that the internal stress in the pure c BN sample sintered with particle size of W15-25 and W8-12 was increased by 210.0%?331.7%and 168.0%?266.67%respectively compared to that of the W1-2 particle size sintered sample.The performance test results showed that the Vick hardness,compress strength and heat resistant temperature of the W15-25 sintered pure c BN sample improved 23.41%,58.02%and 118.9%respectively compared to that of the W1-2 sintered sample.Therefore,it is concluded that the difference in microstructure and performance of different particle size c BN sintered bodies may be caused by the difference of plastic deformation caused by the stress difference in c BN crystals during high-pressure sintering.The micro-strain?,probability of deformation stacking fault f Dand twin crystal fault probability f Tof different particle size c BN sintering bodies were calculated by using X-ray diffraction analysis technique,and the plastic deformation of different particle size c BN sintered bodies were analyzed by high-resolution transmission microscope(HRTEM).XRD analysis results showed that the micro-strain?and the probability of twinning stacking fault f Tof the coarse grain W15-25 sintered sample was-0.43×10-3and 1.89×10-3,respectively,which is only about 1/3 and1/12 of that of the?(-1.4×10-3)and f T(23.93×10-3)in fine grain W1-2 sintered sample,but the probability of deformation stacking fault f D(6.54×10-3)was about 10 times higher than that of the f D(0.646×10-3),while the medium-grain W8-12 sintered sample also had both a high micro-strain?(-1.5×10-3)and probability of twinning stacking fault f T(14.18×10-3)and a high probability of deformation stacking fault f D(6.28×10-3),showing significant critical particle size effect.This shows that the plastic deformation in coarse grain W15-25 sintered sample may be in the form of dislocation and stacking fault movement,and the fine particle size W1-2 sintered sample in the form of twinning stacking fault movement,while the medium-grain W8-12 sintered sample take into account both the deformation of dislocation and stacking fault movement and twin crystal fault plastic deformation.The above conclusion was tested by further HRTEM observation of the three kind of sintered pure c BN samples.Based on the above results,the particle size-induced c BN plastic deformation and its bonding mechanism under ultra-high-pressure sintering condition is proposed.That was,c BN particles were combined together in coarse grain size sintered W15-20sample by recrystallization of plastic flow in the form of dislocation and stacking fault,and the fine grain size W1-2 sintered sample by recrystallization of twinning stacking fault deformation,while in medium-grain W8-12 sintered sample take into account both the deformation of dislocation and stacking fault movement and twin crystal fault plastic deformation.This is the physical essence behind the c BN ultra-high-pressure sintering effect in this experiment.Combined with the above results,a pure Pc BN tool materials with a mixing particle size(W8-12,W3-6 and W1-2 ratio 7:2:1)was developed under the best sintering conditions of 6.2GPa,2000?and 240s for industrial application,with a Vickers hardness of 4232.7and heat resistance temperature 1236.7?respectively.XRD analysis results showed that the probability of deformation stacking fault f D(2.01×10-3)was low and the probability of twinning stacking fault f T(19.79×10-3)was high,indicating that the sample is of both the deformation of dislocation and stacking fault in coarse grain and twin crystal fault plastic deformation in fine grain size c BN sintered sample.HRTEM observation found that there are both high dislocation and stacking fault and twin crystal in the mix-powders sintered sample,thus it is concluded that the high mechanical properties of the mix-powders sintered sample may be dedicated to the bonding mechanism of both dislocation deformation of coarse grain and twin plastic deformation of fine grain c BN sintered sample.High-speed machining hardened steel experiment test was carry out to comparison the tool life and cutting surface roughness of the hybrid grain size sintered pure Pc BN tool and the product of foreign DI company and Korea's IJING company,the results show that when the feed is 0.1mm/r,the depth of cut is 0.1mm,the cutting speed is 150r/min,the flank wear(VB0.172?m)after cutting5min.in the sintered pure Pc BN tool is higher than DI company tool wear(VB0.133mm),but lower than IJING company tool wear(VB0.2mm),continue to machine 30min.the relative flank wear rate in the sintered pure Pc BN tool is 44.54%,that is much lower than the corresponding tool wear rate(155.77%,131.76%)of the two foreign companies.This shows that after the cutting run-in period the cutting performance of the developed pure Pc BN tool is more stable than foreign tools.Its machining surface roughness Ra increases with the increasing cutting speed,but when the cutting speed is greater than 250r/min.,with the increase in the cutting speed but the cutting surface roughness Ra decreases,which is opposite to the processing law of the foreign tools,that is,with the cut speed increases the cutting surface roughness Ra first decline and then rise.When the cutting speed is over 450r/min.,the roughness of the machining surface is the same as that of foreign tool machining Ra,and shows a downward trend.This shows that the developed pure Pc BN tool is more suitable for cutting speed above 450r/min.in hardened steel dry cutting. |
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