HPHT Synthesis And Characterization Of CBN-based Composite Materials

cBN is synthetic crystal with hardness next to diamond.Although the hardness of cBN is lower than that of diamond,its thermal stability is higher than that of diamond.In addition,PcBN tool has an absolute advantage in cutting iron-based metal materials,because in the high temperature environment of cutting,iron-based metal is easy to react with diamond and corrode diamond tool,while cBN has a great chemical inertia to iron-based metal.However,compared with diamond,cBN is more difficult to synthesize large single crystal.Most of the researchers synthesized cBN particles in micron size.Therefore,how to bond cBN particles in micron size and the bonding effect determines the properties of cBN based composite materials.Al₂O₃ is a kind of oxide with high hardness,and Al₂O₃ is stable under HPHT conditions.A series of cBN-Al₂O₃ composites were synthesized under HPHT conditions.Al reacts with cBN and forming AlN under HPHT conditions.A series of cBN?Al?-Al₂O₃ composites were synthesized under HPHT conditions after cBN particles were coated with Al film.B₂O₃ is the oxide of cBN,and cBN is superhard material with hardness next to diamond.What about the hardness and other properties of B₂O₃ after HPHT treatment?Through literature search,the research reported in this area is still very little.Therefore,we have done a series of research on B₂O₃ under HPHT conditions.Through the research,this thesis obtains the following findings and conclusions.1.We synthesized cBN-Al₂O₃ composites in NaCl environment under HPHT conditions,and we characterized the thermogravimetry and Vickers hardness of cBN-Al₂O₃ composites.The results show that the porosity of cBN-Al₂O₃ composites decreases with the increase of Al₂O₃.Al₂O₃ inhibited the appearance of porosity between cBN particles,and Al₂O₃ as support material makes it possible to form chemical bond at the tip of cBN particles.The oxidation resistance of cBN-Al₂O₃composites reaches 1300?,which is 200?higher than that of raw cBN powder and100?higher than that of cBN+0 vol%Al₂O₃.The hardness of cBN+30 vol%Al₂O₃composite is 21.6 GPa.A new method of indirectly controlling the porosity of cBN-Al₂O₃ composites by adjusting the content of NaCl was proposed.2.A series of cBN?Al?-Al₂O₃ composites were synthesized under HPHT conditions of the pressure of 5.0 Gpa and temperature of 1200-1500?after coating cBN particles with Al.Under the pressure of 5.0 Gpa and the temperature of1200-1500?,NaCl began to infiltrate into the cBN?Al?-Al₂O₃ composites,and cBN does not change into hBN.After XRD characterization of cBN?Al?-Al₂O₃ composites,we found that Al reacted with cBN particles under HPHT conditions and generated AlN.AlN phase as bridge phase connects cBN phase and Al₂O₃ phase.Under HPHT experiment conditions,the process can be divided into two parts.On the one hand,Al film reacts with cBN to form AlN,on the other hand,Al film reacts with oxygen which exist in the pores between cBN particles to form Al₂O₃.The hardness of cBN?Al?-Al₂O₃ composites is 37.5%higher than that of cBN-Al₂O₃ composites.The starting oxidation temperature of cBN to B₂O₃ in cBN?Al?-Al₂O₃ composites is about50?lower than that of cBN-Al₂O₃ composites.In addition,the micro amount of NaCl in cBN?Al?-Al₂O₃ composites increase the density of cBN?Al?-Al₂O₃composites.The porosity of cBN?Al?-Al₂O₃ composites can be indirectly controlled by controlling the content of NaCl in the samples.3.We treated glassy B₂O₃ under HPHT conditions and obtained the phase diagram of B₂O₃.We studied the phase diagram of B₂O₃ systematically,the phase diagram of B₂O₃ is mainly divided into three parts:low pressure?-B₂O₃ crystal phase,high pressure?-B₂O₃ crystal phase and high temperature vitreous B₂O₃ phase.We found that in the low-temperature region of B₂O₃ phase diagram,the color,toughness and hardness of B₂O₃ changed obviously due to the grain size of B₂O₃ decreased to nanometer level,and we marked this low-temperature region in B₂O₃ phase diagram.The hardness of?-B₂O₃ which synthesized at 5.5 Gpa and 450?reached 13.9 GPa,this is consistent with the hardness of 16?5?GPa of?-B₂O₃ previously reported.At last,we characterized the B₂O₃ after HPHT treatment by Raman spectrum.Raman spectrum of B₂O₃ indicate that boroxol rings only exist in vitreous B₂O₃ transformed from?-B₂O₃ phase but not in vitreous B₂O₃ transformed from?-B₂O₃ phase.This occurs because?-B₂O₃ phase is composed of ribbons formed by planar BO₃ triangles and?-B₂O₃ phase is composed of BO₄ tetrahedrons.As the temperature increases to the glass transition temperature,the ribbon chains gradually break up and the planar BO₃ triangles form the more stable boroxol rings.The?-B₂O₃ phase is composed of BO₄ tetrahedrons;therefore,no boroxol rings are produced.In conclusion,cBN-Al₂O₃ composites and cBN?Al?-Al₂O₃ composites were successfully synthesized by HPHT method.Under HPHT conditions,Al₂O₃ can fill the pores between cBN particles and support cBN particles.The properties of cBN-Al₂O₃ composite and cBN?Al?-Al₂O₃ composite were fully analyzed and discussed by means of XRD,SEM,Vickers hardness and TGA.Compared with cBN-Al₂O₃ composite,the Vickers hardness of cBN?Al?-Al₂O₃ composite is increased by 37.5%,and its thermal stability is reduced by 50?.At last,we obtained the phase diagram of B₂O₃ which is the oxide of cBN,and studied the hardening mechanism of B₂O₃ and the formation mechanism of boroxol rings.This paper provides an important reference value for the further study of cBN-based composite materials under HPHT conditions.