Study On Microstructures Of OLC Sintered With CBN Or Carbon Nanotubes Under High Temperature And High Pressure

Diamond is the hardest crystalline material with exceptionally high thermal conductivity. However, diamond has an inherent draw-back of poor thermal stability, resulting in the severe wear of diamond tools at high temperatures. Polycrystalline cubic boron nitride possesses high thermal stability and is the best-known material for cutting ferrous and carbide-forming hard substances where diamond completely fails. However, polycrystalline cBNs have a Vickers hardness(HV) of only 33~45 GPa.We use onion-like carbon(OLC) mixed with cBNs as raw material to sinter OLC-cBN at high temperature(HT) and high pressure(HP). The purpose of this thesis is to manufacturea kind of diamond/c BN composites which are expected to possess better thermostability, abrasive resistance, hardness and fracture toughness. Researching how the volume of cBN’s addition, the sintering conditionand and the surfacestates of cBN.affect the microstructures and mechanical properties of OLC-cBN. Analyzing phase composition, microstructure morphology and hardness of the sample by XRD, FESEM, HRTEM and Hardness Test. Finally, it is concluded that, the content of cBN mixed with OLC is 30 wt.% and the process conditions for sintering are 6 GPa, 1300 ℃ and heating 15 min is more outstanding. Under this condition, the average Vickers hardness of OLC-cBN is 36.66 GPa and the percent conversion of OLC to diamond is 46.66%. And the diamond generated by OLC particles can be combined closely with added cBNs.OLC is mixed with MWCNTs and then are sintered at HT and HP in my experiment. It is expected that the pressure and temperature will be further reduced, the percent conversioncan of OLC to diamond will be further increased and the mechanical properties of the sample will be better. Researching how sintering condition affect the microstructures and mechanical properties of OLC-MWCNT. The content of MWCNTs mixed with OLC is 10 wt.% and the best process conditions for sintering are 5.5 GPa, 1150 ℃, and heating 20 min. Under this condition, the average Vickers hardness of sample is 18.84 GPa and the percent conversion of OLC to diamond is 25.96%.