Preparation And Properties Of Ceramic-based Superhard Composite By Spark Plasma Sintering

In this thesis,spark plasma sintering(SPS)technology was employed to prepare polycrystalline superhard composites,which could be a feasible alternative approach to overcome the disadvantages occurred in the conventional high temperature and high pressure sintering process,such as the sample sizes and shapes limited by the high pressure chamber,the high production cost as well as the complicated sintering conditions.The rapid sintering process with relative lower temperature of SPS system can effectively restrain the high-temperature graphitization of diamond and the hexagonal BN(hBN)transformation from cubic BN(cBN).Meanwhile,some specific elements with high reactivity to graphite or hBN were added to consume the newly formed graphite or hBN phase and produce hard phases.Finally,ceramic-based polycrystalline superhard composites have been obtained,which were bonded by the newly generated hard phases without residual graphite or hBN phases.The elements Si and Ti were selected to be the active elements for diamond,as they can react with carbon to form new hard phase that chemically bonded with diamond;which improves the density of the composites and enhances the bonding strength between the hard phase binders and the diamond particles.When the components of Si,Ti and diamond were 20 wt.%,30 wt.% and 50 wt.%,the fabricated polycrystalline diamond(PCD)composites possessed optimized performance with respect to the relative density,the bending strength,and the micro-hardness.In this formula,when the sintering temperature was lower than 1600 °C,the Si/Ti binders cannot be adequately molten,and the component particles cannot be effectively activated to react with diamond chemically,and thus the bonding strength between the binders and diamond was low.When the sintering temperature was 1650 °C,the relative density,bending strength and micro-hardness of the obtained PCD composites achieved their maximum of 99.36%,448.5 MPa and 29.7 GPa,respectively.In other words,ceramic-based PCD composites had been successfully prepared,which possessed the best comprehensive mechanical properties and were free of residual graphite,Si,and Ti phases.When the sintering temperature increased to 1700 ?C,the additional graphite phase was appeared in the prepared PCD composites.Due to the high reactivity with hBN,the elements Al and Ti were added as binders to prepare polycrystalline cBN(PCBN)composites by SPS.When the components of Al,Ti and cBN were 10 wt.%,35 wt.% and 55 wt.%,the fabricated PCBN composites possessed optimized performances with respect to the relative density,the bending strength,and the micro-hardness.In this formula,when the sintering temperature is lower than 1300 °C,the Al/Ti binders cannot be adequately molten,the component particles cannot be effectively activated to react with h BN chemically so that the holding force between the binders and cBN particles was low.When the sintering temperature was 1400 ?C,the relative density,bending strength and micro-hardness of the prepared PCBN composites achieve their highest values of 99.12%,395.6 MPa,and 14.1 GPa,respectively.In this case,no residual hBN,Al,and Ti were found in the ceramic matrix bonded PCBN composites.When the sintering temperature increased to 1500 ?C,the additional hBN phase appeared in the PCBN composites.By raising the sintering temperature to 1600 ?C,the cBN phase in the composites had been completely converted to h BN.In addition,zinc oxide(ZnO)coated micro-diamond powders were successfully prepared by an urea precipitation method and used to prepare vitrified-bonded diamond composites.The ZnO coatings can isolate the contact directly between diamond and the binder,resulting in an increase of the initial oxidation temperature of diamond by 70 ?C.Meanwhile,the bending strength of the vitrified-bonded diamond composites prepared by ZnO-coated diamond was increased by 24% in comparison with that prepared by pristine diamond.Moreover,Zn O coatings can effectively suppress the oxidation of diamond and the binder corrosion effects,which promoted the melting of oxide in the binder,increased a strong combination between the diamond particles and the binder matrix,and thus improved the comprehensive mechanical properties of the prepared vitrified-bonded diamond composites.