Preparation Of Super Hard Particles/alumina Composites By Spark Plasma Sintering

Vitrified bond diamond or cubic boron nitride composite materials are commonlyused in super-hard tools of industrial products. This article first synthesized nano-aluminapowder by microwave method, and then using spark plasma sintering process, withalumina binder prepared diamond/alumina(D/AL₂O₃) composite and cubic boronnitride/alumina(cBN/AL₂O₃) composite. The results show that diamond and cubic boronnitride grits in the composites may Graphite and hexagonal transformation separately athigh temperature sintering process which seriously affected the composite strength andgrinding performance. To solve this problem,several ways are adopted such as coating thediamond surface with a protective layer, add the active absorption elements or somesintering additives in order to improve the lower overall performance of the compositeswhich caused by the phase transition of diamond and cBN during the sintering process.In this paper the protective layer used is Cr coating by vacuum slow vapor deposition,the active absorption elements is Ti and the sintering additive is MgO. The results show:1. The nano AL₂O₃powder which synthesized by microwave method can transform toα-AL₂O₃with the SPS temperature of1200°C. The density and three-point bendingstrength of sintered AL₂O₃are proportional to the SPS temperature, time, and pressure.Therelative density and three-point bengding strength of sintered AL₂O₃reached to99.78%and357.54MPa separately with the SPS condition of1500°C,20min and50MPa。2. Diamond and cBN particles of different Crain size were sintered which usingAL₂O₃as binder agent, and the range of sintering temperature is form1200°C to1450°C.Raman and XRD results show that the diamond particles will occur graphite transitionunder the SPS conditions of1350°C, and the cBN particles also will occur phasetransform as the elevate of sintering temperature, and the higher of the sinteringtemperature, the more obvious of the phase transform of cBN.3. Cr coating on diamond surface can effectively protect diamond form trace oxygen,thereby slowing the process of Craphitizaion of diamond, Cr coated diamond compositehas higher bending strength than those with any coating material, and the value up to 56.83MPa.4. The adding of active absorption elements Ti to D/AL₂O₃composite can effectivelyabsorb the graphite produced by diamond at high temperature and form into hard TiC.5. The adding of active absorption element Ti to cBN/AL₂O₃composite can consumemost of the hBN produced by cBN at high temperature and form into hard TiB2and TiN,2μm cBN/AL₂O₃/Ti composite with15wt.%Ti and100/120cBN/AL₂O₃/Ti composite with20wt.%Ti have the highest bending strength value, which was253.2MPa and164.5MPa, respectively.6. MgO additive to cBN/AL₂O₃can improve the sintering process and bendingstrength of the composite, cBN/AL₂O₃/MgO composite with0.5wt.%MgO has the highestbengding strength of55.24MPa.