Synthesis Of Diamond And Its In-situ Reinforced Iron-nickel Composites By Spark Plasma Sintering

Diamond is one of the allotrope of carbon,with the highest hardness strength in all materials,excellent mechanical properties,good thermal stability and chemical stability.It is widely used in the industry.Spark Plasma Sintering(SPS)is a rapid prototyping technique for materials and is a promising new technology for diamond synthesis.According to the transition phase diagram of graphite and diamond,the temperature and pressure are two important factors influencing synthetic diamond.In this paper,carbon nanotubes are used as carbon source,FeNi30 is the catalyst,using the spark plasma sintering to compound diamond,studying the optimal ratio of carbon source and catalyst;then the influence of different temperature on synthetic diamond was studied and the best sintering process was obtained.In addition,by improving the experiment device for high pressure,this paper discussed the different pressure(no pressure,low pressure and high pressure)have an effect on synthetic diamond.Through XRD,Raman,TEM and SEM to characterize the synthesis of nano diamond.Based on the successful synthesis of diamond,the preparation and properties of the in-situ synthetic nano-diamond reinforced Fe-Ni matrix composites were studied.The results show that carbon nanotubes can be transformed into diamond under the catalysis of FeNi30 powder.When the mass ratio of carbon nanotubes to catalyst is 7:3,nano-diamond can be produced.When the sintering temperature is 1200 ?,sintering pressure of 80 MPa,the crystal type of nano diamond is complete,and the diamond conversion rate is higher;For adding 0.7 wt.% Ce synthetic diamond,the best sintering temperature is 1100 ?,and diamond conversion rate increased;The best temperature of pressureless sintering is 1350 ?,the size of nano diamond particle becomes bigger,but CNTs could prompt graphitized to form carbon onion when the temperature is higher than 1300 ?.At high pressure sintering,the conversion rate of diamond is obviously improved,and the crystallinity of nano-diamond is better,when the pressure is greater than 100 MPa,the fractured carbon nanotube will be graphitized to form carbon onion.Finally,the mechanism of synthesis of diamond by SPS is discussed.Synthesis of diamond from Carbon nanotubes mainly experienced the following stages: carbon nanotubes were spread out open ?structure broken----C-C bond is broken----C-C bond rearranges to form the core of diamond----Diamond core continues to grow----eventually form the nano diamond.In addition,0.25-1.0 wt.%CNTs were added in the iron-nickel alloy powder matrix,and in-situ synthesis of nanodiamond reinforced iron-nickel composites was obtained by spark plasma sintering at 1050 ?,80 MPa.Thehardness and yield strength of composites increased with the increasing of the carbon nanotubes.When 1.0 wt.%CNTs were added,the microhardness decreased obviously.A large number of dimples were observed by scanning electron microscopy in fracture morphology,indicating that fracture mode of the composites is ductile fracture;The coefficient of thermal expansion decreased significantly with the addition of the carbon nanotubes.Based on the above results,0.75 wt.%CNTs is the best ratio of carbon nanotubes for iron-nickel composites.