| TiB2 is a kind of promising engineering ceramic because of its high hardness,high modulus,extreme wear resistance and good thermal and electrical conductivity.It has been extensively employed in cutting tools,ceramic armors,grain refiners,high-temperature coating,cathodes for aluminum electrolysis cells and so on.Commonly,H3BO3 and B4C are widely utilized as the boron source in traditional carbothermal reduction method.However,the H3BO3 can volatilize easily at high temperatures,leading to formation of impurities,which causes the difficulty in determination of formula and acid corrosion on the furnace.Meanwhile,powders prepared via expensive B4C will result in the non-uniform particle distribution and extra process is required to refine particles.In conclusion,how to decrease the cost in purification and grinding becomes a challenge for realizing industrial preparation.It is difficult to obtain dense titanium diboride ceramics by pressureless sintering due to the low self-diffusion coefficient.Meanwhile,silicon carbide has superior mechanical and thermal properties and the technique for its pressureless sintering is relatively mature.Therefore,silicon carbide is added into titanium diboride matrix as the second phase along with optimal sintering aids to prepare composites,which can relieve the diffusion resistance and help to promote the densification.This paper optimized the traditional carbothermal reduction method using Ti02,HBO2,B4C and carbon black as raw materials.The influence of boron source contents,calcination system and the variety of carbon sources on the phase composition and microstructure of final products prepared by TiO2-HBO2-C system and TiO2-B4C-C system was studied.Results showed that typical hexagonal TiB2 grains could be obtained when heated at 1700? for 30min with the mass ratio of TiO2,HBO2 and carbon black being fixed at 30.4:46.7:22.9.Resultant powders with high crystallinity and purity were estimated to be 10?m.Plate-like TiB2 particles ranging from 0.5-1?m with uniform distribution could be prepared when heated at 1600? for 30min with the mass ratio of Ti02,B4C and carbon black being fixed at 61.0:25.3:13.7.In addition,compared to powders synthesized via graphite and activated carbon,TiB2 grains showed ideal crystal shape and better crystallinity when carbon black was used as the carbon source in both systems.TiB2-SiC composites were successfully fabricated via pressureless sintering using phenolic resin(PF)and polyvinylpyrrolidone(PVP)as additives.The optimal formula of ingredients was determined.The effects of sintering system on mechanical and thermal properties of composites were investigated.Results exhibited that the ideal mass ratio of TiB2,SiC,PF and PVP was 75:25:15:0.2.The bending strength and fracture toughness could reach the maximum at 209.5MPa and 4.6MPa·m1/2 when heated at 2100? for 60min.Meanwhile,the hardness and thermal conductivity of composites were 29,3GPa and 32.3W/(m-K),respectively.When the sintering temperature is below 2100? and the holding time is shorter than 60min,it is difficult to get enough energy to overcome the grain boundary resistance and the temperature filed tends to be nonuniform,resulting in difficulty in acheving complete densification.By contrast,when the sintering temperature is higher than 2100? and the holding time exceeds 60min,the abnormal growth of grains and oversintering can occur,which can deteriorate the mechanical and thermal properties of composites.Based on the optimal sintering system and formula,the effects of graphene contents on mechanical and thermal properties of composites were investigated.The toughening mechanisms and heat conduction mechanism were also discussed.The graphene contents varied from 0 to 5wt%.When graphene contents were 2wt%and 3wt%,the bending strength and fracture toughness peaked at 301.2MPa and 5.7MPa·m1/2,respectively.And the thermal conductivity of composites reached the maximum at 35.4W/(m·K)when 2wt%graphene was added.Judging from the fractured surfaces of composites with different graphene contents,it was obvious that the lamellar tearing structure formed as result of the addition of graphene,which can lead to the interlayer sliding,retarding the crack growth.When the graphene content was more than 3wt%,the compact structure between grains was destroyed by dispersive graphene,resulting in internal defects that could deteriorates mechanical properties.The thermal conductivity of composites showed a rise followed by a decline,reaching its peak when 2 wt%graphene was added.This was attributed to the increase in numbers of free electrons accompanied by graphene contents rising,which can create a new equivalent phonon channel.When the graphene content was over 2wt%,the porosity of samples went up and the phonon scattering became more noticeable. |
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