Microstructure And Properties Of TiB₂ Composites Fabricated By Silicon Infiltration

Due to its high melting point,high hardness,wear resistance,corrosion resistance,oxidation resistance,good electrical conductivity and thermal conductivity,etc.,titanium diboride(TiB2)ceramic has found applications in various areas,such as military,aerospace,metallurgy,machinery,chemical industry and electronic industry.Currently,TiB2 ceramics are prepared by hot pressed sintering or pressureless sintering,with low fracture toughness and high fabrication costs hampering its applications.Therefore,based on silicon infiltration,a simple and low-temperature-sintering approach to fabricating TiB2 composite without deformation in shapes and compromise in properties is developed by our group.This work,is to fabricate dense TiB2-SiC and TiB2-MoSi2 composites and to study the influence of fabrication parameters on the phase compositions,microstructure,density,mechanical properties,electrical conductivity,oxidation resistance and corrosion resistance.This work is thus believed to be of high importance in exploiting the potential in applications of TiB2 ceramics.The main results are:(1)TiB2-SiC composites with high densities has been successfully synthesized by infiltration of molten silicon under vacuum into a porous compact composed of TiB2 and phenolic resin powders.Phase composition of the composite are:TiB2,SiC,Si and TiSi2.The microstructure analysis shows that:the TiB2 phases,SiC phases and TiSi2 phases distribute in the composite uniformly,while free Si fills in the continuous gaps between TiB2,SiC and TiSi2 phases.(2)With the increase of infiltration temperatures,the Vickers hardness and flexure strength of composites improve initially and then deteriorate,while the fracture toughness decrease.Both the grain size of TiB2 and the content of SiC increase,while that of free Si decrease with the increase of infiltration temperatures.The optimum relative density,open porosity,Vickers-hardness,flexure strength,fracture toughness and resistivity of the obtained TiB2-SiC composites infiltrated at 1550? for 30 min were 97.60%,0.14%,22 GPa,372 MPa,6.4 MPa-m1/2 and 6,02×10-7 ?·m,respectively.(3)TiB2-MoSi2 composites with high densities has been successfully synthesized by infiltration of molten silicon at 1550? for 30 min under vacuum into a porous compact composed of TiB2 and Mo powders.Phase composition of the composite are:TiB2,MoSi2,Si and(Ti0.8Mo0.2)Si2 phases.The TiB2 phases,MoSi2 phases and(Ti0.8Mo0.2)Si2 phases distribute in the composite uniformly,while free Si fills in the continuous gaps between TiB2,MoSi2 and(Ti0.8Mo0.2)Si2 phases.(4)With the increase of Mo contents,the Vickers hardness and fracture toughness of the composites increase,while the flexure strength improve initially and then deteriorate.The content of MoSi2 increase,while that of free Si decrease with the increase of infiltration temperatures.When the Mo content equals to 25 wt.%,overall properties of the composite reach the maxima.The volume density,open porosity,Vickers-hardness,flexure strength,fracture toughness and resistivity were 3.74 g·cm-3,0.19%,18 GPa,238 MPa,4.5 MPa·m1/2 and 8.69×10-7?·m,respectively.(5)Both the TiB2-SiC and TiB2-MoSi2 composites show excellent oxidation resistance at 1200?.The thickness of the erosion layer by electrolyte of TiB2-SiC composites is about 100?m,while that of TiB2-MoSi2 composites is about 20-30 ?m.