Effects Of Elements (Carbide) Doping On Mechanical And Friction/Wear Properties Of Silicon Carbide

Silicon carbide（SiC） is a kind of important material.The dissertation concentrated on studying the effects of elements（carbide） doping on mechanical and friction/wear properties of SiC.The obtained innovative conclusions are as follows:（1） Molecular dynamics（MD） method was used to investigate the mechanical behaviors and intrinsic deformation mechanisms of the monocrystalline face-centered cubic silicon carbide（3C-SiC） and the 3C-SiC doped with carbon（C） under tension and compression(at the strain rate of 10⁹ s^-1) conditions at room temperature.The results show that under tension conditions,Si-C sp³ and C-C sp³ bonds will respectively transform to Si-C sp² and C-C sp² bonds as soon as the deformation strains reach certain critical values.After forming certain amounts of sp² bonds,instable fractures will emerge in the 3C-SiC.Since the fracture of the C-C sp² bonds occurs earlier than that of the Si-C sp² bonds,the doping of C elements lead to the decreases of strength,elastic modulus and tension fracture strain of 3C-SiC.On the other hand,under compression conditions,Octahedron weak bonds will emerge within certain strain-stress ranges.This was accompanied with the strain-stress variations,which increase first and then decrease with the emergence and disappear of Octahedron weak bonds.Because C-C sp³ are harder than Si-C sp³ bonds,which promote the deformations of the surrounding Si-C sp³ bonds and transformations to Octahedron weak bonds,leading to a lower yield stress of SiC under compression conditions.Besides the Octahedron weak bonds,the compression strain increases to another certain critical value,Si-Si weak bonds will come out and then increase with the increase of the compression strain.Moreover,the doping of C element can accelerate the formation of Si-Si weak bonds,which reduce the ascending trend of the elastic modulus of 3C-SiC.In addition,the three tension strain rates of 5×10⁸ s^-1、1×10⁹ s^-1、1×10¹⁰ s^-1 have effects on the tensile-strength,but no on elastic modulus.The study not only helping understand the deformation behaviors under high-rate deformations,but also provides scientific basis for the material designs in the protection domains of military and aviation.（2） MD method was used to investigate the effects of elements doping on the elastic modulus of monocrystalline 3C-SiC and amorphous SiC bulks.The results show that the elastic modulus of monocrystalline 3C-SiC and amorphous SiC are respectively 551 GPa and 645 GPa. Upon doping the four elements He,Li,Be,Ar（concentrations 0～20 atm.%） the elastic modulus are approximately linearly decreasing,with increasing of the elements concentrations,Li and Be elements lead to the greatest and smallest decreases,respectively.On the other hand upon doping elements H,B,C,F（concentrations 0～20 atm.%）,the elastic modulus are approximately linearly increasing,with increasing of the elements concentrations,B and C elements lead to the smallest and greatest increases,respectively.Doping the same element has the similar effects on monocrystalline 3C-SiC and amorphous SiC bulks.The differences behaviors of elements doping should be the interactions between the doping elements and atoms in SiC,such as the transformation betweenσandÏ€bonds and so on.The study provides not only feasible way but also scientific foundation to adjust and control the elastic modulus of SiC for various requirements in the domains of biomedical materials and cutting tools.（3） The microstructures,nano-indentation and friction/wear properties of C-doped SiC thin films（three kinds of C concentrations） deposited on conventional-Ti substrates through magnetron sputtered techniques were investigated.The results show that the doping of C elements can decrease the hardness,tensile strength and improve the instability-resistance of SiC under compression（nano-indetation and friction/wear） conditions.Besides,doping a proper amount of concentrations of C can reduce the friction coefficient of SiC both at dry and Kokubo simulation body fluid（SBF） conditions.The changes of hardness and friction coefficient are probably because of the changes of the type of chemical bonds in SiC,namly,with the doping of C,the C-C sp³ bonds increase（ID/IG values decrease）,as C-C sp³ bonds induced transformations to Octahedron weak bonds of the surrounding Si-C sp³ bonds more easily, which lead to a lower yield stress of SiC under compressing deformation.In addition,the elastic modulus of 3C-SiC also decreases with the increases of C elements,which has the same reason like mentioned before.The test results are in agreement with the MD results.（4） The microstructures,nano-indentation and friction/wear properties of CNx film in CNx/SiC double layer thin film（SiC films as interlayer） deposited on nano-crystalline Ti substrate using magnetron sputtered techniques were investigated.The results show that there are gradient variations of element dissolutions among SiC thin film,CNx thin film and Ti substrates,which ensure the good elastic modulus matching around the interface areas.This is in accordance with the linear varation,which we found in the study of the effects on the elastic modulus of SiC of doping C concentrations...