Growth Behaviour Of Transition Metal Carbides And Dibordes During Combustion Synthesis

Due to many excellent physical and chemical properties, such as high melting point,high hardness, high modulus, high thermal conductivity and high chemical stability,transition metal carbides and borides are widely used in mechanical, chemical andmicroelectronics applications. They are generally used as high–temperature ceramics,high–performance cutting and wear–resistant workpiece, especially reinforcing particles inthe cemented carbide or composite materials. As known, the morphology of the reinforcingceramic particles in the ceramic reinforced metal matrix composites has a critical impact ontheir properties. The matrix composites with round reinforcing particles have betterproperties than those with irregular shaped reinforcing particles. Therefore, it is of greatpractical significance to study the growth morphology, growth mechanism and thecontrolling factors for the ceramic particles. Moreover, the non–equilibrium growth kinetics(and mechanism) of the ceramics is also an important exploration and research direction inthe theoretical field of crystal growth. In fact, the combustion synthesis of ceramics is a hightemperature, fast, non–linear, non–steady–state transfer process, accompanied by numerouschemical and physical phenomena. Therefore, the factors influencing the growth behavior ofthe ceramics are very complicated. So far, there have been some work on the growthmorphology of the ceramic particles formed during combustion synthesis. However, thesework are mainly focused on the influencing regularity of the combustion kinetics on thegrowth morphology of the ceramic particles. The study on the underlying key theoreticalquestions during the fabrication of the ceramic–metal composites, especially the growthbehavior and mechanism of the ceramic particles, is rather limited.Therefore, in this thesis, the growth behaviors of the ceramic particles (TiC_x, ZrC_x,NbC_x, TaC_x, TiB2, ZrB2, NbB2xand TaB2x) formed in the combustion systems ofAl–Ti/Zr/Nb/Ta–C/B were studied; the influencing regularity of the ceramic structures andthe kinetics conditions on the growth behavior of the ceramic particles were revealed; thegrowth mechanisms of different ceramic particles and their commonness and individuality were discussed.The main results are as follows:1) It was found that the Al content, carbon source and reactant C/Ti molar ratio haveimportant effects on the growth morphology of the TiC_xparticles formed in the Al–Ti–Csystem; The morphology evolution regularity of the TiC_xparticles with the increasingstoichiometry during the SHS was suggested as from octahedron→truncated–octahedron→sphere–like→sphere; The relationship between the growth morphology of the TiC_xand its stoichiometry was established, that is, in the case of quite low stoichiometry, theTiC_xparticles grow to octahedron, while in the case of relatively high stoichiometry, theTiC_xparticles grow to truncated–octahedron, and when the stoichiometry furtherincreases, the TiC_xparticles grow to sphere–like and sphere.2) The growth mode of the TiC_xparticles during the SHS was suggested as thetwo–dimensional layer growth on the {111} surfaces. The main controlling factor of thegrowth morphology of the TiC_xparticles was suggested as the stoichiometry: When thestoichiometry of the formed TiC_xis very low, the {111} surfaces are quite stable, and theTiC_xparticles nucleate and grow as octahedron. With the increase in the stoichiometry,the {111} surfaces become less–stable, and the {100} surfaces begain to be exposed.Then, the shapes of the TiC_xparticles change from octahedron to truncated–octahedron.If the combustion temperature is higher than TR≈1800°C, the {100} surfaces undergoa roughening transition and become round, which makes the TiC_xbecome thespherical–like particles. With the further increase in the stoichiometry or combustiontemperature, the {100} surface grow up gradually, and the TiC_xparticles become moreand more round.3) It was found firstly that necessary condition for the existence of the morphologyevolution and spherical growth shape for the transition metal diborides formed duringthe SHS in Al–Ti/Zr/Nb–B systems is there must exist a wide stoichiometric range. Forthe borides (TiB₂and ZrB₂) with no wide stoichiometric ranges, even the combustiontemperature becomes quite high, their shapes don't change considerably and keep beingthe hexagonal–prisms, while for NbB_2xwith a wide stoichiometric range, itsmorphology changes significantly with the Al content and the reactant B/Nb molar ratio;The morphology evolution regularity of the NbB_2xparticles during the SHS with theincreasing stoichiometry was suggested as from hexagonal–prisms→polyhedron→sphere–like→sphere; The relationship between the growth morphology of the NbB_2xand its stoichiometry was established, that is, in the case of low stoichiometry, the NbB_2xparticles grow to hexagonal–prisms, while in the case of high stoichiometry, the NbB_2xparticles grow to sphere–like and sphere. The growth mode of the TiB₂, ZrB₂and NbB_2xparticles during the SHS was suggested as the two–dimensional layer growth on the {0001}and {1010}surfaces.4) The main controlling factor of the growth morphology of the NbB_2xparticles wassuggested as the stoichiometry, and the spherical growth mechanism of the NbB_2xparticles was firstly proposed: When the stoichiometry is low, the {0001}and {1010}surfaces are very stable, and the NbB_2xparticles grow to the hexagonal–prisms. With theincrease in the stoichiometry, and stability of the {0001}and {1010}surfaces isgradually decreased; while the {1101}surfaces gradually emerge. Then, the NbB_2xparticles change their shapes from hexagonal–prism to polyhedron. When thecombustion temperature is high and exceeds a certain value, the {1101}surfacescomposed with the side–facets of the growth steps are rourhed and become round,inducing the formation of the spherical–like NbB_2xparticles. With the further increase inthe stoichiometry, the NbB_2xparticles become more and more round.5) The commonness and individuality of the morphology evolution of the transition metalcarbides (TiC_x, ZrC_x, NbC_xand TaC_x), nitrides (TiNx) and borides (TiB₂, ZrB₂, NbB_2xand TaC2x) during the SHS were summarized.â…°) Commonness:The necessary condition for the morphology evolution of the TMCs, TMNs andTMDs particles during the SHS was suggested as the wide stoichiometric ranges; Therelationship between the growth morphology and their stoichiometry was establishedand the effect of the combustion temperature on the spherical kinetics of the growthmorphology was revealed, that is, with the increase in the combustion temperature andstoichiometry, the morphology of the TMCs and TMNs evolves from octahedron→truncated–octahedron→sphere–like→sphere, and that of the TMDs evolves fromhexagonal–prisms→polyhedron→sphere–like→sphere.â…±) Individuality:For the borides (TiB₂and ZrB₂) with no wide stoichiometric ranges, there is nomorphology evolution for them during the SHS.6) It was disclosed that the reactive activity of the Me (Cu/Al/Fe)–Ti–CNTs systems can begreatly increased with using CNTs as carbon source, and the higheat metal contents forthe complete raction during the SHS were increased from67.12wt.%Cu,46.65wt.%Aland77.4wt.%Fe to80wt.%Cu,70wt.%Al and80wt.%Fe, respectively; With usingthe CNTs as carbon source, the nano TiC_xparticles with different shapes can besynthesized in the Me (Cu/Al/Fe)–Ti–CNTs systems with high metal contents.7) The influence regularity and affect mechanism of the metal component on the size andmorphology of the TiC_xparticles formed in the Me (Cu/Al/Fe)–Ti–CNTs systems weredisclosed: In the Fe–Ti–CNTs system, because of the fast dissolution rate but the low diffusion rate of carbon in Fe, the TiC_xparticles form in the [C]–rich regions and growrapidly to relatively larger sizes. Moreover, the stoichiometry of these formed TiC_xparticles in the Fe melt is relatively high and thus their growth shapes are cube andsphere–like. In the Cu–and Al–Ti–CNTs systems, the CNTs dissolve more slowlybecause of the poor chemical reactivity. In this case, the TiC_xforms and grows under acondition of [C] scarcity. Hence, the TiC_xparticles grown in these two melts are withrelatively small sizes, and the TiC_xstoichiometry formed at the combustion stage is low,and accordingly, the TiC_xgrowth shape is octahedron.In summary, in this thesis, the growth behaviors of the ceramic particles (TiC_x, ZrC_x,NbC_x, TaC_x, TiB₂, ZrB₂, NbB_2xand TaB_2x) formed in the combustion systems ofAl–Ti/Zr/Nb/Ta–C/B were studied; the influencing regularity of the ceramic structures andthe kinetics conditions on the growth behavior of the ceramic particles was revealed; thegrowth mechanisms of different ceramic particles and their commonness and individualitywere suggested, which lays the necessary theoretical basis for the active control of theceramic particle morphology and the development of advanced ceramic particles reinforcedmetal matrix composites.