| Titanium matrix composite reinforced with continuous SiC fiber (SiCf/Ti) hasexcellent mechanical properties and thermal stability. The interface between SiC fiberand titanium matrix has significant influence on the whole performance of thecomposite. Because of the large reactivity between Ti metal and C, Si non-metalelements, the interfacial reaction layer tends to form on the interface between SiCfiber and titanium matrix during the composite’s preparation and service under hightemperature. Nowadays, the experimental studies of this interface mainly focus on theinterfacial reaction mechanism, the reaction layer’s phase composition andmicrostructrue, as well as the experimental and theoretical investigations of theinterfacial mechanics. However, these studies mainly focused on the macroscopic andmesoscopic scale, a further microscopic insight should be carried out to make acomprehensive understanding for this interface. Moreover, for the composites, themathematic or numerical models were anticipated to predict the composites’macroscopic effective properties by considering their composition and micro-structure.A practical approach is the multi-scale modeling, which combines the micro-andmacro-information together in some way. The research contents in this dissertation areexactly the parts on the atomic (or electronic) scale for the multi-scal modeling ofSiCf/Ti-based composites’ interface.Although the interfacial reaction products between SiC and Ti are rathercomplicated, the most basic composition is titanium carbide (TiC), and the carboncoating is usually deposited on the surface of SiC fiber, thus, the fiber-matrix interfacecan be perceived as SiC/C/TiC/Ti structure. Accordingly, by using first-principlecalculations of plane wave and ultrasoft pseudopotential which base on densityfunctional theory, the SiC/Ti, Ti/TiC and SiC/TiC interfaces, as well as the DLC/SiC,DLC/TiC interfaces by modeling the carbon coating as diamond-like carbon (DLC),are investigated at atomic and even electronic scale. The work of adhesion, interfaceenergy, interfacial electronic structures are calculated, the equilibrium (or the moststable) atomic configuration and interfacial bonding nature are clarified, someinterfaces’ fracture toughness are theoretically estimated, and the carbon atomdeposition on SiC(111) is simulated also. The main content and the results of thisthesis are summarized as below: (1) For the β-SiC(111)/α-Ti(0001) interface formed with C-terminated SiC(111),three different interfacial stacking sites (center-, hollow-,and top-sites) and two kindstilt directions of Ti atoms’ stacking are considered. In total, six differentβ-SiC(111)/α-Ti(0001) models are investigated. The hollow-site-stacked interfaces(i.e. Ti atom locates on the center of C atoms’ hollow site) have larger adhesion work,smaller interface energy, larger interfacial fracture toughness, and larger stability inthermodynamics. The bonding between interfacial atoms of C, Si and Ti areconfirmed on the hollow-site-stacked interface, and the contributions are mainly fromC-Ti covalent bonding. The Ti stacking tilt direction, namely the stacking styles of Tiatoms, has subtle effects on the interface’s stability and interfacial bonding strength.(2) For the α-Ti(0001)/TiC(111) interface, two kinds TiC(111) surfaces and threedifferent interfacial stacking sites are considered, and totally six different models areinvestigated. After fully-relaxation, both the C-terminated hollow-site stacked andTi-terminated center-site stacked models exhibit identical epitaxial stacking style, andtheir interfacial atom configurations are basically same, thus, the both models can beregarded as the two side beside a same interface. This interface is the mostthermodynamic stable one with the largest work of adhesion and smallest interfaceenergy. Its negative interface energy suggests the atom diffusion is likely happenacross this interface, even forming new interfacial phase. The largest interfacialtoughness of this interface is estimated as4.8MPa m1/2. According to the analysis ofelectron density distribution and partial density of states (PDOS), the interfacialbonding is mainly contributed from the covalent C-Ti and metallic Ti-Ti interactions(3) For the β-SiC/TiC(111) interface, two terminations of TiC(111) surface, twoterminations of SiC(111), two different carbon sublattices, and three differentinterfacial stacking sites are considered, and twenty-four different β-SiC/TiC(111)interface models are investigated. C/C-terminated and top-site-stacked interface hasthe largest work of adhesion and smallest interface energy, and it is the most stableinterfacial configuration. The interface’s fracture toughness is estimated as3.6~4.3MPa m1/2. The interfacial bonding is mainly from C-C covalent interaction caused bythe hybridization of C-2p orbitals. Comparing with the interior bonds, the interfacialbonds of Si-C, Ti-C have less covalent features; the interfacial bonds are weaker thanthe interior ones. So, the interfacial bonds have larger probability to decompose. Thedecomposed carbon atoms will segregate at the interface and form a carbon layer.This prediction is consistent with other researcher’s experimental results.(4) The deposition process of carbon atoms adsorbed on C-terminated and Si-terminated SiC(111) surfaces is simulated. The preferential deposition site (or themost stable atomic configuration) is investigated by calculating the binding energy.On the both surfaces with different terminations, the first-layer carbon atoms tend todeposit on the top-site. The carbon atoms adjacent to the SiC(111) surface willarrange as the stacking sequence of SiC (also same as diamond). Consequently, an“epitaxial” interface forms between the deposited diamond-like carbon layer andSiC(111). However, the "priority" of the best deposition configuration is very tiny.With the increasing of atom layers, the carbon atoms are likely to deposit as thesecond-best stacking sequence (or even the non-stable stacking sequence), especiallyunder the perturbation of the circumstance. Consequently, the amorphous carbonlayer will form.(5) Based on the simulation results of carbon atoms’ deposition, the C-terminatedand Si-terminated DLC/SiC(111) interfaces are modelled and calculated. Theequilibrium interfacial separation are1.542and1.875for C-and Si-terminatedinterfaces, and these values are very close to the C-C and C-Si bond lengths ofdiamond and SiC, respectively. The density of DLC layer increases to about2.5g/cm3after fully relaxation. The work of adhesion are8.86J/m2and8.64J/m2for C-andSi-terminated models. The both values are less (or larger) than the correspondingvalue of diamond (or β-SiC). For the C-terminated interface, the interfacial C-C bondis similar to σ+π covalent bonds, and is predominated by the σ bond (which mainlycomes from s-p electronic interactions). Furthermore, the interfacial C-Si bond inSi-terminated interface is rather weaker than the C-C bond in C-terminated one.(6) Three different stacking sites and two kinds carbon sublattice are considered,and six models of C-terminated DLC/TiC(111) interface are investigated. Thestacking site has a significant influence on the interfaces’ stability. Top-site stackedDLC/TiC(111) models have larger work of adhesion (8.76~8.99J/m2), they are morestable. The influence of carbon atom sublattice is rath insignificant: for the modelswith same stacking site, the work of adhesion and interface stability is slightly largerfor the model with twinned carbon sublattice. For the model with top-site-stackingand twinned-carbon-sublattice, its interfacial bonding mainly comes from the s-pcovalent interaction of C-C atom pairs, and the p-d, s-d covalent interactions of C-Tiatom pairs. |
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