Theoretical Study On Interfacial Properties Of Silicon Carbide Reinforced Aluminum Matrix Composites

The interfacial structures and properties of SiC practical reinforced Al matrix composite have been calculated by first-principles density functional theory（DFT）. The surface structures of interfacial product MgAl₂O₄, SiC（0001）/MgAl₂O₄（111） interfaces and Al（001）/MgAl₂O₄（001） interfaces have been established and studied in detail. The surface relaxation, cleavage energies, surface energies, surface grand potential and surface electronic structures of different terminations of MgAl₂O₄（111） surface and the interface relaxation, ideal work of adhesion and interfacial electronic structure of different SiC（0001）/MgAl₂O₄（111） and Al（001）/MgAl₂O₄（001） interfaces have been calculated.The atomic structures and electronic structures of MgAl₂O₄, MgO and Al2O3 have been studied in the Section Third. MgAl₂O₄ has an indirect band gap of 5.594 eV in the middle of the direct band gaps of Al2O3（6.747eV） and MgO（4.748eV）. The conduct band width of MgAl₂O₄ is narrower than that of MgO and Al2O3 and its peak slightly moves to the left, leading to the worse electron transport capability. The value of the formation energy is-0.20 eV, proving that MgAl₂O₄ can be obtained from MgO and Al2O3. The surface properties of MgAl₂O₄（111）, MgO（111） and Al2O3（001） have been compared in the Section Fourth. The surface relaxation, cleavage energies and surface energies, surface grand potential and surface electronic structure of eight different terminations of MgAl₂O₄（111） surfaces indicate that, for every termination, the layer distances near the surface oscillate in a damping style with a conserved profile. By comparing the surface energies of the eight terminations, Mg（O）- and Al（Mg）-terminations have relatively small surface energies. Taking into account the influence of the chemical environment as a function of the relative richness in O and Al, Mg（Al）-, Al（Mg）-, O（Al）- and O2（Al）-terminations are more stable, in particular, Mg（Al）-termination is the most stable in O- and Al-rich environments, and O2（Al）-termination is the most stable in O- and Al-poor conditions. According to the calculated electronic properties, the four terminations are metallic, and their Fermi levels move up. Due to the special crystal structure of MgAl₂O₄, its（111） surfaces exhibit some properties similar to those of Al2O3（001） and MgO（111） surfaces.The atomic structures, ideal work of adhesion（Wad） and electronic structures of twelve SiC（0001）/MgAl₂O₄（111） interfacial structures have been systematically investigated in Section Five. The results show that not only the interfacial atomic layers but also the sub-interfacial atomic layers affect the values of Wad, and Wad of C-terminated interfacial structures are larger than this of Si-terminated interfacial structures. The C/O（Mg） and Si/O（Mg） interfacial structures for C- and Si-terminated interfacial structures respectively have the biggest Wad. The interfacial atoms have large lateral movements and the sub-interface atoms have slight lateral movements besides the perpendicular movements in SiC side. The C/O（Mg） interface has the largest Wad, which means it is the most stable interfacial structure. The lengths of interfacial C-O bonds of C/O（Mg） interfacial structures are shorter than that of Si-O bonds of Si/O（Mg） interfacial structures. The interfacial electron density, electron density difference and density of states（DOS） have been analyzed. Two kinds of polar covalent bonds are formed between C and O atoms for C/O（Mg） interfacial structure. The relatively weak covalent and metallic bonds are formed between O and Si atoms for Si/O（Mg） interfacial structure.The ideal work of adhesion（Wad） and electronic structures of the Al（001）/MgAl₂O₄（001） interfaces are investigated in Section Sixth. Four different interfacial models are considered in detail. The AlO-Al top site interface exhibits the most serious atom reconstruction at interfacial region and has the strongest Wad, indicating that the AlO-Al top site interface has the strong interfacial stability in comparison with other interfacial models. The ionic and weak covalent bonds between Al and O atoms are formed in the AlO-Al center interface, the stronger polar covalent bonds and the weak ionic bonds are formed in the AlO-Al top interface, and the metallic bonds are formed in the Mg-Al center and Mg-Al top interface.The C/O（Mg） and Si/Mg（Al） interfaces respectively have the higthest value of Wad（13.48J/m2） and the smallest one（2.81J/m2） for SiC（0001）/MgAl₂O₄（111） interfaces, and the AlO/Al interface has the higthest value of Wad（2.54 J/m2） for Al/MgAl₂O₄ interfaces. So, the bonding strength of SiC/MgAl₂O₄/Al interface is mainly ditermined by the Al（001）/MgAl₂O₄（001） interface.