Quantum molecular dynamics simulations of α-Al2O3(0001)/3C-SiC(111) interfaces reveal profound effects of thermal annealing for producing strong interfaces consisting solely of cation-anion bonds and their consequence on interfacial structures. A Si-terminated SiC surface and Al2O3 form a stronger interface (Si-interface) with a Si-O bond density of 12.2 nm(-2), whereas the C interface has an Al-C bond density of 9.46 nm(-2). The interfacial bond strengthening is accompanied by the formation of an Al2O3 interphase with a thickness of 2-8 Å. Such atomistic understanding may help rational interfacial design of high-temperature ceramic composites for broad applications such as power generation systems.