Microstructural development in the interface region of (alpha -Al2O3 bilayer composites has been systematically investigated in terms of the sintering additive CaO-SiO2 residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO-SiO2 doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO-SiO2Al2O3 liquid at the interface region, thereby strongly influencing alpha -Al2O3 grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO-SiO2 dopant in the alpha -Al2O3 bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.