Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.