Structure transition of BN layers and its influences on the mechanical properties of AlN/BN nanomultilayers

AlN/BN nanomultilayers and monolithic films were deposited by a radio frequency magnetron sputtering system. The films were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nanoindentation. XRD studies revealed w-AlN phase with texture (002) in AlN films and amorphous BN films. The HRTEM structural investigation exhibited that w-BN layer was crystallized for layer thickness less than 0.55 nm, and amorphous for layer thickness larger than 0.74 nm in AlN/BN nanomultilayers. The thickness-dependent structural transition can be understood through thermodynamic and elastic considerations. Theoretical calculation and experimental results confirmed the assumption that the equilibrium between strain energies produced in w-BN and coherent interfacial energies between w-AlN/w-BN leads to the crystalline-to-amorphous transition in the BN phase. The hardness of AlN/BN multilayers depends on the thickness of BN layers, as measured with nanoindentation. The hardness stays around 38.0 GPa when BN layer thickness is 1 to 2 molecular layers, however, decreases to 27.0 GPa, close to the hardness of monolithic AlN films, as BN thickness increases to 0.74 nm. The hardness enhancement mechanism of AlN/BN multilayers is mainly attributed to coherent interface stresses and modulus difference between crystallized BN layer and AlN layer.