Mechanical properties and microstructures of metal/ceramic microlaminates: Part II. A Mo/Al₂O₃ system

Synthetic multilayers consisting of periodic layers of the refractory metal Mo and the oxide ceramic Al₂O₃ have been produced by alternating d.c. and r.f. reactive sputter deposition. Microlaminates with four different modulation wavelength—5, 20, 30, and 100 nm—were investigated in this study. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy, low-angle x-ray diffraction, and transmission electron microscopy, including high resolution lattice imaging and microdiffraction. Transmission electron microscopy from the microlaminates indicated that the as-deposited Mo layers are polycrystalline, while the as-deposited Al₂O₃ layers are primarily amorphous. The Mo and Al₂O₃ layers are thermally compatible at 800 °C for 6 h, showing no evidence of atomic interdiffusion between the layers. The mechanical properties of the microlaminates, as well as those of monolithic films of Mo and Al₂O₃ (i.e., the baseline materials), were investigated using nanoindentation methods. A higher than expected modulus and hardness were observed for the microlaminate with the longest wavelength (100 nm); otherwise the mechanical properties are explainable by a rule-of-mixtures. The enhanced mechanical properties of the 100 nm microlaminate may be attributed to crystallization of the amorphous Al₂O₃ layers and the evolution of a structural texture within this phase.