The hardness and modulus of molybdenum/aluminum oxide microlaminates as determined by nanoindentation

Abstract

The elastic modulus and hardness of 1.2 $\mu$m thick films made of alternating and equal layers of sputter deposited Mo and Al$\sb2$O$\sb3$ were measured by a depth-sensing indentation apparatus which can test the properties of submicron volumes of material. Microlaminates with bilayer thicknesses ($\lambda$) of 5, 20, 30, and 100 nm were prepared and tested. The moduli of the microlaminates were found to follow a rule-of-mixtures. No "supermodulus effect" was observed. The hardness of the 100 nm microlaminate was observed to be almost twice as high as the hardness of either as-deposited Mo or Al$\sb2$O$\sb3$. This may be evidence of the "superstrength effect." A new method for determining the area of contact between an indenting diamond and the tested sample as a function of the plastic depth of indentation was established. An accurate area function is necessary for deriving mechanical properties from indentation data.