Enhanced mechanical hardness in compositionally modulated and epitaxial thin films

Abstract

The hardnesses and elastic moduli of sputter-deposited epitaxial Fe(001)Pt(001) and Fe(001)Cr(001) multilayers grown on MgO(001) were evaluated as a function of composition wavelength Λ. X-ray diffraction was used to characterize the structure of these multilayers, allowing for the examination of the structural underpinnings of the mechanical properties in these systems. For both Fe/Pt and Fe/Cr multilayers, nanoindentation results reveal no appreciable enhancement in the elastic modulus (the so-called supermodulus effect) over a broad range of Λ. A reduced modulus is observed at small Λ in the Fe/Pt multilayer films, which can be attributed to interfacial bonds which are weaker than those in the bulk. Nanoindentation data reveal that for Fe/Pt multilayers, the hardness is enhanced over that expected from a simple rule of mixtures by a factor of approximately 2.5. This enhancement in hardness occurs over a considerable range in Λ (2–10 nm) and is not a function of Λ in this wavelength regime. Preliminary results indicate that the hardness of Fe/Cr multilayers obtained via nanoindentation is also enhanced over the rule of mixtures value by a slightly smaller amount than observed in the Fe/Pt system. The high hardness may arise from structure modulated strengthening (f.c.c./b.c.c.) in the Fe/Pt system. The structural difference between Fe and Pt is a barrier to dislocation motion between the two materials, and this contributes to the hardness of these multilayer films. However, since a large enhancement is seen in the Fe/Cr system, where no structure modulated strengthening occurs, this structural effect may be minor. The dominant mechanism responsible for the hardness enhancement in Fe(001)Pt(001) and Fe(001)Cr(001) multilayers has not yet been identified. Experiments are underway to determine whether the strength enhancement arises from the number of interfaces, the stress state, the shear modulus discontinuity, or other effects in these multilayer thin films.