ENHANCED MECHANICAL HARDNESS IN COMPOSITIONALLY MODULATED FE(001) PT(001) AND FE(001)/CR(001) EPITAXIAL THIN-FILMS/

The hardnesses and elastic moduli of sputter-deposited epitaxial Fe(00 1)/Pt(001) and Fe(001)/Cr(001) multilayers grown on MgO(001) were eval uated as a function of composition wavelength Lambda. X-ray diffractio n was used to characterize the structure of these multilayers, allowin g for the examination of the structural underpinnings of the mechanica l properties in these systems. For both Fe/Pt and Fe/Cr multilayers, n anoindentation results reveal no appreciable enhancement in the elasti c modulus (the so-called supermodulus effect) over a broad range of La mbda. A reduced modulus is observed at small Lambda in the Fe/Pt multi layer films, which can be attributed to interfacial bonds which are we aker than those in the bulk. Nanoindentation data reveal that for Fe/P t multilayers, the hardness is enhanced over that expected from a simp le rule of mixtures by a factor of approximately 2.5. This enhancement in hardness occurs over a considerable range in Lambda (2-10 nm) and is not a function of Lambda in this wavelength regime. Preliminary res ults indicate that the hardness of Fe/Cr multilayers obtained via nano indentation is also enhanced over the rule of mixtures value by a slig htly smaller amount than observed in the Fe/Pt system. The high hardne ss 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 ba rrier to dislocation motion between the two materials, and this contri butes to the hardness of these multilayer films. However, since a larg e enhancement is seen in the Fe/Cr system, where no structure modulate d strengthening occurs, this structural effect may be minor. The domin ant mechanism responsible for the hardness enhancement in Fe(001)/Pt(0 01) and Fe(001)/Cr(001) multilayers has not yet been identified. Exper iments are underway to determine whether the strength enhancement aris es from the number of interfaces, the stress state, the shear modulus discontinuity, or other effects in these multilayer thin films.