Mechanical properties of compositionally modulated Au-Ni thin films: Nanoindentation and microcantilever deflection expe
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The mechanical properties of compositionally modulated Au-Ni films were investigated by submicrometer depth-sensing indentation and by deflection of micrometer-scale cantilever beams. Films prepared by sputter deposition with composition wavelengths between 0.9 and 4.0 nm were investigated. Strength was found to be high and invariant with composition wavelength. Experimental and data analysis methods were developed to provide more accurate and precise measurements of elastic stiffness. Large enhancements in stiffness (the "supermodulus effect") were not observed. Rather, relatively small but significant minima were observed at a composition wavelength of about 1.6 nm by both techniques. These variations were found to be strongly correlated with variations in the average lattice parameter normal to the plane of the film. Both structural and mechanical property variations are consistent with a simple model in which the film consists of bulk-like Au and Ni layers with interfaces of constant thickness.
I. INTRODUCTION
Thin films in which composition is periodic in the direction normal to the film plane, known as compositionally modulated films or CMF's, are expected to exhibit interesting mechanical properties when the composition wavelength, A, becomes small. Dislocation motion may be restricted by the modulated structure1'2 leading to very high strength. Variations in stiffness arising from interface effects in films where the modulation takes the form of a square wave (multilayers) might also be predicted (e.g., references in Ref. 3). The elastic properties of a material are generally considered to be structure-insensitive; thus, any stiffness effects are expected to be small. The strength properties of a few modulated film systems have been examined and the expected behavior has been observed; i.e., rule-of-mixtures (ROM) behavior in the long-wavelength limit with strength increasing as A decreases.4 The situation regarding elastic properties is not so clear. Very large (>100%) increases in the elastic stiffness of certain CMF's as A decreases to near 2 nm have been reported.5"10 These variations are truly anomalous in that they are very difficult to explain by any established model of the behavior of solids. This socalled "supermodulus effect" has thus been controversial
a) Present
address: Max-Planck-Institut fur Metallforschung, Institut fur Werkstoffwissenschaft, SeestraBe 92, 70174 Stuttgart, Germany. J. Mater. Res., Vol. 9, No. 12, Dec 1994 http://journals.cambridge.org
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and has generated much interest in the elastic properties of CMF's. At this writing, several dozen additional experimental studies have been reported (for recent reviews, see Refs. 3 and 11). Although consistent patterns are emerging, taken altogether these reports disagree regarding the existence and magnitude of elastic stiffness variations with A in metal CMF's. To clarify the situation, we divide these reports into two groups based on the fundamental method by which the elastic properties were determined and i
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