Practical and Fundamental Studies of Nanocrystalline Composite Thin Films
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The University of Wisconsin, Madison, Wisconsin 53706 Packard Electric Division, General Motors, Warren, Ohio 44486
ABSTRACT Nanocrytslline composite films of Ag-Mo and Ag-Ni have been made by a co-deposition technique in UHV. The structure and composition have been studied by x-ray diffraction (XRD), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA). For practical applications, the friction coefficient and wear rate were measured using a. pin-on-plate machine for Ag-Mo composites deposited on steel. For fundamental studies, the hardness of the Ag-Ni composites deposited on oxidized Si wafers was measured using a nanoindenter. Experiments show that (1) reduction of friction and wear rate can be achieved using these nanocomposite coatings and (2) the hardness of the nanocomposites depends on the grain size. As the grain size of the Ag decreases from 100 to 10 nm, the hardness increases about 4 times.
INTRODUCTION Although nanocrystalline materials have been studied for many years, most of these materials were produced by inert gas condensation methods.1 From a practical point of view, these nanocrystalline powders are difficult to use in coating applications. From a fundamental research point of view, their mechanical properties can be difficult to measure because of porosity, which exists even after compaction. Recently, we have made several nanocrystalline and microcrystalline composite thin films by co-deposition in UHV of two elements with a positive heat of mixing.2',' 4 The positive heat of mixing is the thermodynamic driving force for phase separation after the two elements are simultaneously deposited onto substrates. The degree of phase separation, such as the domain and grain size of the phase-separated regions, can be controlled by varying the substrate temperature. The Ag-Mo and Ag-Ni systems were selected because their positive heats of mixing, 5 differences in mechanical properties (i.e., soft-hard materials),6 and potential tribological applications. This paper summarizes their preparation, characterization, tribological, and mechanical properties. Detailed reports may be found in Refs. 3 and 4. EXPERIMENTAL Thin films were prepared by electron beam evaporation in ultrahigh vacuum. The two elements with a positive heat of mixing were simultaneously evaporated from two electron beam evaporation sources onto substrates. Two quartz crystal thickness/rate monitors, one for each source, were used to control the evaporation rates to achieve the desired overall composition. The substrate temperature was controlled to adjust the grain size. The base pressure was in the 10-9 torr range and the pressure during deposition was in the 10-8 torr range. The low pressure during deposition ensures the high purity of the films. The substrates, either steel or oxidized Si wafers, were cleaned ultrasonically in acetone, then 875 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
in methanol for more than 10 min each before being loaded into the deposition system. Immedi
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