Mechanical Properties of Ag/Cr Multilayered Epitaxial thin Films

Abstract

AbstractMetallic multilayer thin film structures exhibit very high hardnesses. The epitaxial Ag/Cr system was chosen to investigate the mechanisms responsible for these high hardnesses. The interfaces of this multilayer structure are expected to be chemically abrupt since silver and chromium are nearly immiscible and do not form intermetallic compounds. The crystallographic structure formed by epitaxial growth isolates the slip systems of each layer.The samples were deposited on (001) MgO substrates by sputter deposition at room temperature. The bilayer spacing of the samples ranges from 1 nm to 20 nm. X-ray diffraction results indicate that the multilayer films exhibit the rotated (001) crystal structure. Hardness and indentation modulus were measured by depth-sensing indentation experiments using a Nanoindenter. The maximum hardness is roughly four times the weighted average of the single component films. The indentation modulus at small bilayer periods varies by ±15% from the value at large bilayer periods, but all of the values fall between the moduli measured for the single component films.Decreasing the bilayer period increases the hardness. At bilayer spacings smaller than 4 nm, the hardness remains constant. Decreasing the thickness of the Cr layer while keeping the thickness of the Ag layer at 5 nm, increases the strength of the multilayered film. At Cr thicknesses smaller than 5 nm, the hardness decreases as the Cr thickness decreases. The effect of the volume fraction of Cr on film strength was also studied.The results are discussed in the context of a deformation model in which the dislocations responsible for deformation are constrained to move in the individual layers. From this model, a lower bound on the strength of the multilayered film can be calculated if plastic deformation occurs in only one layer. An upper bound can be calculated if plastic deformation occurs in both layers. The data fall between the upper and lower bounds and lie closer to the lower bound.