Combinatorial Study of Gradient Ag–Al Thin Films: Microstructure, Phase Formation, Mechanical and Electrical Properties

Abstract

A combinatorial approach is applied to rapidly deposit and screen Ag-Al thin films to evaluate the mechanical, tribological, and electrical properties as a function of chemical composition. Ag-Al thin films with large continuous composition gradients (6-60 atom % Al) were deposited by a custom-designed combinatorial magnetron sputtering system. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning and transmission electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS), nanoindentation, and four-point electrical resistance screening were employed to characterize the chemical composition, structure, and physical properties of the films in a time-efficient way. For low Al contents (<13 atom %), a highly (111)-textured fcc phase was formed. At higher Al contents, a (002)-textured hcp solid solution phase was formed followed by a fcc phase in the most Al-rich regions. No indication of a μ phase was observed. The Ag-Al films with fcc-Ag matrix is prone to adhesive material transfer leading to a high friction coefficient (>1) and adhesive wear, similar to the behavior of pure Ag. In contrast, the hexagonal solid solution phase (from ca. 15 atom %Al) exhibited dramatically reduced friction coefficients (about 15% of that of the fcc phase) and dramatically reduced adhesive wear when tested against the pure Ag counter surface. The increase in contact resistance of the Ag-Al films is limited to only 50% higher than a pure Ag reference sample at the low friction and low wear region (19-27 atom %). This suggests that a hcp Ag-Al alloy can have a potential use in sliding electrical contact applications and in the future will replace pure Ag in specific electromechanical applications.

Effects of Bi Addition on the Microstructure and Mechanical Properties of Nanocrystalline Ag Coatings

“…The two images in Figure 6 are quite similar to the TEM images of the mechanically-milled Ag-5.1% Bi alloys as reported by Chithra et al [ 11 ]. The existence of Bi in the Ag matrix cannot be easily distinguished from XRD patterns as compared to other Ag-based alloy systems, i.e., Ag-Al [ 9 ], since no new phase was generated. With EDS, the average at% of Bi across the coating thickness would be ~1%.…”

“…The low electrical resistivity and the high hardness of Ag-Bi coating indicate that the current Ag-Bi nanocomposite is a good candidate for electrical contacting materials. In addition to Ag-Bi alloy, four other Ag alloys were prepared before, and the electrical resistivity of these alloys were measured [ 1 , 9 , 10 , 13 ]. The relationship between normalized electrical resistivity at room temperature (the electrical resistivity of Ag alloys, , over the electrical resistivity of pure Ag, and alloy at% is shown in Figure 10 .…”

“… The relationships of alloy at% in Ag matrix versus normalized (Norm.) electrical resistivity ( ) of Ag-Bi, Ag-Al [ 9 ], Ag-Pd [ 10 ], Ag-Ti [ 12 ], and Ag-Au [ 13 ] alloys. …”

“…However, the wear resistance of Ag is not as desirable as those of nickel (Ni) or copper (Cu) [ 3 , 4 , 5 ] due to the fact that bulk Ag or Ag coatings are soft, with low hardness. Various methods were attempted to improve the mechanical performance of Ag coatings, such as introducing hard nanoparticles to form Ag-based composites [ 6 , 7 , 8 ], creating new Ag-based alloys with different crystal structures [ 1 , 5 , 9 , 10 , 11 , 12 , 13 ], and reducing the grain size to the hundred nanometer scale [ 9 , 14 , 15 , 16 , 17 , 18 ]. In addition, different coating/film deposition techniques, i.e., electrodeposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), and magnetron sputtering, have been employed to prepare Ag coatings with improved properties [ 19 ].…”

Development and characterization of magnetron sputtered self-lubricating Au-Ni/a-C nano-composite coating on CuCrZr alloy substrate

“…8, without C strengthening, the average Vickers hardness of the Au-Ni alloy coating is 200. XRD and TEM analyses confirmed the presence of the Au-Ni hcp phase, which can generate an additional precipitation hardening effect on the coating[43]. However, the magnetron-sputtered Au-Ni coating is still softer than the electroplated Au-Ni (fcc structure and Ni content of 2 wt.%) coating in which the hardness has been reported to be 267.7 HV[8].C atoms play an important role in improving the hardness of the Au-Ni coating because all Au-Ni/a-C coatings with different C contents were observed to be harder than the Au-Ni reference coating.…”