Amorphous and alloy film formation in sliding of silver on copper

• Published in: Wear Vol. 181; no. II; pp. 922 - 937
• Main Authors: Bednar, M.S., Kuhlmann-Wilsdorf, D.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1995
• Subjects: Alloy formation; Copper; Film formation; Silver; Sliding wear; Film formation; Silver; Alloy formation; Sliding wear; Copper
• ISSN: 0043-1648; 1873-2577
• DOI: 10.1016/0043-1648(95)90216-3

The plastic behavior at contact spots and the formation of tribosurface films in sliding wear was simulated through severe shearing, under pressure, of flat disks made up of stacks of thin metal foils, alternating between substrate and slider material. The samples were investigated after various degrees of shearing under pressures which simulate conditions at contact spots, i.e. approximately the indentation hardness of the softer of the two sliding materials. Interest has focused on anomalous alloy formation of the two materials sliding against each other and, especially, on the formation of an amorphous phase. The practical impetus for this study is the hypothesis that glassy metal surface films may form through sliding and, if so, confer reduced friction as well as wear rates. The model system used in the present study is the copper-silver combination. It exhibits very limited equilibrium solubility at either end of the constitution diagram, and one single eutectic. Results obtained by means of X-ray diffraction reveal that deformation above about one hundred shear strain causes formation of an anomalous alloy, with always more or less in accordance with the averaged composition, at least between 19 and 65 at.% Ag. The alloy is normally crystalline and deforms in the usual manner, but it forms via a presumably very hard and brittle amorphous phase. The X-ray evidence strongly suggests that the alloy formation is not due to anomalous diffusion, e.g. mediated through the large excess vacancy concentration during shearing, but takes place rather abruptly through nucleation from the amorphous alloy that forms when locally the phase boundary spacing decreases below some critical limit, estimated at about 3.5 nm. The Cu-Ag alloys are visually indistinguishable from gold. Indeed, such gold-colored alloys were shown to also form routinely in the rapid wear regime during simple dry sliding of silver against copper and were observed at very low wear rates in a protective atmosphere after sliding silver-plated copper fibers on a copper substrate. The results have evident great importance, also, for mechanical alloying.