Structure and Mechanical Properties of Gold-Cobalt Electrodeposits

Abstract

Gold-cobalt electrodeposits produced from an acid citrate bath have been examined by X-ray diffraction, and transmission and scanning electron microscopy. Measurements have been made of grain size, internal stress and microhardness and a codeposited polymer film has been isolated by leaching the deposits in aqua regia. It has been shown that the extremely fine grain size (~between 100 and 300 Å) varies with current density. However, an attempt to correlate grain diameter with microhardness using a Petch-Hall type relationship indicated that other strengthening mechanisms are responsible for the relatively high hardness of these deposits. There is some evidence to suggest from constitutional data, stress measurements and electron microscopy that these deposits may be strengthened by fine dispersed cobalt precipitates. A technique of using partial dissolution of the gold by aqua regia on the fracture surface revealed that the ‘organic’ polymer forms in a layer structure throughout the deposit. Hence, it is suggested that these deposits consist of sandwiches of fine grained, precipitation hardened gold lying between polymer films which considerably improve the wear properties of these materials.