The micro-hardness of metals at very low loads

Abstract

The micro-hardness of gold has been investigated over a range of indentation sizes from 0.5 mm to 0.2 μm using loads down to 1.0 mg. The indentations were made using a series of spherical indenters of varying radius. In this way it was possible to keep the shape of indentations geometrically similar over a wide range of indentation sizes and so obtain comparable results. The smallest indenters (0.5 μm radius) were prepared using techniques similar to those used for making field-ion microscope tips and the experiments at the small sizes were carried out inside a scanning electron microscope. The measurements were made on specimens in the annealed condition and in the heavily cold-worked condition; considerable care was taken to exclude any possible effect arising from the preparative treatment of the surface. The results show that the hardness is increased by a factor of between two and three at sizes down to 0.2 μm in both the annealed and cold-worked conditions. Possible reasons for this increase are discussed and it is suggested that it is connected with an increase in the stress necessary to operate dislocation sources. Considerable significance is attached to the observation that the increase in hardness due to the size effect and the increase due to cold work appear to be additive, suggesting that the mechanism responsible for the increase at small size is different to that responsible for work hardening. The use of spherical indenters also makes it possible to obtain information on the work-hardening characteristics at widely differing sizes. In common with other observations of strength properties on a small scale, it was found that the rate of work hardening decreased to zero at small sizes. It is suggested that this is due to the inability of defects to remain in the stressed volume when it is very small.