Field ion microscopy of metal-metal contacts

Tungsten-platinum and platinum-platinum mechanical contacts were investigated using field ion microscopy. A clean field evaporated field ion microscope tip served as one contact member, and a fiat outgassed platinum foil was used as the other. The experiments were conducted in-situ using an ultra-high-vacuum field ion microscope pumped to 2 × 10 −10 Torr before being backfilled to 5 × 10 −4 Torr of helium or neon for imaging. A refined contact device was employed having an ultimate applied load resolution of 5 micromg. In contrast to previous results, gross tip deformation could be avoided, while the contact area could clearly be seen against the untouched parts of the tip. It was found in most cases that the damage to the tip at the contact interface extended only 5–15 Å deep, and that this damage depth was independent of both the magnitude of the applied load and the size of the contact area. The functional variation of the contact area with the applied load was found to be in basic agreement with the Hertz theory of elastic deformation. Fundamental limitations on the minimum observable size of the contact area due to impact are discussed.