Field Ion Microscopy of Contacts

A procedure for making contacts between a field ion microscope emitter tip and a fiat surface is described. Iron, tungsten, platinum, and iridium tips were contacted with glass, graphite, nickel, tungsten, gold, or platinum plates. The maximum permissible values for the contact pressure and the current density of the four metals were estimated from the contact load and the current through the contact point, respectively. A contact pressure or a current density above these critical values caused severe plastic deformation of a tip. The measured elastic limits of these metals are quite high and close to the theoretically expected values of the maximum shear stress. The current density through a Contact point is also higher than the permissible current density for the bulk materials. Contaminating surface layers on the nickel plates prevent damage to the tips due to cold welding and yet allow a very high contact current density. The transfer of materials by cold welding or by an arcing between the two metals is described.