Field-Ion Microscopic Study of Surface Damage of Contacts Caused by Closing Voltages

The minimum closing voltage to cause damage on the metal surfaces of an electric contact was studied using the atomic resolution of the field-ion microscope. A tungsten field-ion emitter tip served as a single asperity of one contact member and was contacted with either a flat platinum plate or liquid gallium or tin which served as the other member in high vacuum. The results indicate the existence of a threshold closing voltage to cause damage which increases approximately by 1 V for every 100 A of tip radius. The observed minimum voltage, 1 to 8 V for tip radii of 100 to 800 A, is significantly lower than the known minimum voltage required to make a short arc which is known to depend on the work function and the ionization potential of the contacting materials. Computer calculations support the explanation that the cause of tip damage is the result of electron bombardment heating from the field-emission discharge of the capacitance around the tip assembly (~10 pF) for the case of a positive tip, and heating by bombardment of field ionized metal atoms from the other electrode for the case of a negative tip. The calculations also suggest that covering the electrodes with a low work function metal may reduce the thermally induced lattice disorders caused by this precontact field emission discharge.