A new technique is described for measuring the adhesion between a sharply curved metal point (platinum, tungsten, titanium, nickel or gold, with a radius of curvature typically 300 nm) and a flat surface (platinum, tungsten, cobalt, silicon or graphite). Adhesion forces as small as 100nN can be measured with a sensitivity (limited by friction and vibration) of 20 nN, and the electrical resistance of the contact region is measured as a function of positive and negative load. Any plastic deformation of the metal point, as well as its cleanliness, is monitored by current and voltage measurements on electrons drawn by field emission from the point before and after contact.For platinum and tungsten surfaces placed in contact with near-zero loads, high values of adhesion (up to 55 pN) were measured provided that both surfaces were sufficiently clean; there is evidence that one area of 'single-asperity' contact is obtained, Ductile extension was occasionally observed, but the experimental curves of resistance against load were otherwise reversible for both positive and negative loads, and unaffected by the magnitude of any previously applied positive load. Measurements of adhesion, resistance and change in resistance as a function of load may be divided into two categories according to whether separation took place at the interface or within the bulk of one of the samples. Generally the results are not consistent with the elastic theory of Johnson er al or with the model of Gane et al. Differences in the behaviour of contacts between the other materials are described. Further analysis may enable the method to be used for measuring values of work of adhesion (change in surface energy) for interfaces between stiff solids.
