Understanding of both low load Nanoindentation and Atomic Force Microscopy relies on a knowledge of the mechanical response of contacts at the few nanometre scale. To investigate this area we have developed a new type of AFM/Indenter which applies forces (<1nN up to 1mN) to a tip mounted on a cantilever. The displacement of the cantilever is detected using laser heterodyne interferometry. The combination gives a direct measurement of contact compliance at very low loads. Contacts of diameter below 100Å can be repeatably made.
We show that a water film is generally present in the contact during operation in air, and show how its size can be estimated from our data. One implication is that ambient operation of AFM in both repulsive and attractive modes may necessitate the presence of a liquid lubricating film, explaining the relative lack of damage in many AFM measurements. The estimation of contact area from stiffness is complicated by this layer. We show that the contact stresses are likely to be very high.