Contact of rough surfaces: A review of experimental work

In recent years engineers have come to realize that the determination of the contact geometry of rough surfaces is of critical importance in the solution of many tribological and conduction problems. The theoretical study of the contact of rough surfaces is well advanced whereas the experimental work on this topic has not received the attention that was due. This paper presents a review of the experimental investigations that have been carried out so far. Their findings and experimental methods are discussed. The main topics of experimental study are the separation of the surfaces, the real area of contact, the number of contact spots, the spatial distribution of the contact spots and the distribution of their sizes, and the relation of all these to roughness and to the normal load. Experimental techniques have included total internal reflection, Nomarski interferometry, neutrography, thermal and electrical resistance and computer simulation. The consensus of experimental results is as follows: the real area of contact increases with the load and this increase is due mainly to an increase in the number of micro-contacts, their mean size remaining approximately constant; the separation of the contacting surfaces is approximately inversely proportional to the logarithm of the load; the distribution of contact spot sizes is approximately log normal. However, it appears that the density and average size of micro-contacts can vary over several orders of magnitude for different surfaces at the same load. It also appears that, contrary to established belief, the real area of contact does not vary as the load but increases as its 0.8 power.