PRESSURE EFFECTS ON THE SHEARING OF A COLLOIDAL THIN-FILM

The pressure effect in the frictional behaviour of a colloidal suspension of over-based calcium carbonate stabilized in pure dodecane has been investigated. With a surface force apparatus, we have observed the squeeze effect and the frictional resistance of the suspension using sphere- and plane-geometry on a nanometric scale and for low contact pressure (10(4) to 10(6) Pa). With tribometers, which can support heavy loading, we have analysed the high pressure domain (10(4) to 10(9) Pa). After a critical pressure, evaluated at 10(6) Pa, the colloidal film does not flow in the contact of the solids, but forms a compacted 'mattress' sliding on the plane surface, and squeezing a hydrocarbon layer. The mean shear strength of the interface, tau, can then be expressed by a relation of the form tau = tau(0) + alpha-p, where p is the mean contact pressure. The meaning of tau(0) and alpha is discussed.