Experimental investigation of the effect of temperature on friction in gabbro and granite

High speed (~1 m/s) rotary shear experiments on gabbro samples initially at room temperature have exhibited a complex evolution of the apparent coefficient of friction (Hirose and Shimamoto, 2003, 2005). Friction initially drops with increasing displacement, then rises to a peak value and, finally drops to a residual level as the sample develops a continuous molten layer, and viscosity dominates the frictional behavior. Because friction is a result of the interaction of asperities in contact between the two surfaces, such that most of the normal load is borne by these contacts, the latter experience high local normal and shear stresses. Sliding of the interface leads to momentary contacts between pairs of asperities, which may under certain conditions result in a momentary softening or melting of a thin layer of material at the asperity tips (Rice, 1999). The result would be a weakening of the asperity tips, but no change in contact area. Further sliding ends individual contacts, allowing the layer to again solidify or harden. At high slip velocities typical of earthquakes, this mechanism could act to lower friction initially. However, as sliding continues, heat produced by the high stresses at the asperity contacts will diffuse further into the asperities and to the adjacent area to create a more uniform heating that affects the rock surface along the entire interface. Thermally-activated plasticity or fracture may flatten and broaden the asperities, increasing the total area of contact, thus also increasing the force required to maintain sliding. In an attempt to examine the role of temperature on friction, we conducted a series of direct shear experiments on prepared samples of gabbro, granite and quartzite of uniform roughness under normal stresses ranging from 2-23 MPa and at imposed temperatures ranging from room temperature to 400C. The experiments were conducted at velocities between 10-4 to 10-1 mm/sec in steps of 1-2 orders of magnitude. Temperature was controlled to within .5C, and normal stress to within .1 MPa. The geometry of the samples allowed 40 mm of sliding contact over a constant area of 32cm2. This experimental setup is unique in that it permits direct shear tests both over longer distances and at higher temperatures than previously reported. Results suggest a temperature hardening in both granitic and gabbroic samples, although the data reveal a considerable scatter. We interpret these data as indicating that the temperature-induced changes in the contact topology that lead to increases in the effective contact area offset the effects of thermal weakening of the asperities.