Experimental measurements of surface structure evolution

To evaluate an evolving asperity structure, discs of 25 mm diameter and 3 mm thickness were prepared from aluminium Al-2011 and homogenous isotropic surface roughness was imparted through the high-velocity spraying of 250–300 μm size glass beads. Asperity deformation was achieved by compressing pairs of roughened surfaces against each other using a hydraulic uniaxial press at constant loads for a duration of 10 min with applied stresses of 20·37, 40·74, 61·11, 101·86, 152·78 and 203·72 MPa. Although a nano-scale brittle oxide layer is likely to be present on the material surface and exhibit breakage, asperity deformation is assumed to be predominantly plastic. Applied stresses were found to be sufficient to impart surface-structure flattening without the onset of bulk deformation. In a separate series of samples, compression at 152·78 MPa was applied in multiple events, with samples rotated with respect to one another between events. This was designed to give rise to new asperity contact distributions.

Surface structures were evaluated using a stylus profilometer (Tencor P-11) with a 2 μm radius stylus point. Scans were carried out over 2000 μm lengths at 50 μm/s with data acquisition at 200 Hz, giving a maximal lateral resolution of 0·25 μm. Scans were repeated five times per specimen. These parameters were found to be suitable for obtaining representative surface profiles. Fractal dimensions of one-dimensional profiles were evaluated using methods similar to those used elsewhere, utilising the log–log change of apparent profile length relative to the resolution of measured points (Hasegawa et al., 1996; Sun & Xu, 2005). As fractal dimensions were recorded from linear scans, they lie in the range 1–2. The resolution of measured points was decreased by selecting data points with increasing separation, from a point spacing of 0·25 μm (full resolution) to 250 μm.

Subsequent to surface deformation, static frictional interactions between roughened aluminium surfaces and a rigid flat surface were measured using a single crystal α-quartz [0001] substrate, which is considered as an atomically flat rigid counter surface. Frictional measurements were based on gravity-driven slipping of aluminium discs on the substrate. This was carried out without the application of an additional normal load beyond the normal component of the weight of the specimen (4·3±0·3 g). In this experimental setup, normal loads are sufficiently low to avoid abrasion of surfaces and adhesion-based friction dominates as asperity–asperity interactions are essentially absent even at nanometre scales. To minimise humidity effects, physically adsorbed water was removed by drying the samples and quartz substrates at 110°C prior to frictional experimentation. Friction angles were increased using a screw-driven tilting hinged stage to determine the maximal achievable angle of adhesion of the aluminium discs to the quartz substrates. Using this approach was found to give more consistent results than using minimum angles of slip, as contact ageing effects are thus minimised. Friction measurements were repeated five times and exhibited a variance range of around ±20%.