Sliding-induced microstructure: what forms under a worn surface

Copper under a sapphire sphere

Haug et al. (2020) report that their group earlier found a horizontal line of dislocations at a uniform depth of about 150 nm under copper slid dry against a sapphire sphere (Greiner et al. 2016, under mild reciprocating loading), and named it the dislocation trace line. This page did not read Greiner et al. (2016) itself. The group's DFG project report says the line forms at the very start of sliding and later becomes a small angle grain boundary; deeper down, networks of geometrically necessary dislocations form sub-grains. The same report gives copper oxide clusters on the surface after roughly 100 cycles, and says the line was found over a wide range of speeds, loads and sphere sizes.

Haug et al. (2020) used a 10 mm sphere, 26 N (1.25 GPa peak Hertz pressure at the start), one pass at 0.5 mm/s, 22 °C and 52% humidity, on copper with 14 µm grains. They found two lines, at about 185 and 415 nm, and three processes: simple shear along the sliding direction, local shear at the lower line, and rotation of the crystal above and between the lines about the transverse direction. At a twin boundary the lower line took up 28 to 33° of misorientation and the upper one 5 to 8°. Defect density fell with depth. Sub-grains after one pass were linked to the high load and read as a precursor of the micro- or nanocrystalline layers seen after more cycles.

Ruebeling et al. (2021) varied load (2 to 100 N) and sphere size (1 to 10 mm), giving 530 to 1953 MPa peak Hertz pressure. After one pass the line sat at about 90 to 470 nm (their abstract rounds this to 100 to 400 nm), deeper at higher load and, at equal pressure, deeper under a larger sphere. Hertz theory puts the peak shear stress at about 7 to 76 µm, much deeper than the line. They propose that dislocations are pushed down at the front of the contact, move back up behind it, and stop where the stress falls to the critical resolved shear stress (the shear stress needed to move them on their slip plane). Below 6.75 N, dislocation features also formed below the line. The later stages on this page carry no depths because none were read in a source.

Graded layers in copper

Deng et al. (2016) pressed and slid a platen over copper at room temperature (44 MPa, 6.7 mm/s, four passes of 60 mm). Region I (0 to 15 µm) holds nanoscale lamellae 50 to 100 nm apart, region II (15 to 50 µm) fine grains stretched along the sliding direction, region III (50 to 120 µm) a regular lamellar deformation structure. The deformed zone reaches more than 1 mm. Hardness fell from about 2.28 GPa at the surface to about 1.04 GPa at 100 µm. A larger load gave a larger strain at every depth and a deeper zone. Their strain-depth profile is shown only as a figure, so this page does not plot strain. Hughes and Hansen (2001) found 10 nm structures near a heavily slid copper surface that coarsen with depth.

Grain growth under sliding

Rupert and Schuh (2010) slid a 6 mm tungsten carbide sphere at 5 N for 10,000 cycles (0.015 to 0.15 m/s, about 500 m) on Ni-W with 3 to 47 nm grains; finer grains came with more tungsten (3 to 27.9 at.%). In the 3 nm alloy a 100 to 300 nm thick top layer grew to 20 nm grains; the 25 nm alloy showed very little obvious change. Their upper bound for frictional heating was 500 °C, not high enough for long enough to explain the growth, and they linked it to local plasticity. Grain growth plus boundary relaxation hardened the track, so the finest alloys wore less than hardness predicts. Rupert et al. (2009) found, in tensile tests with no sliding, that shear stress drives grain boundary migration in nanocrystalline aluminium films that were stable at room temperature; growth tracked the distortional (shear) energy, not the volumetric energy.