Effect of test scale on the friction properties of pure and internal-lubricated cast polyamides at running-in

Friction and wear tests for polymers are mostly done on small-scale tribotesters. The effect of different test scales on friction and efficiency of internal lubricant is investigated in this paper. Three scales are used with different contact area sizes, contact geometries and normal loads: flat-on-flat meso-scale tests, cylinder-on-plate small-scale tests and flat-on-flat large-scale tests. Selection tests are presented for (i) pure sodium-catalysed polyamides PA-Na, (ii) pure magnesium-catalysed polyamides PA-Mg, oil-filled polyamides with (iii) homogeneous oil dispersions in the bulk or (iv) holes in the surface with oil lubricant and (v) thermoplastic solid-lubricated polyamides. Running-in against steel counterfaces is examined over one single or 10 reciprocating sliding strokes, neglecting thermal effects and transfer film formation. The effects of mechanical contact and intrinsic material properties are detailed. No unique extrapolation as a function of pv parameters can be made. On every scale, PA-Na has higher coefficients of friction than PA-Mg due to ductile bulk properties for PA-Mg. Elastic contact corresponds to decreasing coefficients of friction with normal load. A transition into high and unstable friction on small- and large-scale tests coincides with the onset of plastic deformation. The test scale and contact surface area mainly affects the efficiency of internal lubricants. Solid lubricants are most favourable for low friction on large-scale tests, while oil lubricants are most efficient in meso-scale tests. This is explained by the oil supply mechanisms of diffusion and migration. The lower coefficients of friction for filled polyamides compared to pure polyamides are mainly attributed to lower surface energies and lower adhesion.