POLYMER TESTING Polymer Testing 26 (2007) 660–675 Test Method Effect of test scale on the friction properties of pure and internal-lubricated cast polyamides at running-in Pieter Samyn à , Tony M. Tuzolana Ghent University, Laboratory Soete, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Received 21 February 2007; accepted 2 April 2007 Abstract 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. r 2007 Elsevier Ltd. All rights reserved. Keywords: Polyamide; Internal lubricant; Friction; Scaling 1. Introduction Friction and wear testing of polymers is done on laboratory scale to determine sliding stability and wear life-time of machine components such as bearings, cams or rollers. Small-scale tests are preferred in conventional tribotesting [1] due to cost- or time-efficiency and flexible handling of test specimens (apparent contact area 1–10 2 mm 2 ). An appropriate test set-up for each wear mode can be selected from ASTM and some DIN or ISO standards. The National Physics Laboratory (UK) identified worldwide more than 400 test methods with scientific or industrial interest [2]. Pin-on-disc (ASTM G99 or ISO 7148-2) [3] or pin-on-plate (ASTM G133) [4] tests are commonly used in continuous or reciprocating motion under adhesive conditions. Block-on-ring (ASTM G88) [5] tests are ARTICLE IN PRESS www.elsevier.com/locate/polytest 0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2007.04.002 à Corresponding author. Tel.: +32 3 264 33 08; fax: +32 9 264 32 95. E-mail address: Pieter.Samyn@UGent.be (P. Samyn).