Effect of counterface surface roughness and its evolution on the wear and friction of PEEK and PEEK-bonded carbon fibre composites on stainless steel

Abstract

The time-dependent variations in the dry sliding wear rate and friction of PEEK (450G and 100P) and carbon fibre-bonded PEEK (APC2) on stainless steel (316S16) have been investigated and the evolution of wear scratches measured with optical profilometry. A three-pin-on-disk tester was used, operating at 0.18 m s −1, with a pressure of 1.0 MPa on 5 mm diameter pin. Specific wear rates were calculated from mass lost by the pins at 15 km intervals and friction force was monitored by a strain-gauged torque beam. Two series of 90-km tests were performed for each pin/disk combination. The stainless steel disks had either a fine-ground or polished surface. The roughness of the ground stainless steel surface was chosen to coincide with the optimum value for low wear rate of the PEEK pins. To maximise pin/disk contact area, the PEEK and APC2 pin surfaces were run-in on 1000-grit silicon carbide paper and ultrasonically cleaned in acetone before the first wear test. There was no significant difference in steady-state wear between these pins and those tested with as-moulded surfaces. The disks were analysed at 15 km intervals with the UBM profilometer, providing 3D surface scans and 2D profiles for roughness parameters without damaging the wear track. All the surface roughness parameters reported in this work were produced from thhe UBM non-contacting optical profilometer. A marked difference in specific wear rate was observed between the two types of PEEK. The 450G had the lowest wear of 6 × 10 −6 mm 3 N −1 m −1 on both the polished and ground counterface, whereas the 100P had wear factors of 1 × 10 −4 mm 3 N −1 m −1 on ground and 2 × 10 −5 mm 3 N −1 m −1 on polished stainless steel. However, the friction forces measured between the 450G PEEK and the polished disk were up to three times higher than those against the ground disk, (0.35±0.01), decreasing to similar values as the polished surface became scratched. The APC2 pins showed very little difference in wear rate between the ground and polished disks. However, a steady decrease from 1 × 10 −6 to 1 × 10 −7 mm 3 N −1 m −1 over a distance of 90 km was measured as more fibres were ground into a graphite transfer film lubricant and the coefficient of friction dropped from 0.35 to 0.18. The specific wear rates and coefficients of friction of the APC2 samples were lower than those for PEEK, however, the APC2 caused more scratch deformation of the polished steel counterface.