Friction, wear and transfer of pure and internally lubricated cast polyamides at various testing scales

Friction and wear of different cast polyamide composites (including two pure polyamides with sodium or magnesium catalysers, oil filled and solid lubricant filled polyamides) is evaluated on a small-scale and a large-scale tribotester to determine transitions in tribological behaviour and application limits. Small-scale tests are done at 50-200 N and 0.3 m/s on a cylinder-on-plate configuration (contact area <10 mm2) and compared to sliding tests at 10-40 MPa and 0.005 m/s on a flat-on-flat configuration (contact area 22,500 mm2) for investigating the efficiency of internal lubricants and reproducibility at different testing scales. Frictional heating and transfer are important and interfere with the sliding or lubricant mechanisms. The bulk and flash temperatures during sliding are calculated and the softening and/or melting mechanisms for the different polymers are closely related to differences in transfer, either on small-scale or large-scale tests. For pure polyamides, the magnesium catalysed polyamide (PA-Mg) has different softening behaviour compared to sodium catalysed polyamide (PA-Na) as revealed from thermal analysis and Raman spectroscopy. Therefore, sliding instabilities attributed to stick-slip for PA-Na do not occur for PA-Mg at low loads. Oil lubricated composites are not able to remove the sliding instabilities as oil supply to the sliding interface is controlled by migration effects that are restricted by deformation or softening of the polyamide matrix. A relatively thick and brittle transfer film develops when the polyamide bulk melts. Solid lubricants are able to stabilise friction and lower wear own to the formation of a thin and coherent transfer film. However, increasing the amount of lubricants induces lower mechanical properties and higher deformation. The friction coefficients for pure polyamides can be extrapolated as a function of a scaling factor. The wear behaviour of tests samples with small and large contact areas is significantly different for solid lubricated composites, with lower specific wear for large-scale samples. Transitions in lubrication mechanism due to softening and melting do not allow for extrapolation and justifies the use of large-scale tests. Only pure polyamides have identical specific wear rates on small-scale and large-scale under mild conditions.