Wear transitions and stability of polyoxymethylene homopolymer in highly loaded applications compared to small-scale testing

Abstract

Wear and failure mechanisms of polyoxymethylene homopolymer (POM-H) loaded above its yield strength are studied on test samples with a 22500 mm 2 sliding area at 8–150 MPa contact pressures. Test results are compared to small-scale cylinder-on-plate tests. Plastification of the sliding surfaces at high loads is favourable for low friction, while different wear mechanisms compared to small-scale testing are induced. Small-scale tests show a transition from mild adhesive/abrasive wear to severe wear due to softening, which is characterised by the formation of shear lips. Softening of large-scale sliding surfaces does not cause overload but it contributes to stable wear rates. Overload of large-scale samples is characterised by the transition from softening to melting and degradation. The dimensional stability of polymer elements is influenced by creep and it is verified that deformation of small-scale samples and large-scale samples loaded at 8 MPa is recovered after sliding, while it remains as permanent deformation for large-scale tests at 16–150 MPa. The wear transitions are further analysed by optical microscopy and available temperature models. The flash temperature concept can be applied for small-scale tests and large-scale tests up to 8 MPa. Calculated flash temperatures indicate softening and are in agreement with visual observations of the polymer surfaces. Flash temperatures for large-scale tests at 16–150 MPa indicate melting and degradation that was not visually observed on the polymer surfaces. The bulk temperature model prevails during large-scale sliding and only indicates melting at 150 MPa. Thermal analysis of the worn polymer surfaces confirms that crystallisation happened during small-scale sliding and large-scale sliding up to 55 MPa, occurring between 120 and 150 °C. Thermo-oxidative degradation above 200 °C is evidenced at 150 MPa.