Semi-analytical calculation model for friction of polymers on the example of POM | PE-UHMW and steel | PE-UHMW

In this paper, a semi-analytical calculation model for the coefficient of friction (COF) of single spherical protrusions is presented. It allows the prediction of the deformative friction part $({\mu }_{\text{def}})$ and adhesive friction part $({\mu }_{\text{adh}})$ of the friction pairings steel | polyethylene with ultra-high molecular weight (PE-UHMW) and polyoxymethylene (POM) | PE-UHMW. The experimental studies included unlubricated friction tests, which served to determine the total COF $({\mu }_{\text{tot}}),$ as well as tests being lubricated with silicone oil, from which ${\mu }_{\text{def}}$ is obtained. Based on the verification tests, it could be shown that both states of lubrication result in the same deformation and that the relationship between the rear angle $(\omega )$ and ${\mu }_{\text{def}}$ postulated in the calculation model is valid. Therefore, friction tests with segmented spheres were carried out, which allow a specific variation of the $\omega$.

It can be concluded that for both pairings the ${\mu }_{\text{def}}$ is generally of minor significance (approx. 1/3 ${\mu }_{\text{tot}})$ and the influence of the ${\mu }_{\text{adh}}$ predominates (approx. 2/3 ${\mu }_{\text{tot}})$ the friction process. Furthermore the ${\mu }_{\text{tot}}$ decreases with increasing contact pressure especially in the low pressure range and depends on the form of motion (continuous and discontinuous).