Characterisation of the fretting wear of unlubricated steel surfaces based on the comparison of wear results obtained by different methods

Fretting wear is the typical wear form caused by small, oscillatory, tangential movements between two normally loaded surfaces. In the present research a bearing ball is subjected to fretting against a flat surface in gross-slip conditions. Because of the small wear volumes it is very difficult to quantify fretting wear accurately. In the present research, fretting wear measurements on the fretting of a bearing ball against cold-drawn steel are carried out by various methods: thin layer activation (TLA), spherical cap modelling (SCM), normal displacement measurement (ND), three-dimensional surface topography (3DST) and scanning electron microscopy (SEM). It is concluded that TLA gives the most accurate measurements. SCM largely overestimates and 3DST underestimates the fretting wear. ND is mainly influenced by particle escape from the contact zone. It is shown that the fretting process consists of two stages. During a running-in stage of about 200 000 cycles the wear rate and the coefficient of friction are high. The wear particles remain trapped in the contact zone for about 8000 cycles. After 200 000 cycles a steady state stage of the fretting process starts, characterised by an increase of the wear scar area rather than the wear scar depth, an increase in the wear of the harder, spherical specimen and readhesion of the wear particles to the surface of the flat specimen.