GALLING MECHANISMS IN LUBRICATED SYSTEMS - A STUDY OF SHEET-METAL FORMING

In many sliding systems, transfer of material occurs between the conta cting surfaces. Since the transfer modifies the contacting surfaces, t he friction and wear behaviour of the system are also affected. A basi c understanding of transfer mechanisms is therefore important, not onl y for specific applications, but also for a general understanding of f riction and wear. The initiation of galling is proposed to be primaril y due to tool surface defects. The defects are either present from the surface preparation of the tool (as grinding marks), or are produced during testing, owing to scratching/cutting of hard sheet fragments (o r occasionally other hard foreign particles) into the tool surface. Th e transfer proceeds with an accumulation of sheet debris on some of th e initiation sites, where the contact stresses become more and more se vere, owing to the hard tool surface irregularities produced during th e accumulation. At later stages, cracking occurs in the transfer layer , and larger fragments move over the tool surface, causing even more s evere tool surface irregularities. At this stage, back transfer of she et fragments to the sheet surface is observed occasionally. The action of the lubricant seems to be mainly a delay of the transfer process c ompared with testing without lubrication. Similar mechanisms with prod uction of hard sheet fragments that plough the tool surface and create surface defects and with a following build-up of larger lumps on thes e defects are observed for non-lubricated testing. However, the proces s is speeded up through a much larger number of fragments and a more r apid build-up of the transfer layer.