Galling mechanisms in lubricated systems: A study of sheet metal forming

Abstract

In many sliding systems, transfer of material occurs between the contacting surfaces. Since the transfer modifies the contacting surfaces, the friction and wear behaviour of the system are also affected. A basic understanding of transfer mechanisms is therefore important, not only for specific applications, but also for a general understanding of friction and wear. The initiation of galling is proposed to be primarily 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 (or occasionally other hard foreign particles) into the tool surface. The transfer proceeds with an accumulation of sheet debris on some of the initiation sites, where the contact stresses become more and more severe, owing to the hard tool surface irregularities produced during the accumulation. At later stages, cracking occurs in the transfer layer, and larger fragments move over the tool surface, causing even more severe tool surface irregularities. At this stage, back transfer of sheet fragments to the sheet surface is observed occasionally. The action of the lubricant seems to be mainly a delay of the transfer process compared with testing without lubrication. Similar mechanisms with production of hard sheet fragments that plough the tool surface and create surface defects and with a following build-up of larger lumps on these defects are observed for non-lubricated testing. However, the process is speeded up through a much larger number of fragments and a more rapid build-up of the transfer layer.