Nanoscale Evolution of Sliding Metal Surfaces During Running-in

Abstract

We studied the formation of tribologically induced nanocrystalline layers in lubricated contacts during running-in using a pin-on-disk tribometer coupled to a radionuclide high-resolution wear measurement system. The experiments were stopped after different running times and the sliding surfaces were analysed by atomic force microscopy, auger electron spectroscopy and focused ion beam analysis. Initially, at very high wear rates, zinc phosphate anti-wear films are formed in the contact regions of the surfaces, which slow down the wear process. During rubbing the formed films will submerge below the surface of the materials as result of large plastic deformations. This process affects the friction coefficient and the wear rate and is considered to be a key mechanism for the running-in.

Origin of Friction in Running-in Sliding Wear of Nitride Coatings

“…the attached fish-scale-like material. These evolutions have been reported in the tribofilm formation of metallic and ceramic materials [2][3][4][5][6][7][8][9][10][11][12][13][19][20][21][22][23][24]. In brief, the external frictional energy provided to the sliding wear results in (1) powdering and eventually amorphorization of the entrapped wear debris agglomerates, (2) shearing deformation/flow within the agglomerates and (3) tribo-oxidation.…”

“…4b, c and 6a). Such wear particle generation through brittle fracture was widely reported in the wear of many other ceramics [4][5][6][7][8][9][10][11][12][13].…”

“…Fundamental studies that attempt to interpret the details of running-in friction are relatively rare. For the running-in period, research has been conducted to investigate the generation of wear particles of various materials, as well as the subsequent fragmentation, adhesive agglomeration and tribo-chemical reactions [4][5][6][7][8][9][10][11][12][13]. For example, the wear particle generation and agglomeration in the sliding wear of ceramics were observed by using an in situ scanning electron microscopy (SEM) [4].…”

“…For example, the wear particle generation and agglomeration in the sliding wear of ceramics were observed by using an in situ scanning electron microscopy (SEM) [4]. More recently, a method of high-resolution worn surface characterization has been reported in the study of nano-scale tribofilm formation in lubricated sliding wear of zinc phosphate [11].…”

Tribofilms Forming in Oil-Lubricated Contacts

“…As Dienwiebel et al showed for a lubricated chromium-cast iron contact with a contact pressure in the MPa range, that the question whether the film is on top or underneath can be answered by analyzing the energy consumption during film formation-very often during running-in-and by observing the changes of surface, microstructure and chemical composition of the near-surface as function of time [1]. For MPa systems in the initial stage of their running-in and highly-loaded contacts (GPa) like gears, the materials tend to develop protective films, commonly containing Zn, P and S from extreme pressure additives.…”