Development of detection techniques for investigating scuffing mechanisms of automotive diesel cast irons

Abstract

Increasingly stringent environmental legislation has led automotive manufacturers to focus engine development on achieving greater fuel efficiency by friction reduction. Engine friction losses between 4 and 15 % are typical for diesel engines, with approximately 50 % occurring at the piston assembly. Viscous losses associated with shearing lubricant films may be reduced by using oil with a lower viscosity, however this also results in the contact surfaces being separated by thinner lubricant films, which makes them more susceptible to scuffing. Improving the scuffing resistance of engine materials requires a detailed understanding of the contact conditions that lead to scuffing. However this is a complicated process that is often characterised by a rapid progression and therefore it is difficult to detect accurately and repeatedly the onset of scuffing failure.

  This thesis explores the possibility of using deviations in the instantaneous friction force measured using a laboratory tribometer to assess whether the onset and progression of scuffing can be repeatedly determined for cast iron diesel cylinder liner materials. Two techniques were developed that assessed the deviation in spatially resolved friction signal as a function of stroke length as well asthe level of disorder in the instantaneous root mean squared friction force. This enabled transitions in scuffing behaviour to be assessed for a Grade 250 flake graphite and a designation 400‐15 nodular cast iron cylinder liner materials under lubricated reciprocating sliding at increasing contact pressure.

The detection techniques allowed tests to be interrupted during scuffing transitions for the analysis of surfaces using a combination of profilometry, scanning electron microscopy, energy dispersive X‐ray spectroscopy and focused ion beam microscopy. The results showed that once sufficient energy was available for the sliding contact to degrade the lubricant, progression to mild scuffing occurred. This was characterised for Grade 250 cast iron by surface platelet formation and subsurface crack networks associated with the de‐cohesion of the flake graphite phase from the pearlitic matrix. Progression to severe scuffing was characterised by adhesive transfer and back transfer of these platelets resulting in catastrophic wear of the cast iron surface.

Compared to flake graphite cast iron, nodular cast iron transitioned to mild scuffing at lower contact pressures, but exhibited an intermediate scuffing stage that prolonged the transition to severe scuffing. This was due to the formation of a comprehensive oxide film associated with the ferritic matrix, but also because the adhesive removal of material was associated with angular dross inclusions from the nodularisation process.

  The techniques were applied to a conformal ring liner geometry that exhibited velocity dependant friction behaviour and therefore the detection techniques were adapted to include contact potential and disorderly friction data to detect scuffing transitions. The failure mechanisms were confirmed to be similar to that observed for both line contact and fired engine scuffing tests and showed that these techniques could be used to repeatedly detect scuffing transitions within 18 % maximum absolute deviation. Recommendations for testing of future engine materials for scuffing resistance are discussed as well as potential areas of further research.

More information

Identifiers

Catalogue record

Export record