Dynamic pressure scuffing initiation of a grade 250 flake graphite cast iron

Abstract

The dynamic contact pressures experienced at the ring-liner interface of an engine are difficult to replicate in laboratory based reciprocating tribometers utilising a sliding Hertzian line contact, not least due to wear of the mating surfaces. A grade 250 flake graphite cast iron flat plate test geometry was designed to allow dynamic contact pressures between 8 and 62 MPa experienced in a heavy-duty diesel engine liner to be achieved on a reciprocating tribometer during the compression and expansion stroke.A constant normal load of 311 N was applied by sliding at 25 mm stroke length in PAO4 at 15 Hz against a 52100 rectangular contact area (2 × 20 mm). The temperature was slowly increased at 4 °C/min to initiate scuffing between the mating surfaces. Severe scuffing initiated at 250.8 ± 2.6 °C for all specimens. The spatially resolved friction force and non-contact optical profilometry suggested scuffing did not always initiate in the region of the highest contact pressure, whilst electron microscopy revealed that subsequent to lubricant failure, a transient low friction iron oxide layer was formed prior to removal and catastrophic adhesive wear.