Reproducing automotive engine scuffing using a lubricated reciprocating contact

Abstract

The frequency and severity of scuffing in automotive engines has the potential to increase due to new low-viscosity lubricants for fuel efficiency and increased cylinder power output. This work is to understand the fundamental causes and events resulting in piston ring and liner scuffing. A TE-77 high frequency reciprocating tribometer was used with a synthetic PAO base oil (4cSt) to reciprocate a 52100 G5 barrel against a ground pearlitic Grade 250 grey cast iron.Samples were run-in at 50N and 10Hz prior to a temperature ramp to 150°C followed by a discreet load ramp to 1kN (0.49GPa). The tests were terminated when a sharp increase in the average friction force was observed indicating that scuffing had occurred. 3D optical profilometry showed that the scuffed cast iron surface consisted of smeared platelets and craters of 35μm depth. SEM and EDX analyses suggested adhesive transfer of cast iron material to the counter-surface was occurring by failure along lamellar graphite interfaces. Tests were repeated using instantaneous high-speed friction data and indicated that micro-scuffing initiated at a load of 620N. Focused ion beam cross-sections of the mildly scuffed surface confirmed the mechanism of sub-surface crack initiation occurring along lamellar graphite boundaries.