Low friction glaze layer formation during lubricated dynamic pressure sliding of grade 250 flake graphite cast iron

Evaluating scuffing resistance of new materials for reciprocating engine components can be achieved using dynamic pressure sliding. Surface species responsible for low friction behaviour observed prior to scuffing initiation in a Grade 250 flake graphite cast iron under dynamic pressure (8–62 MPa) sliding conditions experienced by a heavy-duty diesel engine top compression ring were evaluated. Lubricated reciprocating tribometer tests were performed in a polyalphaolefin base oil with temperature incremented to 250 °C to induce scuffing. Friction and contact potential measurements revealed a transition from mixed to boundary lubrication, followed by a period of low friction preceding scuffing initiation. Post-test surface analysis was conducted using a range of advanced microscopy techniques (Raman, FIB, STEM, EDX and EELS) and suggested a complex series of tribochemical reactions are responsible for formation and removal of phases such as siloxanes and ferrous oxides that lead to the initiation of scuffing.