Tribological performance assessment of lamellar and compacted graphite irons in lubricated ring-on-cylinder test

Abstract

A constant challenge in the automotive industry is to find ways to reduce the friction. Reductions in the weight of components can play this role, but for using smaller thickness in the engine blocks more resistant materials should be employed. The present investigation discusses the tribological performance of two cast irons used for manufacturing engine blocks, grey cast iron (GCI) grade FC250 and compacted graphite iron (CGI) grade GJV450, and the effect of two different thicknesses – 3.9 ( ± 0.6) and 7.4 ( ± 0.4) mm – on their friction and wear behaviors assessed in a ring-on-cylinder test under mixed lubrication. The tests were 8-h long with a normal force of 75 N, a stroke of 10 mm and a frequency of reciprocating movement of 5 Hz. A new parameter was introduced for evaluating the tribological performance, called Tribological Merit Index (TMI), which considers the coefficient of friction (COF) and the changes in the surface topography, determined using a 3D optical interferometry. The thickness of the samples did not affect significantly the COF values. Nevertheless, the best result in terms of the friction behavior was obtained for the CGI-thin sample (0.111). For the contact area estimated by the topographical parameters, the thickness was not relevant just for the samples of lamellar cast iron; and the best result,was obtained for the CGI-thin. Considering the TMI parameter, a ranking could be established, from the best to the worst tribological performance: i) CGI-thin; ii) CGI-thick; iii) GCI-thick; and iv) GCI-thin.