Evaluation of friction coefficient of lamellar and compacted graphite irons in lubricated ring-on-cylinder system

The current investigation compares the friction behavior between specimens extracted from engine blocks of grey cast iron (class FC 250 ABNT NBR 6589 standard) and compacted graphite iron (class GJV450 ISO 16112 standard). The effect of wall thickness was evaluated, by extracting samples from regions equivalent to two levels of thickness. The piston ring used was a nitrided martensitic stainless steel with an asymmetric profile, and the lubricant oil was the SAE 30 CF. The tribological tests were conducted at 75 N normal load for a duration of 1 hour, a frequency of 5 Hz and a stroke of 10 mm. The tests were carried out at a controlled temperature of 40° C. In a general way, the compacted graphite irons presented smaller friction coefficients when compared to that observed for the lamellar ones; and the wall thickness showed relevance only for the compacted graphite iron, being the lowest values found for the thinner wall. On the other hand, for the grey cast iron, the wall thickness did not affect significantly the friction behavior. Finally, all materials presented a tendency to a smoothing of the surface after tests.