Electrolytic polishing of compacted graphite iron for revealing subsurface graphite of honed cylinders

Resumo

The superficial characteristics of honed surfaces are relevant for the tribological performance of cylinders of
internal combustion engines (MCI). Recent studies found a correlation between the occurrence of the phenomenon
called folded metal and the presence of graphite of cast irons. However, the honing process tends to
overlap the graphite, such that an application of a technique to remove a thin layer of material, exposing the
graphite, would be useful. This paper aims to determine the best condition, of electric potential and time, for
the exposition of graphite in honed surfaces using a technique of electrolyte polishing. The polishing was
made on the concave surface with aid of a polishing mask developed for this matter. Samples of honed cylinders
were removed from MCI manufactured using a compacted graphite iron. The characterizations were
done through metallography and white light interferometry. To determine the ideal condition, several conditions
of polishing were tested. For the verification of the repeatability, five repetitions were performed with
the best condition. The results showed satisfactory repeatability, with an exposition of graphite and the matrix
etching. The roughness of the polished region presented considerable uniformity, with a significant
change of roughness in comparison to the honed surface. Furthermore, removal of the characteristic topography
of the honed surface was observed. The layer removed showed little variation in depth along the polished
region, with values close to 5 μm. This value accomplishes the demand of layer removal for the exposition of
subsurface graphite.
Keywords: Electrolyte polishing; Compacted graphite iron; Honing.