Enhanced tribological performance of cylinder liners made of cast aluminum alloy with high silicon content through plasma electrolytic oxidation

Abstract

Aluminum-silicon (Al-Si) alloys have been increasingly used in several industries to develop novel lighter and high-performance components. Nevertheless, these novel advanced materials display poor wear resistance which hamper their application in several critical tribological components. This investigation is focused on the surface modification of Al-Si alloys with high Si content, through a breakthrough green coating technology named plasma electrolytic oxidation (PEO). The main aim is, for the first time, to study the tribological behavior of PEO-treated cylinder liners made of cast Al with high Si content, in a configuration mimicking the real contact between a cylinder liner and a piston ring. Additionally, this study is based on the development of a novel electrolyte especially designed to generate PEO films with improved tribological properties.Novel films were successfully synthesized by PEO in an innovative aluminate-based electrolyte. The morphological, topographical, and chemical features of the films were assessed through scanning electron microscopy (SEM), thickness and average roughness (Ra) measurements, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Cross-sectional SEM images showed that PEO films developed in the novel electrolyte are denser and thicker. Additionally, the novel PEO-treated cylinder liners, presented a significantly improved resistance to wear damage when submitted to tribological actions simulating the cylinder liner/piston ring real contact movements, which was linked with the extremely higher hardness of the films. These cylinder liners presented a lower COF compared to the commercial reference, along the whole duration of sliding. Finally, tribological tests showed that PEO-treated Al cylinder liners displayed a similar level of mechanical wear damage that steel cylinder liners.The results presented in this investigation envisage the feasibility of PEO to be applied on Al-Si engine components submitted to aggressive tribological conditions, enhancing their durability requirements to levels similar to components made of steel.