Subsurface sliding wear damage characterization in Al–Si alloys using focused ion beam and cross-sectional TEM techniques

This paper illustrates the distinctive capabilities of using focused ion beam (FIB) techniques to elucidate the microstructure that forms beneath the worn regions of samples subjected to laboratory tests under ultra-mild wear (UMW) conditions and those extracted from specific regions of a worn surface of engine blocks. TEM investigations of cross-sectional samples excised with a focused gallium-ion beam to obtain electron transparency of the first few micrometers of the contact surface were effective in providing novel information on the micromechanisms of UMW that are not readily obtained with conventional microscopy. Two FIB sample preparation techniques, namely the lift-out and H-bar techniques were used to prepare site-specific cross-sectional TEM specimens from Al–11% Si alloys subjected to pin-on-disk and dynamometer tests. In both cases, the aluminum matrix was heavily deformed in the top few micrometers below the contact surface, which resulted in the formation of ultrafine grains. Damage to the silicon particles was also observed. Additionally, a tribolayer (or an oil-residue layer) whose thickness increased with sliding cycles, was developed on the contact surfaces. It was concluded that the development of the oil residue layer supported by a layer of ultrafine grain structure was necessary to maintain the low wear rates.