FIB and TEM characterization of subsurfaces of an Al-Si alloy (A390) subjected to sliding wear

The material layers underneath the worn surfaces of a hypereutectic Al-Si alloy (A390) subjected to dry sliding wear in air and argon atmospheres were characterized. The samples were tested at a constant load of 10 N and a sliding velocity of 1 m/s using a block-on-ring tribometer. The counterface material was a SAE 52100 bearing steel. The wear rate of the alloy tested in an argon atmosphere (3.05 x 10-5 mm3/m) was 10 times lower compared to that of the sample tested in air (2.96 x 10-4 mm3/m). The subsurface microstructures generated under the two different test environments were characterized using a scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), focused ion beam (FIB) microscope and transmission electron microscope (TEM). Cross-sectional TEM specimens were prepared using a FIB 'lift-out' technique. TEM analysis indicated that the tribolayers formed on the sample tested in air contained significant amounts of iron, aluminum and oxygen. In addition, the tribolayers formed in air were hard and appeared to be severely fractured as an indication of their brittleness due to the large amount of oxide present. On the contrary, a much lower amount of iron and oxygen were found in the tribolayers formed in argon, which were a mechanical mixture of mainly ultra-fine grained aluminum (∼100 nm) and silicon. The tribolayers formed in argon were more stable on the contact surfaces, which reduced the wear rates of A390