Sliding wear of an Al–18.5 wt.% Si alloy tested in an argon atmosphere and against DLC coated counterfaces

Wear rates and coefficients of friction (COF) of an Al–18.5 wt.% Si alloy (A390) were measured in dry air and argon environments at different test loads using a block-on-ring (SAE 52100 type steel) configuration. The wear rates of the A390/52100 steel pair tested in argon were always lower compared to those measured in dry air with 5% RH. The difference was especially significant at low loads: a 10-fold reduction in the magnitude of the wear rates occurred at 10 N. Similarly, the tests performed in argon resulted in lower steady-state COF values of 0.29 ± 0.02 compared to 0.57 ± 0.08 measured in air.Progression of the sliding wear process in A390 tested in an argon atmosphere was studied as a function of the sliding distance: the process initiated with the formation of iron-rich transfer films on the surfaces of the exposed Si particles. During sliding the composition of the layers changed. Aluminum-rich tribolayers were formed during the steady-state wear at long sliding distances. These layers were compacted and provided continuous coverage of contact surfaces. In contrast, the surfaces of the samples tested in air were covered by iron-rich tribolayers that were oxidized and showed evidence for easy spallation. The depth of the damaged zone in the material underneath the tribolayers was smaller in the samples tested in argon compared to those tested in air, in which higher subsurface plastic strains were generated during sliding wear.Wear tests were also performed in air (5% RH) on A390 samples put in sliding contact against graphitic diamond-like carbon (DLC) coatings. The DLC coating prevented direct contact between the steel counterface and the A390 that resulted formation of worn surface morphologies that were free of iron-rich oxidized tribolayers. The corresponding wear rates and COF values were lower than those obtained for the A390/52100 steel pair tested in argon, e.g., a wear rate of 9.26 × 10−6 mm3/m and a steady-state COF value of 0.24 ± 0.02 were attained against the DLC coatings at 10 N.