Progression of wear in the mild wear regime of an Al-18.5% Si (A390) alloy

The mild wear regime of a cast Al-18.5% Si (A390), a lightweight alloy used in automotive components requiring wear resistance, was investigated in order to characterize the progression of the sliding wear processes. Block-on-ring (SAE 52100 steel) type sliding wear tests were conducted under a controlled dry air environment with 5% relative humidity. It was observed that the mild wear regime consisted of two sub-regimes: The first sub-regime of mild wear (MW-1) occurred at loads between 0.2 N and 35 N, and the second sub-regime of mild wear (MW-2) between 60 N and 150 N. A common characteristic of MW-1 and MW-2 was the attainment of steady-state wear conditions. The load (L) dependence of the steady-state wear rates (W) in both sub-regimes was expressed as W = C(L)n, where C1 = 1.08 × 10−4, n1 = 0.56 for MW-1 and C2 = 2.18 × 10−4, n2 = 0.67 for MW-2. A transition regime, where the wear rates of MW-1 increased by 270%, occurred in the 35–60 N load range. The transition between MW-1 and MW-2 was accompanied by a rapid increase (25%) in the amount of material transferred to the counterface.Sliding wear in both sub-regimes proceeded by the formation of tribolayers that were initiated by iron transfer from the steel counterface to the silicon particles on the contact surfaces. Compared to MW-1, tribolayers were formed at a faster rate in MW-2 and the amount of material transferred to the counterface was larger. Also, in MW-2 the magnitudes of plastic strains (ɛ) in the deformed aluminum subsurfaces below the tribolayers were higher, e.g., at 40 μm below the surface ɛ = 3 at 60 N, compared to ɛ = 0.1 at 10 N at the same depth. In addition, in MW-2, both the tribolayers and the material transferred to the counterface contained layers of aluminum, implying that the aluminum matrix became in contact with the counterface. Spallation of thick tribolayers formed in MW-2 as well as extrusion of exposed aluminum surfaces over the tribolayers were among the main reasons for the higher wear rates in this regime compared to MW-1.