Fretting wear behaviour of surface mechanical attrition treated alloy 718

Alloy 718 was subjected to surface mechanical attrition treatment (SMAT) using SAE 52100 steel balls of a 5 mm diameter for four different treatment durations (15, 30, 45 and 60 min). Fretting wear tests were conducted at different normal loads on untreated and treated samples for 25,000 cycles using alumina as a counterbody material. Microstructural features of the surface layer of samples treated by SMAT were characterized by cross-sectional optical microscopy and transmission electron microscopy. Hardness, surface roughness and residual stress were determined using a nano-indenter, surface roughness tester and X-ray residual stress analyzer respectively. SMAT resulted in the formation of nanocrystallites on the surface and near surface regions, increased hardness, increased surface roughness and compressive residual stress at the surface. Treated samples exhibited lower tangential force coefficient (TFC) compared to untreated samples. Samples treated for 60 min exhibited higher grain refinement, higher hardness, lower surface roughness and higher TFC compared to the samples treated for 30 min. The wear volume and wear rate of samples treated for 30 min were lower compared to those of the untreated samples, which may be attributed to an optimum combination of hardness and toughness and a low work hardening rate of the nanocrystalline structure at the surface of the treated samples. In contrast, the wear volume and wear rate of the samples treated for 60 min were higher than those of untreated samples, presumably due to the higher hardness and reduced toughness of the samples treated for 60 min.