Applied Surface Science 258 (2012) 4538–4543 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Effects of surface mechanical attrition treatment (SMAT) on a rough surface of AISI 316L stainless steel B. Arifvianto ∗ , Suyitno, M. Mahardika Center for Innovation of Medical Equipments and Devices (CIMEDs), Department of Mechanical and Industrial Engineering, Faculty of Engineering, Gadjah Mada University, Jl. Grafika 2, Yogyakarta 55281, Indonesia article info Article history: Received 6 August 2011 Received in revised form 20 December 2011 Accepted 3 January 2012 Available online 28 January 2012 Keywords: Surface Roughness Morphology AISI 316L SMAT abstract Surface mechanical attrition treatment (SMAT) improves mechanical properties of metallic materials through the formation of nanocrystallites at their surface layer. It also modifies the morphology and roughness of the work surface. Surface roughening by the SMAT has been reported previously in a smooth specimen, however in this study the starting point was a rough surface and a smoothening phenomenon is observed. In this paper, the mechanisms involved in the surface smoothening of AISI 316L stainless steel during the SMAT are elucidated. The SMAT was conducted on a specimen with a roughness of R a = 3.98 m for 0–20 min. The size of milling balls used in the SMAT was varied from 3.18 mm to 6.35 mm. The modification of subsurface microhardness, surface morphology, roughness and mass reduction of the specimen due to the SMAT were studied. The result shows the increasing microhardness of the surface and subsurface of the steel due to the SMAT. The impacts of milling balls deform the surface and produce a flat- like structure at this layer. Surface roughness decreases until its saturation is achieved in the SMAT. The mass reduction of the specimens is also detected and may indicate material removal or surface erosion by the SMAT. The size of milling ball is found to be the important feature determining the pattern of roughness evolution and material removal during the SMAT. From this study, two principal mechanisms in the evolution of surface morphology and roughness during the SMAT are proposed, i.e. indentation and surface erosion by the multiple impacts of milling balls. A comparative study with the results of the previous experiment indicates that the initial surface roughness has no influence in the work hardening by the SMAT but it does slightly on the saturated roughness value obtained by this treatment. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Surface mechanical treatments have received much attention due to their versatility in improving mechanical properties of metallic materials. In principle, the treatments improve surface structures and properties of metals by impacting milling balls or particles onto the work surface repeatedly [1–12]. Sandblast- ing [1–3], shot peening [4–6], ultrasonic shot peening (USSP) [7], surface nanocrystallization and hardening (SNH) [8] and sur- face mechanical attrition treatment (SMAT) [9–12] are among the typical surface treatments which have been already reported in literatures. The novel SMAT evidently improves tensile strength [10,12] and fatigue resistance of metals [11]. During the SMAT, the multiple and random impacts of smooth milling balls are introduced onto the work surface [9]. The impacts refine the grains size until nanometer scale and induce the formation of residual stress and martensite at ∗ Corresponding author. Fax: +62 274521673. E-mail address: b.arifvianto@gmail.com (B. Arifvianto). the work surface and subsurface [9–13]. The nanocrystallites, resid- ual stress and martensite are responsible to increase the strength and fatigue resistance of metals [9]. The SMAT utilizes smooth and spherical shots [9] rather than particles with angular shapes such as used in the common sandblasting treatment [1–3,15]. Millling balls with a dimension of >1 mm are also used in the SMAT instead of micrometer-size particles as in sandblasting [1–3,15] or smaller shots as in shot peening [9]. The SMAT generates random direc- tional impacts [9] instead of single ones such as in the shot peening and sandblasting [4,15]. The SMAT also modifies the morphology and roughness of the work surface [14] as well as the other surface mechanical treatments [1–4,15]. The modification of surface morphology and roughness during the SMAT has been previously reported in Ref. [14]. The SMAT enhances the average roughness of a smooth AISI 316L stainless steel from R a = 0.04 m to the values spanning from R a = 0.7 m to 0.9 m. The roughness enhancement is attributed to the impacts or indentations of milling balls which result in the formation of craters and pile-up at the steel surface. The uppermost points of the pile- up and the lowest points of the craters become the new peaks and valleys at the treated surface, respectively. The more irregular and 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.021