Effect of surface mechanical attrition treatment (SMAT) on microhardness, surface roughness and wettability of AISI 316L

Surface roughness and wettability are among the surface properties which determine the service lifetime of materials. Mechanical treatments subjected to the surface layer of materials are often performed to obtain the desired surface properties and to enhance the mechanical strength of materials. In this paper, the surface microhardness, roughness and wettability of AISI 316L stainless steel resulting from surface mechanical attrition treatment (SMAT) are discussed. The SMAT was conducted with various processing parameters, including the duration of treatment, the number and diameter of milling ball, and the motor speed of the SMAT machine. The result indicates an increasing surface microhardness due to the SMAT. A harder surface is yielded by the SMAT with a longer duration, a bigger and a larger number of milling balls, and a higher vibration frequency. The SMAT also creates craters on the steel surfaces which correspond to the increasing roughness from 0.046 μm to the values in ranging from 0.681 to 0.909 μm. The change on the surface roughness by the SMAT does not only depend on the duration of treatment, but also the other processing parameters. In addition, the wettability of AISI 316L surface slightly increases by the SMAT as seen on the decreasing droplet contact angle from 88.6° to the values ranging from 74.4° to 87.0°. Such a droplet contact angle reduction is related to the increasing surface roughness after the SMAT. In conclusion, this study reveals the possibility of the SMAT to be used for surface properties optimization in addition to the strength enhancement of stainless steel.