Modeling of Microindentation with Consideration of the Surface Roughness
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TRODUCTION
THE hardness measurement of a surface or coating on a surface using a microindentation technique is based on the resistance against plastic deformation. The hardness value obtained from the indentation test is defined as the ratio of the applied force to the contact area (the contact area with the indenter facet or the contact area projected to the basal plane of the indenter) of the residual indent.[1] The indentation size effect (ISE) refers to the phenomenon where the hardness of a material increases with the decreases in the penetrating load[2–4] and friction.[5,6] Both alter the plastic deformation of the material. The increase in the indent size is explained by the decrease in frictional effect on the contact which induces an outward flow of material and subsequent widening of the projected contact area, leading to a decrease in hardness value.[7,8] Nevertheless, the studies on the friction level studies rarely use a series of penetrating loads. The geometrically necessary dislocation (GND) theory can be used to explain the indent size change based on the density of the dislocated material within a circular dislocation loop. Nix–Gao[9] proposed a mechanistic model that was based on the concept of GNDs with the coupling of the Taylor relation, von Mises flow stress rule, and Tabor equation. This model suggests that the additional hardening component becomes larger with a decrease in the indenting impression. It should be noted that the hardening effect induces changes in the deformation response and the friction level for the contact interface between the indenter and the material. Many studies have assumed that the material surface is smooth, with the surfaces modeled as flat platforms for indentation.[10–13] In reality, surfaces are characterHUN GUAN CHUAH, Student, and ZAIDI BIN MOHD RIPIN, Professor, are with the School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, SPS, 14300, Pulau Pinang, Malaysia. Contact e-mail: [email protected] Manuscript submitted May 24, 2012. Article published online August 13, 2013 5676—VOLUME 44A, DECEMBER 2013
ized by topographical configurations,[5,11] with the surface roughness affecting the force required to deform the asperities[14,15] and the size of the projected area of the residual indent. Walter and Mitterer[16] analyzed a material surface using the actual three-dimensional (3D) surface topography taken from atomic force microscopy data. The 3D indenting model showed less scatter response in the load–displacement results than the 2D model. For a shallow indentation, a higher load is required to deform and flatten the asperities before the indenter makes full contact with the surface material.[6] The normalized hardness value is proportional to the surface roughness for a sharp conical indenter.[17] As the surface-roughness effect is inevitable in the indentation test, the contribution of the surface to the ISE is significant. Unlike a flat surface, a surface topography with surface roughness provides resistance against the sliding motion, which is
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