Residual stresses in DLC/Si and Au/Si systems: Application of a stress-relaxation model to the nanoindentation technique
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Residual stress in a thin film was analyzed by the nanoindentation technique. Two dominant effects of residual stress to indentation were summarized as the slope change in loading curve and the invariant value of intrinsic hardness. A stress-sensitive reversibly deformed zone around contact was modeled to explain the indentation behaviors under a residually stressed state. Finally, the residual stress was evaluated from the changes in contact shape and applied load during stress relaxation under the condition of constant indentation depth. The residual stresses in diamond-like carbon and Au films analyzed from this model agreed well with the average values measured by the curvature method. I. INTRODUCTION
Residual stress generated by local plastic deformation or inhomogeneous heat treatment reduces the fatigue strength and fracture properties of bulk materials.1 In the case of thin films, the residual stress dominates the mechanical properties.2–5 However, current stress-measuring techniques1,6–8 have some problems with stress relaxation during specimen sampling, removal of the local material without damage,6 and complex and time-consuming testing procedures.1,6,7 The conventional curvature method using a laser beam7,9 often employed to measure residual stress in thin films cannot be used for a rough film surface and cannot identify local variations in residual stress. Nanoindentation was proposed as an in situ and nanometer-order local stress-analysis technique. In particular, the nanoindentation technique has significance because it has been widely applied in characterizing the basic mechanical properties of thin films.10,11 The results of previous studies on residual stress using macroindentation were empirical measurements of the variations in the apparent hardness12 or the shape of the residual indentation mark.13 Another study14 attempted to determine a relationship between the apparent hardness from the analysis of the load–depth curve and the residual stress. However, recent accurate observation15 of the indentation mark and indentation simulation16 using the finite element analysis verified the independence of the residual stress and the intrinsic hardness. Furthermore, a theoretical analysis17 was already made of the effect of stress on the shape variation of the indentation curve. However, the distinct stress effects on indentation curve and
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 4, Apr 2002
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contact morphology were not proposed in the analysis. Thus, more fundamental studies on the residual stress effect to nanoindentation are still required. Therefore, the thin-film residual stress was evaluated quantitatively by a theoretical analysis of the stress-influential nanoindentation factors. The changes in the contact morphologies were modeled on the basis of the assumption of an invariant intrinsic hardness regardless of the residual stress beneath a sharp indenter. The morphology ch
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