On the indentation recovery and fleeting hardness of polymers
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Accurate mechanical characterization of viscoelastoplastic materials in small volumes is required for the development of polymeric thin film, nanocomposite, and biomedical applications. Instrumented indentation-based approaches are increasingly implemented to quantify the resistance to permanent deformation of such systems via time-independent analyses. Here, we quantify the significant post-indentation recovery of several bulk polymers via time-lapsed scanning-probe microscopy under ambient conditions, indicating up to 80% recovery of both indentation depth and volume within 48 h. This viscoelastic response demonstrates that indentation hardness values for these polymers are accurate within 10% for less than 5 min to 3.5 days post-indentation, neglecting any other analytical or experimental errors. Further, although the extent and rates of volumetric recovery depend strongly on loading history and polymer structure/physical properties, deformation resistance inferred from indentation hardness does not quantitatively or qualitatively predict recoverable work or residual deformation of polymer surfaces.
I. INTRODUCTION
Mechanical characterization of small-volume and thin film polymers via instrumented indentation is frequently applied to estimate mechanical properties such as the Young’s modulus, E, and semi-quantitative metrics of resistance to plastic deformation such as indentation hardness, Hi.1 Despite the prevalence of such experiments in the literature and in industrial application, the attainment and interpretation of polymer nanoindentation is often based on a framework developed for timeindependent materials. That is, load-displacement data are analyzed following closed-form, semi-empirical equations based in contact mechanics for linear elastic, von Mises yielding materials such as metals.2–4 This compromise is accepted for convenient metrics such as Hi without a quantitative understanding of postindentation polymer recovery rates at room temperature, which would appraise the applicability of Hi: Hi =
Pmax , Ac共hc兲
(1)
where Pmax is the maximum load applied during indentation and Ac(hc) is the calculated contact area at that a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0377 J. Mater. Res., Vol. 21, No. 12, Dec 2006
http://journals.cambridge.org
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load.5 Indentation hardness is therefore dependent on the contact depth, hc, which is a function of the maximum depth at complete unloading of the surface, ho.5 Recovery of polymer surfaces has been the focus of several previous studies.6–12 Lorenzo et al.8 related the change in uninstrumented Vickers microhardness depth (hmax 艌 10 m) determined through interference microscopy post-indentation to variations in weight-average molecular weight, Mw, and %-crystallinity for bulk polyethylene. The authors observed a negative correlation between extent of indentation depth recovery and both %-crystallinity and yield stress. However, these experiments were limited to discrete depth measuremen
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