Stuttgart Mechanics of Soft Polymer Indentation

3 Julia Deuschle Mechanics of Soft Polymer Indentation 189 pages, 67 figures, 8 tables Abstract Nanoindentation has become a fast and reliable technique for the mechanical characterization of engineering materials. The applicability of this technique to small or confined volumes and the subμN and subnm resolution of the instruments are unique features in the field of mechanical testing and make nanoindentation a promising, yet not fully accepted tool for the investigation of soft materials. Since these materials are of high importance for technological and biomedical purposes, a strong interest in the application of nanoindentation to this class of materials exists. The present work deals with nanoindentation studies on various polymers in order to address the two crucial factors for testing of soft materials, which are surface detection and contact area determination. An improved surface detection criterion was established, which allows testing of materials with elastic moduli below 1 MPa, whereas before only materials stiffer than several GPa could be tested. The advances in the surface identification are based on the usage of dynamically acquired instead of quasi-static quantities, which exhibit a much better signal-to-noise ratio, thus allow a more accurate surface detection. The improvements of this method were successfully demonstrated for polymers ranging over 4 orders of magnitude in modulus. For a quantitative determination of the contact area, comparative finite element simulations, in-situ indentation tests and tensile tests were performed. Several factors like viscoelasticity and adhesion can influence the contact area of polymeric materials; thus they must be taken into consideration, which is not done in the widely used Oliver & Pharr method for contact area determination. Through this comparative approach, individual sources of error were identified and their contributions quantified. The Oliver & Pharr method was found to underestimate the contact area for shallow indentations, because the contact increase due to adhesive forces is neglected. For high penetrations, the sink-in effect is underestimated slightly. This leads to the conclusion that common indentation techniques are not applicable to soft polymeric materials without modifications. A methodology with alterations necessary for achieving accurate indentation results is provided in the present study. Thus, nanoindentation is confirmed as a usable tool for the mechanical characterization of materials with elastic moduli below 1 MPa.Nanoindentation has become a fast and reliable technique for the mechanical characterization of engineering materials. The applicability of this technique to small or confined volumes and the subμN and subnm resolution of the instruments are unique features in the field of mechanical testing and make nanoindentation a promising, yet not fully accepted tool for the investigation of soft materials. Since these materials are of high importance for technological and biomedical purposes, a strong interest in the application of nanoindentation to this class of materials exists. The present work deals with nanoindentation studies on various polymers in order to address the two crucial factors for testing of soft materials, which are surface detection and contact area determination. An improved surface detection criterion was established, which allows testing of materials with elastic moduli below 1 MPa, whereas before only materials stiffer than several GPa could be tested. The advances in the surface identification are based on the usage of dynamically acquired instead of quasi-static quantities, which exhibit a much better signal-to-noise ratio, thus allow a more accurate surface detection. The improvements of this method were successfully demonstrated for polymers ranging over 4 orders of magnitude in modulus. For a quantitative determination of the contact area, comparative finite element simulations, in-situ indentation tests and tensile tests were performed. Several factors like viscoelasticity and adhesion can influence the contact area of polymeric materials; thus they must be taken into consideration, which is not done in the widely used Oliver & Pharr method for contact area determination. Through this comparative approach, individual sources of error were identified and their contributions quantified. The Oliver & Pharr method was found to underestimate the contact area for shallow indentations, because the contact increase due to adhesive forces is neglected. For high penetrations, the sink-in effect is underestimated slightly. This leads to the conclusion that common indentation techniques are not applicable to soft polymeric materials without modifications. A methodology with alterations necessary for achieving accurate indentation results is provided in the present study. Thus, nanoindentation is confirmed as a usable tool for the mechanical characterization of materials with elastic moduli below 1 MPa. Kurzzusammenfassung 5 Julia Deuschle Mechanik der Indentation von weichen Polymeren 189 Seiten, 67 Abbildungen, 8 Tabellen Kurzzusammenfassung Nanoindentation als Methode zur Bestimmung mechanischer Kenngrößen wird heute in verschiedensten Bereichen angewendet. Vorteilhaft gegenüber konventionellen Prüfmethoden sind dabei die hohe Kraftund Wegauflösung und die kleinen Probenvolumina, die zur Messung ausreichen. Diese Eigenschaften haben, bedingt durch die rasche Entwicklung von Medizinund Biotechnologie, ein starkes Interesse an der Anwendung von Nanoindentation auf weiche Materialien hervorgerufen. In der vorliegenden Arbeit wurden systematische Untersuchungen an Polymeren durchgeführt. Es zeigte sich, dass die Oberflächenfindung und die Kontaktflächenbestimmung die beiden entscheidenden Faktoren für eine verlässliche Bestimmung der mechanischen Eigenschaften weicher Materialien darstellen. Zunächst konnte die Oberflächendetektion wesentlich verbessert werden, wodurch die Basis für die Untersuchung von Proben mit E-Moduli unter 1 MPa geschaffen wurde. Im Vergleich dazu konnten vorher nur Proben mit E-Moduli von einigen GPa getestet werden. Die neue Methode verwendet die dynamische Kontaktsteifigkeit, die im Gegensatz zur sonst verwendeten statischen Kontaktsteifigkeit ein deutlich geringeres Rauschen aufweist und somit eine sensiblere Oberflächendetektion ermöglicht. Die weiteren Arbeiten konzentrierten sich auf eine quantitative Ermittlung der Kontaktfläche und eine Quantifizierung der Effekte, die die Kontaktfläche verändern können. Bei Polymeren sind hier vor allem Viskoelastizität und Adhäsionseffekte zu nennen. Durch Vergleiche zwischen Nanoindentation, In-Situ Indentation, Finite Elemente Simulationen und Zugversuchen konnte herausgefunden werden, dass die Kontaktflächen, die üblicherweise mittels der Oliver & Pharr Methode bestimmt werden, deutlich von den tatsächlichen Werten abweichen. Für geringe Eindringtiefen werden adhäsive Kräfte vernachlässigt, daher sind die Kontaktflächen bis zu 40 % zu klein; bei Eindringtiefen oberhalb einiger μm ergeben sich um ca. 5 % zu große Kontaktflächen. Diese Abweichungen sind zurückzuführen auf das Deformationsverhalten der Elastomere, das sich klar von der Verformung elastisch-plastischer Materialien unterscheidet. Diese im Rahmen der vorliegenden Arbeit erzielten Erkenntnisse erlauben nun eine geeignete Wahl der Indentationsmethodik für Polymere und biologischen Materialien und bestätigen somit, dass eine adäquate mechanische Charakterisierung solcher Materialien mittels Nanoindentation möglich ist.