Original in situ observations of creep during indentation and recovery of the residual imprint on amorphous polymer
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Creep during loading and recovery phases after load removal are studied using a homemade experimental device that allows us to record in situ the evolution of the true contact area and of the residual imprint versus the time. Indentation tests are performed using a spherical indenter with a tip radius R 5 400 lm onto amorphous polymeric surface poly(methylmethacrylate) (PMMA) at different contact durations (10–105 s) and controlled temperatures varying between 20 and 100 °C. Original experimental results are presented about the true evolution of the contact area during creep and recovery phases. An interesting experimental parameter, defined by the ratio a(t)/a0, (with a(t), evolution of the contact radius with creep or relaxation time, and a0, the initial value of the contact radius at the end of the loading phase or at the end of the creep phase) has been introduced to describe the evolution of imposed strain during indentation. As a function of the temperature and of the initial average strain imposed at the end of the loading phase, some nonlinear phenomena can be observed. Using two-dimensional axisymmetric finite element modeling, assuming only viscoelastic behavior, creep and recovering phases during indentation have been reproduced. The simulation results indicate that (i) the test is mainly controlled by the imposed strain and not by the contact pressure, and (ii) some plasticity could appear in the contact zone and as a function of the location and the size of the volume where the strain is maximal, the recovery is more or less limited.
I. INTRODUCTION
Depth-sensing indentation at micro- and nanoscale levels is the most advanced experimental method for the determination of mechanical properties such as hardness, elastic modulus, or fracture toughness of very small volumes of materials and in particular of thin films.1 The well-established procedure of Oliver and Pharr2,3 can be applied to load–displacement curves to deduce hardness and elastic modulus. This standard method is based on the assumption of purely elastic unloading phase for an elastic–plastic contact. Many materials, however, exhibit under certain conditions (loading and unloading rates, long holding time, temperature) a time-dependent behavior, and as a result, conventional methods may not provide an adequate determination of material properties. Indeed, the unloading behavior can be viscoelastic, and if the viscosity effects are not corrected, the calculated modulus can be seriously erroneous and dependent on the testing conditions. Many studies dealing with the investigation of creep under load during indentation tests can be reported for both sharp and spherical indenter geometries.4–14 These different a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.400 12
J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
http://journals.cambridge.org
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works are focused on the use of indentation to assess creep (stress exponent in the case of metallic surfaces9) or the viscoelasti
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