Measurement of creep compliance of solid polymers by nanoindentation

Methods to measure the local surface creep compliance of time-dependent materials are proposed and validated in the regime of linear viscoelasticity using nanoindentation. Two different bulk polymers, Polymethyl Methacrylate (PMMA) and Polycarbonate (PC), were employed in the validation study; though it is expected that the methods developed herein can be applied for very small amounts of materials and heterogeneous materials. Both Berkovich and spherical nanoindenters were used to indent into the material in nanoindentation tests. Two loading histories were used: (1) a ramp loading history, in which the indentation load and displacement were recorded; and (2) a step loading history, in which the indentation displacement was recorded as a function of time. Analysis of the linearly viscoelastic material response was performed to measure the creep compliance functions for the two materials under two different loading histories. The limit of linearly viscoelastic behavior for each of the two materials was determined through the observation of the indent impression recovery after complete unloading; it is postulated that linearity is achieved if indentation impression is fully recovered after unloading. Results from nanoindentation tests generally agree well with data from conventional tension and shear tests. It has thus validated the techniques of measuring linear creep compliance in the glassy state using nanoindentation with the Berkovich and spherical indenter tips.