Abstract
The goal of this paper is to study the creep behaviour of multi-walls carbon nanotubes (1, 3, 5 wt.% MWCNTs) filled polypropylene (PP) through the instrumented indentation technique. Two types of the 3-step indentation test were considered to analyse viscoelastic behaviour through sharp indentation at room temperature. Under the loading conditions, the PP/MWCNTs samples present a time-independent plastic behaviour which must be considered not to under/overestimate the viscoelastic properties. The ratio of residual indentation depth to the maximum indentation depth at peak load calculated from the initial portion of the unloading curve was over 0.7 and demonstrated the plastic character of tested materials. The indentation hardness and modulus calculated from the slope at the unloading curve were changed due to the presence of plastic deformation from the loading phase, to the load intensity and hold time. The local distribution of carbon nanotubes lead to the variation in creep response and also, in the mechanical properties of material. Calculating the creep rate, a better creep response was offered by the higher concentration of carbon nanotubes during the indentation test at peak load of 1 N. A 3-step standard indentation test (Oliver and Pharr method) was considered to avoid the influence of the duration from the holding phase on the mechanical parameters. We found that a holding phase of 40 seconds was enough to have a negligible influence of creep on the indentation modulus for 5 wt.% of MWCNTs sample. Keywords: indentation, creep, carbon nanotubes, indentation hardness, indentation modulus