Proceedings of SerbiaTrib '13

4.3 NanoindentationFig. 6 illustrates typical load–displacementcurves of indentationsmade at a peak indentationload of 4.8mN on the pure epoxyresins and theMWCNT nanocomposites. No cracks were formedduring indentation as no steps orr discontinuitieswere found on the loading curves.The indentation depths at the peak load rangefrom around 0.5 to 0.6 μm. Lower indentationdepths areobserved for the t MWCNTnanocomposites as compared with the pure epoxysamples. The hardness and elastic moduluss isincreased as the concentration is increased. It iswell documented in the literature that the elasticmodulus has an increasing trend ass the percentageloading of MWCNTs is increasing [ 13].There is a significant difference in elasticmodulus as obtained from the nanoindentationtesting compared to theone of thetensile testss asshown in Table 1.Clearly the elastic modulus obtained from thenanoindentation testingtechniquee was 14-18%higher than the one obtained from the tensile tests.intrinsic errorss may leadd to results which aredifficult to explain in the case of softer, viscoelasticcsurfaces like the solidified epoxy resin r in thecurrent case.Table 1. Elastic moduli values as a derived fromexperiments.Material0% CNT0,5%CNT1%CNTE tensile(GPa)3,3 ± 0,124,5 ± 0,154,64 ± 0,18E nanoindentati ionE(GPa) modified (GPa)3,9 ± 0,122 3,375,22 ±,0,18 4,575,31 ±,0,22 4,75The process of nanoindentation measurementts isa relativelycomplicateprocedure, , especially forpolymeric materials asit has been reportedd invarious studies [11, 14]. The system compliancemay be tooo low to measure the material responseproperty for ‘soft’ materials like the epoxy resin.Also, the nanoindentation techniquee is based onn theelastic behaviour of the test material; thereby theviscoelasticc behaviour may cause an error in thecalculation of the elastic modulus. Moreover, thereare uncertainties in tip shape calibrationthatdirectly relate to the area functionn (A) whichh ismaterial dependent in most cases. The tip defect,which is always presentdue to technical limitationsin the fabrication of the indenter, may greatly affectthe assessment of the mechanical properties of f thetested surface at the first material layers. This isexacerbatedby the calibration procedure whichrequires a series of indentations upon the referencematerial at various depths and produces an intrinsicblunting effect on the calibrated tipat the deepestpenetrations, which donot correspond with thetip/machinebehaviour at theshallowestindentationsand so the final area functionextrapolatedmay not be exact. Therefore, the368Figure 5. SEM micrographs m of typical fracture surfacesof a) pure epoxy resin, b) MWCNT, c) MWCNT athigher magnificationAlso, for an epoxy resinn material, pile-ups and adistorted surface are usually observedaround thecrater of the nanoindentation. It is evident thereforeethat the typical calibration procedure whichhinvolves calibration on a reference material of awell-defined elastic modulus such as fused silica isnotsuitable for polymer materials. This isdocumented byy the observed differences in elasticmodulus between the nanoindentationresults andthe uniaxial tensile test measurements.13 th International Conference C onn Tribology – Serbiatrib’13