Proceedings of SerbiaTrib '13

Figure 6. Loading and unloadin versuss depth profiles ofpure epoxy resin and MWCNT nanocomposites.Figure 7.Hardness versus of pure epoxy resin anddMWCNT nanocomposites.Nevertheless the elastic modulus results asmeasured byboth techniques revealed similar trends.Subsequently as suggested by other researchers [10,11] a material depending calibrationprocedure hasbeen utilized for the current measurements. Usingequations (1-6) from the Oliver and Pharr [12, 15], , themodified area function related to indentation depthwas obtained using the elastic modulus from a tensiletest of the pure epoxy resin which was 3.3 GPa. Usingthe new calibrated area function the elastic moduli ofthe nanocomposites wascalculated. The result off theelastic modulus based onthe modifiedd area function ismarked as modified nanoindentation. Clearly, themodified elastic modulusvalues shown in Table 1 arein good agreement with the elastic modulus fromtheuniaxial tensile tests. For MWCNTs nanocompositesthe elastic modulus is increasing ass measured fromboth the tensile tests and from the nanoindentationexperimentswith the proposed calibration technique.Fig. 7 also shows the hardnessof thenanocompositesas a functionMWCNTconcentration. In agreement withthe previousoutcomes the hardness follows the elastic modulustrend and increases in the case of MWCNTs ass theconcentration increases from 0.5%wt to 1%wt. Itshould be noted that when measuredat small scales,13 th International Conference on Tribology – Serbiatrib’13the hardness is larger than at largerr scales. Anexample of this phenomenon is theso called‘indentation sizee effect’ which can be observed as anincrease in hardness with decreasing indentation depth[16]. This effectt complicatess the determination of thematerial hardness at low indentation depths, given thesmall remaining impression. However, , the resultssobtained in the current study lie within valuesobtained from other studiess investigating MWCNTepoxy nanocomposites [17, 18].The hardness of the carbon nanotubes themselvessis higher than the t one fromm the epoxy resin r therebythisexplains the t small increase noticed in thepresented results.5. ELASTIC MODULUS PREDICTIONSDespite the outstanding mechanical properties ofnanotubes, thee nanocomposites involving suchnanofillers exhibit a very limited improvement ofmechanical performances,if compared to conventionaladvanced composites.Thisopposingbehavior canbeexplained byy considering that the reinforcingcontribution of MWCNTs iss yieldednot only by theiramount within the material, , but also bythe state ofdispersion, orientation,shapee and number of contactswithin the matrix system. All these features play acriticalrole on the t final reinforcement enhancement,andthey shouldd be taken into account if i possible inorder todevelopp reliable models for prediction ofnanocomposite effective e properties.The classicall micromechanics approaches for shortfibre reinforcedd compositess were employed in thiswork in order todevelop predictive models for theMWCNT nanocomposites.AA popular and widelyadopted model to predict the stiffness of MWCNTsnanocompositess is the Halpin-Tsaimodel.TheHalpin–Tsai model m [19] iss widely used in manyliterature references.It is based on a force f balancemodel and empirical data and it isusedwidely formacroscopic composites.Forr the moduli of randomlyoriented MWCNTs in the epoxy matrix, theHalpin–Tsai model may predictthe elastic modulusof the nanocomposites, E N NC,which is governed bythe following set of equations: 3 1 8 1 5 12 8 1 1 (8) 369(7)