Carbon nanotube/polymer nanocomposites: a study on mechanical ...

Carbon nanotube/polymer nanocomposites: a study on mechanical integrity through nanoindentation Elias. P. Koumoulosa, Pravin Jagdaleb, Malamatenia A. Kokliotia, Ioannis A. Kartsonakisa, Mauro Giorcellib, Alberto Tagliaferrob, Constantinos A. Charitidisa,* a

National Technical University of Athens, School of Chemical Engineering 9 Heroon, Polytechniou st., Zografos, Athens, GR-157 80, Greece b Politecnico di Torino, Applied Sciences and Technology Department, Corso Duca degli Abruzzi 24, Torino, IT10129, Italy *Author’s e-mail: [email protected] Abstract Carbon nanotubes (CNTs) are under intense investigation in materials science owing to their potential for modifying the mechanical proprieties of the composites. In this work, nanomechanical and nanotribological properties of polymer composites, reinforced with multiwall carbon nanotubes (MWCNTs) and single wall carbon nanotubes (SWCNTs), have been studied using the nanoindentation and nanotribological technique. In particular three different epoxy resins reinforced using several percentage of two different types of MWCNTs have been studied (range 0-7 wt%). Another resin was reinforced using SWCNTs as filler (range 0-5wt%). Hardness and elastic modulus using nanoindenter instrument have been evaluated, while the coefficient of friction of the nanocomposites is obtained using nanoscratch. The results show an evident dependence with the percentage of CNTs. For all types of resins an optimum in nanomechanical properties is found at intermediate levels of CNTs filling. 1. Introduction Polymer composites, consisting of additives and polymer matrices, including thermoplastics, thermosets and elastomers, are considered as an important group of relatively inexpensive materials for many engineering applications. Epoxy resin (EpR) category is one of the most common thermoset polymer categories used in the formation of polymer nanotube composites; these polymers cure when mixed with a crosslinker, a catalyzing agent or hardener. Epoxy resins have been widely used in practical applications such as adhesives, construction materials, composites, laminates and coatings owing to their excellent mechanical properties, low cost, ease of processing, good adhesion to many substrates, and good chemical resistance [1]. The investigation of mechanical behavior of polymeric composites filled with CNTs is a topic of ongoing research, as CNTs are highly desirable candidates for improvement of polymers’ properties. Several properties have been examined, such as elasticity [2, 3], damage [4], buckling [5, 6], tribology [7, 8] and toughness [9] of CNTscomposites. Changes in the glass transition behavior of polymers as a result of adding carbon nanotubes are also reported [10, 11]. A previous study focused on the effect of CNTs reinforcement on the tribological performance of EpR composites; it was found that 1wt% CNTs in EpR yield optimal tribological performance [12]. Among thermosets, epoxy resins have been very often studied as a potential matrix for nanocomposites with CNTs. Small quantity of CNTs, often between 0.1 and 5.0% (w/w), are added to the polymeric matrix aiming to improve mechanical and thermal properties [13]. Li et al. [14] studied the nanomechanical properties of SWCNTs reinforced epoxy composites with varying weight percentage (0, 1, 3, and 5 wt%) via nanoindentation and nanoscratch technique; the addition of 5 wt% SWCNTs increased the elastic modulus by 75% and hardness by 30% when compared to the pure epoxy. An improvement by 80% in tensile modulus was obtained when thermoplastic poly(vinyl alcohol) (PVA) was mixed with only 1wt.% CNTs [15]. An increase of 28% in tensile Young’s modulus was observed in the rubbery system using 1 wt% functionalized nanotubes, compared to the unreinforced rubbery epoxy [16]. Raman spectroscopy has proved to be an invaluable tool for the characterization of various forms of carbon materials and specifically to determine the vibrational modes of carbon nanotubes and their composites [17]. The ﬁrst Raman experiment on carbon nanotubes was reported in 1993 by H. Hiura et al. [18]. This type of technique is based on the inelastic scattering of light and allows non-destructive measurements without any sample preparation. Raman spectroscopy of CNTs-based composite materials provides significant information about the structural, electronic and phonon properties of carbon nanotubes concerning their crystallite size, diameter, defects present, sp2–sp3 hybridization, metallic/semiconducting behavior [19] and dispersion [20] in nanocomposites. 1

Recently, the viscoelastic and creep behavior of CNTs based composites gained momentum toward using them for damping applications. Zhou et al. [21] utilized uniaxial tensile test to measure the loss factor of nanocomposites based on SWCNTs. Alternatively, Sneddon et al. [22] utilized direct viscoelastic shear mode of CNTs–epoxy composite thin films to characterize the complex compliance and material loss factor. The investigators reported an 1400% increase in loss factor (damping ratio) of the baseline epoxy by adding 50% CNTs by volume. Long-term viscoelastic/viscoplastic behavior of CNTs-based composites was investigated by Zhang et al. [23], using uniaxial tensile tests; it was demonstrated that SWCNTs additives in low weight fractions (0.1– 0.25%) are effective in limiting the load induced re-orientation of the epoxy chain, resulting in significant decrease of creep response. 2. Materials and Methods 2.1 Materials Two thermoset epoxy reinforced resins, namely a thermoplastic polymer and an elastomer reinforced resin, were used to produce composites by addition of several varying percentages of two different types of MWCNTs (MW-1, MW-2, range 0-7 wt%) and one type of SWCNTs (SW-1, range 0-5 wt%) and were purchased from Cheap Tubes Inc. Their characteristics are reported in Table 1. Table 1. Description of MWCNTs and SWCNTs

Properties Outer Diameter Inside Diameter Ash Purity Length Specific Surface Area Electrical Conductivity Bulk density True density

MW-1

MW-2

SW-1

30-50nm 5-10nm 95 wt% 10-20um 60 m2/g >100 S/cm 0.28 g/cm3 ~2.1 g/cm3

100 S/cm 0.27 g/cm3 ~2.1 g/cm3

1-2 nm 0.8-1.6nm < 1.5 wt% > 90 wt% 0.5-2 um 407 m2/g > 100 S/cm 0.14 g/cm3 ~ 2.1 g/cm3

1. Thermoset Henkel resin (Hysol EA-9360 Resin and Crosslinker) Henkel (Hysol EA-9360 Part A) is off white viscous paste, with a density of 1.18 g/cm3 (high peel strength, excellent static stress durability and room temperature cure). Henkel (Hysol EA-9360 Part B) is a blue paste, with a density of 1 g/ml. The mixing ratio of resin and crosslinker is 100:43 by weight. The chemical compositions of the Henkel resin and crosslinker are given below (Table 1). Table 2. The chemical composition of the Henkel (resin and crosslinker)

Henkel Resin Hysol (EA-9360) Chemical Composition (wt. %) Resin Crosslinker Epoxy resin 30-60 Piperazine derivative 30-60 Proprietary Proprietary Polyfunctional epoxy 10-30 Butadiene-acrylonitrile 10-30 resin Proprietary copolymer Synthetic rubber 10-30 Silica 5-10 Proprietary amorphous (fumed) Glass spheres 5-10 Benzyl alcohol 5-10 Proprietary Filler 1-5 Cycloaliphatic amine 5-10 Proprietary Proprietary Substituted silane 1-5 Phenol 1-5 Proprietary Diethylene glycol Di-(31-5 aminopropyl)ether Substituted Piperazine 1-5 Proprietary 2

Method of preparation of Henkel/MWCNTs: Resin, crosslinker and MW-2 were thoroughly mixed in specific ratio with mechanical stirring (20,000 RPM for 2 minutes); the composite mixture was then degassed in low vacuum. Before the onset of polymerisation, the polymer was set into the mould. Chemical reaction between resin and crosslinker resulted in exothermic reaction. Handling strength for these composite occured in 24 hours (when temperature >77°F/25°C) and complete curing was achieved after 5-7 days at 25 °C. For faster curing the molds were kept in the oven at 90 °C for 1 hour or 70 °C for 4 hours. Samples were prepared with four different MW-2 concetrations: 0.8, 3, 5 and 7 wt%. 2. Thermoset Epoxy resin (Epilox) Resin (T 19-36/700) is a commercially modified, colorless, low viscosity (650-750 mPa⋅s at 25 °C) epoxy resin with reduced crystallization tendency having density 1.14 g/cm3. Its main components are Bisphenol A (30 - 60%), Crystalline silica (quartz) (1 – 10%), Glycidyl ether (1 – 10%), Inert fillers (10 –60 %). Hardener (H 10-31) is a liquid, colourless, low viscosity (400-600 mPa.s) modified cycloaliphatic polyamine epoxide adduct having density 1 g/cm3. Method of preparation of Epilox/MWCNTs: Samples preparation was carried out as in the previous case using MW1 in concentrations ranging from 2 to 7 wt%. 3. Poly (vinyl butyral) (PVB) PVB is a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces,toughness and flexibility. It is prepared from polyvinyl alcohol by reaction with butyraldehyde. The IUPAC name of the polymer is poly[(2-propyl-1,3-dioxane-4,6-diyl) methylene] with chemical structure as mentioned below. It is in white powder form with specific gravity 1.0830 g/cm3. Method of preparation of PVB/MWCNTs: In preparation of MW-2–PVB (Butvar B-98, Sigma Aldrich) polymer composites, Ethanol (Carlo Erba) and 1-Butanol (Sigma-Aldrich) solvents were used with vigorous stirring and sonication; degassing was important for eliminating the entrapped solvent gas bubbles under vacuum. The composite was placed in oven at 70 °C for curing. Samples were prepared with four different MW-2 concentrations: 0.5, 1, 3 and 5%. 4. Polydimethylsiloxane (PDMS) The resin was the commercially available Silicon Elastomer Sylgard 184 (Dow corning); it is a silicon based clear colorless low viscous liquid having specific gravity 1.11 g/cm3. It is chemically stable and not forming hazardous polymerization. The chemical formula of PDMS is CH3[Si (CH3)2O]nSi(CH3)3. After polymerization and crosslinking, PDMS samples revealed an external hydrophobic surface. Method of preparation of PDMS/M-SWCNTs: The mixing ratio of base and curing agent was 1:1 by weight and curing time for the composite was 48 hours at 25 °C. Composites were prepared with ten different contents of MW2: 0.1, 0.2, 0.3, 0.4, 0.5, 1, 1.5, 2, 2.5 and 3% and seven concentrations of SWCNTs: 0.1, 0.2, 0.5, 1, 3, 4 and 5%. 2.2 Methods 2.1 FE-SEM and Energy Dispersive X-ray Spectroscopy Sample morphology was checked by a scanning field emission electron microscope (FE-SEM, Zeiss Supra 40), connected to an Energy Dispersive X-ray Spectroscopy (Oxford Inca Energy 450), used to determine the average elemental composition, in particular the presence of non carbon elements. 2.2 Raman Spectroscopy The Raman spectra were obtained using a Renishaw MicroRaman system with two different laser lines (wavelengths of 514 and 785 nm) equipped with a Charged-Coupled Device (CCD) as a detector. The microscope used a 50X objective lens to focus the laser beam on sample surface, and the size of the focused laser spot on the sample has a diameter of a few micrometers. All measurements were performed at room temperature and ambient conditions. 2.3 Nanoindentation and nanoscratch testing Nanoindentation testing was performed with Hysitron TriboLab® Nanomechanical Test Instrument, which allows the application of loads from 1 μN to 30 mN and records the displacement as a function of applied loads with a high load resolution (1 nN) and a high displacement resolution (0.04 nm). The TriboLab® employed in this study is equipped with a Scanning Probe Microscope (SPM), in which a sharp probe tip moves in a raster scan pattern across 3

a sample surface using a three-axis piezo positioner. In all nanoindentation tests a total of 10 indents are averaged to determine the mean hardness (H ) and elastic modulus (E ) values for statistical purposes, with a spacing of 50 μm, in a clean area environment with 45 % humidity and 23 °C ambient temperature. In order to operate under closed loop load or displacement control, feedback control option was used. All nanoindentation measurements have been performed with the standard three-sided pyramidal Berkovich probe, with an average radius of curvature of about 100 nm [24]. Nanoscratch testing is a versatile tool for analysis of the mechanical properties of thin films and bulk materials [25]. The scratch tests performed in this work included three main segments. Firstly, a pre-scratch scan under a very small load (1 μN) was carried out. Then, the indenter scrapes the sample under a certain force and a scratch was generated. The normal applied loads used in this work were 0-400 μN. The length of the scratches was 10 μm. Based on the half-space elastic deformation theory, H and E values can be extracted from the experimental data (load displacement curves) using the Oliver-Pharr (O&P) method [26, 27]. The derived expressions for calculating the elastic modulus from indentation experiments are based on Sneddon’s [28] elastic contact theory:

Er 

S  2 Ac

(1)

where S is the unloading stiffness (initial slope of the unloading load-displacement curve at the maximum displacement of penetration (or peak load)), Ac is the projected contact area between the tip and the substrate and



is a constant that depends on the geometry of the indenter (

 =1.167 for Berkovich tip [26, 27]). Conventional

nanoindentation hardness refers to the mean contact pressure; this hardness, which is the contact hardness actually dependent upon the geometry of the indenter (Eq. 2-4).

Hc  F / A

(Hc )

is

(2)

where,

A(hc )  24,5hc 2  a1hc  a1/2hc1/2  ...  a1/16hc1/16

(3)

and

hc  hm  

where

hm

Pm Sm

(4)

is the total penetration displacement of the indenter at peak load,

Pm

is the peak load at the indenter

displacement m , and  is an indenter geometry constant, equal to 0.75 for Berkovich indenter [26-29]. Prior to indentation, the area function of the indenter tip was calibrated in a fused silica, a standard material for this purpose [30, 31].

h

3. Results and discussion 3.1 Structural Characterization FE-SEM images are illustrated in Fig. 1. The impurities have been determined by EDX: Ni 1.87%, Fe 0.56%, 0.21 Cl and S 0.02% in case of MW-1 and Co 1.1%, Cl 1.0%, S 0.3 %, Al 0.2 % in case of MW-2. Representative Raman spectra of Epilox and Epilox with 7% MW-1 as filler are presented in Fig. 2. Epilox resin exhibits characteristic peaks associated with epoxy groups. The peak observed at 639 cm-1 is due to the epoxy ring deformation. The band at 1002 cm-1 is attributed to the stretch of C–O–C and the epoxy ring vibration is evident at 1112 cm-1 [32, 33]. The Raman spectrum of the nanocomposite is dominated by the bands corresponding to the 4

carbon nanotubes; the D band observed at ~1310 cm-1 is related to structural defects of graphite or nanotube, the G peak located at ~1585 cm-1 is assigned to the in-plane stretching vibration of sp2 carbon-carbon bonds. The peak present at ~1610 cm-1 is typical of defective graphite-like materials and the G’ band located at ~2600 cm-1 is approximately twice the wavelength of D band (overtone) [19].

(a)

(b)

(c)

(d)

(e) Figure 1. Scanning electron microscopy (FE-SEM) images of (a) MW-1, (b) MW-2, (c) Epilox–7% SW-1, (d) Henkel–5% MW2 and (e) PVB–5% MW-2

5

G

1311

2620

1585

Intensity (a.u.)

1610

Epilox 7% MW-1

2900

3076

2900

1301

1458

1609

1112 1229

639

MW2

821

G'

D

Intensity (a.u.)

Intensity (a.u.)

1003

Epilox 0% MW-1

MW1 0

500

1000

1500

2000

2500

3000

3500

500

4000

1000

1500

2000

2500

500

3000

1000

(a)

2000

2500

3000

-1

Raman shift (cm )

Raman shift (cm )

1500

Raman shift (cm )

-1

-1

(b)

(c)

Figure 2. Raman spectra of (a) M1-1 and MW-2 obtained by the laser at 514nm, (b) Epilox 0% MW-1 and (c) Epilox 5% MW-1 obtained by the laser at 785nm

3.2 Nanomechanical Characterization The relation (input functions) of displacement change versus time used in experiments is plotted in Figs. 3,4 below (schematic trapezoidal load-time P  P(t ) input function). In the case of creep experiment, loading and unloading times were identical (10 sec), while creep time was set to 100 sec (40 and 3 sec for conventional nanoindentation testing, respectively). 1000

1000

800

Applied load ()

Load (µN)

800

600

400

200

600

400

200

0

0 0

20

40

60

80

Time (s)

Figure 3. Load curves as a function of time

0

20

40

60

80

100

120

Time (sec)

Figure 4. Applied load curves as a function of creep time

The hardness and the elastic modulus as a function of displacement and the plots of the maximum applied force versus contact depth for all samples are presented in Figs. 5-9. Hardness and elastic modulus values of Epilox MW-1 samples were found to deviate at surface region (~0-200nm), probably due to roundness of the tip and Indentation Size Effect (ISE), tending to reach a constant value of 0.2 and 4 GPa, respectively. The same observation can be drawn for Henkel MW-2, PVB MW-2 and PDMS with MWCNTs and SWCNTs samples. The reasons for the wide range in hardness and modulus values obtained from these nanoindentation measurements are currently unknown, but it is likely due to a combination of factors, e.g. graded surface structure due to CNTs concentration, adhesive forces between the tip and the sample etc. Due to the very low contact area between the indenter and the sample, very high stresses can be developed. The high hydrostatic pressure exerted by the surrounding material allows plastic deformation at room temperature when conventional mechanical testing only leads to fracture. It is revealed that some materials exhibit ISE, which shows an increase in hardness with decreasing applied load [34]. Apparently, the existence of ISE may hamper the accurate measurement of hardness value, and is attributed to experimental artifact, a consequence of inadequate measurement capability or presence of oxides on the surface [35]. Other explanations include indenter-specimen friction [36], and changing dislocation density for shallow indents due to the presence, for instance, of geometrically necessary dislocations [37]. The Berkovich indenter generates dislocations organised in a quite complex way during a nanoindentation test, even for very low deformations [38], making difficult the formulation for the stress field generated, even during an elastic deformation, as well as its modelling. Most of the dislocations stay generally confined around the residual imprint in a dense structure [39-41] with many dislocation interactions [42]. 6

Enhanced nanomechanical properties for Henkel MW-2 (Figs. 5 (a,b)) are revealed for the case of 3% MW-2, while samples with 7% MW-2 exhibit low H (for depth below 250nm) and E values. No significant differences are observed between 0 and 0.8% MW-2 (same for 5 and 7% MW-2). Concerning Epilox MW-1 samples (Figs. 6 (a,b)), enhanced nanomechanical properties are revealed for the case of 2% MW-1, while sample with 7% MW-1 exhibit low H (for depth below 250nm) and E values. Increased nanomechanical properties (H, E) are observed in the case of 1% MW-2 addition in the PVB (Figs. 7 (a,b)), while specimens with 5% MW-2 exhibit decreasing H and E values (for depth below 100nm). As for PDMS with MWCNTs (Figs. 8 (a,b)), the main enhancement of hardness is evidenced at concentrations of 0.2-0.4% MW-2 and elastic modulus values are increased for samples with 0.2, 0.3 and 0.5% MW-2. The specimen filled with 0.5% SW-1 exhibited increased hardness; the lowest values of elastic modulus are observed for pure PDMS, as shown in Figs. 9 (a,b). The elastic modulus values for the composites with SW-1 concentrations varying from 0.1-5% did not reveal significant deviation. In Figs. 5-9, the intermediate levels of CNTs filling with optimum in nanomechanical properties are noted with dashed circle, where applicable. In Figs. 5-9 (c), the maximum applied load versus contact depth for all composites is presented. The empirical equation is used for describing the ISE in the Meyer’s law [43, 44], which uses a correlation technique between the applied indentation test load and the resultant indentation size using a simple power law, P max = Chcn, where C and n are constants derived directly from curve fitting of the experimental data. Compared to the definition of the apparent hardness, no ISE would be observed for n = 2. The calculated constants are presented in Table 3.

1200

Henkel 0% MW-2 Henkel 0.8% MW-2 Henkel 3% MW-2 Henkel 5% MW-2 Henkel 7% MW-2


1000

800

600

400

200

0 0

100

200

300

400

500

Contact depth (nm)

(a)

(b)

(c)

Figure 5. (a) Hardness, (b) elastic modulus as a function of displacement and (c) plots of P max versus contact depth according to the Meyer’s law for Henkel MW-2 samples

14

0,8

Epilox 0% MW-1 Epilox 2% MW-1 Epilox 7& MW-1

12 11

Elastic modulus (GPa)

0,6

Hardness (GPa)

1200


13

0,5 0,4 0,3


1000

10

Applied load (N)

0,7

9 8 7 6 5

800

600

400

4

0,2

200

3 2

0,1 0

50

100

150

200

250

300

350

Displacement (nm)

(a)

400

450

500

550

0

50

100

150

200

250

300

350

Displacement (nm)

400

450

500

550

0 0

100

200

300

400

500

Contact depth (nm)

(b)

(c)

Figure 6. (a) Hardness, (b) elastic modulus as a function of displacement and (c) plots of P max versus contact depth according to the Meyer’s law for Epilox MW-1 samples

7

0,6

500

PVB 0% MW-2 PVB 1% MW-2 PVB 5% MW-2


6,0

PVB 0% MW-2 PVB 1% MW-2 PVB 5% MW-2 400

0,4

0,3

0,2


5,5

Elastic modulus (GPa)

Hardness (GPa)

0,5

5,0

4,5

4,0

300

200

100

3,5 0,1 0

100

200

300

400

3,0

500

0

0

100

200

Displacement (nm)

300

400

500

0

50

100

Displacement (nm)

(a)

150

200

250

300

350

400

Displacement (nm)

(b)

(c)

Figure 7. (a) Hardness, (b) elastic modulus as a function of displacement and (c) plots of P max versus contact depth according to the Meyer’s law for PVB MW-2 samples 50

120

Elastic modulus (MPa)

Hardness (MPa)

30

20 PDMS 0% MW-2 PDMS 0.1% MW-2 PDMS 0.2% MW-2 PDMS 0.3% MW-2 PDMS 0.4% MW-2 PDMS 0.5% MW-2 PDMS 1% MW-2 PDMS 1.5% MW-2 PDMS 2% MW-2 PDMS 2.5% MW-2 PDMS 3% MW-2

100

20

10

80

PDMS 0% MW-2 PDMS 0.1% MW-2 PDMS 0.2% MW-2 PDMS 0.3% MW-2 PDMS 0.4% MW-2 PDMS 0.5% MW-2 PDMS 1% MW-2 PDMS 1.5% MW-2 PDMS 2% MW-2 PDMS 2.5% MW-2 PDMS 3% MW-2

15

Applied load (N)


40

60

40

10

5 20

0 0

100

200

300

400

0

500

0 0

Displacement (nm)

100

200

300

400

500

0

100

Displacement (nm)

(a)

200

300

400

500

Contact depth (nm)

(b)

(c)

Figure 8. (a) Hardness, (b) elastic modulus as a function of displacement and (c) plots of P max versus contact depth according to the Meyer’s law for PDMS MW-2 samples 10

12

8

50

Elastic modulus (MPa)

Hardness (MPa)

10

PDMS 0% SW-1 PDMS 0.1% SW-1 PDMS 0.2% SW-1 PDMS 0.5% SW-1 PDMS 1% SW-1 PDMS 3% SW-1 PDMS 4% SW-1 PDMS 5% SW-1

60

6

4

40


8

Applied load (N)


30

20

6

4

2

10

0

100

200

300

400

0

0 100

2 500

200

300

400

0

500

Displacement (nm)

(a)

100

200

300

400

500

Contact depth (nm)

Displacement (nm)

(b)

(c)

Figure 9. (a) Hardness, (b) elastic modulus as a function of displacement and (c) plots of P max versus contact depth according to the Meyer’s law for PDMS SW-1 samples

Resin

Content

MW1

Epilox

0 2 7

2.13 1.46 1.84

Henkel

Table 3. Meyer’s law n constant for all examined samples

0 0.8 3 5 7

MW2

SW1

3.27 2.26 2.17 1.58 2.02

8

PVB PDMS

0 1 5 0 0.1 0.2 0.3 0.4 0.5 1 1.5 2 2.5 3 4 5

1.68 1.29 1.70 1.16 1.09 0.93 0.85 0.64 1.03 1.16 1.21 1.23 1.12 0.97

1.16 1.25 1.13

0.94 1.02

1.26 1.27 0.99

3.3 Wear analysis-deformation mechanism An important feature of indentation experiments is that the material around the contact area tends to deform upwards (pile-up) or downwards (sink-in) with respect to the indented surface plane. The occurrence of such pile-up and sink-in patterns is usually interpreted in terms of the strain-hardening behavior of the indented material [45-48]. According to these studies the surface around indents tends to pile up against the indenter in cases where the indented sample is heavily pre-strained with only little reserves for further work-hardening or has generally a low strain-hardening potential. On the other hand, when the sample is fully annealed and has a high strain-hardening potential, the surface around indents tends to sink in [45-48]. The reason for this relationship between strain hardening behavior and displacement patterns is plausible: well-annealed soft metals which exhibit a high strainhardening rate tend to show far off field plasticity yielding a large lateral smear out of the plastic out-of-plane displacement field. Rapid strain-hardening in the immediate vicinity of the indenter tip will cause plastic deformation to occur gradually further away from the contact region, causing the material to be displaced far away from the indentation entailing sink-in patterns. In contrast, strain-hardened materials as well as alloys and metallic glasses which exhibit a low (residual) strain-hardening rate will reveal a stronger localization of the plastic zone, creating a local pile-up instead of a sink-in displacement pattern around the indent. Good knowledge of the deformation zone around an indent is of considerable importance for nanoindentation testing because the shape of the out-of-plane displacement zone determines the actual contact area between the indenter and the specimen. Sinkin patterns reduce and pile-up patterns increase the contact area. These differences in the surface deformation mode affect the quantitative analysis of the hardness measurements. Not taking the piling-up or sinking-in into account in micro- and nanoindentation hardness tests can result in significant errors when extracting hardness values from the experimental data [45, 46]. The presence of creep during nanoindentation has an effect on pile-up, which results in incorrect measurement of the material properties. Fischer-Cripps observed this behaviour, in case where the measured elastic modulus was much less than expected [49]. Rar et al. observed that the same material when allowed to creep for a long duration produced a higher value of pile-up/sink-in indicating a switch from an initial elastic sink-in to a plastic pile-up [50]. In Figs. 10-14 (a,b), the normalised pile-up/sink-in height hc/hm and the normalized hardness H/E are plotted vs. displacement. In Figs. 10-14 (c), the normalised pile-up/sink-in height hc/hm vs. the normalized hardness is plotted, following the almost linear observed in literature [51, 52]. Higher stresses are expected in high H/E, hard materials, and high stress concentrations develop towards the indenter tip, whereas in case of low H/E, soft materials, the stresses are lower and are distributed more evenly across the cross-section of the material [50]. Rate sensitive materials experience less pile-up compared to rate insensitive materials due strain hardening. Cheng and Cheng reported a 22% pile-up for a work hardening exponent [53]. This is consistent with the fact that when hc/hm approaches 1 for small H/E, deformation is intimately dominated by pile-up [54, 55]. On the other hand, when hc/hm approaches 0 for large H/E, it corresponds to purely elastic deformation and is apparently dominated by sink-in in a manner prescribed by Hertzian contact mechanics [56]. The ratio of hardness/elastic modulus (H/E) is of significant interest in tribology. Higher stresses are expected in high H/E, hard materials, and high stress concentrations develop towards the indenter tip, whereas in the case of low H/E, soft materials, the stresses are lower and are distributed more evenly across the cross-section of the material [57, 58]. The high ratio of H/E is indicative of the good wear resistance in a disparate range of materials [58, 59]: ceramic, metallic and polymeric (e.g. c-BN, tool steel and nylon, respectively), which are equally effective 9

in resisting attrition for their particular intended application. In Figs. 10-14, the change of H/E slope reveals the strengthening of nanocomposites with increasing CNTs concentration (more resistant to wear). In Fig. 11 (a) at low displacement ranges, composites with 0 and 7% exhibit a rather sink-in deformation, which is further switched to pile-up (at~100nm of displacement). Henkel composite with 7% MW-2 revealed higher resistance to wear (H/E) than that of other filler concentrations; the same behavior is presented in the case of Epilox 7% MW-1. Also, it is evident that the PDMS filled with 0.4% MW-2 and 0.1% SW-1 exhibit enhanced wear resistance, as illustrated in Figs. 13-14 (b). The deformation mechanism (hc/hm) for almost all nanocomposites exhibit a linear trend, as is presented in Figs. 10-14 (c). However, the deformation mechanism of PVB during nanoindentation changes significantly with the addition of MW-2, with respect to the H/E ratio. In Fig. 15, the H/E ratio versus normalised pile-up/sink-in height hc/hm of all examined composites is presented. 1,0

Pile-up

0,8 0,7

hc/hm

0,6 0,5 0,4

Sink-in

0,3

Henkel 0% CNTs Henkel 0.8% CNTs Henkel 3% CNTs Henkel 5% CNTs Henkel 7% CNTs

0,2 0,1

1,0 Henkel 0% CNTs Henkel 0.8% CNTs Henkel 3% CNTs Henkel 5% CNTs Henkel 7% CNTs

0,09 0,08

0,9 0,8

0,07 0,7

0,06

hc/hm

Increased resistance to wear H/E

0,10

0,9

0,05 0,04

0,5

0,03

0,0 100

200

300

400

Henkel 0% CNTs Henkel 0.8% CNTs Henkel 3% CNTs Henkel 5% CNTs Henkel 7% CNTs

0,4 0,02 0,3

0,01 0

0,6

500

0,00 0

Displacement (nm)

100

200

300

400

0,2 0,00

500

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

H/E

Displacement (nm)

(a) (b) (c) Figure 10. (a) Normalized pile-up/sink-in height hc/hm versus displacement , (b) wear ratio (H/E) versus displacement and (c) H/E ratio versus normalised pile-up/sink-in height hc/hm of Henkel MW-2 samples

0,8 0,7

hc/hm

0,6 0,5 0,4

0,25

1,0

Epilox 0% MW-1 Epilox 2% MW-1 Epilox 7% MW-1

0,20

0,9 0,8 0,7

0,15

0,6

hc/hm

Pile-up

0,9

H/E

Increased resistance to wear

1,0

0,10

Sink-in

0,3

0,5 0,4 0,3


0,2 0,1

0,05

0

100

200

300

400

0,2


0,1

0,0

0,00

500

0

Displacement (nm)

100

200

300

400

0,0

500

0,05

Displacement (nm)

0,10

0,15

0,20

0,25

0,30

0,35

0,40

H/E

(a) (b) (c) Figure 11. (a) Normalized pile-up/sink-in height hc/hm versus displacement, (b) wear ratio (H/E) versus displacement and (c) H/E ratio versus normalised pile-up/sink-in height hc/hm of Epilox MW-1 samples

Pile-up

0,85 0,80

hc/hm

0,75 0,70

Sink-in

0,65

0,10

0,90

0,09

0,86

0,07 0,06

0,84

0,05


0,55 0,50 0

100

200

300

Displacement (nm)

400

500

0,82

0,04 0,80

0,03

0,60


0,88

0,08

hc/hm

Increased resistance to wear H/E

0,90

0,02


0,01

0,78 0,76

0,00 0

100

200

300

Displacement (nm)

400

500

0,04

0,05

0,06

0,07

0,08

0,09

0,10

H/E

(a) (b) (c) Figure 12. (a) Normalized pile-up/sink-in height hc/hm versus displacement, (b) wear ratio (H/E) versus displacement and (c) H/E ratio versus normalised pile-up/sink-in height hc/hm of PVB MW-2 samples

10

1,0

1,0

0,7

hc/hm

0,6

0,8 0,7 0,6

hc/hm

Pile-up

0,8

0,5 0,4

Sink-in


0,9


0,9

0,5 0,4 0,3

0,3

0,2

0,2

0,1 0,0

0,1

0,0

0,2

0,4

0,0 0

100

200

300

400

0,6

0,8

1,0

H/E

500

Displacement (nm)

(a)

(b)

(c)

Pile-up

Figure 13. (a) Normalized pile-up/sink-in height hc/hm versus displacement, (b) wear ratio (H/E) versus displacement and (c) H/E ratio versus normalised pile-up/sink-in height hc/hm of PDMS MW-2 samples 1.0

1,0

0.9

0,9

0.8

0,8

0.7

0,7 0,6

hc/hm

hc/hm

0.6 0.5 0.4

0,5 0,4 0,3

0.3

Sink-in

PDMS 0% SW1-1 PDMS 0.1% SW-1 PDMS 0.2% SW-1 PDMS0.5% SW-1 PDMS 1% SW-1 PDMS 3% SW-1 PDMS 4% SW-1 PDMS 5% SW-1

PDMS 0% SW1-1 PDMS 0.1% SW-1 PDMS 0.2% SW-1 PDMS0.5% SW-1 PDMS 1% SW-1 PDMS 3% SW-1 PDMS 4% SW-1 PDMS 5% SW-1

0.2 0.1 0

100

200

300

400

0,2 0,1 0,0 0,10

500

0,15

0,20

0,25

0,30

0,35

0,40

H/E

Displacement (nm)

(a)

(b)

(c)

Figure 14. (a) Normalized pile-up/sink-in height hc/hm versus displacement, (b) wear ratio (H/E) versus displacement and (c) H/E ratio versus normalised pile-up/sink-in height hc/hm of PDMS SW-1 samples

1.0 Epilox/Henkel composites

0.9 0.8 0.7 0.6

hc/hm

Henkel 0% MW-2 Henkel 0.8 % MW-2 Henkel 3% MW-2 Henkel 5% MW-2 Henkel 7% MW-2 Epilox 0% MW-1 Epilox 2% MW-1 Epilox 7% MW-1 PVB 0% MW-2 PVB 1% MW-2 PVB 5% MW-2 PDMS 0% MW-2 PDMS 0.1% MW-2 PDMS 0.2% MW-2 PDMS 0.3% MW-2 PDMS 0.4% MW-2 PDMS 0.5% MW-2 PDMS 1% MW-2 PDMS 1.5% MW-2 PDMS 2% MW-2 PDMS 2.5% MW-2 PDMS 3% MW-2 PDMS 0.1% SW-1 PDMS 0.2% SW-1 PDMS 0.5% SW-1 PDMS 1% SW-1 PDMS 2% SW-1 PDMS 4% SW-1 PDMS 5% SW-1

0.5 PDMS composites

0.4 0.3 0.2 0.1 0.0 0.01

0.1

1

H/E

Figure 15. H/E ratio versus normalised pile-up/sink-in height hc/hm of all composites

3.4 Analysis beyond nanomechanical properties 3.4.1 Nanotribological properties

11

Normal load (μN) 200

0

400

1,2


Coefficient of friction (μ)

1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 -6

-4

-2

0

2

4

6

Lateral Displacement (μm)

Figure 16. Coefficients of friction for Epilox nanocomposites

In Fig. 16, coefficients of friction for Epilox composites are presented, as representative for all samples. In the initial stages the CoF is higher; afterwards, it reaches a plateau that represents the effective value. For Epilox MW-1 nanocomposites, the addition of 2% MW-1 increases the CoF compared to plain polymer matrix, while 7% MW-1 sample exhibits lower CoF, which could imply sliding of nanotubes inside the polymer matrix. The behaviour in case of Henkel MW-2 nanocomposites is almost identical; the concentration of 5% MW-2 is a threshold, beyond which the nanocomposite exhibits lower coefficient of friction. PVB MW-2 nanocomposite exhibits decreased CoF for 5% MW-2, compared to the addition of 1% MW-2 in the polymer matrix. 3.4.2 Creep Investigation During nanoindentation, the applied load can be controlled at a constant value, whereas the penetration of the indenter tip into the sample surface is continuously recorded. This is often called constant-load indentation creep test, and it has been widely used to study the time-dependent properties of crystalline materials. The nanoindentation creep consists of two stages, transient (primary creep) and steady state (secondary creep) [60]. The stress exponent, n, of the steady-state creep can be derived from the constant load indentation tests [61-65]. In a nanoindentation creep experiment, the tip is pushed into the surface at a constant rate of indentation until a prefixed load or penetration displacement is plausible, then the load is held constant while the indenter continues to creep into the material. With the indenter tip held fixed at that load or displacement, the material beneath the indenter tip continues to deform in time and finally the indenter tip is retracted from the material. Creep within a specimen occurs during the hold time of the loading phase of nanoindentation testing and manifests itself as a change of indentation displacement with the load, kept constant. It is postulated that the stress fields in the material underneath the indenter develop a chemical potential gradient that lead to a thermally activated diffusional flux of atoms moving from below the indenter to the surface and along the interface between the indenter and the specimen, even under an elastic contact [66-68]. In Fig. 17, the change in depth of indenter as a function of hold time (applied loading rate 10μN/sec) for Henkel MW-2, Epilox MW-1 and PVB MW-2 composites is illustrated. 20

15

8010

more creep deformation

Henkel 0% MW-2 Henkel 0.8% MW-2 Henkel 3% MW-2 Henkel 5% MW-2 Henkel 7% MW-2 Epilox 0% MW-1 Epilox 2% MW-1 Epilox 7% MW-1 PVB 1% MW-2 PVB 5% MW-2

Creep displacement (nm)

5

60 0 0

5

10

15

20

40

20

0 0

20

40

60

80

100

less creep deformation

end of primary creep

Creep time (sec)

Figure 17. Change in depth of indenter as a function of hold time (applied loading rate 10μN/sec) for Henkel MW-2, Epilox MW-1 and PVB MW-2 composites

12

3.4.3 Adhesion energy Mechanical properties of surfaces are significant for the comprehension of behavior of adhesive and sliding contacts. Consideration of the adhesion energy at the tip/ sample interface is a requisite for determining accurate elastic modulus values of PDMS samples and other soft, elastomeric materials from nanoindentation experiments, as the soft material samples are expected to have significant adhesive forces [69]. A characteristic nanoindentation adhesion test is comprised of pressing the tip into the patterned sample, followed by unloading it at a constant rate and finally obtaining a distinctive (and often abrupt) pull-off force representing the adhering surfaces. Adhesion is observed in a load-displacement curve as a region of negative load during unloading [70]. Figs. 18 (a,b) shows two load-unload curves versus displacement of pure PDMS and PDMS incorporated with 5% SW-1 obtained during the nanoindentation, as the tip approached and retracted from the samples. It is evident that the introduction of CNTs into the polymeric matrix decreases the adhesion force.

7

PDMS

6

12

5 4

8

3

Load (µN)

Load (µN)

PDMS 5% SW-1

10

2 1

6 4

0

2 -1

Padh=-2.1N

0

-2

Padh=-1.1N

-3 -1600 -1400 -1200 -1000 -800 -600 -400 -200

Displacement (nm)

(a)

0

200

400

600

-2 -400

-200

0

200

400

600

Displacement (nm)

(b)

Figure 18. Load versus displacement as the tip approached and retracted from (a) PDMS and (b) PDMS 5% SW-1

4. Conclusions In this work, the mechanical integrity of various polymeric matrices reinforced with different concentrations of carbon nanotubes is analysed and discussed. The main findings are summarized below: Raman spectroscopy has proved to be a convenient technique for the characterization of carbon nanotubes, as the spectra changes related to the incorporation of different types of CNTs in the polymeric matrices were studied and revealed useful information about the structural properties of CNTs. As for instrumented nanoindentation, it is a valuable tool to evaluate both the mechanical and time-dependent properties for epoxies and their nanocomposites filled with MWCNTs and SWCNTs. The addition of CNTs resulted in the improvement of nanomechanical properties for Henkel 3% MW-2, Epilox 2% MW-1, PVB 1% MW-2, PDMS 0.2-0.5% MW-2 composites, as PDMS 0.5% SW-1 exhibited increased hardness. Higher resistance to wear (H/E) was observed for Henkel 7% MW-2, Epilox 7% MW-1, PDMS 0.4% MW-2 and PDMS 0.1% SW-1. Nanocomposite samples retained better mechanical properties compared to the neat polymer matrix; however, for each polymer-CNTs system a threshold–concentration was identified, beyond which an obvious deterioration of nanomechanical properties occurs. The introduction of 2% MW-1 into Epilox matrix increased the coefficient of friction in comparison to the pure polymer. The concentration of 5% MW-2 in Henkel and PVB is a threshold, beyond which the nanocomposites exhibited lower coefficient of friction. A decrease in adhesive energy in the case of PDMS filled with 5% SW-1 was revealed, indicating that the adhesive force plays a key role at the nanometer scale in indentation tests. Acknowledgments The support from the EU FP7 Project “Enhancing structural efficiency through novel dissimilar material joining techniques” (SAFEJOINT) under Grant Agreement no. 310498 is greatly acknowledged by EPK, MAK, IAK and CAC, while PJ, MG and AT acknowledge Salvatore Guastella for his support in performing FE-SEM analysis. References 13

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4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

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