Journal of Colloid and Interface Science 360 (2011) 777–784

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Journal of Colloid and Interface Science

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Nanotribology of polyvinylidene difluoride (PVDF) in the presence of electric field

Hyungoo Lee, Bharat Bhushan ⇑Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics, The Ohio State University, Columbus, OH 43210, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 February 2011Accepted 22 April 2011Available online 29 April 2011

Keywords:Polyvinylidene fluorideAtomic force microscopyNanotribologyLubricantAdhesionFrictionWearMEMS/NEMS

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.04.087

⇑ Corresponding author.E-mail address: bhushan.2@osu.edu (B. Bhushan).

Polyvinylidene difluoride (PVDF) is one of the most widely used piezoelectric materials in micro-electro-mechanical systems (MEMS) and nano-electromechanical systems (NEMS) due to its excellent properties.Its applications range from biological to electric devices, such as an artificial hip joint, a microgripper, anda force sensor. It is critical to understand friction, adhesion, and wear mechanisms of this material. In thisstudy, effect of piezoelectricity and lubricant with electric field on tribological properties was investi-gated, using poled and unpoled PVDF. To understand the tribological properties at nano- and macro-scales, scale effect was also studied using an AFM and a tribometer. Relevant mechanisms are discussed.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Piezoelectric materials play an important role in micro-electro-mechanical systems (MEMS) and nano-electromechanical systems(NEMS) [4]. One of the most widely used piezoelectric materials ispolyvinylidene difluoride (PVDF) due to its strong piezoelectricity[18]. Piezoelectricity is defined to the capability to generate elec-tric potentials in response to mechanical pressure applied to thematerials and vice versa.

PVDF is a semicrystalline polymer. A schematic of PVDF struc-ture is shown in Fig. 1. The unit monomer of PVDF is composedof CH2–CF2. In the unit monomer, the fluoride side is polar positive,and the hydrogen side is polar negative, producing a dipole. The se-quence of the monomers forms a linear chain. The portion of thepolymer chain stacked in an orderly manner is called a crystallineregion or lamella; the area with no order in the chain stacks iscalled the amorphous region. The PVDF consists of 50% lamellarcrystals which are embedded in an amorphous region. There arefour types of PVDF phases: a (form II), b (form I), c (form III),and d (form IV). The phases are usually identified by infrared trans-mission or X-ray scattering techniques. To improve the mechanicalor electrical properties of the a-phase PVDF, the b-phase PVDF isneeded in order to align the dipoles, called poling. The b-phase isgenerated by mechanically rolling uniaxially (d11 direction) orbiaxially stretching [28], or exposing in a high electric field [12].The b-phase PVDF is hence called poled, and the a-phase PVDF is

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called unpoled. Most of the chains in the poled PVDF are oriented,pointing in the same direction (TTT conformation). However, in theunpoled PVDF, the dipoles of adjacent units point in differentdirections (TGTG0 conformation). Therefore, the net dipole momentof the unpoled PVDF is lower than that of the poled PVDF; piezo-electric coefficient of the poled PVDF is 23 pC/N [28].

Polyvinylidene difluoride (PVDF) has been widely used as a pie-zoelectric material in applications due to its favorable chemicaland mechanical properties. PVDF has excellent flexibility, biocom-patibility, thermal stability, lightweight, conformability, low cost,and low acoustic and mechanical impedance, making it a favorablematerial for various applications, such as a transducer [15], a loud-speaker [23], a supercapacitor [16], an ultrasonic motor [30], andan electric power generator [21]. It is also possible for PVDF tobe used as an artificial hip joint due to its biocompatibility [13].In artificial hip joints, lubricant plays a critical role against frictionand wear to provide joint durability [17].

Another application for PVDF is a microgripper and associatedsensor [14,19,20]. Fig. 2 shows a schematic of a microgripper.The microgripper can grip a micro/nanoscale object and relocateit to wherever it is needed. Electric potential is applied to themicrogripper hands via the base of the microgripper. Importantfunctioning criteria of a microgripper are friction and adhesion.In terms of gripping and releasing an object, friction and adhesionon the microgrippers should be high to grip and hold an objectstrongly. However, to release and locate the gripped object easily,lower friction and adhesion are expected. It is known that a micro-gripper can manipulate an object by friction, not by texture [10].PVDF can also be used to sense the gripping force of the microgrip-

Fig. 1. Schematic of PVDF structures; a monomer unit and polymer structures as adipole, a chain, a crystalline called lamella and an amorphous region. An unpoled(a-phase) PVDF film is mechanically stretched along the d31 direction to obtain apoled (b-phase) PVDF film of which piezoelectric coefficient is higher than that ofthe unpoled one.

Fig. 2. Schematic of a PVDF microgripper to which electric field is applied tomanipulate a nanowire.

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per. As a transducer, PVDF generates an electric potential whenmechanical force is applied to it. With the generated electric

potential, the gripping force could be measured. In this way, PVDFon the microgripper could work as a finger-skin-like sensor.

Depending on the situation, adequate friction and adhesion arerequired for a microgripper to function properly. In addition, sinceadhesion has hindered the development of MEMS or NEMS devices[4], the study of adhesion with the electric field is needed. How-ever, the effect of piezoelectricity with electric field on frictionand adhesion has yet to be understood. The effect of an electricfield on molecular structure and dipoles in PVDF needs to beexplained. One needs to understand how the change of the mole-cule structure in PVDF affects adhesion and friction. In this study,the effectiveness of a lubricant on adhesion and friction with elec-tric field is investigated. With understanding of these issues, it ispossible to control adhesion and friction on PVDF. In order todevelop a fundamental understanding of friction and adhesionmechanism, it is desirable to perform experiments at the nanoscaleto simulate a single asperity contact. With atomic force microscopy(AFM), it is possible to study friction and adhesion mechanisms in ananoscale contact [5].

2. Experimental

2.1. Materials and sample preparation

The PVDF samples used for this study are shown in Table 1. Thethickness of the PVDF films is 52 lm. The size of all samples was3 cm in length and 1 cm in width. Using a sputtering machine(E3612A, Hewelett Packard), the gold (Au) thin (100 nm) filmswere deposited on the top and the bottom of the poled and unp-oled PVDF films as electrodes. Each lamella in PVDF films is onthe order of 10–20 nm in thickness, and its molecular length ison the order of 100 times of the lamellae thickness [9]. Therefore,the experimental results of Au coated PVDF reflect the tribologicalproperties of PVDF films. The Au coated PVDF films are shown inFig. 3. The positive and negative electrode leads from a DC powersupply (K550X, EMITECH, Kent, UK) are connected to the top andbottom gold films.

For the nanolubrication studies, a lubricant with a hydroxylgroup on one end and a cyclotriphosphazene group on the otherend (Moresco A20H) was applied to the gold coated poled PVDFfilms using a dip-coating technique. The method and the apparatus

Table 152-lm thick PVDF film samples used in this study.

Sample Phase Lubrication RMS roughness (nm)

Unpoled PVDF a (TGTG0) unlubricated 161 (±5)Poled PVDF b (TTT) unlubricated 40 (±1)Lubricated poled PVDF b (TTT) Lubricated (A20H) 38 (±1)

All samples are coated with Au (gold) of 100 nm thickness. Average film thicknessof lubricant deposited on the lubricated PVDF film is 65 nm. ± values represent ±onestandard deviation limits.

Fig. 3. The Au (gold) coated PVDF films used in experiments. The positive andnegative electrode leads from an electrical source are connected to the left and theright side of the Au films on a PVDF film, respectively.

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used have been described elsewhere [25,29]. Briefly, the dip-coaterallows withdrawal of the samples from the lubricant reservoir at aconstant velocity. The withdrawal speed was 6 mm/s. The sampleswere submerged into a beaker containing a dilute solution of lubri-cant with a concentration of 0.4% lubricant in HFE 7100 (3M, St.Paul, MN), which consists of isomers of methoxynonafluorobutane(C4F9OCH3). After 10 min, the samples were withdrawn from thesolution. The thickness of the lubricant on the samples was mea-sured by a thickness mapping technique using AFM [7,8,5]. It isknown that the lubricant film thickness increases with the with-drawal speed as there is less time available for lubricant to drainat higher withdrawal speeds.

2.2. Nanoscale surface roughness, adhesion, friction and lubricantthickness measurements

A commercial AFM (Nanoscope IIIa, Veeco, Santa Barbara, CA,USA) was used for this study [4,5]. Silicon nitride tips of nominal50 nm radius attached to the end of a triangular cantilever beam(DNP, spring constant of 0.12 N/m) were used for surface rough-ness, adhesion, friction, and lubricant film thickness measure-ments. The measurements were conducted against electricpotential (�15 V to +15 V) with respect to the principal direction(d31) of the PVDF films, as shown in Fig. 1. Electric field (E) appliedto the samples in d33 direction was calculated by the followingequation

Ed ¼ V ð1Þ

where E is the applied electric field (unit is V/m), d is the distancebetween the electrode gold films on the samples (=PVDF film thick-ness, 52 lm), and V is the applied electric potential.

Adhesive force and lubricant film thickness on the samples wascalculated using the force distance curve technique [6–8,11,22,26].The experiments were performed at room temperature (21 �C) and45–55% relative humidity.

The force distance curves were collected at the same maximumcantilever deflection of 70 nm (relative trigger mode). In order toobtain a map of adhesive forces and lubricant film thickness, a64 � 64 force distance curve array (total of 4096 measurementpoints) was collected over a scan area of 80 lm � 80 lm with a3 Hz scan rate for a sample. For each force distance curve, thereare 128 sampling points. A custom program coded in Matlab wasused to calculate the lubricant thickness and adhesive force.

The quantitative measurement of friction force was made usingthe method described by Bhushan [1,2,4,5] and Palacio and Bhu-shan [27]. The normal load was varied (300–2500 nN), and a fric-tion force measurement was taken at each increment. By plottingthe friction force as a function of normal load, an average coeffi-cient of friction was obtained from the slope of the best fit lineof the data.

Representative AFM maps for the surface height, friction force,adhesive force, and film thickness map of unlubricated and lubri-cated poled PVDF samples with the applied electric potential of0 V are shown in Fig. 4. Shown above each image is a cross-sectiontaken at a position denoted by the corresponding arrows. It showsthat the characterization techniques for nanoscale analysis usingAFM are feasible to measure adhesive force and lubricant thicknessas well as surface height and friction force. With these AFM tech-niques, every measurement for roughness, adhesive force, andcoefficient of friction was conducted with the various applied elec-tric field to the PVDF samples.

2.3. Nanoscale wear

For wear study on the nanoscale, an AFM diamond tip wasrepeatedly scanned on the sample surfaces [4,5]. The radius ofthe diamond tip is about 100 nm, and the spring constant (k) is10 N/m. The tip was scanned 500 times at 30 lN of normal load.The scan area and rate were 10 lm and 20 Hz, respectively. Thesurface morphology before and after the wear tests for each samplewas measured to obtain wear depth.

2.4. Macroscale friction

To study scale effects on friction for nanoscale and macroscale, amacroscale tribology study for the gold coated poled PVDF sampleswas carried out. For the macroscale tribology study, friction exper-iments were conducted using a pin-on-disk tribometer [1,2]. In thetribometer, a flat PVDF sample was placed on a reciprocating linearstage, and a pin was mounted on a cantilever beam with crossedI-beams mounted with semiconductor strain gauges. Normal loadof 100 mN was applied, loading the pin on the flat sample usinga microactuator. The normal and friction forces were measuredusing strain gauges. The experiments were performed for a certainnumber of cycles, and friction force was monitored during the test.

3. Results and discussion

3.1. Effect of piezoelectricity

Piezoelectricity of the poled (b-phase) PVDF is large (23 pC/N).In this section, the effect of piezoelectricity with an externally ap-plied electric field on the tribological properties of PVDF films is

Fig. 4. Representative AFM maps for surface height, friction force, adhesive force, and film thickness distribution for unlubricated and lubricated (65 nm thickness) poledPVDF samples at 0 V applied. Shown above each image is a cross-section taken at a position denoted by the corresponding arrows.

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studied. RMS roughness, adhesive force, coefficient of friction andwear depth on poled (white dots) and unpoled (black dots) PVDFsamples as a function of the electric field applied to the PVDF sam-ples are shown in Fig. 5.

The RMS roughness of the unpoled PVDF films is four timeshigher than that of the poled PVDF, due to the poled PVDF filmsbeing stretched out during fabrication process. While the rough-ness of the unpoled PVDF is unchanged, roughness of the poledPVDF increases with an increase in the absolute value of electricfield. This is attributed to the piezoelectric coefficient of poledPVDF being higher than that of unpoled PVDF. In poled PVDF, thepolarized elements, such as molecules (dipoles), lamella and thechain elements in the amorphous region, are better aligned thanthose in unpoled PVDF. Consequently, the poled PVDF is sensitiveand responsible for electric field more than the unpoled PVDF. Thismeans that the elements of the poled PVDF are easily moved or ro-tated due to electric field, so that the poled PVDF surface (rough-ness) is more easily changed than the unpoled PVDF due to themoved elements. It is observed that the error bars (standard devi-ation) of the unpoled PVDF in Fig. 5 are five times longer than thatof the poled PVDF. In the poled PVDF, the polarized elements in thewhole area on the surface respond uniformly to electric field. How-ever, in the unpoled PVDF, the polarized elements on some areas ofthe surface respond to electric field, whereas the elements on otherareas of the surface do not due to its low piezoelectricity.

Adhesive force of the unpoled PVDF did not change with theexternally applied electric field, but for the poled PVDF, it increasedslightly with the electric field. As externally applied electric fieldthe attraction force between the AFM tip and the PVDF surface in-creases which is responsible for an increase in the adhesive forceon both of the unpoled and the poled PVDFs, regardless of piezo-electricity. However, adhesive force increased in the poled PVDF,

and did not change in the unpoled PVDF. This indicates that thestrong piezoelectricity of the poled PVDF more strongly attractedthe AFM tip as the externally applied electric field increased fromnegative (�15 V) to positive (+15 V) on the top gold film, and thepolarized elements (molecules, lamella and chains) in the poledPVDF tend to align along the opposite direction of the applied elec-tric field. The negative pole of the polarized elements in the PVDF ispointed toward the positive pole of the applied electric field. As thenegative pole of the polarized elements is aligned to the top sur-face of the poled PVDF, the neutral AFM tip which is relatively po-sitive against the negative pole of the polarized elements in thepoled PVDF is attracted to the polarized elements of the PVDF,inducing an increase of adhesive force with the applied electricfield.

The coefficient of friction on the poled PVDF increased with theapplied electric field, but not on the unpoled PVDF. The change ofthe coefficient of friction is not due to the roughness change, be-cause the trendline of roughness change with the applied electricfield is different. It is possible that the change of adhesive forceon the poled PVDF with the electric field affects coefficient of fric-tion. However, the change of the coefficient of friction with theelectric field is much larger than that of the adhesive force; coeffi-cient of friction for the poled PVDF with an electric field is doubledfrom �15 V to +15 V, indicating that there are other factors affect-ing the increase of coefficient of friction for the poled PVDF. If theapplied electric field affected coefficient of friction, coefficient offriction on both the poled and the unpoled PVDF should have beenaffected. However, coefficient of friction on the unpoled PVDF wasunchanged with the applied electric field. It means that the effectof the applied electric field on coefficient of friction was small ornegligible. It also indicates that the polarized elements in the poledPVDF affect coefficient of friction. In other words, the higher piezo-

Fig. 5. Roughness, adhesive force, coefficient of friction and wear depth on poled(white dots) and unpoled (black dots) PVDF samples as a function of electric fieldapplied to the PVDF samples. Wear tests were conducted using AFM at 30 lN ofnormal load.

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electricity of the PVDF films is, the more coefficient of friction onthe PVDF films is affected by the applied electric field.

The PVDF films experience wear as well as friction and adhesionduring AFM tips sliding over the surface of PVDF samples. It is ob-served that there is no effect of the applied electric field on weardepth on the unpoled PVDF, but wear depth on poled PVDF

decreases with an increase of the applied electric field. It maynot be due to the applied electric field but the piezoelectricity ofthe PVDF films, as mentioned for coefficient of friction and adhe-sive force. The poled PVDF used for this study has a negative piezo-electric coefficient (d33), and the polarization (P) of the poled PVDFis in the same direction of the applied electric field. This meansthat as the applied electric field increases, the poled PVDF filmscontract more. When the electric field is applied, the polarized ele-ments in the poled PVDF move, rotate, and contract. As the electricfield increases, the polarized elements more strongly hold theirdirection and location. It drives an increase in the wear resistanceof the strongly held elements, with the applied electric field. There-fore, as the applied electric field increases, the higher the piezo-electricity is, the smaller the wear depth is.

It is observed that adhesive force of the unpoled PVDF is largerthan that of the poled PVDF in Fig. 5. According to the previous re-search of relationship between roughness and adhesive force [1,2],smooth surfaces have higher adhesive force than rough surfacesdue to the AFM tip contact area being larger on smooth surface.However, in this study, the results on adhesive force are oppositeto the research. The poled PVDF has lower adhesive force thanthe unpoled PVDF, even though roughness of the poled PVDF hasmuch lower than that of the unpoled PVDF. It could be attributedto electrostatic effect. The AFM tip used for the experiments is anelectrical insulator (silicon nitride). During the AFM tip sliding overthe PVDF sample surfaces, charges are built up on the surface of theAFM tip due to the tip contact with the PVDF sample surfaces.Compared to the poled PVDF dipoles, the amount of dipoles onthe unpoled PVDF is too low to discharge the buildup charges onthe AFM tip. Therefore, an electrostatic force is present betweenthe AFM tip and the unpoled PVDF, resulting in higher adhesiveforce on the unpoled (rough) PVDF than that on the poled (smooth)PVDF.

3.2. Lubricant effect

Since the electric field had no effect on the nanotribology of theunpoled PVDF, the effect of lubricant on the tribological propertiesof only the poled PVDF films was studied. A20H lubricant film with65 nm thickness was deposited on the gold coated poled PVDF.RMS roughness, adhesive force, coefficient of friction, and weardepth on unlubricated and lubricated poled PVDF samples as afunction of electric field applied to the PVDF samples are shownin Fig. 6. Electric field does not affect roughness of either lubricatedor unlubricated poled PVDF. However, the lubricated poled PVDFhas lower roughness than the unlubricated poled PVDF. This isattributed to the fact that lubricant molecules tend to fill the lowerareas so that the values of RMS of the lubricated poled PVDF de-crease [7,8].

The lubricated poled PVDF has adhesive force three times largerthan the unlubricated poled PVDF, due to the meniscus formedaround the AFM tip contacting lubricant on the lubricated poledPVDF. It is known that lubricant film thickness affects adhesiveforce as well as lubricant viscosity [1–5]. An increase of lubricantfilm thickness increases meniscus formation, which leads to higheradhesive force [24]. Adhesive force on the unlubricated poled PVDFslightly increases with electric field. On the other hand, it is ob-served that adhesive force on the lubricated poled PVDF decreaseswith an increase in the electric field. On the unlubricated PVDF, thepolarized elements with electric field cause an increase of adhesiveforce by attracting the AFM tip. However, on the lubricated PVDF,lubricant molecules (dipoles) disturb the polarized elements to at-tract the AFM tip, leading to a decrease of adhesive force with elec-tric field. Fig. 7 shows a schematic of the electric field andattraction force generated by the dipoles of the poled PVDF andlubricant. When the external electric field is applied to the poled

Fig. 6. RMS roughness, adhesive force, coefficient of friction and wear depth onunlubricated poled (white dots) and lubricated poled (black dots) PVDF samples asa function of electric field applied to the PVDF samples. Wear tests were conductedusing AFM at 30 lN of normal load.

Fig. 7. Schematic of the Au (gold) coated poled PVDF films without (top) and with(bottom) lubricant film (65 nm) at present of the externally applied electrical fieldand the AFM tip. (Top image) When the external electric field is applied to the poledPVDF, the electric field direction of dipoles in the poled PVDF are aligned to pointthe top of the PVDF surface, attracting the AFM tip, finally increasing adhesive force.(Bottom image) When lubricant is present on the poled PVDF surface, the dipoles oflubricant molecules are attracted by the PVDF dipoles, losing attraction force for theAFM tip, hence reducing adhesive force. However, the net adhesive force on thelubricated poled PVDF is higher than that on the unlubricated PVDF, due tomeniscus formation generated lubricant film.

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PVDF, the electric field direction of dipoles in the poled PVDF alignto point towards the top of the PVDF surface, attracting the AFMtip, which increases adhesive force. When lubricant is present onthe PVDF surface, the dipoles of the lubricant molecules are at-tracted by the PVDF dipoles, losing attraction force for the AFM

tip, hence reducing adhesive force. The net adhesive force on thelubricated PVDF is higher than that on the unlubricated PVDF,due to meniscus formation generated lubricant film.

The coefficient of friction on the lubricated poled PVDF is lowerthan that on the unlubricated poled PVDF. Lubricant film was pres-ent between the AFM tip and the sample surfaces, and acted as afluid bearing surface allowing smoother movement of the AFMtip on the lubricated poled PVDF than on the unlubricated poledPVDF. It reduced friction force and coefficient of friction. Whilethe coefficient of friction on the unlubricated poled PVDF increasedwith electric field, coefficient of friction on the lubricated poledPVDF was unchanged by the electric field. This is attributed tothe fact that lubrication occurs when the AFM tip was separatedfrom the PVDF film surfaces by the lubricant film, allowing theAFM tips to move more smoothly.

Wear depth on the lubricated poled PVDF decreased with elec-tric field, which is consistent with the data for the unlubricatedpoled PVDF presented earlier. It is also observed that wear depthon the lubricated poled PVDF is always lower than that on theunlubricated poled PVDF. This is attributed to lubricant film allow-ing separation between the AFM tip and the lubricated PVDF sur-face, mitigating wear. In other words, the mobile fraction of thelubricant molecules on the lubricated PVDF surface disturbs thecontact between the AFM tip and the PVDF surface. The distur-bance of the lubricant molecules reduces wear on the surface[24]. There are other possible reasons. Wear rate increases withan increase of temperature [1,2]. During sliding of the AFM tip overthe PVDF sample surface, friction occurs, generating heat on thesurface. Since the lubricated PVDF has lower coefficient of friction

Fig. 8. Coefficient of friction taken by AFM (left column) and tribometer (right column) for poled and unpoled PVDF (the top row), and unlubricated and lubricated poledPVDF (the bottom row), as a function of electric field applied to the PVDF samples. The normal loads were 300–2500 nN under AFM and 100 mN under tribometer,respectively.

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than the unlubricated PVDF, the AFM tip generates lower frictionon the lubricated PVDF surface than on the unlubricated PVDF. Itsuggests that lower heat energy occurred on the lubricated PVDFthan on the unlubricated PVDF. In addition, the heat energy gener-ated by friction is absorbed to lubricant molecules as well as thePVDF surface. The heat energy absorbed to the PVDF surface is low-er than the heat energy generated by friction. It means that not allthe heat energy generated by friction is absorbed to the PVDF sam-ple surface. Due to lubricant film, the lower heat energy on thelubricated PVDF than that on the unlubricated PVDF can be anadditional factor to reduce wear depth on the lubricated PVDF.

3.3. Scale effect

Using a tribometer, the coefficient of friction at the macroscalewas obtained for the poled and unpoled PVDF samples without lu-bricant. Both the unpoled and poled unlubricated samples werestudied in order to compare with nanoscale data. However, as indi-cated earlier, since the electric field had no effect on nanotribology,the unpoled lubricated samples were not studied.shows the coeffi-cient of friction taken by AFM (left column) and tribometer (rightcolumn) for poled and unpoled PVDF (the top row), and unlubri-cated and lubricated poled PVDF (the bottom row), as a functionof the electric field applied to the PVDF samples. The normal loadswere 300–2500 nN under AFM and 100 mN under tribometer,respectively. Coefficient of friction taken by AFM for Figs. 5 and 6is shown again here (left column) to compare with the coefficientof friction at the macroscale.

At both nanoscale and macroscale, coefficient of friction on theunpoled PVDF does not change with the electric field, as shown inthe top row of Fig. 8. The coefficient of friction at the nanoscale onthe poled PVDF increases with electric field, whereas at the macro-

scale it does not respond to electric field. As mentioned earlier, atthe nanoscale the polarized elements in the poled PVDF affect coef-ficient of friction with the applied electric field; coefficient of fric-tion increased with electric field. However, for macroscale, thepolarized elements were considered negligible, so they could notaffect coefficient of friction. It is also observed that the coefficientof friction at macroscale on the unpoled PVDF is always higherthan that on the poled PVDF. It could be affected by roughness;RMS roughness of the unpoled PVDF is about three times higherthan that of the poled PVDF, leading to higher friction on the unp-oled PVDF.

In the case of lubricated PVDF (bottom row, Fig. 8) both on themacroscale and nanoscale the coefficient of friction does notchange with the electric field. Trends in macroscale data for lubri-cated PVDF are consistent with that of unlubricated data; whereas,on the nanoscale, trends are different. As explained earlier foradhesion data with references to Fig. 7, in the lubricated PVDF,electric field does not affect friction on the nanoscale.

At both macroscale and nanoscale, coefficient of friction on thelubricated poled PVDF is always lower than that on the unlubri-cated PVDF. Lubricant film on the PVDF hence plays a role indecreasing friction. Due to the reduction of friction by lubricant,the heat energy generated by friction could be also reduced. There-fore, it is also expected that the wear resistance of the lubricatedPVDF is increased by lubricant film reducing friction.

4. Conclusions

Effect of piezoelectricity with electric field on the nanotribolog-ical properties of PVDF films was studied. Piezoelectricity in thepoled PVDF increased roughness, coefficient of friction, and wear

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with electric field. Due to high piezoelectricity of the poled PVDF,RMS roughness on the poled PVDF increased with electric field,rather than being unchanged as on the unpoled PVDF. Adhesiveforce on the poled PVDF increased with electric field, since thepolarized elements attracted the AFM tip with an increase of elec-tric field. The polarized elements in the poled PVDF with electricfield also affected increasing coefficient of friction. It was observedthat wear depth on the poled PVDF decreased as electric field in-creased. The higher electric field was applied, the more firmlythe polarized elements in the poled PVDF held their direction. Inthe case of the unpoled PVDF, roughness, coefficient of friction,and wear did not change with electric field due to its lowpiezoelectricity.

Effectiveness of a lubricant on the poled PVDF films was inves-tigated. Due to the presence of lubricant on the lower surface areasof the poled PVDF, roughness was lower than that of the unlubri-cated PVDF. Meniscus formation of lubricant occurred around theAFM tip, leading to increasing adhesive force. Lubricant film withan increase of electric field decreased adhesive force, because thedipoles of lubricant molecules are attracted by the PVDF dipoles,losing attraction force for the AFM tip. Lubricant film allowedsmoother movement of the AFM tip over the lubricated PVDF byreducing friction force, indicating coefficient of friction decreased.Due to lubricant film, there was a separation between the AFM tipand the lubricated PVDF, disturbing contact between them, result-ing in decreased wear depth.

At the macroscale, effect of piezoelectricity, with the electricfield and lubricant on poled PVDF films was studied using a trib-ometer. The coefficient of friction of poled PVDF both unlubricatedand lubricated does not change with the electric field. Effect of thepolarized elements in the poled PVDF on the coefficient of frictionwas negligible at the macroscale, unlike at the nanoscale. Lubricantfilm reduced the coefficient of friction by decreasing friction gener-ated during sliding as observed at the nanoscale.

In summary, as the electric field increases, adhesion and frictionof piezoelectric materials increase due to the effect of their piezo-electricity with highly piezoelectric materials exhibiting loweradhesion and friction. To reduce adhesion and friction, lubricantcan be applied on their surfaces. Applications such as a microgrip-per should use a piezoelectric polymer material of high piezoelec-tricity. For the microgripper to grip an object, the electric field isapplied so that adhesion and friction increase to grip the objecteasily, and is removed during release. Increasing the electric fieldpromotes wear resistance of piezoelectric materials. Lubricant film

on the surface of the materials provides an additional increasedwear resistance. For MEMS and NEMS applications of which issueis wear, lubricant film on their surfaces can be beneficial. However,an increase of adhesion on the surfaces due to meniscus formationof lubricant film is inevitable.

Acknowledgment

We would like to acknowledge Measurement Specialities (VA,USA) for providing poled and unpoled PVDF sheets of this study.

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