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2016 Jpn. J. Appl. Phys. 55 106602

(http://iopscience.iop.org/1347-4065/55/10/106602)

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Advanced in situ multi-scale characterization of hardness

of carbon-fiber-reinforced plastic

Hongxin Wang1*, Hideki Masuda1, Hideaki Kitazawa1, Keiko Onishi1, Masamichi Kawai2, and Daisuke Fujita1

1Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan2Systems and Information Engineering, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan

*E-mail: WANG.Hongxin@nims.go.jp

Received June 7, 2016; revised July 25, 2016; accepted July 29, 2016; published online September 9, 2016

In situ multi-scale characterization of hardness of carbon-fiber-reinforced plastic (CFRP) is demonstrated by a traditional hardness tester,instrumented indentation tester and atomic-force-microscope (AFM)-based nanoindentation. In particular, due to the large residual indentation andnonuniform distribution of the microscale carbon fibers, the Vickers hardness could not be calculated by the traditional hardness tester. In addition,the clear residual microindentation could not be formed on the CFRP by instrumented indentation tester because of the large tip half angle of theBerkovich indenter. Therefore, an efficient technique for characterizing the true nanoscale hardness of CFRP was proposed and evaluated. Thelocal hardness of the carbon fibers or plastic matrix on the nanoscale did not vary with nanoindentation location. The Vickers hardnesses of thecarbon fiber and plastic matrix determined by AFM-based nanoindentation were 340 + 30 and 40 + 2 kgf/mm2, respectively.

© 2016 The Japan Society of Applied Physics

1. Introduction

Hardness is a functional indicator of characterization of softor hard materials. Generally, it is the local area mechanicalproperties of materials response to the overall performancesunder certain conditions. Hardness is a measurable parameter,mainly used to test the mechanical quality of materials and todetermine a reasonable processing technology. Hardness canbe measured by a traditional hardness tester1) or instrumentedindentation tester.2) Because of residual indentation atmicrons to millimeter range, surface is slightly damaged bythe traditional hardness tester. According to internationalstandards of the instrumented indentation technology, thehardness measurement range is divided into micro and nanorange. Surface is not damaged within this range.

A technique called “nanoindentation”3)— using a standardnanoscale diamond probe— is an effective approach forunderstanding the nanoscale mechanical properties ofmaterials.4) Atomic-force-microscope (AFM)-based nano-indentation has been successfully used to determine thenanoscale mechanical properties of silicon.5,6) In particular,the mechanical properties of monolayer graphene flakes aremeasured by AFM-based nanoindentation.7,8) Since AFM hasa high longitudinal and lateral resolution, it is suitable formeasuring nanoscale surface topography. Thus the surfacearea of residual nanoindentation can be calculated directlyby AFM.

Carbon-fiber-reinforced plastic (CFRP)9) is one of themost-advanced composite materials with a high-strengthstructure. This kind of composite material contains two parts:a carbon-fiber-reinforcement part (which provides strength)and a plastic-matrix part (which binds the reinforcementtogether). Since CFRP is lighter than aluminum, strongerthan iron, and has higher elasticity than titanium, it is widelyused for industrial applications such as aerospace,10–12)

automobile,13–15) civil engineering,16,17) and sports goods.18)

The achievements of modern industry have benefited greatlyfrom testing and evaluation of the mechanical properties ofmaterials. In practice, testing and evaluation of the hardnessof materials are a direct bridge connecting material designand industrial applications. Consequently, for developing

CFRP in various industrial fields, it is very important tounderstand the hardness of CFRP.

As commonly known, because single-crystal silicon (001)has high flatness and low surface roughness, it is often usedas a standard material for characterizing mechanical proper-ties.19,20) However, composite materials such as CFRP arecomposed of complex multiphase systems of heterogeneousmaterials. The hardness of CFRP are related not only to theintrinsic properties of the components, namely, carbon fibersand plastic matrix, but also to the characteristics of the inter-face region between the carbon fibers and plastic matrix.21–24)

In the present research, the hardness of CFRP are char-acterized on a multi-scale for the first time. Importantly, thegoal of this research is to develop an efficient and reliabletechnique for characterizing the true hardness of compositematerials.

2. Experimental methods

Before the mechanical properties were characterized, CFRPsamples (TORAY P2053-17) were cut into rectangular sheetswith length of 10mm, width of 3mm, and thickness of 1mm.The cross sections of the samples were then polished by Ar+

beam (a cross-section polisher, JEOL IB-09020CP, 8 kV) for15 h. Next, the hardness of CFRP were in situ multi-scalecharacterized in the following steps.

First, the macroscale hardness of CFRP was characterizedby a micro Vickers hardness tester (Shimadzu HMV-G21series). The micro Vickers hardness tester has a standardizedautomatic length measurement function using a CCD camera.The indentation measurement resolution is 0.09 µm with 40times objective lens. The tip half angle of a diamond Vickerswas 68°. The loading forces were from 9.8 to 245mN, andwere applied for 10 s in each indentation test.

Second, the microscale hardness of CFRP was character-ized by the instrumented indentation tester (Hysitron TI950Triboindenter). The indenter was a diamond Berkovich withtip half angle of 65.3°. The loading forces were from 1.5 to7.5mN and applied for 30 s in each indentation test.

Third, the nanoscale hardness of CFRP was characterizedby AFM-based nanoindentation (Bruker AXS Multimode 8),which was set in a glove box filled with Ar. The diamond

Japanese Journal of Applied Physics 55, 106602 (2016)

http://doi.org/10.7567/JJAP.55.106602

REGULAR PAPER

106602-1 © 2016 The Japan Society of Applied Physics

probe glued onto a rectangular silicon-etched cantilever wasused from Artech Carbon. The sample was scanned so thatsmooth carbon fibers or plastic matrix could be selected. Acarbon fiber or plastic matrix were nanoindentation tested.Sample images after the nanoindentation tests were obtainedin contact mode. The loading forces applied were from 0.7to 8 µN.

3. Results and discussion

3.1 Characterization of macroscale hardness by atraditional hardness testerAs shown in Fig. 1(a), marks with different sizes wereformed on the silicon. The marks looked like a quadrangularpyramid, namely, the shape of the Vickers indenter. Theloading forces were 9.8, 19, 49, and 98mN, which corre-spond to the marks from left to right in Fig. 1(a).

Since silicon is very brittle, it is easy to produce a stressconcentration on the edge of the pyramid. Two phenomenon,namely, chipping and radial cracks, occurred at the corners ofthe pyramid. The standard mark without any stress concen-tration was selected as the one for calculating the Vickershardness, HV (kgf=mm2), of silicon. The Vickers hardnesswas determined as loading force, F (kgf), divided by thesurface area, S (mm2), of indentation as

HV ¼ F

S: ð1Þ

If the unit of force is the newton (N), the Vickers hardnesscan be expressed as

HV ¼ 0:102F

S: ð2Þ

Kilogram-force is the gravitational unit of force. The averagelength of the diagonal line of the marks d (mm), could beread directly by the software of the micro-Vickers-hardnesstester. Lastly, the Vickers hardness was calculated as

HV ¼ 0:1022F sin �

d2¼ 0:1891

F

d2; ð3Þ

where θ is the tip half angle (68°) of the Vickers indenter,which is the angle between the axis of pyramid and its face.The indentation test on the silicon was done for 15 times. TheVickers hardness of silicon was determined from Eq. (3) as1050 ± 20 kgf=mm2. This value fits into a range of Vickershardness given in the literature, namely, from 918 to 1173kgf=mm2.25)

Cross sections of the CFRP samples are shown in Figs. 1(b)and 1(c). The bright white circles are carbon fibers; other(gray) areas are plastic matrix. A typical diameter of carbonfibers is about 7 µm. The packed carbon fibers after indenta-tion test is shown in Fig. 1(b) under a loading force of245mN. The surface area of indentation could not becalculated because there was no a quadrangular-pyramid holeformed on the CFRP. Although the loading force wasdecreased to 98mN or even small, the quadrangular-pyramidhole could not be formed yet in Fig. 1(c). The carbon fibersdebonded from the plastic matrix at the millinewton range ofthe applied force.

Due to the large residual indentation and nonuniformdistribution of the microscale carbon fibers, it was difficultto measure the hardness at macro scale by the traditional

hardness tester. The hardness of CFRP were therefore triedto be characterized at micro scale by the instrumentedindentation tester.

3.2 Characterization of microscale hardness by theinstrumented indentation testerAs shown in Fig. 2(a), the triangular-pyramid marks formedon the silicon represent the shape of the Berkovich indenter.The marks become enlarged with increasing applied forcesup to the millinewton range. The Vickers hardness of silicon(HV ) are calculated as 1180 ± 30 kgf=mm2 by the Oliver–Pharr method,26,27) as follows:

HV ¼ Pmax

Ac; ð4Þ

Ac ¼ 3ffiffiffi3

ph2c tan

2 �� ¼ 24:56h2c ; ð5Þhc ¼ hmax � "Pmax

S; ð6Þ

S ¼ dP

dh

� �h¼hmax

: ð7ÞThe relationship between loading force (P) and displace-

ment (h) is shown in Fig. 3. The slope of the curve in thefigure (black-dotted line) is indicative of the contact stiffness

(a)

chipping radial cracks9.8 mN

Si(001)

49 mN19 mN 98 mN

(b) CFRP

245 mN

(c)

98 mN

CFRP

Fig. 1. (Color online) (a) Optical-microscope images of single-crystalsilicon (001) and cross sections of CFRP after macroscale-indentation test(b, c) on the carbon fibers.

Jpn. J. Appl. Phys. 55, 106602 (2016) H. Wang et al.

106602-2 © 2016 The Japan Society of Applied Physics

(S) [given by Eq. (7)] upon unloading and was used tocalculate the depth of indentation (hc) [given by Eq. (6)].Constant ε (which is related to the shape of the Berkovichindenter) was 0.75. The depth was used to calculate theprojected area (Ac) of indentation [Eq. (5)], where θ is the tiphalf angle (65.3°) of the Berkovich indenter, which wasbetween the axis of pyramid and its face. The Ac (hc) functionof the Berkovich indenter is calibrated according to astandard procedure on a reference fused silica sample.28)

The value of the Vickers hardness of silicon is fairlyconsistent at macro- (1050 ± 20 kgf=mm2) and microscale(1180 ± 30 kgf=mm2) characterization and fits into the rangeof Vickers hardness given in the literature from 918 to1173 kgf=mm2.25) Duo to the different measurement tech-niques and calculation methods for the Vickers hardness ofsilicon between macro- and microscale characterization, 13%difference was reasonable and acceptable.

In order to get clear holes on the carbon fibers, the loadingforce was decreased from 98 to 4.5mN [Fig. 2(c)], whichwas the maximum of the indenting apparatus. As shown inFigs. 2(b) and 2(c), the marks of a cross formed on the plasticmatrix and carbon fiber are the diagonal lines of a triangularpyramid. Duo to the approximate size of the tip half anglebetween the Vickers and Berkovich indenter, the cleartriangular marks could not be formed on the carbon fiber

under a loading force of 4.5mN [Fig. 2(c)]. Therefore theVickers hardness of CFRP could not be calculated at microscale by the instrumented indentation tester when no holeswere formed.

To produce a clear mark on the CFRP, a sharp nanoscalediamond probe of AFM (see Fig. 4) was used in place ofthe Berkovich indenter. The tip apex was a spherical shapewith radius of about 20 nm after the nanoindentation test.

3.3 Characterization of nanoscale hardness by AFM-based nanoindentationAs shown in an AFM image [Fig. 5(a)] of the standardsample of silicon, the surface was smooth and highly con-sistent so that uniform shape of nano-holes could be obtainedand were enlarged with increasing loading force. Thus thehardness of silicon could be compared between AFM-basednanoindentation and the hardness tester. To calculate thecontact area of the nano-holes, it was assumed that the shapewas circular.

An AFM image of the cross section of CFRP is shown inFig. 5(b). The bright circles (black-dotted circle) is carbonfiber; the other parts are plastic matrix. As shown inFig. 5(b), the carbon fiber was significantly higher thanplastic matrix, which was formed during polishing by Ar+.Since the hardness of plastic matrix was much smaller thancarbon fiber, it was worn and removed easily. Thus thecarbon fiber was raised from the surface. Because the raisedcarbon fiber blocked the polishing of the neighboring plasticmatrix, a height difference was formed around the carbon

Si(001)(a)1.5 mN3.0 mN4.5 mN6.0 mN7.5 mN

(b)

CF

CF PM

PM

0.5 mN

CF

PM

(c)

4.5 mN

Fig. 2. (Color online) (a) AFM images of single-crystal Si(001), (b) plastic matrix (PM), and (c) carbon fiber (CF) after microscale indentation test using adiamond Berkovich indenter. Loading force: (a) 1.5, 3, 4.5, 6, and 7.5mN; (b) 0.5mN; (c) 4.5mN. The bright circles (in the black-dotted circle) are carbonfibers. Plastic matrix surrounds the carbon fibers.

0

200

400

600

800

1000

1200

1400

1600

0 20 40 60

Forc

e (μ

N)

Displacement (nm)

S

hc

hmax

Pmax

Fig. 3. Dependence on applied loading force of displacement of atriangular-pyramid hole formed on the silicon.

cantilever

(a) (b)

Fig. 4. (Color online) Helium-ion-microscope (HIM) images of nanoscalediamond probe. (a) and (b): side views of the probe after nanoindentationtest.

Jpn. J. Appl. Phys. 55, 106602 (2016) H. Wang et al.

106602-3 © 2016 The Japan Society of Applied Physics

fiber. Although the polishing caused a nanoscale heightdifference, it did not have a strong influence on the shape ofresidual nanoindentation.

Because the hardnesses of the plastic matrix and carbonfibers differ significantly, a nanoindentation test was per-formed on the plastic matrix and carbon fibers separately.The nanoscale holes formed on the plastic matrix [image (c)]and on the carbon fiber [image (d)] by a diamond AFM probebecome enlarged with increasing applied force up tomicronewton range. The nano-holes close to and away fromthe interface (interface region) do not obviously change shapewhen a constant loading force is applied. In other words, thelocal hardness of the carbon fiber or plastic matrix on thenanoscale do not vary with nanoindentation location.

The Vickers hardness can be expressed by Eq. (4) as theinverse of the slope in Fig. 6. Vickers hardnesses of silicon(green square), carbon fiber (red circle), and plastic matrix(blue triangle) were successfully determined by AFM-basednanoindentation as 640 ± 60, 340 ± 30, and 40 ± 2 kgf=mm2,respectively. Comparing the literature-quoted value betweenthe graphite (from 7 to 11 kgf=mm2)29) and diamond-likecarbon (from 2000 to 9000 kgf=mm2)30) reveals that thecarbon fibers of CFRP is harder than the graphite, and softerthan the diamond-like carbon.

The Vickers hardness of silicon on the nanoscale is foundto be smaller than that on the macro- and microscales.But roughly speaking, the Vickers hardness of silicon onthe multi-scale was the same reported value in order of

magnitude (103). Therefore, the consistent hardness of thehomogeneous materials could be obtained by a traditionalmethod or AFM-based nanoindentation.

4. Conclusions

In situ multi-scale techniques for characterizing the hardness

CF

PMSi(001)

(a)

8 μN

7 μN

6 μN

5 μN4 μN

(b)

(d)CF PM

5.0 μN5.5 μN

6.0 μN

6.5 μN

6.9 μN

(c)

CF

PM

CF

1.1 μN1.0 μN

0.7 μN0.8 μN

0.9 μN

Fig. 5. (Color online) (a) AFM images of single-crystal silicon (001) and (b) (d) cross sections of CFRP. AFM nanoindentation test on (a) Si(001),(c) plastic matrix (PM), and (d) a carbon fiber (CF). Loading force: (a) 8, 7, 6, 5, and 4 µN; (c) 1.1, 1, 0.7, 0.8, and 0.9 µN; (d) 5, 5.5, 6, 6.5, and 6.9 µN(from top to bottom).

0 2 4 6 8 100

1

2

3

4

Loading Force/ kgf

Con

tact

Are

a / m

m2

[×10-9]

[×10-7]

PM

CF

Si

Fig. 6. (Color online) Dependence of the measured area of nano-holeformed on the silicon (Si, green squares), carbon fiber (CF, red circles), andplastic matrix (PM, blue triangles) with applied load.

Jpn. J. Appl. Phys. 55, 106602 (2016) H. Wang et al.

106602-4 © 2016 The Japan Society of Applied Physics

of composite materials such as CFRP were evaluated. Due tothe large residual indentation and nonuniform distribution ofthe microscale carbon fibers, the Vickers hardness could notbe calculated by a traditional method, such as micro Vickershardness tester. Although the loading force was decreased toget a small size of residual indentation on the carbon fiber,the clear marks of residual indentation could be formed. TheVickers hardness of CFRP could not be calculated by theinstrumented indentation tester. Accordingly, a techniquecalled AFM-based nanoindentation was found to be aneffective approach for measuring the true nanoscale hardnessof CFRP. The local hardness of the carbon fiber or plasticmatrix on the nanoscale did not vary with nanoindentationlocation. The clear holes were made by a nanoscale diamondprobe. The Vickers hardness of the carbon fiber and plasticmatrix were determined by AFM-based nanoindentation as340 ± 30 and 40 ± 2 kgf=mm2, respectively. Our method(AFM-based nanoindentation) can be used for character-ization of hardness of composite materials as a standardmethod. It is no doubt to be used for the homogeneousmaterials.

Acknowledgment

This work was supported by Cross-ministerial StrategicInnovation Promotion Program—Unit D66— InnovativeMeasurement and Analysis for Structural Materials (SIP-IMASM).

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Jpn. J. Appl. Phys. 55, 106602 (2016) H. Wang et al.

106602-5 © 2016 The Japan Society of Applied Physics