1999-Nano Struct Mat-mechanical Properties in Nanometer Scale

8/12/2019 1999-Nano Struct Mat-mechanical Properties in Nanometer Scale

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NaooStmctured Materials, Vol. 12, pp. 1049-1052, 1999Elsevier Science LtdPergamon

PI1 SO 59773(99)00297-4

Q 1999 Acta Metallurgica Inc.tinted in the USA. All rights reserved09659713l996-see front matter

EVALIJATION OF MECHANICAL PROPERTIES IN NANOMETERSCALE USING AFM-BASED NANOINDENTATION TESTER

K. Miyahara, N. Nagashima, T. Ohmura, aud S. MatsuokaNational Research Institute for Metals

1-2-1 Sengen, Tsukuba-shi, Ibaraki 3050047, JapanAbstract -A nanoindentation hardness tester was developed on the basis of an atomic force

microscope (AM) to evaluate mechanical properties of microstructures. Not only the force -penetration depth curves but the topographic images can be obtained by this tester One of newfeatures of the developed tester was modified lever design in order to accommodate the Berkovichindenter into the middle of the lever Furthermore, a piezo-actuator was added to control theappliedforce and the vertical displacement of the lever was monitored by a laser displacementdetector Tlhe diamond tip works as both an AFM tip and indenter

Force -penetration depth curves represent mechanical properties of specimens. However;because of indentation size eflect, the hardness values obtained by nanoindentation testers areusually greater than the bulk harhess. In this paper several metallic single crystals were usedas standard specimens and an empirical model for calculating tickers hardness from force -penetration depth curves is proposed. Finally, mechanical properties of ferrite in pearliticsteels were obtainedusing the instrument. 01999 Acta Metallurgica Inc

INTRODUCTIONThe evaluation of mechanical properties in microstrnctures becomes much more important

as a result of increasing interests in nanostructured materials, semiconductor devices, andsuucmml materials. Nanoindentation hardness testers, which have been developed and studiedintensive nowadays (1,2,3), can provide such information. In this paper a new AFM-basednanoindent,ation tester is introduced and a simple method to evaluate Vickers hardness fromforce - penetration depth curves is proposed.

AFM-BASED NANOINDENTATION TESTERFigure 11 hows a diagram of the AFM-based nanoindentation tester and its special leverdeveloped by the authors. The difference from a conventional atomic force microscope is asfollows: (a) use of a special lever held on both sides, (b) addition of a piezo-actuator for force

control, and (c) measurement of lever displacement instead of its deflection. Further details ofthis instrument are described elsewhere (1). It should be noted that both force - penetration

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depth curves and surface topographic images are easily available with this system.Indentation experiments were carried out under force control mode. Loading and unloading

rate was 10 @J/s for all experiments. AFM images were obtained before and after indentationtest in order to choose flat location for hardness test and observe indentation shape. A Berkovichdiamond indenter with an apical angle of 60 was used, to obtain sharp AFM images.

In hardness tests, the indentation size effect (ISE) is often observed, i.e., the hardness valuesobtained by nanoindentation testers are usually greater than the bulk hardness. In order to applythis instrument to material research, it is desirable to measure an absolute hardness value,which is independent of experimental conditions such as curvature radius of indenters and freefrom the ISE. Empirical methods are necessary for this purpose because whether the ISE is agenuine size effect or not is still under discussion (2) and to estimate effects of indenter roundnessand specimen surface analytically and quantitatively is still not easy when penetration depth isas low as 100 mu. Oliver et al. (3) proposed a new technique for determining hardness andelastic modulus by correcting contact area to reduce the indentation size effect. In this work, asimple empirical method for evaluating hardness with reduced indentation size effect is proposedand described in the following section.

EXPERIMENTAL RESULTS AND DISCUSSIONThe (100) surfaces of single crystals of tungsten, molybdenum, iron and nickel were used as

test specimens. These specimens were polished mechanically and then electrolytically. Theyare used as standard specimens for determining hardness. The basic idea of hardnessdetermination is to establish the relationship between macroscopic hardness and force -penetration depth curves in nanoindentation. The single crystals were used since they have nograin boundaries or precipitates and therefore the same mechanical properties would be expectedin both nanoindentation and macroscopic hardness tests.

Figure 2 shows force - penetration depth curves of the single crystals. It can be seen fromFig. 2 that at the same force level, the depth of indentation decreases as the degree of Vickershardness increases. This means relative hardness can be compared in any case. Figure 3 shows

Laser Displacement etectorh iezo Transducer

Diamond Indenter

\iem Scanner

Fig. 1 AFM-based nanoindentation tester and its special lever.


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the relationship between force necessary to achieve a certain penetration depth and Vickershardness obtained by a conventional Vickers hardness tester at 100 gf. Using standard curvefitting technique, a power function model was found to be the best one to describe the relationship.This power faction is shown in the following equation:

HV = [a(h) FJ n (h >lOO) PIwhere HV is Vickers hardness, F is force in pN, and h is penetration depth in nm. a(h) is h-dependent and n is a constant and n = 1.214 in this case. Plotting F(h)/WV */nagainst h furtherand again using curve-fitting for various type of functions, the best fitted one was found as:

(h > 100) PIwherep = 5.6634 x 10 -3and q = 122.83. Subsequently, Eq. [2] can be rewritten in terms of HVas follows:

= [F/ IS.6634 x 10 -3(h f 122.83))] 1.214 (h > 100) [ 3It should be noted that these parameters may be valid only for the current indenter and theirphysical melmings are not clear yet. However, a possible hypothesis is that q represents thetruncation length of indenter and p and n are related to hardness conversion between 60 indenterand Vickers indenter. Further experiments and discussion will be necessary.

A pearlitic steel was chosen as a test material. The pearlitic steel was etched by 1 nitricacid and 99 alcohol. The microstructure of the pearlitic steel, i.e., ferrite grains and cementitelamellae, is in submicron scale as shown in the AFM image in Fig. 4. The mechanical propertiesof the micro:structure are much of interest in order to establish a model in micromechanics.

ltim 2CCnm

4COnm

Penetration Depth nm) Force p N)Fig. 2 Force - penetration depth curves of Fig. 3 Relationship between force and Vickers

single crystals. hardness of single crystals


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Fig. 4 AFM image of an indentation on ferrite 0 50 loo 150 200 250in pearlitic steels Penetration Depth (run)Fig. 5 Force - penetration depth curves of ferrite.

Figure 4 illustrates an indentation on a ferrite grain. Because of imaging capability of theinstrument, it is easy to choose a specific location for nanoindentation tests even if a specimen isheterogeneous. Figure 5 shows force - penetration depth curves for both pearlitic and ferriticsteels. Both curves showed almost the same properties. A slight difference was observed at 200mn, which might be due to cementite lamellae. From Fig. 5 and Eq. [3], Vickers hardness isestimated as 160 - 170 and it is greater than 98, which is the value of the iron single crystal.This difference may be due to the effect of dislocations and chemical composition. Its worthnoting that there is a discontinuation in force - penetration depth curves shown in Fig. 5. Thisbehavior can be observed in Fig. 2, too. It is considered as a yield point and consistent with theresults reported previously (3,4,5).

CONCLUSIONAn AFM-based nanoindentation tester was successfully developed, which allows both force

- penetration depth curves and AFM topographic images to be obtained. A simple empiricalmethod to evaluate Vickers hardness from force curves was proposed and mechanical propertiesof pearlitic and ferritic steels were obtained, using the developed tester and proposed method.

REFERENCES(1) Miyahara, K., Matsuoka, S., Nagashima, N. and Mishima, S., Trans. Jpn. Sot. M ech. Eng.

A, fl 1995), pp. 2321-2328.(2) Tabor, D., Phil . Mag. A, 74 1996), pp. 1207-1212.(3)Oliver, W. C. and Pharr, G. M., J. Mater. Rex, 1 1992), pp. 1564-1583.(4) Gerberich, W. W., Nelson, J. C., Lilleodden, E. T., Anderson, P. and Wyrobek, J. T.,Acta

metal., 44 1996), pp. 3538-3598.(5) Miyahara, K., Matsuoka, S and Nagashima, N., Tr ans. Jpn. Sot. Mech. Eng. A, 63 (1997),pp. 2220-2227.

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1999-Nano Struct Mat-mechanical Properties in Nanometer Scale