Deformation Structuring of Aluminum Films upon Microindentation

ISSN 1027�4510, Journal of Surface Investigation. X�ray, Synchrotron and Neutron Techniques, 2014, Vol. 8, No. 6, pp. 1275–1285. © Pleiades Publishing, Ltd., 2014.Original Russian Text © T.A. Kuznetsova, S.A. Chizhik, A.L. Khudoley, 2014, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, 2014, No. 12,pp. 46–56.

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INTRODUCTION

Indentation has been widely applied for a long timefor estimation of the mechanical properties of solidbodies [1, 2]. Due to achievements in the field ofinstrument development it allows one to investigatealso nanoscale surface layers [3–5]. The indentationprocess is multifaceted and is now being considered inthe fields of mechanics of continuous mediums, simu�lation, studies on the behavior of individual disloca�tions under the influence of small loads, etc. [6]. Themechanism of the formation of microindentationimprints is connected with the motion and interactionof dislocations and interstitial defects [7]. Therefore,experimental studies on microindentation imprints byhigh�resolution methods are required to explain themechanisms of the plastic deformations of variousmaterials [6]. Such a possibility is provided by atomicforce microscopy (AFM) which is the main methodfor determining the surface topography with nanome�ter resolution [8–10]. In this context, the develop�ment of experimental methods for determination ofthe physical�mechanical characteristics of materialswith the use of widespread standardized methods ofindentation by a hardness microtester and subsequentverification of their results (imprints) by AFM isurgent [11–14]. The indisputable advantages of AFMare the possibility of direct measurements of theindentation�trace depth with a resolution of 1 to 2 nmand determination of the structural component sizeson the imprint slopes that cannot be performed byother methods [12]. Direct observation of the imprintallows one to assess, besides its true geometrical form,the mechanism of plastic deformation of the materialfrom the result of its action on the surface of imprint

slopes which is revealed in the form of tiny changes inthe microstructure under the influence of loading.

A method for the integrated assessment of themechanical characteristics and structure of thin coat�ings that consists in producing imprints using a hard�ness microtester and subsequent scanning of the sur�face by AFM is suggested. Thin films and modifiedlayers are an integral part of modern technology andare widely used in the production of integrated cir�cuits, magnetic and optical devices, and microsensors,and as protective and wear�resistant coatings for thesurfaces of parts and tools [8, 15]. Despite the fact thatthe electrical properties of thin�film materials are pri�mary in the majority of applications, their mechanicalcharacteristics play a significant role. Aluminum filmsare often used as contact pads in microcircuits andmicrosystem switches [16]. The grain size is importantfor the electrical conductivity, and the mechanicalproperties of the contact pads are important uponinteraction with contact surfaces. The most wide�spread mechanical characteristic of thin layers ismicrohardness. The strength properties of films alsodepend on the plasticity and elastic modulus, whichtogether with the microhardness more fully character�ize the operational properties of thin layers. The sug�gested approach allows one to calculate microhard�ness values, and to estimate the plasticity and elasticmoduli of materials from the diagonal sizes and depthof relaxed imprints in the AFM images. The microin�dentation method supplemented by studies on themorphology of imprint faces by the method of high�resolution AFM makes it possible to estimate changesin the structure of aluminum films depending on theload value which leads, in turn, to better understand�ing of deformation processes in the material.

Deformation Structuring of Aluminum Films upon Microindentation

T. A. Kuznetsova, S. A. Chizhik, and A. L. KhudoleyInstitute of Heat and Mass Transfer, National Academy of Sciences of Belarus, ul. Brovki 15, Minsk, 220072 Belarus

e�màil: [email protected] July 19, 2013

Abstract—The elastic moduli, microhardnesses, and deformation coefficients of aluminum films and bulkmaterials (Al, Si, Bi, and Ti) are determined using a combination of indentation and atomic force microscopymethods. The surface morphology of aluminum films with a thickness of 10, 200 and 1000 nm on silicon sub�strates is studied in the initial state and after indentation with loads in the range from 2 to 100 g. The stages ofthe fragmentation and agglomeration of grains under the influence of loads are revealed experimentally. Theeffect of the initial grain size, film substructure, and load values on the processes of deformation structuringis determined.

DOI: 10.1134/S1027451014050115

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KUZNETSOVA et al.

EXPERIMENTAL

Aluminum films with a thickness of 10, 200, and1000 nm produced by the magnetron sputteringmethod on a silicon substrate with the (100) orienta�tion were studied. Bulk Al and Si samples in the formof microsections were used for comparison of the mor�phology of the microindentation imprints. In order toconfirm the reliability of determination of the physi�cal�mechanical characteristics of the materials in thebulk and thin�film states, Ti and TiN films with athickness of 100 nm, and also bulk Bi and Ti sampleswere used.

According to GOST 9450�76, a series of dents wereimprinted on the investigated sample surface underthe influence of a steady�state load (2–100 g) appliedto the diamond tip of a Vickers pyramid (PMT�3hardness microtester, Russia) for 10 s. The distancebetween imprints was no less than two diagonals. Thensites with a series of imprints were scanned using aNT�206 atomic force microscope (Belarus) equippedwith a long�focus optical system with a magnificationrate of ×200 and relaxed imprint images were obtained(Fig. la). The diagonal size and depth of the relaxedimprint were determined in order to estimate thephysical�mechanical characteristics (Fig. 1b). Thediagonals were measured by the profile of the sectionalong the imprint’s internal contour at the initial sur�face level of the material, this way excluding piles, andat the junction point of the initial surface with theimprint face in the absence of a pile. The use of AFMfor visualization of the produced imprints increasesthe accuracy of determination of the microhardnessvalues of thin coatings and small regions of phases,where small loads should be applied. The imprint’simage is surrounded by piles of plastic deformationand can hardly be distinguished from them in an opti�cal microscope. The AFM method solves the issue ofdifferentiation of the imprint’s diagonal and a pile(Fig. 1b). The penetration depth of the indentor is cal�culated from the imprint’s diameter, which barelychanges upon recovery, and the geometrical form ofthe indentor. The microhardness, deformation factor,and elastic modulus of the films and bulk materials arecalculated by the formulas given in [11–14].

The suggested integrated technique combining tra�ditional indentation methods with the modern high�resolution AFM method supplemented by the calcula�tion formulas [12–14], allows one to determine thephysical�mechanical properties of thin films, such asmicrohardness, deformability factor, and the elasticmodulus, by measuring the spatial microscopic geom�etry of an imprint (Fig. 2). Supplementing the micro�hardness tests with the AFM method allows one toinvestigate the surface morphology of thin films in theinitial state and on the imprint’s faces, and to revealchanges in the grain size in the films at different defor�mation stages. A change in the grain size of aluminumdue to the generation and motion of dislocations

under loading with a contact�area size comparablewith the grain size is shown in [7] by the methods oftransmission electron microscopy and nanoindenta�tion in situ. In the present work, the change in thegrain size of aluminum films deposited onto a hardersilicon substrate is investigated by the AFM methodafter the action of a deforming load distributed on thecontact surface, which exceeds the grain size in the filmby 106 times, and subsequent recovery of the surface.

RESULTS AND DISCUSSION

Increasing the Accuracy of Microhardness Determination upon Visualization of an Imprint

by the AFM Method

Determination of the microhardness values of bulkmaterials in the range of loads from 5 to 500 g using anoptical microscope has shown that upon magnifica�tion of the imprint’s image (in the range from ×100 to×400) the accuracy of the microhardness measure�ment increases, depending on the load, in the rangefrom 7 to 28%, and the lower the load the smaller theimage of the imprint, and the lower the accuracy. Thesmall film thickness imposes restrictions on the loadsize and, as a consequence, on the imprint size. Theuse of AFM in this case raises the determination accu�racy (by 153% for a load of 20 g and by 201% for a loadof 5 g) as compared to optical microscopy. Beside sim�ple magnification of the image, AFM allows one toexclude the pile region with a length of 1.4 µm fromthe Si imprint diagonal at a diagonal length of 4.0 µm(Fig. 1b).

Studies on the Morphology of Aluminum Filmsby the AFM Method

The film surface morphology which reveals thegrain size, contour and roughness is a significant char�acteristic when studying films. The film’s electricalconductivity, strength, tribological, and optical prop�erties depend on the grain size. The surface morphol�ogy of the investigated aluminum films is shown in Fig. 3.The grain size of the 10�nm aluminum film is compa�rable with its thickness and is 10 to 50 nm at a heightof grains in the range from 1 to 3 nm. The shape of thegrains is roundish (Fig. 3a). Inconsistency in the shapeand size of grains is characteristic for a film with athickness of 200 nm. Grains with flat faces but roundedvertices and edges have a size of 500 to 800 nm. Theroundish grains are much smaller, 200 to 500 nm insize. The roundish grains have subgrains with a size of10 nm. The size, at which grains protrude from thefilm surface, is in the range 80 to 100 nm (Fig. 3b). Thefilm with a thickness of 1000 nm differs from theother aluminum films. The grain size in the film is100 to 300 nm, whereas an oriented nanoscale striatedsubstructure with a distance between stripes of 10 nmis observed inside the grains. The grains protrude fromthe film surface by 10 to 20 nm (Fig. 3c).


DEFORMATION STRUCTURING OF ALUMINUM FILMS UPON MICROINDENTATION 1277

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x: 2.5 μmz: 191.1 nm

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x: 6.5 μmz: 187.5 nm

Fig. 1. AFM image of the imprint on the surface of silicon with the (100) orientation at a microhardness measurement with a loadof 5 g: (a) 3D image; (b) imprint profile.

Results of AFM Studies of the Geometryof the Microhardness Imprints

AFM images of the microhardness imprints areshown in Fig. 4. Their geometrical parameters, i.e.,the imprint diagonals, relaxed imprint depth, and fullindentation depth, and the microhardness, deforma�tion coefficient, and elastic modulus calculated ontheir basis, are given in the table.

The AFM images of the microhardness imprintsobtained on the surface of a 10�nm aluminum film ona silicon substrate with loads of 5, 20, and 100 g showthat the imprint geometry at such a thickness is deter�mined by the substrate. The form of the imprint withplain faces is typical for silicon but not for the ductilealuminum coating. Among all three imprints, a frag�ment of the rounded contour typical for a plastic

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material is observed only on the left corner of theimprint produced by a 5�g load. The coating with athickness of 10 nm at a load of 20 and 100 g repeats theindentation of the silicon substrates, and film breaklines parallel to the imprint contour with an intervalbetween breaks of about 1 µm are seen on the slopes.Along the imprint contour, there are almost no pileswhich are typical for plastic materials such as alumi�num. It is impossible to correctly calculate the micro�hardness values, elastic modulus, and other parame�ters from the given imprints due to the presence ofcracks beginning at the corners of the imprints alreadyat a load of 5 g (Fig. 4a). At loads of 20 and 100 g,cracks are present also on the slopes, since the energyof the indentor is spent not only on the formation of amicrohardnes imprint, but also on the formation ofcracks.

The external shape of the microhardness imprintsin the 200�nm and 1000�nm aluminum films at loadvalues in the range of from 2 to 100 g entirely corre�sponds to the imprint’s shape in a plastic material andappears as a circle of aluminum�film piles, insidewhich a concave imprint that is typical for an elastic

material is often observed (Figs. 4b, 4c). Only theexternal contour remains visible upon studying theimprint’s shape using an optical microscope. Thus,AFM allows one to more fully characterize the proper�ties both of the coatings and substrates. The internalshape of an imprint typical for elastic materials isrevealed especially clearly in the case when the indenta�tion depth considerably exceeds the coating thickness.

AFM detects breaks in the film at a load value of100 g and an aluminum coating thickness of 200 nm.A crack in the coating that comes out to the surface isobserved also on the top right corner of the imprint.Among all the imprints, only the imprint obtained onthe 1000�nm aluminum film with a load of 2 g does notreach the substrate and has a depth of about 600 nm(Fig. 4c). The imprint’s shape is typical for a plasticmaterial and appears as an almost perfect circle. Aconvex contour is noted not only on the sample sur�face, but also in the imprint’s profile measured via thevertical diagonal (Fig. 4).

The 1000�nm aluminum film is shifted at a load of20 g, and a step on the film surface is produced due tothe formation of an imprint in the silicon substrate. If the

Stage 1Indenting a series

of imprints using a PMT�3

microhardness tester

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Calculationof the HV, f, and E values

[12–14]

HV = 0.189F/d2

f = ((hb1 + hb2)/h) × 100%

E =0.91

hel2

0.227P– 0.001

P = 2–100 g

Advantages of using AFM:⎯accuaracy in determination of the sizes;⎯possibility of determination of the depth;⎯possibility of determination of the pile sizes;

⎯possibility of determination of the elastic modulus;⎯possibility of analysis of the morphology.

⎯possibility of determination of the deformation coefficient;

Fig. 2. Technique of determination of the physical�mechanical properties of thin films using indentation with a microindentorand visualization of the imprints by AFM.



imprint’s external diagonal for an Al film is 16.45 µm andthe microhardness value equals 136.9 HV, then themean diagonal of the internal imprint in silicon is6.25 µm, and the microhardness value in this case is948.3 HV. Thus, the microhardness values of both thefilms and the substrates can be estimated and com�pared from one imprint upon its visualization byAFM. For silicon with a plastic film on the surface,piles which are not typical for this material areobserved under such indentation conditions.

The imprint’s shape on a bulk aluminum sampleconsiderably differs from that produced on a film. Therhombic shape of the imprint in bulk aluminum, hav�ing correct faces but somewhat displaced along one ofthe diagonals, shows that bulk aluminum is less plasticthan films (Fig. 5). Defects on the faces form rectan�gular indentations on the plane, and their depth corre�

sponds to one or several interplanar distances in alu�minum.

Results of Investigation of the Microhardness�Imprint Faces for Revealing Changes in the Film Grain

Size upon Deformation

The face morphology of the microhardnessimprints in aluminum films shows that the imprint’ssurface microstructure is often rearranged under theinfluence of an indentor, and the degree of changes inthe grain size depends on the applied load value and onthe initial grain size. As shown in [6], both fragmenta�tion of the initial grain due to the formation of a networkof dislocations typical for grains with a size of 400 nm andmore, and the agglomeration of grains as a result of ashift of the small�grain (of about 150 nm) boundarycomprised of a cluster of dislocations are possible as a

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consequence of mechanical deformation. In the 10�nmaluminum film with initial grain sizes in the range 20to 60 nm (Fig. 6a), grains on the horizontal regions ofthe sliding steps formed on the silicon�substrate sur�face enlarge up to a size of 50 to 100 nm upon inden�tation with a load of 5 g (Fig. 6b). An increase in theload up to 20 g gives rise to an increase in the height ofthe sliding steps in the silicon substrate and to theinverse effect of decreasing grain size of a film on thestep surface by 10 to 30 nm. The initial grain size ispreserved on the lateral faces of the steps, but theirrearrangement occurs, i.e., the grains align in the direc�tion perpendicular to the step’s lateral face (Fig. 6c). The

grains in the specified films were of nanometer sizes.This facilitated their reorganization under the influ�ence of load and promoted the alignment of grains.Thus, the agglomeration and fragmentation of grainsin the 10�nm aluminum film with an initial grain size inthe range from 20 to 60 nm depends on the load value andsubsequently take place on the horizontal sliding steps ofsilicon with the maximum level of loading.

The disintegration of grains to the nanometer rangeunder load more clearly proceeds in aluminum filmswith a thickness of 200 and 1000 nm, and initial grain sizesranging from 200 to 600 nm and from 100 to 300 nm,respectively. Grains in the 200�nm film plastically

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Fig. 4. AFM images of the microhardness imprints on aluminum films of different thicknesses obtained under the following con�ditions: (a) 10�nm film, a load of 5 g, an imprint depth of 330 nm, and the area is 7 × 7 µm; (b) 200�nm film, a load of 2 g, animprint depth of 450 nm, and the area is 13 × 13 µm; (c) 1000�nm film, a load of 2 g, an imprint depth of 330 nm, and the areais 7 × 7 µm; (d) 1000�nm film, a load of 20 g, an imprint depth of 800 nm into the coating and 600 nm into silicon, and the areais 20 × 20 µm.



deform and elongate along the imprint’s edge underthe influence of a load of 5 g. The length of such grainsis about 1 µm at a width ranging from 300 to 400 nm.Some grains in the specified film initially had a nanoscalesubstructure with a subgrain size of 10 nm (Fig. 7a). Allgrains after indentation exhibit identical subgrainstructure, specifically, the striated structure with the

width of stripes in the range from 10 to 20 nm. Manystripe boundaries are oriented along the imprint’s edge(Fig. 7b). The obtained film structure fully corre�sponds to the fact of the appearance of a dislocationnetwork inside the grain with a size of 400 nm and itsfragmentation described in [6]. This result shows thatthe initial stages of changes under the influence of

Parameters of the Vickers imprints and material characteristics determined from them

Material

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Bulk Al 19.6 9.50 0.85 1.34 0.49 36 64 78

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19.6 10.15 0.92 1.42 0.50 35 65 69 36

49.0 10.90 0.91 1.53 0.62 40 60 76 77.9

196.0 11.80 0.95 1.65 0.70 42 58 204 266

980.0 14.45 2.07 2.02 – – 103 – 887.1

1000�nm Al film

19.6 10.00 0.82 1.40 0.58 41 59 39 37

49.0 8.75 0.87 1.22 0.35 29 71 2127 121

196.0 16.45 1.42 2.30 0.88 38 62 74 136.9

98.0 29.90 – 4.19 – – – – –

Bulk Bi 49.0 24.10 2.62 3.37 0.75 22 78 33 32–34 15.9

Bulk Si (100) 19.6 4.65 0.20 0.65 0.45 69 31 115 109–130 171.3

Bulk Ti 19.6 10.05 0.76 1.40 0.64 46 54 24 108–112 36.7

100�nm Ti film 19.6 5.85 0.34 0.82 0.48 58 42 84 112 108.2

100�nm TiN film 19.6 5.65 0.40 0.79 0.40 48 52 236 250–440 116

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mechanical loading on aluminum films with a grain sizeof more than 200 nm are the fragmentation of a graininto subgrains with nanometer diameter and alignmentof the subgrain borders in a certain direction.

The following stage of structural change under theinfluence of loading on the 200�nm aluminum film isthe disappearance of the borders of the initial grains.The border fragments of the initial grains are barelydistinguishable after the influence of a load of 20 g.Grains with a size of about 50 nm (Fig. 7c), which arealigned into arrays parallel to the larger axis of the ini�tial grain, are observed on the imprint surface. A fur�ther increase in the load up to 100 g breaks the linearalignment of the grains. Inconsistency in the aggregatesize increases, and then the grain�agglomeration stagebegins. Along with small grains with sizes in the rangefrom 20 to 30 nm, large blocks with a size of up to 400 nm

are present (Fig. 7d). The results of investigation of thedeformed surfaces of the 200�nm aluminum film showthat the stages of the fragmentation of large grains, dis�integration of the initial grain to nanometer size, andsubsequent merging the nanoscale grains into largerblocks proceed consecutively upon an increase in theload value.

The behavior of the 1000�nm aluminum film underload is similar to the behavior of the 200�nm film, butthe stages of plastic deformation of the initial grainsand the formation of disintegrated (nanoscale) grainsin it occur at lower load values. Probably, this isexplained by the smaller size of the initial grain and bythe presence of the nanoscale striated substructure inall the initial grains facilitating the rearrangement ofdislocations, of which the subgrain borders consist(Fig. 8a). The effect of the initial subgrain structure on

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the grain�agglomeration process is confirmed by thesurface topography of the lateral face of the imprintobtained at a load of 2 g. Not only plastic deformationof the initial grains, at which the length of one of thegrain axes considerably increases and the length of theother perpendicular axis decreases to the same extent,is observed in this case, but also the real consolidationof grains takes place (Fig. 8b), i.e., the maximum grainsize increases from 200 to 800 nm, while the minimumsize, which is perpendicular to the maximum size,increases from 50 to 150 nm. The degree of theiragglomeration at a load of 2 g considerably exceeds thedegree of deformation of the grains in the 200�nm filmunder a load of 5 g. Except agglomeration, the grainsshow the striated substructure as in the Al film with athickness of 200 nm. But its alignment is stricter, i.e.,all borders are arranged in the grain�elongation direc�tion, which coincides with the imprint’s edge (Fig. 8b).

The grains become smaller under the influence of aload of 5 g. Their size is 50 to 300 nm, and they arealigned perpendicularly to the imprint’s edge. Theimprint’s structural rearrangement under the influ�ence of a load of 100 g is fuller than in the 200�nm film.The grains are enlarged, their width is 500 nm andlength several micrometers, and the grain borders areparallel to the imprint’s edges. A nanoscale surfacestructure on extensive areas of the imprint’s surface isnot observed, and no substructure is present in thegrain (Fig. 8d). It was revealed upon studying the1000�nm film that the grain�agglomeration and disin�tegration stages alternate during the course of anincrease in the load values, whereas smaller initial grainswith the presence of nanoscale substructure facilitatestheir deformational rearrangement, and all stages pro�ceed more completely and at lower load values.

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For monocrystalline silicon of the (100) orienta�tion, sliding steps of a height of 20 nm are detected inthe imprint at a load of 100 g.

CONCLUSIONS

The advantages of using AFM methods for thevisualization of imprints upon determination of themicrohardness of thin films are shown experimentally.The application of AFM raises the accuracy of deter�mination of the microhardness value as compared tooptical microscopy by 1.5 times at a load of 20 g and by2 times at a load of 5 g due to higher magnification ofthe imprint’s image and possibility of ignoring piles.

The physical�mechanical properties of thin filmsand bulk materials are studied by the indentationmethod using a Vickers indentor, and the producedimprints are visualized by means of AFM. The elastic

modulus, deformation coefficient, and microhardness ofaluminum films with a thickness of 200 and 1000 nm andbulk materials, such as Аl, Ti, Si, and Вi, are deter�mined as a result.

The morphology of the aluminum films is investi�gated by the AFM method, and the shape and size ofgrains in the films are established. The size of thegrains in the 10�nm�thick film is comparable with itsthickness and is 10 to 50 nm; the shape of grains isroundish. The 200�nm�thick film is characterized byinhomogeneity in the shape and size of the grains.Grains with flat faces but smoothed vertices and edgeswith a size in the range from 500 to 800 nm, and roundishgrains with a size in the range from 200 to 500 nm arerevealed. There is a subgrain structure with a subgrainsize of 10 nm inside the roundish grains. The grain sizein the 1000�nm�thick film is 100 to 300 nm. At the

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160

120

80

40

0

1

0

10

Y, µ

m

X, μm

z, nm

35

30

25

20

15

10

5

0

1

010

Y, µ

m

X, μm

z, nm

30

10

–10

–30

–50

–70

(a) (b)

(c) (d)

Fig. 8. Comparison of (a) the initial morphology of the 1000�nm aluminum film with the morphology of the imprint face pro�duced by loads of (b) 2, (c) 5, and (d) 100 g; the area is 1 × 1 µm.



same time, an oriented nanoscale substructure with agrain size of 10 nm is observed inside the grain.

A change in the grain size in aluminum films underthe influence of a distributed load applied to the cen�tral field of the imprint’s face is established. The stagesof fragmentation of large grains, disintegration of theinitial grain to the nanometer range, and the subse�quent merging of nanoscale grains into larger blocksand then to grains also proceed successively along withan increase in the load value. The effects of the initialgrain size in aluminum films and the presence of thegrain substructure on the processes of deformationstructuring are shown.

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Translated by O. Kadkin

Deformation Structuring of Aluminum Films upon Microindentation

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