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Microstructure and morphology of electrodeposited NieP alloys treated by highenergy surface mechanical attrition

Ádám Révész a,*, László Péter b, Péter J. Szabó c, Péter Szommer a, Imre Bakonyi b

aDepartment of Materials Physics, Eötvös University, Budapest, H-1518, P.O.B. 32, Budapest, HungarybResearch Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525, Budapest, P.O.B. 49, HungarycDepartment of Materials Science and Engineering, University of Technology and Economy, Budapest, H-1111, Hungary

a r t i c l e i n f o

Article history:Received 18 November 2010Received in revised form28 April 2011Accepted 9 May 2011Available online 30 May 2011

Keywords:CoatingsAbrasionSurface propertiesHardness

a b s t r a c t

Fully amorphous NieP layer electrodeposited onto a Cu plate was subjected to severe plastic deformationusing surface mechanical attrition treatment in a high energy SPEX 8000 shaker mill. Two series ofexperiments using different milling conditions (series I: 20 6.35-mm balls; series II: 100 1.59-mm balls)were carried out to explore the mechanism of the process and to investigate the structure of thedeveloped coatings. The evolution of the microstructure and mechanical properties of the NieP layerafter the deformation process was studied by x-ray diffraction, scanning electronmicroscopy and hard-ness measurements. We demonstrate that the different mechanical treatments controllably influence themechanical behavior of the NieP metallic glass coating. When the vial of the mill is loaded with largerballs, deformation-induced Ni3P compound particles form in the amorphous matrix resulting in a hard(HV ¼ 17 GPa) but non-uniform coating. In the case of milling with many small balls, the increase in thesurface hardness is considerably lower (7 GPa) as a consequence of reduced impact energy.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Failure of engineering materials often begins at the surface, thusoptimizing the structure and properties of the surface layer maysubstantially improve the overall behavior. Surface mechanicalattrition treatment (SMAT) uses repeated multidirectional impactsby flying balls to induce surface hardening of bulk samples [1,2].The impacts cause plastic deformation in the surface layer of thetreated sample, leading to grain refinement and large grainboundary misorientation, dislocation blocks and microbands [3]. Itwas shown that the surface becomes chemically active, promotingthe effectiveness of subsequent treatments such as nitriding [1].Interdiffusion of alloying elements during the SMAT technique canlead to improved surface properties [4,5].

Mechanical alloying (MA), a solid-state powder processingtechnique based on severe plastic deformation, has been used forseveral decades [6,7]. In MA, the energetic collisions in a ball millare used to produce homogeneous alloys or compounds startingfrom blended elemental powder mixtures [6]. It is a well-established method to refine microstructure, and to preparemetastable phases such as supersaturated solid solutions [8] and

amorphous alloys [9]. Generally, the MA process involves repeatedcold welding and fracturing of powder particles in a high energyball mill at a low working temperature, with the maximummacroscopic temperature of the grinding vial not exceeding 50 OC[1]. Meanwhile, the inner surfaces of the grinding vial are alsocontinuously impacted by a number of flying balls carrying somepowder, resulting in the deposition of the treated powder on theinner surfaces. In general, such a phenomenon is regardedundesirable.

Nevertheless, from an opposite view, it may be potentiallyuseful to combine SMAT with MA to produce coatings on the innersurfaces of ball milled components [10e15]. Experience shows thatthe balls and the container walls always become coated with theprocessed powder during MA [16]. This phenomenon can beutilized for intentionally coating a disk attached to the containerwall [17e20]. The impacts of the milling balls activate the surface,deliver particles from the powder charge and attach them to thesurface. The primary benefit of using mechanical deposition is theenhanced bonding between substrate and coating due tomechanical activation. When a hardmaterial is coated with a softermetal, the process is dominated by simple deposition, as it wasdemonstrated for the deposition of Al onto steel [20]. The process ismore complex, if a soft surface is coated with a harder material,such as aluminum with nickel [19] or hard oxides [21]. In sucha case, the particles of the coating powder are pressed into the

* Corresponding author.E-mail address: reveszadam@ludens.elte.hu (Á. Révész).

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surface, causing substantial plastic deformation and mixingbetween the target and the particles. If the properties of thecomponents permit, new and potentially favorable phases, such ashard intermetallic compounds, may form [19] and even partialsolid-state amorphization can take place [22].

Recently, NieP coatings were prepared by mechanically assistedelectrodeposition [23] and electroless plating [24] by applyinga gentle ball-rolling during deposition. In the case of this low-energy technique, the sample is impacted by small glass ballssupplied by a continuous stirring of the solution. Experiments haveshown that the corrosion behavior and hardness of the NieP layertreated mechanically during plating increased considerablycompared to the corresponding parameters of the coating withoutmechanical assistance [23,24].

In this investigation, a continuous amorphous NieP metallicglass coating was initially electrodeposited onto a Cu plate and thenmechanically treated using the SMAT technique by attaching theplate as a target to the end surface of the milling vial of a highenergy SPEX 8000 shaker mill. For comparison, two series ofexperiments using different milling conditions were carried out toexplore the mechanism of the process and to investigate thestructure of the developed coatings.

2. Experimental

2.1. Electrochemical deposition

NieP amorphous layers were electrodeposited at 70 �C witha current density of 0.4 A/cm2 for 30 min onto a Cu plate witha diameter of 50mm and thickness 5mm. The Cu cathode platewaswashed with a detergent then cleaned with a sulfuric acid solutionof 1 mol/L concentration in an ultrasonic bath for about 3min. Afterthis procedure, the Cu plate was rinsed subsequently with ultra-pure water and ethanol, and it was left to dry in air. The electrolytesolution for the deposition contained nickel sulfate (fromNiSO4$7H2O) 0.54 mol/L, sodium hypophosphite (from NaH2-PO2$H2O) 0.18 mol/L, sodium sulfate (from Na2SO4$10H2O)0.31mol/L and formic acid (from 97% solution of HCOOH) 1.18mol/L[25]. Although the current efficiency of the depositionwas typically0.2 due to the intensive hydrogen evolution, formic acid served asa pH stabilizer. During the electrodeposition process, a Ni plateserved as the anode (counter electrode).

The cathode plate was placed into the electrochemical cell withthe active surface facing upward and filling the whole cross-sectionof the cell in order to avoid edge effects. Although the electrolytewas not stirred mechanically, the hydrogen evolution during theplating procedure agitated the electrolyte intensively. The electrodearrangement and this spontaneous stirring together ensureda uniform NieP deposit, see the disk in Fig. 1a.

2.2. Mechanical treatment

The subsequent mechanical treatment of the NieP deposits wascarried out using a SPEX 8000 Mixer Mill. It is a high energy shakermill that swings the hardened steel cylindrical milling vial (insidediameter 38 mm and length 57 mm) in a complex three-dimensional pattern. The main component of the vial motion isa vibration with a frequency of 17.6 Hz, approximate amplitude of25.6 mm, and a maximum velocity of about 2.8 m/s [26]. The Cutargets (disks) replacing the end cap of the steel milling vial werefixed tightly to the vial. In order to ensure an approximatelyuniform distribution of impacts, the container was filled witha large number of hardened steel milling balls. In the case of thefirst milling series (series I), 20 balls with a diameter of 1/400

(6.35 mm) were used for different treatment times (15 s, 1 min,2 min, 5 min, 30 min and 90 min). Another set of milling experi-ments (series II) was carried out by using 1300 balls with a diam-eter of 1/1600 (1.59 mm) for 5 min, 30 min and 90 min. For bothseries, the total mass of the impacting balls was about 21 g. Anoptical micrograph abundant in ball imprints envisages the effect ofthe heavy mechanical deformation occurring during the treatment(Fig. 1b).

2.3. Microstructural characterization

The as-deposited and mechanically treated NieP layers elec-trodeposited onto the Cu targets were used as samples for powderX-ray diffraction (XRD) measurements directly. The diffractogramswere collected with a Philips X’pert diffractometer, using Cu-Karadiation inQ�2Q geometry, in steps of 0.02�. For an analysis of themicrostructure, cross-sections of the treated disks were embeddedin epoxy and carefully polished mechanically. The morphology wasstudied by a Philips XL 30 scanning electron microscope (SEM) inbackscattered electronmode (BSE). Quantitative elemental analysis

Fig. 1. Optical micrograph of the as-deposited fully amorphous NieP layer on a copper plate (a) and of the mechanically treated layer of series I (b).

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was carried out by energy dispersive X-ray (EDX) analysis witha relative accuracy of 1%.

2.4. Mechanical characterization

Mechanical tests were carried out on the cross-section of thelayers using a CSIRO UMIS 2 device at constant load rate with anapplied maximum load of 1 mN. The dynamic hardness (HV) wascalculated using the OliverePharr method [27].

3. Results and discussion

A general view of the deposition process for series I and II can beinferred fromX-ray diffractionmeasured on the surface of the platesprocessed for different lengths of time (Fig. 2). The as-depositedspecimen is characterized by a broad halo corresponding to a fullyamorphous nature of the NieP layer. The observed sharp Bragg-peaks correspond solely to the Cu plate indicating that the depositis not thick enough to block diffraction from the target. When largerballs are used during the mechanical treatment (series I), theamorphous halo still dominates the pattern after 2 min of milling,however, some changes occur already after 5 min of deformation(Fig. 2a). At this stage, some crystalline peaks evolve superimposedon the spectrum. Longer milling results in a considerable change ofthe spectrum. The halo is less dominant and is accompanied withthe development of more intense peaks at 30 min and 90 min. Theinset depicts that the as-deposited fully amorphous NieP layerundergoes some (nano)crystallization during the heavymechanicaltreatment which results in the formation of Ni3P compound. Thegradual increase of the Cu-peaks is induced by the continuousimpact of the flying balls which can homogeneously compress theNieP layer and/or abrade some of the material, leading to a non-continuous coating.

Conversely, when the vial is loaded by smaller balls (series II)with similar total mass, the change in the XRD patterns is definitelyless pronounced. As seen in Fig. 2b, the crystallization after even90 min of milling is still a minor effect, although the formation ofNi3P is evident. The relatively smaller intensity of Cu-peakscompared to series I indicates less abrasion which is in accor-dance with the smaller energy delivery occurring during the indi-vidual impacts.

Note that the amount of Fe contamination originating from themilling media is negligible after each processing time according tothe XRD patterns.

SEM images taken from the cross-section of the specimens(Fig. 3) explain some aspects of the XRD patterns and exhibitseveral interesting features concerning the morphology of thetreated NieP layers. The as-deposited coating is absolutely uniformwith an average thickness of 35e40 microns. As also seen, thereexists a thin and uneven interface between the amorphous NiePlayer and the copper disk. The complementary EDX analysis takenat several points of the coating revealed that the layer compositionis Ni70P30.

As the impact of the 1/400 steel balls starts up (Fig. 3a), somecracks perpendicular to the surface develop after only 15 s ofmilling; however, the coating is still contiguous. The observedchange in the coating thickness can be attributed to variation of thecurrent density along the diameter of the disk [28] and to therobust plastic deformation. We can observe drastic changes up to2 min of deformation: the initially uniform coating breaks up intosmaller blocks, which develop across thewhole layer thickness. Theaverage composition of these blocks is identical to that of the initialstate.

At an intermediate stage (5 min) the embrittlement of thesurface layer goes on, nevertheless, the average thickness decreases

substantially. The lamellar blocks are clear sign of surface shearingand folding. Already at this point some of the amorphous materialis removed from the plate, resulting in some debris in the vial andan uneven and inhomogeneous coating in accordance with theincreased reflection of X-rays from the Cu target (Fig. 2a).

At the final stage of the milling process (30e90 min),a complete break-up of the NieP layer occurs, while some indi-vidual particles are visible as deep as 60e70 microns form thesurface. It seems that the continued impacts thoroughly generatea dynamic equilibrium of abrasion and deposition. The removedpieces of the NieP layer are powderized inside the vial and, lateron, these particles adhere to the surface of the balls whichre-transfer them to the target. These micron-scale processes are

Fig. 2. XRD patterns corresponding to series I (a) and II (b) obtained on the surface ofthe as-deposited and mechanically treated disks.

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supplemented by defect-enhanced diffusion and rearrangementof clusters on the atomic scale. As a consequence, almost nodebris was visible inside the vial. As the NieP fragments areharder than Cu, they get pressed into the target by the impactingballs through the intensive mechanical re-deposition. Accordingto the EDX measurements, these particles have a slightly differentconcentration (Ni74P26) which practically coincides with the Ni3Pcompound. It is evident that high energy milling results in notjust mixing but some nanocrystallization as well, see Fig. 2a.

A considerably different evolution takes place when muchsmaller balls fill up the container (Fig. 3b). A treatment of 5 mindestroys the NieP deposit much less. The average block size isabout double than for series I at a similar stage. Their shape is alsodifferent, i.e. the lack of the lamellar structure is an indication ofless shearing. At the end of themilling process (90min), we still canobserve some untreated NieP region at a depth of about20e30 microns from the surface. The top layer is somewhat

deformed; however, this structure is a consequence of the reducedimpact. The average composition of the top darker zone is Ni73P27,which probably contains some nanocrystalline inclusions;however, it is not really confirmed by the corresponding X-raydiffractogram (Fig. 2b). Also note that the lack of particles presseddeep inside the target indicates that the abrasion and re-depositionof the NieP material by the many tiny balls to the copper plate isnot dominant. In spite of the similar total ball mass for series I andII, the reduced impact originates from the lower energy of theindividual balls and from the toomany ball-to-ball collisions for thesmall balls.

Fig. 4 presents the hardness of the cross-section as a function ofdepth for the coatings. Indentations separated by 2 microns werecarried out along three parallel lines perpendicular to the coatingsurface. In order to reduce the effect of the rough and unevensurfaces, the data were averaged for the three lines in the case of allspecimens. As seen, the continuous as-deposited amorphous layer

Fig. 3. (a) SEM images taken on the cross section of the coated disks for series I. (b) SEM images taken on the cross section of the coated disks for series II.

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exhibits an average HV of 6 GPa which value is comparable to thatof electrodeposited nanocrystalline, amorphous and amorphousnanocomposite [29,30] NieP coatings and slightly higher than thatof the substrate made of hardened Cu. The obtained thickness(35 microns) is in line with the SEM image (see Fig. 3a).

The general behavior for both series shows similar features. Asthe heavy deformation is introduced by the impacting balls, theamorphous NieP surface region becomes considerably harder.After 2 min of milling, the hardness of the coating increases up to10 GPa for series I, while it reaches exceptionally high values(18 GPa) in some positions after 90 min. It has been demonstratedthat the hardness of as-plated NieP coatings can be increased byheat treatment resulting in the formation of Ni3P precipitates [29].Similarly, the remarkably high surface hardness values in thepresent experiment can correspond to the (nano)crystallized Ni3Pregions which formed during the heavy deformation. The observedlarge scatter of the data points even after averaging is due to thenon-uniformmicrostructure, including regions of much smaller HVvalues (7 GPa). Apart from this, we can observe some outstandingpoints deeper than 40 microns from the surface, in correlationwiththe pressed in NieP particles shown in the SEM images (Fig. 3a).Deeper than 50 microns from the surface, the measured HV valuesare practically identical to that of the copper substrate.

For series II, the increase in the HV values is considerably lower(7 GPa on average) due to the much smaller individual impactenergy. Consequently, the large number of repeated impactsproduced by the tiny stainless steel balls cannot compensate theeffect of the much less but larger balls, which is supported by thelack of any deformation-induced (nano)crystallization for series II.

As the high energy mill is running, the steel balls inside the vialimpact the as-deposited NieP layer repeatedly, abrading it, whileplastically deforming the upper-most surface layer. The change inthe morphology, structure and mechanical behavior developingduring the deformation treatment is a series of complex events.Understanding the details will require further investigations.

At the beginning of the process, the balls and the walls of thecontainer are free. The early impacts on the NieP target lead tosurface roughening and significant wear resulting in shearing andfolding and some debris. At this stage, the powderized amorphousNieP particles are mixed, even on atomic scale if the millingintensity is high enough (series I). In a short time, these highlydeformed fragments adhere to the surface of the milling balls andthe vial wall and the balls begin to transfer some material to the

already destroyed NieP layer, leading to further mixing on the topand below the surface, resulting in the formation of thedeformation-induced crystalline Ni3P inclusions of high hardness.After about 30 min of milling, a dynamic equilibrium betweenabrasion and re-deposition can set in. When the impact energy ismuch lower, this dynamic process is less pronounced, in accor-dance with the undeformed bottom layer of the NieP coating, thesmall hardness increase and the lack of crystallization.

A significantly different mechanism takes place in the work ofPing and coworkers, when the coating is much less impacted by theballs moving in the electrolyte during the mechanically assistedelectroplating technique [23]. As a result, the coating is stillcontinuous showing no evidence of shearing and folding.

4. Conclusions

An electrodepostied fully amorphous NieP layer was subjectedto severe plastic deformation using surface mechanical attritiontreatment in a SPEX 8000 shaker mill. Two series of experimentsusing different milling conditions were carried, i.e. in series Itwenty 1/400 (6.35-mm) balls were used, while series II was carriedout with one thousand and three hundred 1/1600 (1.59-mm) balls.

1. When the vial is loaded with the larger balls, the as-depositedfully amorphous NieP layer first breaks up into blocks and isabraded from the target. Later on, these abraded particlesadhere to the surface of the balls which re-transfer them to theactivated target. As the NieP fragments are harder than Cu,they get pressed into the target by the impacting balls throughthe intensive mechanical re-deposition, resulting in a hard(HV¼ 17 GPa on average) but non-uniform coating after 90minof treatment. During the milling, deformation-induced Ni3Pcompound particles form in the amorphous matrix.

2. In the case of milling with many tiny balls, the microstructuralchanges associated with the deformation are not so drastic. Atthe end of the milling process, some undeformed NieP regionscan still be observed. The top layer is somewhat deformed;however, this structure is a consequence of the reduced impact,in accordance with the lack of any (nano)crystallization. Theincrease in surface hardness is considerably lower (7 GPa onaverage).

Acknowledgment

Á. R. and P.J. Szabó are indebted for the János Bolyai ResearchScholarship of the Hungarian Academy of Sciences and for thesupport of the Hungarian Scientific Research Fund under grant No.OTKA F67893. The European Union and the European Social Fundhave provided financial support to the project under the grantagreement no. TÁMOP 4.2.1./B-09/1/KMR-2010-0003.

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