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Scripta Materialia 69 (2013) 732–735

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Wettability of nanotextured metallic glass surfaces

Harpreet Singh Arora,a Quan Xu,a Zhenhai Xia,a Yee-Hsien Ho,a

Narendra B. Dahotre,a Jan Schroersb and Sundeep Mukherjeea,⇑aDepartment of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA

bDepartment of Mechanical Engineering, Center for Research on Interface Structures and Phenomena, Yale University,

New Haven, CT 06511, USA

Received 27 July 2013; revised 18 August 2013; accepted 19 August 2013Available online 27 August 2013

The wettability of different nanotextured metallic glass surfaces is investigated. Wettability is quantified by the sessile drop tech-nique using a distilled water droplet. It is demonstrated that hydrophilic–hydrophobic nature of the metallic glass surface can becontrolled through nanotopography. The contact angle was found to increase from 70� for the flat metallic glass surface to 110�for a nanorod patterned surface. The difference in contact angle is explained in terms of the surface topography/roughness measuredusing atomic force microscopy.� 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Bulk metallic glass; Wettability; Atomic force microscopy (AFM); Texture

Metallic glasses are multi-component metallic al-loys that have exceptional stability against crystalliza-tion and remarkable properties, such as high strength(�2 GPa) and a high elastic strain limit (�2%) [1–4].Surface wettability of metallic glasses plays a major rolein determining their usefulness in a number of applica-tions, such as corrosion resistant coatings [5], bio-im-plants [6] and catalysis [7]. Metallic glasses can bethermoplastically processed across multiple length scales(macro/micro/nano) to fabricate complex shapes andsurface patterns with great precision and on the smallestlength scales [4,8,9]. Pairing desired wetting characteris-tics of metallic glass surfaces with other properties en-hances their potential application range.

Wettability has been widely studied as a function of amaterial’s chemistry, surface texture and processing con-ditions. Surface energy and morphology are believed tobe the main factors affecting wettability [10]. The influ-ence of surface topography on wettability has been stud-ied for polymeric materials with varied nanotextures [11]and different processing conditions [12]. Wettability hasalso been investigated for lithographically patterned sil-icon surfaces with different sizes, shapes and spacing ofposts [13]. While there are number of studies on wetta-bility of metallic alloys [14–16], the influence of differentnanotextures has not yet been thoroughly investigated

1359-6462/$ - see front matter � 2013 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2013.08.014

⇑Corresponding author. Tel.: +1 940 565 4170; fax: +1 940 5652944; e-mail: sundeep.mukherjee@unt.edu

[17,18]. Wettability of different metals with water hasbeen reported under continuous condensing conditions[19]. Most noble metals (including Au, Pd and Pt) exhi-bit a high contact angle compared to the less noble met-als. Other studies have demonstrated that metal surfacesfree from surface contaminants exhibit a very small(close to 0�) contact angle [20,21]. Metallic glasses withtunable surface characteristics offer the opportunity tostudy wettability as a function of nanotexture in metals.Thermoplastic forming and electrochemical processingenable controlled surface texturing of metallic glassesat the nanometer length scale.

In the present study, the effect of nanotopography onthe wettability of Pd-rich metallic glass surfaces has beeninvestigated. Periodic nanorod patterns were fabricatedby thermoplastic processing of Pd43Ni10Cu27P20 metallicglass [4,8,9]. In addition, Pd-rich anisotropic nano-morphologies were obtained by electrochemical process-ing of Ni60Pd20P17B3 metallic glass [22,23]. Wettabilitystudies were done using a sessile drop experiment witha droplet of distilled water. The contact angles on thenanotextured surfaces were compared with those on flatPd43Ni10Cu27P20 metallic glass and a pure palladiumsurface. The contact angle was found to be directly cor-related to the surface roughness, measured using atomicforce microscopy (AFM).

Amorphous alloys of desired compositions were pre-pared in vacuum-sealed silica tubes by melting high-pur-ity constituents. Fully amorphous rods were achieved by

sevier Ltd. All rights reserved.

Figure 1. SEM images for palladium-rich metallic glass surfaces with (a) a nanorod structure, (b) a nanoporous morphology and (c) nanodendrites.The periodic nanorod pattern was fabricated by thermoplastic processing, while the nanoporous/nanodendritic morphologies were obtained byelectrochemical processing of metallic glass [23].

H. S. Arora et al. / Scripta Materialia 69 (2013) 732–735 733

water quenching molten samples from a temperature of1000 �C after appropriate B2O3 fluxing. To obtain Pd-rich nanorod structures, Pd43Ni10Cu27P20 amorphousalloy was thermoplastically processed in the supercooledliquid region above its glass transition temperature(Fig. 1a). The metallic glass softened into a viscous li-quid at the thermo-plastic forming temperature and flo-wed into a nanomold under a controlled appliedpressure. Commercially available anodized aluminamolds were used as a template for the nanoforming.To obtain Pd-rich nanostructures by electrochemicalprocessing, Ni60Pd20P17B3 metallic glass was used in aclassical three-electrode setup with an acidic medium.Depending on the range of applied potential, widely dif-ferent structures were obtained [22,23]. At potentials lessthan a critical value, enhanced surface diffusion of thenoble metal atoms at the alloy–electrolyte interface re-sulted in a nanoporous network, as shown in Figure 1b.Similar nanoporous structures have been obtained forother metallic glasses by electrochemical dealloying[24,25]. For potentials greater than a critical value, dis-solution and redeposition of palladium atoms resultedin highly branched dendrites with nanoscale substruc-tures, as shown in Figure 1c.

Figure 2. Contact angle measurement using a distilled water droplet for: (a)morphology and (d) a nanodendritic morphology.

Wettability studies were performed for the differentnanostructured metallic glass surfaces by the sessile droptechnique using a distilled water droplet. The contactangles of distilled water droplets on different metallicglass surfaces are shown in Figure 2a–d. The contact an-gle increases from 70� (Fig. 2a) for a flat Pd43Ni10-

Cu27P20 metallic glass surface to 110� (Fig. 2b) for thesame material having a nanorod pattern with rod diam-eter of 200 nm. The contact angle is 84� for the Pd-richnanoporous structure (Fig. 2c), while it increases to 112�for the nanodendritic structure (Fig. 2d). The contactangle on a flat pure-palladium surface was found to be60� (not shown). Both the flat metallic glass and the purepalladium surfaces were diamond polished in the sameway prior to wettability testing.

The topography and roughness of the surfaces weredetermined by AFM using an Si tip. Figure 3a–c showsthe topography of the different surfaces. The surface ofthe flat Pd43Ni10Cu27P20 metallic glass shown in Fig-ure 3a was very smooth, with an average roughness ofabout 2 ± 0.36 nm. Similar to the flat metallic glass,the roughness of the pure Pd surface had an average va-lue of 1.3 ± 0.35 nm. The average roughness for thenanoporous specimen, shown in Figure 3b, was found

a flat metallic glass surface, (b) a nanorod surface, (c) a nanoporous

Figure 3. AFM images showing the surface topography for (a) a flat metallic glass surface, (b) a nanoporous surface and (c) a nanorod surface; (d)plot showing topographic variation in the nanorod surface. The average roughness values (Ra) for the different surfaces are indicated.

734 H. S. Arora et al. / Scripta Materialia 69 (2013) 732–735

to be nearly 7.05 ± 0.20 nm. Figure 3c represents thetopography of the Pd43Ni10Cu27P20 metallic glass sur-face with nanorods. The roughness of this specimenwas found to be nearly 58.4 ± 0.26 nm. The topographychanges on the surface of the nanorod specimen are rep-resented in Figure 3d. It can be observed from this plotthat the rods are uniformly spaced, with small variationsin their peak heights. The roughness of the nanodendrit-ic surface could not be obtained reliably due to its highlybranched and complex structure.

In addition to the average roughness, other param-eters characterizing the roughness at the nanometerlength scale were obtained, including the root-mean-square roughness (r), the in-plane roughness correla-tion length (n) and the roughness exponent (c) [26].The correlation length (n) describes the average dis-tance between consecutive hills or valleys on the sur-face. On the other hand, the roughness exponent (c)is a measure of the degree of surface irregularity atlength scales smaller than the correlation length. Theheight-to-height correlation function (HHCF), givenas Hðr; r0Þ ¼< hðrÞ � hðr0Þ½ �2 >¼ 2r2½1� Rðr; r0Þ�, wasused to determine n, where R(r,r0) is the autocorrela-tion function [27]. For a homogeneous and isotropicsurface, the HHCF depends on the distance,z = |r � r0|, between the two points, which can beexpressed as Hðr; r0Þ ¼ ½HðzÞ� ¼ 2r2½1� RðzÞ�. In termsof n and c, the preceding equation can be written as

Table 1. Roughness parameters for different surfaces obtained using atomic

Sample Flat Pd Flat m

Average roughness, Ra (nm) 1.3 ± 0.35 2 ± 0.RMS roughness, r (nm) 10.53 ± 0.33 13.39Correlation length, n (nm) 442.41 ± 1.9 534.67Roughness exponent, c 0.77 0.72Long wavelength roughness ratio, r/n 0.02 0.02

HðzÞ ¼ 2r2f1� exp ½� z=fð Þ2c�g [28]. Height-to-heightcorrelation curves obtained for the nanotextured andflat surfaces were used to obtain the values of r andn (Table 1). At small distances (z� n), HðzÞ is directlyproportional to z2c. The slope of the straight line of theH(z) vs. z curve on the log–log scale gives c (Table 1)[29]. The long wavelength roughness ratio (r/n) forall the surfaces is also included in Table 1.

The contact angle (h) of a liquid droplet on a flat sur-face is given by Young’s equation [30],cosðhÞ ¼ ðcsa � cslÞ=cla, where csa, csl and cla are the inter-facial tension of solid–air, solid–liquid and liquid–aircontact. Wenzel [31] modified the relation given byYoung and proposed an equation, cos hrough ¼ f cos h,for the determination of the contact angle on a rough sur-face. In the Wenzel equation, hrough is the contact angleon a rough surface, h is the contact angle on the samematerial with a smooth surface and f is the roughness fac-tor (actual surface area/geometric surface area). Assum-ing that a liquid droplet in contact with a micro/nanotextured surface has a composite interface, Cassieand Baxter [32] proposed the relation cos hcomposite sur-

face = f1 cos h � f2, where f1 is the fraction of the solid–li-quid interface area and f2 is the liquid–air interface area.

From the quantitative roughness measurements, it isclear that nanotextured surfaces have greater roughness.Liquid penetration into crevices is possible for weakroughness surfaces, i.e. for c P 0.5 and r/n� 1 [26].

force microscope.

etallic glass Nanoporous texture Nano-rod texture

36 7.05 ± 0.20 58.4 ± 0.26± 0.50 30.82 ± 0.23 74.46 ± 0.20± 25 362.3 ± 4.2 85.65 ± 2.3

0.75 0.720.08 0.87

H. S. Arora et al. / Scripta Materialia 69 (2013) 732–735 735

The roughness exponent, c, is nearly the same for all ofthe surfaces (Table 1). However, the value of r/n is high-er for nanotextured surfaces than for flat surfaces. Asmall n or large r/n indicates the presence of sharp localirregularities on the surface, where the liquid fails topenetrate the air pockets (Cassie and Baxter regime).However, for surfaces with a small r/n, the liquid caneasily flow/spill into valleys/hills (Wenzel regime) [33].Thus, the higher contact angle for nanotextured surfacescan be explained on the basis of their greater roughness(r) and shorter correlation length (n). Further, the flatPd and metallic glass surfaces show comparable contactangle values, indicating that the surface chemistry doesnot have a pronounced influence.

To eliminate the influence of adsorbed contaminants,all nanostructured surfaces were ultrasonically cleanedfor several minutes prior to the contact angle measure-ments. After cleaning, the surface atomic layers were ana-lyzed using X-ray photoelectron spectroscopy (XPS). TheXPS analysis of the flat Pd metallic glass surface is givenin Figure 4. It shows the presence of carbon and oxygenalong with the constituent elements of the metallic glass(Pd, Ni, Cu, P). Similar peaks for carbon and oxygen werefound for all the nanostructured surfaces. This indicatesthat ultrasonic cleaning is ineffective in completelyremoving all of the contaminants from the specimen sur-face. These contaminants may be present in the form ofoxides and/or adsorbed monolayers on the surface. Ithas been reported in earlier studies [20,21] that metal sur-faces devoid of any surface contamination are hydro-philic, with a contact angle close to 0�. The surfaces inthese studies were cleaned with cold argon plasma or anopen flame [20,21]. The presence of surface contaminantstends to increase the contact angle. In the current study,all the specimens were cleaned by exactly the same meth-od prior to contact angle measurement. The degree andnature of contamination is the same across all the nano-structured surfaces. Therefore, the observed variation incontact angle is attributed entirely to the difference in sur-face topography and roughness.

In summary, it is demonstrated that the wetting ofwater on metallic glass surfaces can be influenced by

Figure 4. XPS analysis of the flat Pd43Ni10Cu27P20 metallic glasssurface. The presence of carbon and oxygen impurities can be seen inaddition to the constituent elements of the metallic glass.

nanotopography, which can be obtained by a varietyof processing techniques. The changes in the wetting an-gle are significant; wetting behavior untypical for metalscan be achieved solely by changes in topography.

J.S was supported by the National ScienceFoundation under MRSEC DMR-1119826.

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