Letter pubs.acs.org/NanoLett
Functionalization of Metallic Glasses through Hierarchical Patterning Molla Hasan,† Jan Schroers,‡ and Golden Kumar*,† †
Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06511, United States
‡
S Supporting Information *
ABSTRACT: Surface engineering over multiple length scales is critical for electronics, photonics, and enabling multifunctionality in synthetic materials. Here, we demonstrate a sequential embossing technique for building multitier patterns in metals by controlling the size-dependent thermoplastic forming of metallic glasses. Sub-100 nm to millimeter sized features are sculpted sequentially to allow an exquisite control of surface properties. The process can be integrated with net-shaping to transfer functional patterns on three-dimensional metal parts. KEYWORDS: Metallic glass, thermoplastic embossing, hierarchical patterns, wetting, functional surfaces
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plastic fabrication of multifunctional structures4,17,49 has been challenging in metallic glasses. This limitation stems from high surface tension (∼1 N m−1 vs ∼0.03 N m−1), viscosity (∼107 Pa·s vs ∼105 Pa·s), and thermal conductivity (∼15 Wm−1K−1 vs ∼0.2 Wm−1K−1) of metallic glasses compared to polymers in the supercooled liquid state. Low surface tension and viscosity of thermoplastic polymers result in low processing pressure for replica molding of nano- to macro-sized features of any shape.20,50,51 In contrast, high surface tension and viscosity of metallic supercooled liquids cause a strong size-dependent pressure requirement for thermoplastic molding.28 Additionally, low thermal conductivity and viscosity of polymers allow localized heating and restructuring of preformed features to build hierarchical architectures.52,53 Such controlled large thermal gradients are not feasible in metallic glasses because of their high thermal conductivity and risk of heating-induced devitrification. To overcome these hurdles, we developed a sequential thermoplastic embossing of metallic glasses which enables deterministic fabrication of hierarchical structures without use of hierarchical molds or temperature gradients. The pressure required for thermoplastic embossing of metallic glasses can be quantified by using creeping flow conditions and capillary contributions.27 For a cylindrical feature, it becomes
ature often employs topography in tandem with chemistry to achieve desirable properties in living organs and biological materials.1−4 Structural hierarchy has been recognized as the vital aspect in a broad range of examples, such as the exceptional adhesive properties of gecko feet,5−7 the reduced drag of shark skin,1,8 and the self-cleaning of lotus leafs.8−10 Despite the remarkable success of bioinspired approach in polymers and semiconductors,11−18 this avenue of surface engineering is less explored in metallic materials. This disparity is directly related to the fabrication capabilities for different material classes. A wide range of techniques capable of surface patterning such as lithography, imprinting, selfassembly, selective etching, and electrospinning have been developed for semiconductors and polymers.19,20 Many of these techniques are not suitable and scalable for metals due to their high surface energy, oxidation, and isotropic etch rates. Thin film deposition or electroplating on lithographically structured templates can create patterned metal surfaces,21 but building hierarchical structures through these techniques is challenging. Selective etching or dealloying is also being explored for creating hierarchical patterns from noble metals.22−25 The inherent challenges to these existing techniques are complexity, cost, independent control over different length scales, and limited application to practical 3D parts. In recent years, metallic glasses have been explored to bridge the processing gap between metals and polymers.26−30 Metallic glasses exhibit high strength and elasticity due to their amorphous structures31−35 and retain the thermal,36,37 the electrical,38,39 and the optical40,41 properties of metals due to metallic bonding. Their unique structure and composition enable access to a supercooled liquid state at temperatures much lower than the melting temperature. This low temperature supercooled liquid state of metallic glasses has been used in developing a wide range of thermoplastic shaping operations inspired from processing of polymers.42−48 Despite a similar flow mechanism, adapting polymer techniques for thermo© 2015 American Chemical Society
32η ⎛ l ⎞ 4γ cos θ ⎜ ⎟ − ⎝ ⎠ t d d 2
P=
(1)
where P is the applied pressure, η is the viscosity of the metallic glass supercooled liquid, t is the molding time, l is the length and d is the diameter of cylindrical feature, γ is the surface tension, and θ is the dynamic contact-angle between the Received: September 29, 2014 Revised: December 29, 2014 Published: January 5, 2015 963
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metallic glass and the mold. Figure 1 plots the required pressure for embossing of cylindrical features with diameters ranging from sub-100 nm to 1 mm under typical thermoplastic conditions and interfacial properties of metallic glasses. The embossing pressure exhibits strong size dependence due to high surface tension (second term in eq 1), which can become comparable or even higher than the viscous pressure (first term in eq 1). In polymers, the pressure is typically in the range of 1−10 MPa for embossing of any sized features.50,51,54 For metallic glasses, the molding pressure increases from below 1 MPa for macro-scale features to above 100 MPa for nanoscale features (Figure 1). This large variation in embossing pressure requires different mold materials for different length scales. We decouple the size-dependent pressure and mold requirements by sequential embossing of metallic glass features of different sizes. This allows fabrication of complex hierarchical structures that are challenging with single step embossing and other metal processing techniques. Most of the results presented here are from Pt57.5Cu14.3Ni5.7P22.5 metallic glass but the methodology is applicable to other metallic glasses and polymers suitable for thermoplastic forming. Typical thermoplastic processing temperatures for the considered metallic glass are in the range of 250−280 °C and the corresponding viscosity values are between 109 to 106 Pa·s.55 Experimental details are provided in the Supporting Information document. Figure 2 shows a schematic illustration of a sequential embossing scheme and the resulting multi-tier patterns formed on metallic glass surfaces. In the first step, metallic glass is patterned by embossing against a mold that consists of features of one length scale. Due to a size-dependent pressure requirement (Figure 1), the first embossing step is performed using a mold of the smallest feature size. To add a second length scale, the patterned metallic glass is used as a feedstock for the second embossing, which is carried out using a mold with larger features. The outcome after the second embossing is a metallic glass textured with 2-tier patterns. As demonstrated by examples shown in Figure 2, features ranging from sub-100 nm to 500 μm can be incorporated in two-tier hierarchical patterns. Subsequent embossing by using the metallic glass textured with 2-tier features results in formation of a hierarchical surface composed of three length scales. This fabrication sequence can be continued as long as the metallic glass can deform thermoplastically. The key requirement for sequential thermoplastic embossing with metallic glasses is to retain the surface features during multiple operations. This can be ensured by selecting feature size and processing conditions based on guidelines depicted in Figure 1. There are two modes by which the metallic glass surface features can distort during thermoplastic embossing: (i) physical contact with the mold can smear out the features and (ii) Laplace pressure (∝ surface tension/radii of curvature) driven viscous flow can alter the shape of features.42 Our approach to minimize the direct contact with mold is by embossing progressively larger features and by utilization of trapped air as a protective cushion (Figure 2). The increase in feature size lowers the required molding pressure and, hence, reduces the smearing effect. Before embossing, the mold features are occupied by air that remains trapped during mold filling and prevents squashing of the metallic glass surface against the mold. It was observed that the presence of air also reduced the surface-tension-driven deformation of features. The effectiveness of the air-cushion was investigated in detail by studying the shape of metallic glass surface features annealed in
Figure 1. Thermoplastic forming pressure for cylindrical metallic glass features of diameters ranging from sub-100 nm to 1 mm. The pressure was calculated from eq 1 using a viscosity of 107 Pa·s, a molding time of 100 s, a surface tension of 1 N m−1, and a wetting angle of 180°. The pressure requirements are distinct in macro- to nanoscale regions because of strong contributions from the capillary pressure.
Figure 2. Schematic illustration of sequential embossing of metallic glasses at T > Tg. In the first step, metallic glass is patterned with the smallest sized features by embossing against a mold. The resulting patterned metallic glass is subsequently embossed onto a mold with larger features. This sequential patterning can be continued to add more length scales. The key requirement is to prevent the direct contact between the existing pattern features during subsequent molding operations. The trapped air in the mold cavities is utilized as a protective cushion between the metallic glass and the mold.
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Figure 3. Hierarchical patterns composed of various length scales and shapes fabricated by sequential embossing of metallic glasses. (a) Hemispherical blobs of 200 μm diameters are covered with rectangular microholes. (b) Holes of 50 μm diameters are surrounded by 200 nm diameter nanopillars. (c) Square and cylindrical shaped microposts are patterned with 100 nm diameter nanopillars.
any size if a physical contact between the pattern features and the heating media is prevented. By a suitable choice of size and morphology of mold features, a broad range of multi-tier hierarchical patterns can be fabricated through sequential embossing of metallic glasses. Figure 3 shows examples of hierarchical patterns consisting of different combinations such as microblobs decorated with microholes (Figure 3a), microholes surrounded by cylindrical nanopillars (Figure 3b), and square-shaped microposts covered with cylindrical nanopillars (Figure 3c). Similar structures in thermoplastic and thermosetting polymers are fabricated by either replication of hierarchical templates56,57 or sequential stamping of progressively smaller features. 52,53,58 These techniques are not practical for metallic alloys because of strong capillary effects and stringent template requirements. However, the approach presented here (additive embossing of larger features) is very versatile and can be applied to any thermoplastic material. We have successfully fabricated dualscale patterns in PMMA (poly(methyl methacrylate)) without using hierarchical templates through this methodology (Figure S3 in Supporting Information). In polymers, patterning can be even applied to nonplanar surfaces through room temperature replication of deformed elastomeric templates.19,59 Such soft templates will not withstand high temperature and pressure required for embossing of metallic glasses. Instead, we incorporate blow molding47 in the sequential process to generate nonplanar patterned surfaces from metallic glasses (Figure 4). Patterns can be placed on either side or even on both sides because both
Figure 4. Schematic illustration and experimental demonstration of fabrication of nonplanar metallic glass surface covered with hierarchical patterns. Pre-patterned metallic glass is thermoplastically blown out-ofplan by air pressure (a). Top (b) and cross-sectional (c) views of a hollow metallic glass dome decorated with multiscale features spanning from 150 nm to 500 μm (d).
air and oil (see Figures S1 and S2 in Supporting Information). These results suggest that metallic glasses will retain patterns of 965
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view, and the details of hierarchical features (Figure 4d) demonstrate the feasibility of our approach for hierarchical patterning of nonplanar surfaces. A common challenge of surface patterning techniques is their limited applicability to actual 3D parts. The practicality of sequential patterning described here allows one to integrate patterning and net-shaping techniques to fabricate microparts with desirable topography. This can be achieved by forming pre-patterned metallic glass into a mold of desirable feature shapes (Figure 5). The excess metallic glass is removed by a planarization process reported in previous studies.27,60 Subsequently, separation of metallic glass features from the mold yields 3D objects which are textured with hierarchical features. Figure 5 shows SEM images of various patterned metallic glass microparts. Figure 5(b−d) shows a close up of the surface patterns in selected 3D objects. Though, examples shown here are for generic shapes, the process can be tuned for more complex geometries such as implants61 or MEMS (microelectromechanical systems) components.46,62 Surface patterns consisting of different length scales and shapes are of considerable interest in technologies because of their proven effectiveness in Nature.1,6,63−66 Ability to control topography of polymers at any scale has enabled studies of bioinspired structures1,2,5 as well as their applications in cell biology,49 microfabrication67 and optics.68,69 Similar studies and utilization of metallic structures are deficient because of fabrication challenges. We evaluate the potential of proposed processing in functionalization of metals by characterizing the wetting properties of textured metallic glasses. Figure 6 shows the effect of topography on static contact angle of flat and textured Pt57.5Cu14.3Ni5.7P22.5 metallic glass. This metallic glass is intrinsically hydrophilic with a contact angle of 58 ± 5° (Figure 6a). An increase in surface roughness through patterning with single length scale features (microholes, microrods, and nanorods) reduces the contact angle (Figure 6b−d), indicating the formation of Wenzel state. In contrast,
Figure 5. Integration of sequential embossing and net-shaping for applying hierarchical patterns on 3D objects. A pre-patterned metallic glass is formed into a mold with desirable shape (e.g., gear) and followed by established planarization to obtain patterned 3D object (a). Overview of various microparts (b) and their surface textures (c− e) demonstrate the feasibility of pattern application on net-shaped metallic glass parts.
surfaces remain mostly contact-free during out-of-plane deformation. The geometry of a curved metallic glass can be tailored by selecting an appropriate anchoring support.47 In the present case, a circular anchoring support was used to produce a metallic glass dome patterned with multiscale features (Figure 4). The top view (Figure 4b), the cross-sectional (Figure 4c)
Figure 6. Effect of surface patterning on wetting behavior of Pt57.5Cu14.3Ni5.7P22.5 metallic glass. The flat sample is hydrophilic (a: contact angle = 58°) and remains hydrophilic upon patterning with single length scale features such as microholes (b: contact angle = 23°), microrods (c: contact angle = 48°), and nanorods (d: contact angle = 13°). Addition of second length scale to the nanopatterned metallic glass turns it hydrophobic with contact angle above 130° (e−g). The topography controlled wetting can be used to create hybrid metal surface covered with hydrophobic and hydrophilic domains (g). The measurement error in contact angle is ±5°. 966
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Nano Letters the formation of dual-scale texture renders the metallic surface hydrophobic with contact angle exceeding 130° (Figure 6e−f). Even higher contact angles were possible by incorporating an additional level of texture. These results support the existing models for stabilization of Cassie−Baxter state through hierarchical roughness.15,53,70,71 The transition from Wenzel to Cassie−Baxter shown here is purely due to topography because the Pt57.5Cu14.3Ni5.7P22.5 metallic glass is chemically inert. In previous reports on enhancement of hydrophobicity of Pd-rich72,73 and Zr-rich74 metallic glasses, the effect of topography and composition cannot be unambiguously discerned due to possibility of oxidation. As shown in Figure 6g, topographic control alone can be used to engineer hydrophobic and hydrophilic domains on the same metal. Such hybrid surfaces with coexistence of hydrophobic and hydrophilic regions are relevant for directed-assembly75 and water-harvesting76 applications and for fundamental studies on droplet condensation.77 Sequential patterning technique introduced here is not limited to the amorphous state of glass forming alloys. The hierarchically patterned metallic glasses can be annealed to create crystalline or composite structure. We studied the effect of annealing induced crystallization on various metallic glass patterns (see Figure S4 in Supporting Information). Overall, the results showed that patterned crystalline surfaces can be obtained by a careful selection of annealing conditions. To our knowledge, there is no scalable processing technique for fabrication of controlled surface topography in multicomponent crystalline metallic alloys. The characterization of amorphous and crystalline samples with comparable patterns provides a novel tool to understand the effect of atomic structure on the properties of textured metals. In summary, we demonstrate that nano-, micro- and macrosized features can be added in a sequential order to build hierarchical structures using thermoplastic embossing of metallic glasses. The independent control over size, shape, periodicity, and crystallinity of pattern features can enable engineering of metallic surfaces with controlled functionality such as wetting, cellular response, electrochemical activity, and optical properties. Versatility and practicality of this processing approach suggest that it can be applied to any thermoplastic material.
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ACKNOWLEDGMENTS
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REFERENCES
The work of M.H. and G.K. was supported by National Science Foundation through CMMI-1266277. J.S. would like to acknowledge the financial support from National Science Foundation under Grant No. MRSEC DMR 1119826.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, effect of air-cushion on sequential embossing, fabrication of patterned crystalline metals, and application of sequential embossing to thermoplastic polymers. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
G.K. and J.S. conceived the idea. M.H. and G.K. designed and conducted the experiments and analyzed the data. M.H., G.K., and J.S. wrote the manuscript. Notes
The authors declare no competing financial interest. 967
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