Electroplating in the Context of Worldwide Nanotechnology Initiatives: A Heritage Paper

Editor’s Note: At the turn of the 21^st century, nanotechnology was coming into its own and about to become a vital technological field, central to many aspects of life in the world today.  Originally published in the inaugural issue of the AESF Journal of Applied Surface Finishing, 1 (1), 16-22 (2006), this paper considered the outlook in the surface finishing industry for nanotechnology as it was on the verge of the transformation from a scientific curiosity to a significant technology.  A printable PDF version of this report is available by clicking HERE.

ABSTRACT

This paper deals with two important issues of interest to the electroplating and surface finishing industries. In the first part, a summary is presented on recently established nanotechnology initiatives in various countries around the world. Program funding levels and core activities will be compared to provide a basis for assessing business opportunities for various industries. The second part of the paper looks at specific examples of nanostructures made by electrochemical methods currently at various stages in their development, or already in use. These include electrodeposition of monolithic pure metals and alloys, multi-phase composites, compositionally modulated multilayers, template manufacture by anodizing for nanodot and nanowire synthesis and electroplating of components for microelectromechanical systems (MEMS).

Worldwide nanotechnology activities

Nanotechnology is a relatively new multidisciplinary field in which many of the unusual physical and chemical effects observed only on the nanoscale (typically less than 100 nm) are utilized to create new materials, devices and objects with outstanding properties and functions never seen before.  Nanotechnology includes several subdisciplines which can be summarized as shown in Fig. 1. The various subfields have their origin in different disciplines and were initially developed by scientists and engineers with different backgrounds.  For example, nanostructured materials were originally developed by physicists and materials scientists, while bio-inspired nanotechnology was the domain of chemists and biologists.

Today’s’ nanotechnology approach requires the breakdown of traditional barriers between these fields and strong collaborative efforts of people with quite different but complementary skill sets.  

The enormous long term potential of nanotechnology in revolutionizing our approach to technology, which would ultimately result in increased productivity, a deeper understanding of nature and a better quality of life, was recognized in the late 1990s in many different countries. Table 1¹ summarizes various comprehensive research programs in nanotechnology with government funding levels in excess of US$100 million per year, established after 2000.

President Clinton’s initial nanotechnology initiative was launched in 2000, and was later enhanced under President Bush to become the 21st Century Nanotechnology Research and Development Act.² Table 2¹ shows a breakdown of funding levels for various Federal Departments or Agencies for 2000-2005 within the U.S. National Nanotechnology Initiative (NNI).  Over this period the total funding level increased by more than 360% to close to US$1 billion for 2005.

Similar growth rates were observed in other countries such as Western Europe and Japan (Table 3¹).  The total worldwide spending on nanotechnology reached US$3 billion by the year 2003.

The funding comparison for Asia-Pacific countries presented in Table 4³ clearly shows that even smaller countries such as Australia, New Zealand or Singapore invest heavily in selected areas of nanotechnology R & D.  China, Taiwan and South Korea have established very aggressive programs with South Korea alone contributing US$1 billion over the five year period 2003-2007.

The expected returns on investment in nanotechnology are huge. Table 5² gives a breakdown by area of the projected market size by 2015, totaling US$1 trillion. The largest sectors are nanostructured materials and electronic components which probably reflects the relatively long nano research activity in these areas. For example, nanostructured materials were already introduced in the early 1980s⁴ with considerable research efforts throughout the 1980s and 1990s. It was also during this period when most of the early fundamental work on nanostructured electrodeposits was carried out.⁵

In view of the great potential for new business opportunities arising for the electroplating industries from the worldwide activities in nanotechnology, in particular nanostructured materials, AESF established the Nanomaterials Subcommittee in 2002.  A first comprehensive paper, on “Nanotechnology Opportunities for Electroplating Industries,” was published in Plating & Surface Finishing in 2003⁶ and full day sessions on Nanomaterials were held at the SUR/FIN 2003 in Milwaukee and SUR/FIN 2004 in Chicago.  It was also in 2004 that the Nanomaterials Subcommittee was accorded full Committee status.

The remainder of this paper deals with specific examples which look at current and future application areas for electroplating and surface finishing industries in nanotechnology.

Monolithic metal nanostructures

Synthesis methods for the production of monolithic electrodeposits with crystal sizes less than 100 nm have been developed since the early 1980s. One of the earliest systems that was extensively studied was Ni-P,^7,8 using bath formulations very similar to the ones developed by Brenner, et al.,⁹ and direct current plating. Particular emphasis was on electrodeposits with very narrow grain size distributions and average grain sizes in the 5 to 50 nm range. An example of such a structure is shown in Fig. 2. In the late 1980s, emphasis shifted towards more complex alloy systems (e.g., Ni-Fe, Co-Fe, Ni-Mo, Zn-Ni and Ni-Fe-Cr) and the application of pulsed current deposition of pure metals such as Ni, Co or Cu.^10,11

During electrodeposition, two competing factors control the grain size of the electrodeposit. When a metallic ion in solution is reduced on a substrate, it can either contribute to the nucleation of a new crystal or the growth of an existing crystal. Nanocrystal formation requires a set of operating parameters (e.g.. bath composition, pH, temperature, current density, current on and off times during pulse plating and agitation) which promote massive nucleation throughout the entire electrodeposition process. What must be avoided are conditions that would result in a transition with increasing deposit thickness from initially fine grained structure to coarse columnar structure often observed in conventional electrodeposits.¹²

Electrodeposition processes have been developed for the synthesis of nanocrystalline deposits in many different shapes and forms.^5,6 These range from thin and thick corrosion and wear-resistant coatings to electroformed sheet and plate products for structural, magnetic or electronic applications. This technology can be easily incorporated with little extra cost in conventional electroplating facilities using a variety of different plating approaches, including rack plating, reel-to-reel plating, barrel plating, brush plating or continuous strip/foil plating.

As a result of their small grain size, monolithic electrodeposits show significant improvements in many properties compared to their conventional counterparts. These include considerable increases in hardness, tensile strength, wear resistance and corrosion resistance all of which are well understood in terms of the underlying physical and chemical principles of nanomaterials.¹³ The earliest applications of this type of nanocrystalline deposits was the in-situ electrosleeve nuclear reactor steam generator tubing repair technology developed in Canada since the early 1990s for both Canadian and U.S. nuclear reactors.¹⁴ In this process, steam generator tubes (e.g., Alloys 600 or 400) whose structural integrity was compromised by localized degradation phenomena (e.g., by intergranular corrosion or pitting) were repaired by electroplating their insides with a 1 mm thick nanocrystalline Ni-P alloy to restore a complete pressure boundary.  In order to give the required combination of strength, ductility, corrosion resistance and long term stability at temperatures in the 280 to 350°C range, the grain size of the electrosleeve was adjusted in the 50 to 100 nm range. It should be noted that, in 1994, this electrodeposited nanostructure was the first large scale industrial application of any structural nanomaterial in the world. It saved the Ontario utilities alone several hundred million dollars.

Structurally graded nanometals

Monolithic nanometals with narrow grain size distributions exhibit remarkable mechanical properties in terms of hardness, tensile strength and wear resistance. However, their ductility is usually compromised, regardless of the processing route.¹⁵ This is mainly the result of restricted dislocation activity at such small grain sizes.  In recent years it has been recognized that considerable ductility can be restored in the materials through broader or bi-modal grain size distributions.¹⁵ In view of this, we have developed electroplating conditions for a variety of structurally graded nanomaterials (Fig. 3). These include bi-modal distributions [Fig. 3(a)], alternate layers of large and small grain sizes [Fig. 3(b)] and grain size gradient deposits [Fig. 3(c)]. In such structures, the small grain size regions provide the outstanding strength while the larger grains allow for sufficient dislocation activity to result in reasonable ductility values.

Nanocomposites

Composite coatings consisting of conventional polycrystalline metal electrodeposits with second phase particles co-deposited during

electroplating have been in use for many years for applications such as increased wear resistance or reduced surface friction coatings.¹² Over the past several years this concept has been extended to nanocrystalline materials.^6,13,16  Fig. 4 shows several examples of submicrocrystalline / nanocrystalline second phase particles or fibers embedded in a nanocrystalline metal matrix.  Examples of second phase particles/fibers include Al₂O₃, TiO₂, SiC or B₄C for improved wear resistance applications, carbon nanotubes for increased strength or Teflon^® and MoS₂ for low friction coating applications. An alternative route to particle co-deposition is first to produce an alloy deposit as a supersaturated solid solution.  Subsequent heat treating of such heat-treatable coatings produce the second phase particles by a precipitation reaction.⁵

Compositionally modulated alloy (CMA) nanostructures

Compositionally modulated nanostructures consist of alternating layers of deposits with different chemical composition as schematically shown for Ni-Cu multilayers in Fig. 5. The critical nano-dimension in this case is the thickness of the individual layers. Because of their unusual microstructure these materials exhibit unexpected properties such as increased strength, elastic constants and tribological properties, as well as outstanding electromagnetic properties. In fact, one of the most advanced applications of CMA’s are alternating layers of ferromagnetic and non-magnetic layers to produce the giant magnetoresistance effect which has already resulted in widespread industrial applications in the recording head industry.¹⁷ CMAs can be produced by many different methods including physical vapor deposition, chemical vapor deposition and electrodeposition. Electrodeposition approaches include rotating substrate methods and potential-stepping methods. In the rotating substrate method, two physically separated baths are used, and the cathode is rotated between the two baths.  In the potential stepping methods, ions of the two metallic species are contained in the same plating bath. By periodically stepping the potential between predetermined values, the relative deposition rates of the two species create the required concentration changes in the multilayers. Using these approaches, many CMA structures have been produced by electrodeposition such as Ag-Pd, Cu-Ni, Cu-Pb and Ni-P with modulation wavelengths down to few nm.^18-21 Even nanomodulated ceramic structures have been produced by this technique.²²

Template nanotechnology by anodizing

Anodizing of aluminum is a widely used finishing process to improve the surface characteristics of aluminum parts.^23-25 This process involves the artificial build-up of the natural protective aluminum oxide

layer from a few nanometers in thickness to tens or even hundreds of micrometers by making the aluminum part the anode in a suitable

electrolyte. Under anodizing conditions, a porous oxide layer with regular or irregular arrangements of hexagonal columns with pores extending from the free surface down to the initial barrier layer is produced [see top and side views in Fig. 6(a,b)]. For requirements such as scratch or corrosion resistance, the porous aluminum oxide is sealed, e.g., by boiling in water which hydrates and expands the oxide to effectively close the pores.  In color anodizing, e.g., for architectural applications, the pores are impregnated with dyeing compounds or mineral pigments of different color, prior to sealing [Fig. 6(c)].

Over the past 10 years the basic anodizing approach has been further developed to produce highly ordered nanoporous honeycomb oxide structures for use in nanostructure synthesis by the template approach.²⁶ In this process, the porous alumina film is extracted by dissolution of the aluminum substrate and used as a free-standing mask to produce, by vapor deposition, structures such as nanodots or nanowires of various materials, in particular semiconductors (Fig. 7). For example, regular arrays of so-called quantum dots produced by this method have very interesting electrical properties resulting from electron state confinement in crystals with very small external sizes.

Microsystem technology

Microsystems can be described as intelligent miniaturized systems that combine sensing and/or actuating functions with processing functions. Such systems are typically multifunctional, combining two or more electrical, mechanical, optical, chemical, biological or magnetic properties such as in microelectromechanical systems (MEMS).  Many microsystems are produced by various micromachining methods on a single substrate in the form of a monolithic system (for example an all-silicon device).  However, other designs require metallic components which, in most cases, are produced by electrodeposition methods (e.g., the so called LIGA process).²⁷ Figure 8 shows a series of schematic diagrams of the various process steps involved in making a microgear by the LIGA process. The first steps are preparation of mask and mold into which the metal is to be deposited. Traditional LIGA parts have a columnar grain structure with very high grain shape anisotropy.  As the size of the microsystem parts are getting smaller and smaller, this structure becomes unacceptable because it leads to loss in overall strength, non-uniform properties and unreliable system performance.²⁸ We have recently shown that these problems can be alleviated by adopting nanostructured electrodeposits for microsystem components.^29,30 By using equiaxed nanocrystalline Ni, Co or Ni-Fe deposits, property variations due to grain size/shape anisotropies have been eliminated and the overall performance of the components in terms of specific strength, elastic energy storage capability, thermal shock resistance and reduced eddy current losses were improved enormously.

Summary

Nanotechnology is one of the fastest growing areas in science, engineering, biology and medicine and is expected to have a tremendous impact on society over the next few decades. In view of the expected returns on investment, many countries have established their own nanotechnology platforms with substantial government funding. Many industries have already formulated their plans for strategic positioning in this very broad field. In particular, electroplating and surface finishing industries have much to offer in this area, when it comes to meeting the challenges in providing new nanostructured materials of improved performance or manufacturing capabilities to produce structures, devices and systems on a micro/nanoscale.

Acknowledgments

Financial support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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About the author

Dr. Uwe Erb is a Professor Emeritus in the Department of Materials Science & Engineering, University of Toronto, Ontario, Canada. His research group at the University has been widely recognized as a world leader in the field of electroplated nanostructures. He and his research team were the first to synthesize fully dense nanostructured materials in 1985 and developed numerous industrial applications for these materials with various companies.  More recently, his research has been broadened to include bio-inspired nanostructures. He is the co-author of numerous patents related to the production of electroplated nanostructures and has co-authored nearly 300 scientific papers in the field of materials science. Dr. Erb holds a Ph.D. in Materials Science (1980) from the University of the Saarland, Germany. He  has served as an editorial member and reviewer of several international reputed journals. Dr. Uwe Erb is a member of many international affiliations.