Review pubs.acs.org/IECR
On the Influence of Surfactant Incorporation during Electroless Nickel Plating Nkem Nwosu,* Alan Davidson, Colin Hindle, and Mike Barker School of Engineering and the Built Environment, Edinburgh Napier University, EH10 5DT, U.K. ABSTRACT: The effects of surfactants on the dynamics and characteristics of plain and composite electroless nickel plating (ENP) are highlighted and discussed herein. An outline of the rudiments of surfactants and some underlying selection criteria is presented at the outset, before emphasis is laid on the salient benefits of introducing such external agents during the plating process. Evidence suggests that their influence on key coating characteristics such as the rate of deposition, structure, corrosion resistance, and the degree of second-phase particle incorporation into the nickel matrix is significant, and depending on the nature and concentration of surfactant employed, can be of immense value to the system.
1. INTRODUCTION Patented by Brenner and Riddle in 1950,1 electroless nickel plating (ENP) is a highly desirable autocatalytic inorganic surface coating procedure. Its ability to produce uniform surface deposits with superior wear, corrosion, fatigue2 (depending on substrate material), and abrasion resistance has made it an attractive surface finishing technique actively employed in several industries today.3−6 In line with efforts to diversify and maximize the potential of such a versatile engineering process, various types of additives are often introduced during the plating process. Surfactants are one of such additives, often added to ENP baths. With molecules consisting of hydrophobic−hydrophilic moieties (i.e., amphiphilic), surfactants are surface active substances. Their unique tendency to accumulate at surfaces (condensed phase and a gas phase) or interfaces (two condensed phases) due to their polar−apolar nature, characteristically results in a reduction of the surface or interfacial free energy present at such boundaries. With a lower surface energy encouraging a liquid to better “wet” or spread over the surface of a solid of which it is in contact with (solid−liquid interface interactions), additives of this nature are commonly referred to as wetting agents. As will be expected, studies and application of their inherent amphiphilic nature which enables them to also act as detergents, emulsifiers, and dispersants to mention a few, have already been extensively carried out in several fields and industries as of date. May et al. investigated the classic oil and water mixture and found that with the addition of both ionic surfactants sodium dodecyl sulfate (SDS) and tetra-decyl trimethyl ammonium bromide (TDTMAB), the separation time of the mixture was over twice as fast as that observed under the influence of gravity alone.7 In a study conducted by Gungor et al.,8 it was found that the introduction of surfactants initiated particle flocculation with a resultant increase in viscosity observed. Alsari et al. evaluated the effect of SDS solutions as gelation media on the formation of polyethersulfone (PES) membranes and found that the pore size of the membrane varied with the concentration of the surfactant in the medium.9 Clearly, it can be deduced from these studies that © 2012 American Chemical Society
the physicochemical characteristics of surfactants possess the ability to alter the ordering of a system. This review highlights and discusses the influence of surfactants on plain and composite ENP. It deals with in particular, the effects of such external agents on the rate of deposition, corrosion resistance of the resultant coatings, structure, and degree of second-phase particle incorporation into the nickel matrix.
2. OVERVIEW OF ELECTROLESS NICKEL PLATING The mechanism of ENP involves the deposition of nickel onto a substrate without an electric current. The process starts by nucleation in catalytically sensitive areas on the substrate, before autocatalytically growing in an isotropic pattern.10−12 A much desired uniform thickness regardless of recesses and bores is subsequently observed. To ensure adequate catalytic activity on the surface of the substrate, preplating treatment activities are often carried out with substances such as palladium (especially for metals like copper or materials with passive surfaces which lack catalytic properties such as ceramics13,14). In an ENP solution, the metal ions are maintained in suspension by chelating agents. Reduction of nickel from Ni2+ to a zerovalent metallic state, Ni0, before its subsequent adhesion to the substrate is achieved via the supply of electrons from reducing agents such as hydrazine (N2H4), borohydride (BH4−), or hypophosphite (H2PO2−). While it has been suggested that the adhesion in the case of metallic substrates is of an atomic nature,13 the bond between the coating and nonmetallic materials such as ceramics is purely mechanical.13,15,16 Alloys of nickel and elements such as phosphorus (i.e., Ni−P4,17) or boron (i.e., Ni−B18,19) which originate respectively from the reducing agents used, are generally known as binary alloys. When second phase particles are introduced and depending on the composition, they are classed as ternary (e.g., Ni−P−Al2O320−23) or quaternary alloys Received: Revised: Accepted: Published: 5635
November 14, 2011 March 31, 2012 April 1, 2012 April 2, 2012 dx.doi.org/10.1021/ie202625n | Ind. Eng. Chem. Res. 2012, 51, 5635−5644
Industrial & Engineering Chemistry Research
Review
Table 1. Basic Components of an Electroless Nickel Bath role
activity
complexing agent/ chelators stabilizing agent
binds to the nickel ion; temporarily inactivating and preventing it from reacting with other bath components
carboxylates such as citric acid are commonly used
example
prevents bath decomposition; though an essential component of the bath, exceeding certain concentrations may result in complete inhibition of the deposition process
buffers
helps regulate ph of the solution
heavy metal ions such as lead, mercury, tin or unsaturated organic compounds such as maleic acid are prime candidates here typically of the borate or succinate group
(e.g., Ni−W−Cu-P24/Ni−W−P-ZrO225). Though the borohydride ion is the fastest reducing agent available, the hypophosphite ion remains the most popular and widely used (over 70%) due to its lower costs and greater ease of process control.4 Various compoundsa few of which are summarized in Table 1are often added to electroless nickel baths to aid the deposition process. Acidic and alkaline baths existthe latter capable of coating materials with lower temperature thresholds such as polymers. Typical deposition rates achievable with the process lie in the range of about 20 μm h−1, and the resultant coatings are normally silver−gray in appearance.
CMC, which directly affects the performance of a surfactant, is unique to every surfactant and is affected by many factors. For example increasing the number of ethylene oxide molecules for a given group of nonionic surfactants of equal alkyl chain length produces higher CMC values for that surfactant and in turn results in the achievement of lower surface tensions. Nonionic surfactants generally have significantly lower CMC’s of about 2 orders of magnitude less than those for similar ionics of same alkyl chain length.26 An associated effect of increasing surfactant concentration is also a reduction of the solution’s surface tension (Figure 2).
3. SURFACTANT CHEMISTRY AND SOME CRITERIA FOR SELECTION IN ELECTROLESS NICKEL PLATING PROCESSES A surfactant’s molecule consists of a hydrophobic hydrocarbon or fluorocarbon tail of 8−18 carbon atoms26 and a hydrophilic part having an affinity for water (Figure 1a). When it carries a
Figure 2. Surface tension vs surfactant concentration. Reprinted with permission from www.kruss.de. Figure 1. Schematic representation of (a) a single surfactant molecule and (b) a spherical micelle structure.
When combined with agitation and hydrogen evolution during ENP, foaming of the solution often occurs. The point at which no further reduction in surface tension is observed also denotes the CMC. 3.2. Some Selection Criteria. Although the selection of a surfactant for use in ENP is an open choice, for optimum performance of the additive, certain rules are applicable. Primarily, a surfactant should be partially soluble and stable in the medium, possess substantial adhesion tension and have good leveling power. 3.2.1. Compatibility: Hydrophile−Lipophile Balance. The first and foremost condition in surfactant selection is the surfactant’s compatibility with the solution. Compatibility here refers to the ability of a surfactant to achieve adequate solubility in the solution. Having a good knowledge and understanding of the structure of a surfactant always provides an insight to its expected solubility in any solution. On the back of the fact that the methylene group (CH2) in organic surfactants is hydrophobic, as a rule of the thumb, the longer the hydrocarbon chain length of the surfactant, the more insoluble it will be in
negative charge, it is classed as anionic. Those that dissociate to yield positively charged amphiphilic ions are referred to as cationic. Amphoteric surfactants, also known as zwitterionic, are a class of surfactants which may exhibit either negative or positive charges depending on factors such the pH of the solution. Nonionic surfactants do not dissociate in a solution and as such bear no charge. 3.1. General Property: Critical Micelle Concentration. The concentration of a surfactant in a solution is of utmost importance. At very low concentrations in a bath, surfactants tend to exist as single molecules (Figure 1a). As the concentration increases, the molecules begin to transform into aggregates known as micelles (Figure 1b). The concentration above which the micelles are first detected is known as the critical micelle concentration (CMC). Micelles, which can be lamellar, cylindrical, or spherical in shape, generally contain aggregates of about 10−100 molecules and possess hydrophobic core diameters of about 10−30 Å.27 The 5636
dx.doi.org/10.1021/ie202625n | Ind. Eng. Chem. Res. 2012, 51, 5635−5644
Industrial & Engineering Chemistry Research
Review
negative charge and are adversely affected by the presence of cations such as nickel in the solution. Su et al. investigated the interaction between the anionic surfactant SDS and divalent copper ions and found that the sodium ions were displaced from the micelle surface by copper ions due to a stronger coloumbic force of attraction between the copper ions and the micelle.41 The result was a reduction of the surface charge of the micelle structure and a corresponding drop in its CMC. Newberry42 measured the CMC of SDS in the presence of nickel ions and found that it changed from 8 × 10−3 M in pure water to as low as 4 × 10−4 M depending on concentration. This finding which has immense implications for the use of anionic surfactants during ENP is in agreement with studies43−45 that have highlighted the ionic strength of a solution as a factor that affects the CMC of a surfactant. A decrease of the concentration at which a surfactant achieves its CMC is an undesired feature since it indicates a lower effectiveness of the surfactant or a probable need for a higher concentration of it. Unlike anionic surfactants though, positively charged cationic surfactants are generally compatible with metallic cations. Besides being a property that makes cationic surfactants such as CTAB specifically provide good corrosion inhibition propertiesan attribute said to improve with increasing lengths of their alkyl chain at concentrations above their CMC,46 the positive charge of cationic surfactants provides good adhesion to substrates which are mostly negatively charged in electroless nickel solutions. With respect to the behavior of fluorosurfactants (surfactants in which their hydrogen atoms have been replaced by fluorine atoms), this group of additives appear to be in a different class. Properties such as stability against acidic, alkaline, oxidative, and reducing agents as well as elevated temperatures makes them well-suited for not only ENP processes but also for special applications in the industry.47 Furthermore and quite uniquely, fluorosurfactants possess the ability to reduce the air−aqueous solution surface tension to about half of the value reachable by the best tension reducing hydrocarbon surfactant.40 While nonionic, cationic, and fluorosurfactants may appear to be better candidates in this department than their anionic counterparts, it should be noted that anionic surfactants are much less expensive.40
water. Conversely, if its molecules are too soluble in water, it will dissolve completely into that phase or cannot suitably perform due to poor surface activity (below 10 carbon atoms for anionic surfactants28). Closely related to this phenomenon also is Traube’s rule which states that each methylene unit added will result in a reduction of the CMC of the surfactant by a factor of 3.29 To help identify appropriate surfactants for given solutions, the hydrophile−lipophile balance (HLB) number was introduced by Griffin.30,31 Though commonly used to select suitable nonionic surfactants for emulsification purposes, HLB numbers are calculated on a molecular weight basis and generally range from 0 to 20although some, such as those of ionic surfactants, do extend to 40 and above.32 Surfactants with low HLB numbers (i.e.,
