Electroless Deposition of Alloys

Electroless deposition, or autocatalytic plating, may be defined as deposition of a metal coating by a controlled chemical reduction, catalyzed by the metal or alloy being deposited. Electroless deposition has been known for a long time. One of its early uses was the deposition of a mirror-like layer of silver on the internal surfaces of Dewar flasks for improved thermal isolation, and as the back coating of mirrors. Later, it was used for deposition of different metals and alloys, and even for induced codeposition of alloys. 

Electroless deposition is usually a slow process, and it is therefore limited to formation of relatively thin layers. Since the development of surface roughness in metal deposition has a built-in positive feedback effect, causing the roughness to increase with increasing thickness of the deposit, this is less of a problem in the case of electroless deposition. 

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The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

For practical as well as fundamental reasons, there has been considerable interest in the deposition of alloys containing metals such as W, Mo, and Sn. In their pure forms, these metals do not catalyze the oxidation of the usual electroless reducing agents. Therefore, their mechanism of codeposition is intriguing, and developing an understanding of it should help to better understand the mechanism of electroless deposition as whole. Obvious questions in ternary and quaternary alloy deposition include the effect of the third or fourth element containing ions in solution on the rate of electroless deposition, as well as on the P and B contents in the case of alloys such as Ni-P and Ni-B. 

A. Molenaar, in Proc. Symp. Electroless Deposition of Metals and Alloys, M. Paunovic and I. Ohno, (eds.), The Electrochemical Society, Inc., Pennington, New Jersey 88-12, 37 (1988). 

It is interesting to note that Brenner and Riddell (2-4) accidentally encountered electroless deposition of nickel and cobalt during electrodeposition of nickel-tungsten and cobalt-tungsten alloys (in the presence of sodium hypophosphite) on steel tubes in order to produce material with better hardness than that of steel. They found deposition efficiency higher than 100%, which was explained by an electroless deposition contribution to the electrodeposition process. 

Electroless plating — An autocatalytic process of metal deposition on a substrate by reduction of metal ions from solution without using an external source of electrons. It is promoted by specific reductants, namely formaldehyde, sodium hypophosphide, sodium boro-hydride, dialkylamine borane, and hydrazine. Electroless deposition has been used to produce different metal (e.g., nickel, cobalt, copper, gold, platinum, palladium, silver) and alloy coatings. It can be applied to any type of substrate including non-conductors. Some substrates are intrinsic catalytic for the electroless deposition other can be catalyzed usually by sensibilization followed by Pd nucleation also, in some non-catalytic metallic substrates the electroless process can be induced by an initial application of an appropriate potential pulse. In practical terms, the evaluation of the catalytic activity of a substrate for the electroless deposition of a given metal is... 

A1) Applied and hot-pressed Pt /PTFE (A2) Electroless deposition of Pt on membrane (A3) Applied and hot-pressed Pt/C(or Pt)//ionomer within ionomer-impregnated carbon paper (A4) Alloy catalyst sputtered onto array of nanowhiskers and the catalyzed array embedded into the membrane surface (Bl) Pt/C//PTFE applied to substrate and impregnated with recast ionomer (B2) (Bl) + sputtered Pt layer (B3) Pt catalyst electrodeposited... 

Studies and practices of plating of different types of coatings from aqueous solutions without an external current source were described in the literature as electroless deposition. The research was predominantly related to the deposition of metals and/or alloys. However, it seems that the deposition of compounds without an external current source was in a way neglected. It is to be noted that electroless deposition of oxides, salts, polymers, etc. is also possible. 

Electroless deposition of Ni-P alloys using hypophosphite as a reducing agent ... 

To find the conditions in which the electroless deposition of gold at Ni-P surfaces with an acceptable appearance and adhesion would be possible, the activation of Ni-P substrates with 10% HC1 solution or a mixture of 0.1 M NH4F and sodium sulfamate was investigated. The results showed that only low phosphorus alloys (P content less than 5 wt%) pretreated with ammonium fluoride/sodium sulfamate mixture can be used for a successful electroless gold plating from cyanide solution and hydrazine as a reducing agent.31... 

Electroless deposition of the catalytic Pt or Pt-Ru layer was proposed for the preparation of electrodes in microdirect methanol fuel cells.53 A porous silicone substrate is prepared by the anodic etching in HF-ethanol-water (1 1 1) solution. After the etching, at the surface of porous silicon substrate, a thin film of titanium is sputtered and then a film of Pt or Pt-Ru alloy with thickness of about 150-200 nm was electroless deposited. The electrodes prepared in this way helped in minimization of the fuel cell size and increased the reactive area of the catalyst over the silicon electrode surface. 

Owing to the simplicity of electroless deposition of metals, alloys, and compounds, this method offers huge advantages for the... 

Electroless deposition of thin films of Ni-P alloys has found numerous applications in many fields due to their excellent corrosion resistance, high wear resistance, high hardness, and acceptable ductility. 

S. S. Djokic, Electroless Deposition of Metals and Alloys", in Modern Aspects of Electrochemistry, Ed. by B. E. Conway and R.E. White, Kluwer Academic/Plenum Publishers, New York, 2002, p.51. 

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