Nickel-Phosphorus Alloys (Electroless Nickel)

Nickel-Phosphorus Alloys (Electroless Nickel) Nickel-phosphorus alloy coatings are deposited primarily through electroless processes in which hypophosphites function as the reducing agent. By contrast, electroless nickel-phosphorus deposition from baths containing phosphite or phosphate has no significant technical importance. Phosphorus-modified electroless nickel coating is advantageous wherever hard coatings must be deposited uniformly, even in greater thickness, on components with complex surface geometry such as perforated or hollow parts. Nickel-phosphorus coatings offer advantages when coating the cutting surfaces of metal cutting tools such as turning tools and twist drills. This not only extends the service life of these tools but also improves surface quality of workpieces and increases work performance. Chemical reduction deposition with hypophosphite has also proven successful for producing nickel dispersion layers. To achieve the smoothest possible deposition of both the nickel matrix and the interkalation phase, it is recommended to maintain the material to be deposited as an intercalation in a uniform suspension throughout the entire electroless metal deposition process in the nickel bath. The incorporation rate of solid particles into the layer depends both on the transport of particles to the cathode/electrolyte phase boundary and on the passage and adhesion of particles through the boundary layer in front of the cathode; therefore, in addition to particle concentration in solution, bath agitation is also important. Dispersion coatings can fulfill various functions. For example, through inclusion of hard materials such as silicon carbide or diamond, it is possible to produce nickel coatings that are particularly resistant to wear and abrasion, suitable for press dies, cylinder pins, cutting tools, etc. High heat-resistant coatings are obtained through inclusion of aluminum oxide, as used in the production of press dies for plastics and plates, printing cylinders in the graphic industry, and contacts and waveguides in electrical engineering. Additionally, metallic particles such as chromium, molybdenum, and titanium can be deposited to improve corrosion behavior. One method of producing porous bodies is based on the inclusion of plastics or other organic materials that are dissolved from the layer after deposition through thermal decomposition or with suitable solvents. Finally, dry lubricating layers can also be produced by adding molybdenum disulfide or graphite. Another specialized application of the process is the deposition of thin conductive layers on plastic parts, which are subsequently electroplated decoratively. Another application involves deposition of non-magnetic nickel-phosphorus layers for electronic applications. On such smooth, electrically conductive substrates, thin planar permalloy structures for data storage elements, for example, can be advantageously deposited. Another application for nickel-phosphorus layers is thin-film resistors in stripline circuits and other miniaturized circuits. In the production of such resistive layers, uniform layer thickness is particularly important to maintain the desired sheet resistance values within required tolerances. Additionally, ternary nickel-phosphorus alloys deposited electrolessly, containing 11 to 12 percent phosphorus and 1 percent copper, have gained importance. These alloys are characterized by high corrosion resistance and hardness coupled with excellent ductility and can be used wherever quality requirements such as those of chromium-vanadium or chromium-molybdenum steels are demanded. Ternary nickel-copper-phosphorus coatings with high copper content have recently become of interest both for corrosion protection, particularly at elevated temperatures, and for production of high-voltage low-resistance coatings with low temperature coefficients. With copper contents reaching 70 percent, the phosphorus contents of such layers are typically approximately 2 to 8 percent lower than in alloy layers with no copper content or low copper content. The sequence of chemical reactions in the deposition of nickel-phosphorus alloys using sodium hypophosphite as the reducing agent can be represented approximately by the following simplified overall equation: Newly deposited phosphorus-rich nickel layers should be understood as X-ray amorphous, metastable, supersaturated solid solutions of phosphorus in nickel. In part, nickel phosphite Ni₂P precipitates from this, which reacts with nickel at higher temperatures to form crystalline Ni₃P and is thus incorporated into the nickel matrix. Baths used for electroless deposition of nickel-phosphorus layers contain, in addition to nickel and hypophosphite ions, organic complexing agents such as hydroxy acids or their salts. Commercial baths typically contain 19–23 g/l nickel(II) sulfate and 23–30 g/l sodium hypophosphite. In addition, 30–50 g/l complexing agents and buffering agents are usually present, along with accelerators, stabilizers, and wetting agents. Additionally, a number of other formulations of only theoretical significance exist. Chemical deposition of ternary nickel-copper-phosphorus alloy coatings using sodium hypophosphite as the reducing agent is carried out essentially from ammonia-free, weakly alkaline citrate baths in which nickel and copper are present as sulfates. In addition, organic complexing agents such as acetates and/or borates as well as ethylenediaminetetraacetic acid are frequently present.

Bath Analysis

It is important to determine the quantity of reducing agent, in this case hypophosphite, for monitoring electroless nickel coating baths, as this has a significant effect on the phosphorus content in the layer. The dependence of phosphorus content on hypophosphite concentration in an acetate-containing electrolyte is shown in Figure 1. Additionally, the quantity of decomposition products of the reducing agent, particularly phosphite content, is of interest. In copper-containing electrolytes, analysis of copper content may also be necessary. Since copper content is usually quite low relative to nickel content, photometric methods are preferred for copper determination. Hypophosphite and phosphite contents are determined by oxidimetric titrations. Bromine and iodine are particularly suitable oxidizing agents for this purpose. Use of stronger oxidizing agents such as potassium permanganate, cerium(IV) sulfate, or potassium dichromate is prohibited due to the presence of organic compounds in the electrolyte that could also be attacked by these oxidizing agents. The combined bromatometric determination of hypophosphite and phosphite has proven to be the most suitable method. In a second bath sample, phosphite content is subsequently determined iodometrically. The difference between the two titrations can then be used to calculate the hypophosphite quantity. In principle, however, direct determination of hypophosphite is also possible. For example, this can be accomplished through prior extraction as potassium or sodium hypophosphite using ethanol, followed by oxidimetric titration, or through oxidation with silver perchlorate and subsequent potentiometric back-titration of excess silver with sodium chloride. However, these methods not only require greater experimental effort but are also more difficult to use due to the high flammability of absolute alcohol and the explosive tendency of dry silver perchlorate. Furthermore, iodometric determination of hypophosphite in hydrochloric acid solution does not yield accurate results because phosphite ions are also partially oxidized.

Layer Analysis

Examination of electrolessly deposited nickel-phosphorus coatings is generally limited to accurate determination of phosphorus content. The nickel content of the alloy is then calculated from the difference to 100 percent. Direct quantitative nickel determination, which is basically possible in the coating solution, does not give very accurate results due to the high nickel content, normally exceeding 90 percent. Layers intended for analysis are suitably deposited galvanically on substrate layers that can be chemically dissolved easily or that can be selectively dissolved while retaining the layers. The first group includes, for example, a gold layer from which the layer can be separated with dilute nitric acid. The second group includes, for example, aluminum, which can be dissolved with 25 percent sodium hydroxide solution without noticeably attacking nickel-phosphorus alloy coatings. Another possibility is deposition onto polished stainless steel, for which a thin albumin interlayer must first be applied if necessary. After filing one edge, layers can be easily removed mechanically from the substrate or lifted with a scalpel, thereby obtaining a complete sample weight in this manner. For determination of approximately 5 to 10 percent phosphorus content, a sample quantity of approximately 200 mg is required. A layer thickness of approximately 15 μm on an area of 15 to 20 cm² is sufficient for this purpose. Polarographic and photometric analytical methods have proven particularly suitable for quantitative analysis of ternary nickel-copper-phosphorus layers. The photometric method for phosphorus determination can also be used for quantitative analysis of binary nickel-phosphorus layers, particularly for small layer quantities or contents. Sources Die kompl. Titration 7. Aufl. – Anal.Chem. M. Theis Metalloberflaeche – 26 Aufl. Analyse d. Metalle – Springer Verlag Prakt. Galv.technik – Leuze Verlag Electr.plating Eng. – Van Nostrand Ausführung quant. Analysen – Hirzel İzzet Aydın General Manager Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.