Surface Coating Technologies

Besides manufacturing contact materials from the solid phase, i.e. by melt or powder metallurgy, the production starting in the liquid or gaseous phase is generally preferred when thin layers in the μm range are required which cannot be obtained economically by conventional cladding methods. Such coatings fulfill different requirements depending on their composition and thickness. They can serve as corrosion or wear protection or can fulfill the need for thin contact layers for certain technical applications. In addition they serve for decorative purposes as a pleasing and wear resistant surface coating.

Table 7.1: Overview of Important Properties of Electroplated Coatings and their Applications

To reduce the mechanical wear of thin surface layers on sliding and connector contacts additional lubricants in liquid form are often used. On silver contacts passivation coatings are applied as protection against silver sulfide formation.

7.1 Coatings from the Liquid Phase

For thin coatings starting from the liquid phase two processes are used differentiated by the metallic deposition being performed either with or without the use of an external electrical current source. The first one is electroplating while the second one is a chemical deposition process.

7.1.1 Electroplating (or Galvanic Deposition)

For electroplating of metals, especially precious metals, water based solutions (electrolytes) are used which contain the metals to be deposited as ions (i.e. dissolved metal salts). An electric field between the anode and the work pieces as the cathode forces the positively charged metal ions to move to the cathode where they give up their charge and deposit themselves as metal on the surface of the work piece. Depending on the application, for electric and electronic or decorative end use, different electrolytic bath solutions (electrolytes) are used. The electroplating equipment used for precious metal plating and its complexity varies widely depending on the process technologies employed. Electroplating processes are encompassing besides the pure metal deposition also preparative and post treatments of the goods to be coated. An important parameter for creating strongly adhering deposits is the surface of the goods to be metallic clean without oily or oxide film residues. This is achieved through various pre-treatment processes specifically developed for the types of material and surface conditions of the goods to be plated. In the following segments electrolytes – both precious and non-precious – as well as the most widely used electroplating processes are described.

7.1.1.1 Electroplating Solutions – Electrolytes

The actual metal deposition occurs in the electrolytic solution which contains the plating material as metal ions. Besides this basic ingredient, the electrolytes contain additional components depending on the processes used, such as for example conduction salts, brighteners, and organic additives which are codeposited into the coatings, influencing the final properties of the electroplating deposit.

7.1.1.1.1 Precious Metal Electrolytes

All precious metals can be electroplated with silver and gold by far the most widely used ones (Tables 7.1 and 7.2). The following precious metal electrolytes are the most important ones:

• Gold electrolytes For functional and decorative purposes pure gold, hard gold, low-karat gold, or colored gold coatings are deposited. Depending on the requirements, acidic, neutral, or cyanide electrolytes based on potassium gold cyanide or cyanide free and neutral electrolytes based on gold sulfite complexes are used.

• Palladium and Platinum electrolytes Palladium is mostly deposited as a pure metal, for applications in electrical contacts however also as palladium nickel. For higher value jewelry allergy protective palladium intermediate layers are used as a diffusion barrier over copper alloy substrate materials. Platinum is mostly used as a surface layer on jewelry items.

• Ruthenium electrolytes Ruthenium coatings are mostly used for decorative purposes creating a fashionable “grey” ruthenium color on the surface. An additional color variation is created by using “ruthenium-black” deposits which are mainly used in bi-color decorative articles.

• Rhodium electrolytes Rhodium deposits are extremely hard (HV 700 – 1000) and wear resistant. They also excel in light reflection. Both properties are of value for technical as well as decorative applications. While technical applications mainly require hard, stress and crack free coatings, the jewelry industry takes advantage of the light whitish deposits with high corrosion resistance.

• Silver electrolytes Silver electrolytes without additives generate dull soft deposits (HV ~ 80) which are mainly used as contact layers on connectors with limited insertion and withdrawal cycles. Properties required for decorative purposes such as shiny bright surfaces and higher wear resistance are achieved through various additives to the basic Ag electrolyte.

Table 7.2: Precious Metal Electrolytes for Technical Applications