Types of Electroless Alloy Plating Systems

Most electroless alloy systems are based on nickel alloys and have been developed from basic electroless nickel-plating technology. Many alloy systems are documented in the literature and several are commercially available. In the following section each coating type is summarized and its availability is indicated as either "production" or "laboratory."

Nickel-Phosphorus. Electroless nickel-phosphorus is the most common nonelectrolytic alloy coating system. Electroless nickel has been used since the early 1950s and continues to show the most growth and development.

These coatings are widely used for control of wear and corrosion. An important property is the amorphous structure in the as-plated condition and the ability to heat treat the deposit by precipitation hardening to produce a crystalline structure.

There are three nonequilibrium phases for the as-plated electroless nickel deposit: beta, beta + gamma, and gamma. The beta phase is present up to a phosphorus content of 4%; above 4%, there is a mix of beta and gamma. When the deposit has more than 11.2% P, the deposit is gamma-phase. Upon heat treating the deposit containing nickel and phosphorus, a precipitation occurs, producing an equilibrium mix of nickel phosphide and alpha-nickel. A coating of unique mechanical and physical qualities can be produced by controlling the phase of metal being deposited, the alloy of phosphorus, and the postheat treatment.

To help classify the different nickel-phosphorus coatings, a system has been developed that groups them into types. This system has been incorporated into national and international draft specifications. The scheme presented in Table 2 follows the current industry convention.

Table 2 Electroless nickel-phosphorus plating systems

Alloy

Hardness,

Environments in which

Significant properties and applications

Availability

HKjoo

plating

has demonstrated

corrosion resistance

Type II: 13% P, bal Ni

435-680

Alkali, brine, strong caustics

Solderability, electrical conductivity1-3"1, wire bonding

Production

Type III: 2-4% P, bal Ni

700-800

Alkali, brine, strong caustics

High as-plated hardness for wear application on aluminum, beryllium, copper, and other base materials that cannot be precipitation hardened

Production

Type IV: 5-9% P, bal Ni

520-650

Alkali, brine, caustic solutions

General wear and corrosion resistance applications, including application in industrial interior and exterior nonmarine environments, undercoatings. Thicknesses for these applications range from 1 to 125 ^m (0.03 to 5 ^in.).

Production

Type V: 10-14% P, bal Ni

430-530

Alkali, brine, mildly acidic solutions, marine environments

Wear and corrosion resistance applications in marine and corrosive industrial environments; nonmagnetic undercoating in memory discs; oil and gas environments

Production

(a) Conductivity, 10-30 cm2

(a) Conductivity, 10-30 cm2

Procedures for the electroless deposition of nickel, nickel-phosphorus, and nickel-boron are described in detail in the article "Electroless Nickel Plating" in this Volume.

Nickel-Boron. Another family of nonelectrolytic alloy coatings uses boron-based reducing agents (Table 3). These are classified by boron alloy content. Generally the low-boron coatings are used in electronic applications as a replacement for gold. The higher-boron coatings are used to provide a hard surface to prevent galling in iron and nickel wear applications.

Table 3 Electroless nickel-boron plating systems

Alloy

Hardness, HK100

Significant properties and applications

Availability

0.1-1% B, bal Ni

520-620

Electronic applications, replacement for gold in microelectronic equipment and printed wire circuit boards

Production

2-4.5% B, bal Ni

750-800

Aircraft engines, landing gear, valves, and pumps; resistance to galling, fretting, and erosion wear

Production

In general, high-boron coatings use sodium borohydride as the reducing agent and operate at a pH of 12.5 and above. Low-boron coatings use dimethyl amino borane (DMAB) as the reducing agent and operate in a neutral pH of 5.5 to 7. These systems are commercially available and provide the chemical basis for many of the nonelectrolytic ternary alloy systems. Both DMAB and sodium borohydride are powerful reducing agents and provide energy to reduce many elements.

Hypophosphite-Reduced Cobalt Alloy Coatings. Nonelectrolytically produced cobalt alloy coatings have been used in a limited number of magnetic and wear applications. These coatings are produced from sodium hypophosphite-based solutions at a slightly alkaline pH range at elevated temperatures. Table 4 lists property and application information for a hypophosphite-reduced cobalt-phosphorus coating.

Table 4 Electroless cobalt alloy plating systems

Alloy

Hardness, HK100

Environments in which plating has demonstrated corrosion resistance

Significant properties and applications

Availability

Hypophosphite-reduced cobalt-phosphorus (3-6% P, bal Co)

550-650

Alkali, brine solutions

Magnetic and medical applications

Production

Boron-reduced cobalt-boron (3-4% B, bal Co)

350-500(a)

Wear resistance in high-temperature applications

Laboratory; limited application

(a) As-plated; 800-1000 HK100 after 30 min at 400 °C (750 °F)

Boron-reduced cobalt alloy coatings (Table 4) are produced by a reduction reaction of sodium borohydride or DMAB reducing agents. The deposits are harder and have higher melting points than those alloyed with phosphorus. These coatings have not been commercially available. They could be used in wear applications where service at higher temperatures is required.

Ternary alloy coatings are used to provide higher performance in specific properties over conventional electroless coating systems. By incorporating a third element in significant or trace levels, the basic structure and physical properties of the coating can be altered. There are many possibilities and systems that have been investigated and are available (Table 5).

Table 5 Electroless ternary alloy plating systems

Alloy

Hardness, HK100

Environments in which plating has demonstrated corrosion resistance

Significant properties and applications

Availability

Hypophosphite-reduced alloys

Nickel-phosphorus-molybdenum (5-9% P, 0.51% Mo, bal Ni)

550-650

Alkali, brine, caustics, weak acid solutions

Pitting corrosion protection

Laboratory

Nickel-copper-phosphorus (4-8% P, 1-3% Cu, bal Ni)

430-520

Alkali, brine, caustic solutions

Nonmagnetic, conductive, high modulus

Production

Nickel-cobalt-phosphorus (15-40% Co, 3-8% P, bal Ni)

High-coercivity coating for use in magnetic memory applications

Laboratory, limited production

Nickel-iron-phosphorus (14% Fe, 2-4% P, bal Ni)

Magnetic applications in electronics

Laboratory

Nickel-rhenium-phosphorus (1-45% Re, 38% P, bal Ni)

High melting point (1700 °C, or 3090 °F); high-temperature wear resistance

Laboratory

Nickel-tungsten-phosphorus (4-8% P, 1-4% W, bal Ni)

550-620

High melting point (1550 °C, or 2820 °F); high-temperature wear resistance

Laboratory

Cobalt-tungsten-phosphorus (4-8% P, 1-5% W, bal Co)

570-640

High-temperature wear resistance

Production

Boron-reduced alloys

Nickel-thallium-boron (35% Tl, 3-5% B, bal Ni)

650-850

Wear applications requiring resistance to galling, fretting, and erosion; coating is self-lubricating in contact with ferrous materials

Production

Nickel-tin-boron (3-5% B, 1-3% Sn, bal Ni)

650-850

Wear applications requiring resistance to galling, fretting, and erosion; this coating is also self-lubricating

Production

Cobalt-tungsten-boron (35% B, 1-5% W, bal Co)

750-850

Wear applications requiring resistance to galling; can be used at higher temperatures than phosphorus systems

Laboratory

Boron-reduced ternary alloy coatings are produced by a reduction reaction of sodium borohydride or DMAB reducing agents. The deposits are harder and have higher melting points than those alloyed with phosphorus. Some of these coatings have not been commercially available. Figure 1 shows the microstructure of an electroless nickel-thallium-boron plating deposit.

Fig. 1 Electroless nickel-thallium-boron deposit. The hard columnar structure increases resistance to fretting wear and the ability of the deposit to retain oil. Additional lubrication is provided with the presence of thallium, which interferes with the galling process between nickel and iron.

0 0

Post a comment