Nickel Cobalt

Some of the earliest solutions for bright nickel plating contained cobalt, formate, and formaldehyde additions, but with the development of modern bright nickel solutions based on organic addition agents only, the cobalt-containing solutions have fallen into disuse. Today, the cobalt additions are used when it is necessary to increase the hardness and strength of nickel plating, especially in electroforming applications.

Advantages. Compared with nickel itself, nickel-cobalt alloys are harder and stronger. In contrast to nickel hardened with conventional organic addition agents such as naphthalene 1:3:6 trisulfonic acid, nickel-cobalt alloys can be heated to high temperatures without embrittlement by sulfur incorporated from addition agents.

Disadvantages. Compared with nickel alone, the need to maintain the level of cobalt ions in solution introduces an additional maintenance requirement. Also, deposit internal stress is moved in the tensile direction, and there is a practical limit to the level of cobalt that can be used. Hence the maximum hardness of about 400 HV is less than the 600 HV that can be attained using conventional organic additives.

Process Description. Most of the published data about nickel-cobalt plating were determined using the 600 g/L nickel sulfamate solution of the Ni-Speed process (Ref 9, 10, 11). The initial charge of cobalt is added to the base solution as cobalt sulfamate. Replenishment during operation of the solution is usually made by metered additions of cobalt sulfamate. It can, however, be achieved by dividing the anodic current between a nickel anode and a cobalt anode, so that the percentage of total current passing to the cobalt is the same as the percentage of cobalt required in the alloy deposited at the cathode. Good control of solution cobalt content by analysis is needed.

Processing Variables and Properties of the Alloy from 600 g/L Nickel Sulfamate Solution. With solution composition and pH standardized, the cobalt content and the properties of the alloys depend on solution temperature and deposition current density. The properties are modified by subsequent heat treatment (Ref 12, 13, 14) and by simultaneous use of sulfur-free organic addition agents (Ref 15). The main results of various studies were drawn together in 1989 (Ref 16).

Alloy Hardness. The relationship between cobalt content and deposit hardness is shown in Fig. 1. The broken curve shows microhardness versus cobalt content in the deposits formed at 5.4 A/dm2 (50 A/ft2), and the solid curve shows microhardness versus cobalt content in the solution. A peak hardness of about 520 HV is attained with 6 g/L Co in the solution, which gives an alloy containing about 34% Co. At peak hardness, internal tensile stress is too high for electroforming applications, although the alloy can be used as a coating on a solid substrate. For electroforming purposes, the limit of tolerable deposit stress is reached with alloys containing about 15% Co that have hardnesses around 350 to 400 HV.

Fig. 1 Relationship between deposit hardness and cobalt concentration in the sulfamate solution and in the deposit

Alloy hardness depends on both the cobalt content of the solution and the deposition current density. Figure 2 shows hardness plotted against current density for different concentrations of cobalt in a solution operated at 60 °C(140 °F), pH 4.0. Deposit stress also depends on deposition current density, and moving left to right along any one of the curves, deposit stress changes from compressive through zero to tensile. The superimposed dotted line is a zero-stress contour linking the combinations of cobalt ion concentration and current density for zero-stress alloys. The corresponding value of deposit hardness can be read from the graph for each combination. The overall relationship is that the higher the cobalt ion concentration, the lower the current density that can be used for zero-stress alloys, but the higher the hardness of the alloy.

Current density, AJdfr?

Current density, AJdfr?

Fig. 2 Relationship between current density and alloy deposit hardness for various cobalt concentrations in solution with superimposed zero stress contour

Effect of Heat. Heating at temperatures up to 300 °C (570 °F) has little effect on the mechanical properties of the alloys. At higher temperatures deposit hardness falls, but nickel-cobalt alloys still retain greater hardness than that of nickel deposits similarly heat treated (Fig. 3).

100 200 300 400 500 eoo Heat treatment temperalure, "C

Fig. 3 Room-temperature hardness of electroformed nickel and nickel-cobalt alloys after heating

Tensile strength, 1034 MN/m2 (150,000 psi) for a 15% Co alloy and 745 MN/m2 (108,000 psi) for a 10% Co alloy (Ref 14), falls progressively as heat-treatment temperature is increased above 300 °C (570 °F) (Fig. 4). The effect is similar with proof stress, 677 MN/m2 (98,000 psi) for an as-deposited 15% Co alloy and 531 MN/m2 (77,000 psi) for a 10% Co alloy (Ref 14).

Fig. 4 Effect of heat treatment on the mechanical properties of Ni-10Co and Ni-15Co alloys

Deposit ductility increases on heating above about 300 °C (570 °F), from a value of 5% elongation as-deposited to about 40% after heating at 600 °C (1100 °F) for both 10% and 15% Co alloys (Fig. 5).

Fig. 5 Hardness and elongation of Ni-10Co and Ni-15Co alloys after heating to various temperatures

Properties of Alloys from Other Nickel Solutions. The above data apply to alloys from a 600 g/L nickel sulfamate solution. A Ni-14Co alloy deposited from a 450 g/L solution without chloride or organic additions, operated at 52 °C (126 °F), pH 4.0, and cathodic current density 2.5 A/dm2 (23 A/ft2) has a hardness of 350 to 400 HV, ductility less than 5%, and internal tensile stress of 50 to 60 MN/m2 (7000 to 9000 psi) (Ref 17).

Measurements of the effects of heat treatment at 204 °C (400 °F) on alloys containing 53 to 55% Co indicate a small increase in yield strength, from 1062 MN/m2 (154,000 psi) to 1124 MN/m2 (163,000 psi), and a substantial increase in elongation, from 10 to 16% (Ref 18). Heat treatment at 260 °C (500 °F) or more reduces yield strength, as with alloys from the 600 g/L nickel sulfamate solution. Prior heat treatment of the 53 to 55% Co alloys at 204 °C (500 °F), 371 °C (700 °F), or 427 °C (800 °F) improves subsequent mechanical properties measured at a test temperature of 260 °C (500 °F). Thus pretreatment for 4 h at 427 °C (800 °F) increases yield strength from 372 to 448 MN/m2 (54,000 to 65,000 psi) and elongation from 28 to 32%.

Alloys deposited from a sulfosalicylate-based solution exhibit a maximum hardness value above 700 HV (Ref 19). To date, this solution has not been used industrially.

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