FEATURES OF ELECTROPLATING MAGNETIC MEASURING SCALES WITH A Co—Ni—P ALLOY

A. A. Asmus, Ya. A. Belikhmaer

(Submitted by the scientific seminar of the Departments of Machine Tools and Metal Cutting
and Mechanical-Engineering Technology)

High requirements are imposed on the coating of measuring scales with the magnetic alloy Co—Ni—P; these requirements can be met only if all the special features of the electroplating technology are observed.

The technology of coating with the Co—Ni—P alloy was developed in detail by Ya. B. Kaznachei and V. M. Zhogina and is given in sources [1, 2, 3]. On the basis of investigations carried out at the institute, the following additions may be made.

The areas to be coated should not be degreased in an alkaline solution, since the high temperature of the alkaline solution (80° ÷ 90°C) makes it difficult to insulate the areas not subject to coating. Quite satisfactory results are obtained by degreasing with Vienna lime at room temperature. For insulating areas not subject to electroplating, the guiding technical material 13—61 provides for a number of materials.

Some enterprises, for example the Chelyabinsk Measuring Instruments Plant, use complex protective fixtures made of Plexiglas.

In this case, in some places small gaps remain between the disk and the fixture, in which solutions from baths preceding the main coating remain. Especially dangerous is the presence of a copper sulfate solution from the preliminary copper-plating bath.

The simplest and most reliable way to insulate areas not subject to coating is with BF-4 glue, without using any additional protective fixtures. BF-4 glue should be applied in two layers, with a drying interval of 30 minutes.

The quality of the electroplated coating with the Co—Ni—P alloy depends on the method of immersing the disk in the electrolyte. An even, nonporous coating is obtained when the disk rotates at a speed of 1 ÷ 1.5 rev/sec, with immersion in the electrolyte of approximately \( \frac{1}{3} \div \frac{1}{4} \) of the surface to be coated (Fig. 1).

In manufacturing kinematometers, in addition to disks it is necessary to coat with the alloy a brass strip 6 ÷ 8 meters long.

Usually, drums made of Plexiglas are used as fixtures; the strip to be coated is wound spirally on their surface.

Contacts from the current collector are brought only to the ends of the strip, which does not ensure a uniform current density along the length of the strip and leads to a nonuniform coating thickness.

Fig. 1. Schematic of an installation for electroplating disks up to 400 mm in diameter: 1 — disk; 2 — combined anode (70% of the area — cobalt, 30% of the area — nickel).

It is most rational to use a fixture in the form of a copper ring (Fig. 2).

The strip 1 to be coated is fastened with two screws to the copper disk 2, which is press-fitted onto the hub 3 made of an aluminum alloy. The current collector is the shaft on which the fixture is mounted.

Owing to good contact along the entire length of strip 1 with copper ring 2, the current density is the same along the entire length of the strip, which ensures a uniform coating thickness.

After strip 1 has been fastened, the entire surface of ring 2 and hub 3 is covered with a film of BF-4 adhesive. The use of such a fixture simplifies polishing of the surface before and after coating.

Polishing is carried out on a lathe using a polishing head.

Particular attention should be paid to the cleanliness of the electroplating baths. Electrolyte solutions were prepared from chemical reagents of grades “Ch” and “ChDA” in distilled water.

To remove organic impurities, the electrolytes were periodically fil—

Fig. 2. Fixture for coating strips.

... were filtered through activated carbon. To obtain a coating of a specified thickness, the dependence of thickness on coating time was determined. This dependence is shown in Fig. 3. The coating time is indicated for the case in which \(1/3\) of the total coating area is immersed in the electrolyte. The current density is \(12\ \mathrm{A/dm^2}\) \((1200\ \mathrm{A/m^2})\). The electrolyte temperature is \(52 \div 56^\circ\mathrm{C}\).

Combined anode: 70% of the area—cobalt \((77\ \mathrm{cm^2})\),
30% of the area—nickel \((33\ \mathrm{cm^2})\).

The distance from the anode to the coating surface is \(85\ \mathrm{mm}\).

Fig. 3. Dependence of thickness on coating time.

Signal reproduction can be carried out by the contact method or with a constant gap between the magnetic head and the disk. With contact reproduction, the design of the transducer is simplified, but the service life of the disk is reduced.

In the study, the amount of disk wear during contact reproduction of signals was determined. A \(60\ \mathrm{mm}\) disk with a Co—Ni—P coating thickness of \(5\ \mu\mathrm{m}\) rotated at a speed of \(380\ \mathrm{mm/s}\) \((2\ \mathrm{rev/s})\) in direct contact with the magnetic head. The force with which the head was pressed against the disk was \(\approx 30\ \mathrm{g}\). After 85 hours 35 minutes, the layer had worn off in individual places, and the signal level had decreased by 20%. The amount of wear was

\[ \frac{85 \cdot 60 \cdot 120}{5} \approx 122\,000 \]

disk revolutions per \(1\ \mu\mathrm{m}\) of coating thickness.

If a transducer with a coating thickness of \(20\ \mu\mathrm{m}\) is used and inspection is carried out at a milling-machine spindle speed of \(60\ \mathrm{rev/min}\) \((1\ \mathrm{rev/s})\), then the service life of the transducer will be

\[ \frac{122\,000 \cdot 20}{60 \cdot 60} \approx 680 \]

hours of continuous operation, which is quite sufficient.

Conclusions

1. To insulate areas not subject to coating, instead of Plexiglas fixtures it is advisable to use a protective coating made of BF-4 adhesive film.

2. To obtain a pore-free coating uniform in thickness, the parts being coated must be rotated according to the scheme in Fig. 1.

3. The best quality of coating on brass tapes is achieved by using a special fixture in the form of a copper ring (Fig. 2). Coating of tapes 8000 mm long and disks up to 400 mm in diameter has been mastered.

4. To obtain a coating of a specified thickness, the coating time is determined from the graph in Fig. 3.

References

1. B. Ya. Kaznachei, V. M. Zhogina. Electrodeposition of a high-coercivity nickel–cobalt alloy. Proceedings of the Institute of Sound Recording, Gosizdat “Iskusstvo,” 1957, issue 1, pp. 91–93.

2. B. Ya. Kaznachei, V. M. Zhogina. Electrolytic production of a nickel–cobalt alloy with specified magnetic characteristics. Proceedings of the Institute of Sound Recording, Gosizdat “Iskusstvo,” 1957, issue 1, pp. 79–90.

3. N. P. Fedot’ev, N. N. Bibikov, P. M. Vyacheslavov, S. Ya. Grilikhes. Electrolytic Alloys. Mashgiz, 1962.