Study of the Structure, Phase Composition, and Thickness of Surface Films at Different Ambient Humidity and Atmospheric Pressure
Abstract Generated abstract
This study examines surface films formed on M1 copper slip rings by MGS-7 brushes under varying ambient humidity and atmospheric pressure, with attention to their role in sliding brush contact behavior in electrical machines. Using electron microscopy, electron diffraction, X-ray diffraction, and interference thickness measurements, the authors relate film structure, phase composition, and thickness to contact current-voltage characteristics. Increasing humidity from about 38 to 98 percent intensifies copper oxidation, raises the cuprous oxide content, produces predominantly polycrystalline films, and increases average film thickness by roughly 1.8 to 2 times. Reduced atmospheric pressure weakens film formation, lowers cuprous oxide content, alters crystal orientation, and diminishes the film’s influence on the electrical characteristics of the contact.
Full Text
INVESTIGATION OF THE STRUCTURE, PHASE COMPOSITION, AND THICKNESS OF SURFACE FILMS AT DIFFERENT AMBIENT HUMIDITY AND ATMOSPHERIC PRESSURE
R. F. Bekishev, B. I. Kostylev, A. I. Skorospeshkin
(Presented by the scientific seminar of the Department of Electrical Machines and General Electrical Engineering)
The reliable operation of a sliding brush contact in commutator-type electrical machines is substantially influenced by the processes that ensure the formation of the surface film produced on commutators and slip rings. The study of the structure, phase composition, and thickness of films formed under various climatic conditions contributes to an understanding of the physical processes occurring in a sliding contact.
In the present work, surface films formed on slip rings (material—M1 copper) by MGS-7 brushes were investigated over a range of relative ambient humidity of 38–98%. The running-in of the surface films was carried out in a humidity chamber at constant current density \((20\ a/cm^2)\), contact-ring temperature \((45^\circ C)\), and atmospheric pressure \((760\ \text{mm Hg})\).
As a result of electron-microscopic, electron-diffraction, and X-ray diffraction studies [1], quantitative data were obtained on the structure and content of the components in the surface films investigated; these data are presented in Table 1.
Table 1
| No. | Relative humidity of the medium, % | Cu₂O content, % | Dimensions of Cu₂O crystals, μm | Graphite content, % |
|---|---|---|---|---|
| 1 | 38–40 | 30 | 0.025–0.1 | 70 |
| 2 | 60 | 50 | 0.05–0.18 | 50 |
| 3 | 70 | 60–65 | 0.05–0.1 | 35–40 |
| 4 | 95–98 | 80–85 | 0.05–0.08 | 15–20 |
Analysis of Table 1 shows that, with increasing ambient humidity, the process of oxidation of the copper of the contact ring is significantly intensified, which leads to an increase in the Cu₂O content in the investigated surface films. It is characteristic that the oxidation process proceeds by the formation of a large number of cuprous-oxide crystals, whereas the increase in the size of the Cu₂O crystals is insignificant. Thus,
surface films formed at elevated humidity have, as a rule, a polycrystalline structure.
Already in electron-microscopic and electron-diffraction studies of these films it was noted that the thickness of surface films produced at different humidities of the surrounding medium differs significantly.
The thickness of the surface films was measured by the interference method after the films had been separated from the substrate [1].
As a result of the measurements it was found that surface films formed at elevated humidity of the surrounding medium have a thickness 1.8–2 times greater than those produced under normal conditions. In some cases, when the film thickness exceeded 10000 Å, it was impossible to observe an interference pattern on the film surface.
The measurement results are given in Table 2.
Table 2
| No. | Relative humidity of the medium, % | Film thickness, $\delta_{\max}$ | Film thickness, $\delta_{\min}$ | Å, $\delta_{\mathrm{av}}$ |
|---|---|---|---|---|
| 1 | 38 | 5000 | 3000 | 4000 |
| 2 | 60 | 7000 | 4000 | 5500 |
| 3 | 70 | 8200 | 5000 | 6600 |
| 4 | 98 | 10600 | 5300 | 7950 |
Table 2 presents the limiting values of the thickness measured in different regions of the film.
Thus, with an increase in the humidity of the surrounding medium, the surface film exerts an ever greater influence on the operation of the sliding contact. The dependences presented in Fig. 1, which characterize the process of formation of the surface film, show that at elevated humidity a normally functioning film can be formed considerably faster than under normal conditions. The family of static current–voltage characteristics (Fig. 2), recorded after complete formation of the surface film under the specified conditions, fully corresponds to changes in the phase composition and thickness of the films. The increase in the level of the contact voltage drop and in the nonlinearity of the current–voltage characteristics is explained by an increase in the percentage content of cuprous oxide in the surface film, as well as by its thickness.
Consequently, there is a direct relationship between the parameters of the equivalent circuit [2], which is described by the current–voltage characteristics of the sliding contact, and the phase composition and thickness of the surface film.
Study of the surface films formed at different atmospheric pressures (760–10$^{-1}$ mm Hg) proved possible by the methods considered above only when the atmospheric pressure was reduced to 10 mm Hg. The film formed at low atmospheric pressure is so thin that it practically cannot be separated from the substrate.
With decreasing atmospheric pressure, the conditions for formation of surface films deteriorate significantly, both as a result of the decrease in the oxygen content necessary for the formation of cuprous oxide Cu$_2$O and in connection with a sharp increase in the coefficient of friction.
Electron-microscopic and electron-diffraction study of similar films showed that, when atmospheric pressure is lowered, the content of cuprous oxide in the surface films decreases substantially, and disorientation of the $\mathrm{Cu}_2\mathrm{O}$ crystals is observed. These reasons probably explain the fact that, with decreasing atmospheric pressure (at constant temperature and humidity), the current–voltage characteristics (Fig. 3) become increasingly straightened and the polarity differences disappear. The influence of the surface film disappears and, already at an atmospheric pressure of $10$ mm Hg, is practically no longer manifested. With the disappearance of the surface film, the current–voltage characteristics are straight lines.
Fig. 1
Fig. 2
Fig. 3
Conclusions
- Quantitative data have been obtained on the content of the components of surface films formed at different humidity of the ambient medium.
-
It has been established that surface films formed at elevated humidity have a thickness 1.8–2 times greater than that of films built up under normal atmospheric conditions.
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When atmospheric pressure is reduced, the structure and phase composition of the surface films change substantially.
REFERENCES
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R. F. Bekishev, B. I. Kostylev, A. I. Skorospeshkin. Investigation of surface films of collectors of electrical machines. Izvestiya TPI, vol. 212, 1971.
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A. I. Skorospeshkin, B. I. Kostylev, R. F. Bekishev. Physical nature of current–voltage characteristics. Proceedings of the Fourth All-Union Conference on Commutation in Electrical Machines, Omsk, 1969.