Investigation of Surface Films under Various Thermal Operating Modes of Sliding Contact
Abstract Generated abstract
This study examines surface films formed on copper contact rings by MGS-7 copper graphite brushes under different thermal conditions in a sliding electrical contact. Films run in at 45 to 95°C were separated from the substrate and analyzed by electron microscopy, electron diffraction, X-ray phase analysis, and interferometric thickness measurement, alongside current-voltage characterization. The films were found to consist mainly of cuprous oxide and graphite, with increasing temperature associated with higher cuprous oxide content, larger oxide crystals, greater film thickness, and faster attainment of steady contact voltage. The results suggest that contact voltage behavior cannot be explained solely by film composition or thickness and should be interpreted using an equivalent circuit approach.
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STUDY OF SURFACE FILMS UNDER VARIOUS THERMAL OPERATING CONDITIONS OF A SLIDING CONTACT
R. F. Bekishev, B. I. Kostylev
(Presented by the scientific seminar of the Department of Electrical Machines and General Electrical Engineering)
The principal tool for studying the physical properties of the surface film formed on commutators and contact rings, and of the sliding brush contact as a whole, is the current-voltage characteristic. It has been established that the contact voltage drop for a brush–copper commutator (ring) pair, in the range of low current densities (up to \(20\ \mathrm{A/cm^2}\)), is determined mainly by the properties of the surface film.
The study of surface films by the modern methods described in [1] makes it possible to assume that the structure, phase composition, and thickness of the surface film depend directly on the running-in conditions and, first of all, on the temperature of the commutator.
Therefore, investigation of the structure, phase composition, and thickness of surface films formed under different thermal operating conditions of the contact is of definite interest.
The running-in of the surface film was carried out on contact rings made of M1 copper with MGS-7 grade brushes at constant current density \(10\ \mathrm{A/cm^2}\) and rotational speed \(15\ \mathrm{m/sec}\).
The surface film was considered to have formed (under the given conditions) when the voltage drop between the brush of each polarity and the contact ring reached a steady value.
After running-in of the surface film and recording the current-voltage characteristics of the sliding contact, the contact rings were removed from the shaft of the setup and prepared for separation of the films according to the procedure described in [1].
The structure, orientation, and phase composition of the constituents of the surface films separated from the copper of the contact rings were studied by electron-microscopic and electron-diffraction methods using a UMB-100K electron microscope, by obtaining microphotographs and microelectron diffraction patterns. Numerous investigations of surface films by these methods showed that, in a number of cases, for films comparatively uniform in thickness, microphotographs can also be used for quantitative determination of the phase composition. For this purpose, the linear method of determining the phase composition of films proposed by Rozival [2] may be used.
In addition, for some films it proved possible to carry out quantitative determination of the phase composition of the constituents by obtaining electron diffraction patterns using the spot method of A. A. Glagolev [2].
In all other cases, the study of the phase composition of the surface films was carried out by the X-ray phase-analysis method on a URS-50 I/M ionization X-ray apparatus; in the quantitative determination of the phase content, the method of homologous pairs [3] was used.
The thickness of the surface films separated from the substrate was measured by an interference method on a Linnik MII-4 microinterferometer. Various areas of the film under study were subjected to measurements, and the limiting values of thickness were determined. The choice of the method for measuring the thickness of surface films and their preparation for these studies are described in [1].
Fig. 1
In studying surface films formed at different ambient temperatures (the range of variation of the contact-ring temperature was \(45 \div 95^\circ\mathrm{C}\)), the atmospheric pressure and humidity of the ambient medium remained constant and were equal, respectively, to 760 mm Hg and \(38 \div 40\%\). The temperature of the ring was monitored by thermocouples built into the contact rings, and the excess of the contact-ring temperature over the ambient temperature was no more than \(2—4^\circ\mathrm{C}\).
Fig. 2
The electron-microscopic, electron-diffraction, and X-ray structural studies of the surface films that were carried out showed,
that the latter consist of crystalline cuprous oxide, \( \mathrm{Cu_2O} \), and graphite, occurring both in the amorphous and in the finely crystalline state.
As an illustration, Figs. 1 and 2 present a light-field micrograph of a surface film formed at a contact-ring temperature of \(45^\circ\mathrm{C}\), and a microelectron diffraction pattern obtained from a region of this film. These images rather fully reflect the structure of the surface film formed at a comparatively low temperature, and the indexing of the microelectron diffraction patterns shows that the diffuse rings and numerous point reflections correspond to the crystal lattice of graphite, which constitutes the greater part of the film, occurring both in a finely crystalline and in an amorphous state. The clearly expressed diffraction rings correspond to finely crystalline cuprous oxide.
Fig. 3
Fig. 4
The micrograph and microelectron diffraction pattern (Figs. 3, 4), obtained from regions of the surface film worked in at a temperature of \(95^\circ\mathrm{C}\), reveal the structure of large, well-oriented crystals of cuprous oxide. Lines of amorphous and crystalline graphite occur much more rarely. Table 1 presents quantitative data on the character of the structure and phase composition of surface films ...
... films formed by MGS-7 brushes in the temperature range 45–95°C, obtained by the methods indicated above.
Table 1
| No. | \(t^\circ C\) | Cu\(_2\)O content, % | Crystal size, μm | Graphite content, % |
|---|---|---|---|---|
| 1 | 45 | 30–35 | 0.025–0.1 | 65–70 |
| 2 | 55 | 50 | 0.1–0.15 | 50 |
| 3 | 70 | 60–65 | 0.2–0.4 | 35–40 |
| 4 | 95 | 70–75 | 0.5–1.0 | 25–30 |
Analyzing the structure and phase composition of the surface films formed by brushes of grade MGS-7 at various temperatures, one should note a clearly expressed tendency toward an increase in the cuprous oxide content in the film as the temperature rises. At the same time, both a significant increase in the size of the Cu\(_2\)O crystals and an increase in their number during crystallization are observed. It should be noted that the surface films studied had a comparatively uniform structure through the thickness and were sufficiently stable under the action of the electron beam. Measurement of the thickness of the surface films by the interferometric method confirmed that the films produced by sliding over the contact ring of copper–graphite brushes (MGS-7) are more uniform in thickness than the films formed by electrographite brushes. Films run in under conditions of elevated temperature of the contact ring have a considerably greater thickness than those formed at low temperature.
Table 2 presents the results of measurements of the thickness of surface films formed at different temperatures of the contact ring.
Table 2
| Brush grade | Ring material | Running-in conditions | \(\delta_{\max}\) | \(\delta_{\min}\) | \(\delta_{\mathrm{avg}}\) |
|---|---|---|---|---|---|
| MGS-7 | M1 | \(t = 45^\circ C\) | 5000 | 4000 | 3000 |
| MGS-7 | M1 | \(t = 55^\circ C\) | 5500 | 3000 | 4250 |
| MGS-7 | M1 | \(t = 70^\circ C\) | 6000 | 3700 | 4850 |
| MGS-7 | M1 | \(t = 95^\circ C\) | 7000 | 4000 | 5500 |
The accuracy of measuring film thickness at a light wavelength
\[ \lambda = 5500 \ \text{Å} \]
is \(\pm 275\) Å. Thus, the surface film formed on collectors and contact rings under different thermal operating conditions of a sliding contact differs substantially in structure, phase composition of the components, and thickness.
With an increase in temperature, the conditions for formation of the surface film improve and the rate of oxidation of the copper of the collector or contact ring increases. The dependences shown in Fig. 5, reflecting the process of formation of the surface film under different thermal operating conditions of the contact, show that the contact drop...
the voltage drop reaches its steady-state value at high temperatures considerably faster than at low ones.
The volt-ampere characteristics, taken in the static mode after running-in of the surface film under various thermal operating conditions of the contact (Fig. 6), show that the level of the contact voltage drop decreases as the temperature increases, whereas the content of cuprous oxide in the surface films and their thickness (according to the data of the investigations presented above) increase. Therefore, the content of the constituents of the surface film and its thickness cannot be unambiguously linked with the form of the volt-ampere characteristics. In our view, the analysis must be carried out through the parameters of the equivalent circuit [4], which, under certain conditions, can be determined directly with allowance for the results of quantitative analysis and measurement of the thickness of the surface films.
Fig. 5.
Fig. 6.
REFERENCES
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A. I. Skorospeshkin, B. I. Kostylev, R. F. Bekishev. The physical nature of volt-ampere characteristics. Proceedings of the IV All-Union Conference on Commutation of Electrical Machines. Omsk, 1969.