INTRODUCTION

With the development of the modern world industry, there has been a tendency to make maintenance-free, high-resource, and high-temperature-resistant friction assemblies. When designing and operating such components, the use of solid lubricants is becoming increasingly popular. For some fields of modern industry, solid lubricants are of particular importance, for example, in robotics, aerospace, electric vehicles, and in the production of devices operating under vacuum or in the range of extreme temperature values. In the case of such systems, friction losses are of particular importance, since they directly affect autonomy and safety.

The main solid lubricants are represented by molybdenum disulfide, tungsten disulfide, graphite, polytetrafluoroethylene, and boron nitride. Most of these materials, owing to their layered crystalline structure, exhibit a low level of shear resistance between the layers of the structure [1–3].

Nowadays, solid lubricants are most often used in the form of special coatings [2, 3]. As a rule, such antifriction solid lubricant coatings (ASLCs) represent mixtures consisting of solid lubricants, binders, solvents, and functional additives [4–7]. Resulting from applying such ASLCs onto the surface of products, a thin composite layer with a thickness of 15–25 µm is formed, which represents a polymeric matrix that retains highly dispersed particles of solid lubricant in the structure thereof.

The type of solid lubrication filler in ASLCs determines their properties, their operating characteristics and the scope of application [7–9]. The ASLCs have a wide operating temperature range (from –210 to +730°C), they cannot be oxidized or evaporated, they exhibit good anticorrosion properties and a high load-bearing capacity (up to 3000 MPa) [2, 10, 11]. The fact that the coatings have small thickness makes it possible to use them with non-subsequent mechanical treatment.

There are many examples of the use of ASLCs in different industries [9–12]. For example, ASLCs are applied to the piston skirts of internal combustion engines, crankshaft liners, pneumatic drive parts, worm transmission pairs, lead screws, rack and pinion gears, ball joints, threaded joints, etc. A wide range of promising compositions for the formation of ASLCs are produced in European countries and the United States. In the Russian market, the Modeling and Engineering enterprise (Russia) is actively developing the field of ASLCs [3, 9, 12].

Thus, the use of ASLCs in metal–polymer friction pairs is of practical interest and is promising, in particular, for parts of spherical elements (fingers) working in conjunction with a plastic liner.

Objective—Revealing the effect of ASLCs and greases exerted on the tribological properties of a metal–polymer friction pair.

MATERIALS AND METHODS

Laboratory-scale testing was carried out according to the ASTM G99 standard with the use of a friction machine that operates according to a sphere-disk pattern at sliding velocity V = 0.8 m/s, load F = 23 N, and rotation frequency n = 310 min–1. The friction machine (Fig. 1) is equipped with an NI LabVIEW automatic control and data registering system. During testing, the value of the sliding friction force was permanently registered, and the value of the friction coefficient was automatically calculated and graphically displayed on the screen. The testing was performed until the accumulation of preset friction path L = 2880 m. After testing, the linear wear level of the spherical counterbody was measured with the use of a MKTs 25 micrometric instrument (Russia) with an accuracy of 1 µm.

Fig. 1.
figure 1

Equipment for tribotechnical testing: (a) a general view of the friction machine; (b) general view of the friction pair; (c) a holder with installed spherical counterbody; and (d) friction pattern during testing.

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The samples for the experiments were made in the form of steel disks with a diameter of 80 mm and a thickness of 0.8 mm made of 40Kh grade steel, and spherical counterbodies 10 mm in diameter made of polyoxymethylene (POM). The working surfaces of the samples, having a roughness of Ra = 0.5 μm, were degreased after which three types of ASLCs, such as M-1014, M-1009, and M-A20 were deposited onto them with the use of spraying. After the coatings based on M-1014 and M-1009 were dried for 10 min at room temperature, hot curing (by means of polymerization) was carried out.

The M-A20 coating was cured at room temperature for 30 min. The characteristics of the ASLCs and polymerization modes under testing are presented in Table 1. The thickness of the deposited coatings has been determined by a Konstanta K5 electronic thickness gauge (Russia) to amount to 25 ± 5 μm. The testing was carried out with the use of different greases the characteristics of which are presented in Table 2. The weight of grease applied onto the disk amounted to 0.25 g.

Table 1.   Characteristics of tested antifriction solid lubricating coatings
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Table 2.   Characteristics of the greases under testing
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RESULTS AND DISCUSSION

The complex of tribotechnical investigations was in two stages. The first stage involved the studies on the samples with and without ASLCs in the absence of grease. The second stage involved studying the combined effect of ASLCs and different greases exerted on the wear processes occurring in the metal–polymer friction pairs under testing. At all stages of the investigation, we measured the linear wear level and the friction coefficient for the counterbodies. Figure 2 shows the results of ASLC testing in comparison with uncoated samples in the absence of a grease lubricant. The graphs plotting the change in the friction coefficient of the ASLCs over the entire testing period are presented in Fig. 3.

Fig. 2.
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Testing results for antifriction solid lubricant coatings: (a) friction coefficient; (b) linear wear of counterbodies.

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Fig. 3.
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Changes in friction coefficients under testing antifriction solid lubricant coatings: (1) uncoated samples; (2) M-1014; (3) M-1009; and (4) M-A20.

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By analyzing the diagrams presented in Fig. 2, one can conclude that the use of all three ASLCs exerts a beneficial effect on the friction and wear processes. All ASLCs, regardless of the composition, provide a greater than sevenfold decrease in the linear wear and a twofold decrease in the friction coefficient. At the same time, samples with ASLC M-A20 based on polytetrafluoroethylene exhibit the lowest friction coefficient (0.10) and the lowest level of linear wear (80 μm). This coating reduced the friction coefficient by 3.8 times, and the linear wear level by more than 9 times as compared to the uncoated sample.

The combined application of greases and ASLCs has made it possible to reveal a synergistic effect that is exhibited by a decrease in the friction coefficient (see Table 3) and linear wear level of counterbodies (see Table 4) when the ASLCs are used together. Moreover, it can be argued that this effect is observed for all combinations of greases and ASLCs. The lowest friction coefficient (0.05) has been registered for the M‑1014 coating in combination with lubricant No. 2 (Fig. 3, curve 4).

Table 3.   Results of determining friction coefficient for MODENGY antifriction solid-lubricating coatings together with greases
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Table 4.   Results of determining linear wear levels for counterbodies upon using antifriction solid-lubricating coatings together with greases
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The synergetic effect is achieved owing to the presence of PTFE in the lubricant and the coating. The smallest linear wear level of the counterbodies (50 µm) was observed in the case of the M-1014 coating in combination with lubricant No. 3 (Fig. 3, curve 2) as well as when combining the M-A20 coating and lubricants No. 2 and No. 3 (Table 4). The presence of MoS2 in the coating can significantly reduce wear level. The combination of ASLCs and grease can provide reducing the linear wear level by up to 14.2 times, and the friction coefficient, by up to 4.4 times.

Based on the analysis of the results of the investigations, it could be noted that ASLCs in general exert a more significant effect on the tribological parameters of the metal–polymer friction pair in comparison with greases. The ASLC coatings are under an active implementation at the enterprises of the Russian automotive industry. In particular, Fig. 4 shows a car steering ball joint.

Fig. 4.
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Ball joint with antifriction solid lubricant coating M-1014.

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CONCLUSIONS

Resulting from the series of conducted experiments, it has been established that:

— The application of modern ASLCs instead of grease makes it possible to provide a 5.1–8.9-fold decrease in the linear wear level of counterbodies, whereas the friction coefficient can be reduced by 1.2–2.2 times, depending on the used coating grade.

— The use of ASLCs studied in this work, makes it possible to provide, in combination with greases, a 5.9–10.1-fold decrease in the linear wear level of counterbodies, and a 1.5–2.3-fold decrease in the friction coefficient, depending on the used coating grade.

— When used in combination with ShRB-4 grease, an M-A20 coating is the best ASLC from the standpoint of wear reduction, whereas an M-1014 coating is the best ASLC in terms of friction reduction.

— By selecting an optimal combination of ASLCs and grease, one can provide a 14.2-fold decrease in the linear wear level of counterbodies (for example, as it is in the case of coatings M-1014 and M-A20, as well as experimental lubricant No. 3), as well as provide a 4.4‑fold decrease in the friction coefficient (for example, as it is in the case of coating M-1014 and experimental lubricant No. 2).

ABBREVIATION AND NOTATION

ASLC antifriction solid lubricant coating

POM polyoxymethylene

PTFE polytetraflioroethylene