Wear products and tribochemical reactions during friction of a brass-steel pair
Introduction
Monitoring the state of friction units becomes relevant with an increase in the resource of high-speed rotating and reciprocating machinery mechanisms [1,2]. An effective approach to analyzing the data obtained is to identify signs of failure by studying the composition of the lubricant, the relationship between the size and concentration of wear product particles in the oil and the level of wear of the mechanisms in the machines. During the stable operation of machines and mechanisms, the lubricating medium contains particles of wear products of small size, usually less than 10–20 μm, in a low concentration. An increase in the concentration and size of particles (in the range of 50–100 μm) is accompanied by a sharp increase in the wear of machine parts [3], and their interaction with the friction surface plays a decisive role in increasing the friction coefficient. In addition, friction and external conditions contribute to the oxidation of the contacting surfaces of metals or alloys, as a result of which not only metal particles, but also their oxides can act as wear products [[4], [5], [6]]. It was found that particles of metal oxides behave like abrasive substances during friction, accelerating the wear of metal surfaces and increasing their content in the lubricating medium. However, it is noted [7,8] that under certain conditions, due to the creation of dense protective oxide layers arising after the compaction of fragments of wear particles on the friction surface, the wear resistance of the tribo-conjugated surfaces increases.
Tribological studies [[7], [8], [9], [10], [11], [12]] show that the service life of friction units at high loads and high rotational speeds can be increased, including due to the formation of antifriction coatings with different physicochemical and mechanical characteristics. For their formation on the surfaces of the tribocontact, base oils are modified with additives, most of which are high molecular weight or organometallic compounds [13,14] containing polar functional groups. In addition, powders of metals (Cu, Mo, Fe, Ni, etc.) or their combinations [[15], [16], [17]], capable of forming antifriction films on friction surfaces as a result of reactions with a tribocontact surface, are used as friction modifiers.
In this work, the influence of the nature of the lubricating medium on the qualitative composition of wear products during prolonged frictional interaction of a brass-steel friction pair in model media is studied. This work is a continuation of our research on the nanotribology of aqueous solutions of monobasic carboxylic acids [18,19].
Section snippets
Experimental
All carboxylic acids R–COOH, where R is H (formic acid), CH3 (acetic acid), C2H5 (propionic acid), C3H7 (butyric acid), C4H9 (valeric acid), C5H11 (caproic acid), were purchased from Aldrich and Merck of reagent grade and were used without purification.
The evolution of the friction coefficient of a brass-steel pair was investigated using an AE-5 type pin-on-disc friction machine. The friction unit (Fig. 1) was a 40X steel disk (chemical composition,%: C 0.407, Si 0.255, Mn 0.597, P < 0.001, S
Results and discussion
The study of the evolution of the friction coefficient of a brass-steel pair for 10 h shows that in the series of acids from propanoic acid to caproic acid, its decrease is noted, which then remains practically constant (Fig. 2) [18].
During the running-in the brass-steel triboconjugation, it was found that the friction coefficient in the “brass – aqueous solution of formic acid – steel” system, as well as in the “brass – aqueous solution of acetic acid – steel” system, is significantly higher
Conclusions
Thus, as a result of the work carried out, the dependence of the tribological and physicochemical characteristics of the friction pair on the lubricant composition was found: a decrease in the concentration of iron in the lubricant after frictional interaction during the transition from formic to caproic acid; the formation of copper nanoclusters upon friction of a friction pair of a copper alloy - steel by the presence of an SPR band in solutions of carboxylic acids; the formation by friction
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We thank the resource center for collective use of the scientific and educational center “Materials” (http://nano.donstu.ru) for conducting sedimentation analysis. B.S. Lukyanov would like to acknowledge the financial support by Ministry of Science and Higher Education of the Russian Federation; project 0852-2020-00-19 for the FT IR spectroscopy investigations.
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