## 1 Introduction
Optimal tribological properties were achieved when the nano-EG content was 15 wt%. The low wear rate was attributed to the smooth and continuous sliding layer formed on the mating surface of the self-lubricating PI/nano-EG composite. The positive effect of the formed sliding layer on tribological phenomena was confirmed by [PERSON] et al. [24] and [PERSON] et al. [25]. It should be noted that the use of expanded graphite in sliding materials does not always imply a decrease in the coefficient of friction and wear of the cooperating materials. [PERSON] et al. [26] showed that the addition of expanded graphite to polyethylene composites under dry conditions increased the coefficient of friction by up to 38% compared to the pure polymer. Changing the operating conditions by introducing water as a lubricant resulted in an increase in friction for the pure polymer, which was attributed to the formation of a polymeric sliding layer. The introduction of expanded graphite hindered the formation of the polymeric sliding layer under water lubrication conditions, resulting in less friction and wear compared to the pure polymer. As reported by [PERSON] et al. [27], the tribological properties of composites containing expanded graphite are also influenced by graphite concentration and particle size. A low friction coefficient of the composite was obtained by adding 10 wt% of expanded graphite. However, this amount of graphite led to an increase in the wear of the composite. A new application of expanded graphite in sliding materials is its use as a component of self-lubricating microcapsules, which allows a slow and uniform release of lubricant between mating surfaces. [PERSON] et al. [28] improved the tribological properties of the polyurethane composite by enriching it with microcapsules containing expanded graphite combined with paraffin. They found that the coefficient of friction reached 0.18 and 0.08 under dry and water lubrication conditions, respectively, which were 77% and 62% lower compared to the pure polyurethane composite. [PERSON] et al. [29] developed a three-dimensional expanded graphite in which they uniformly dispersed SP80/PW microcapsules. The resulting capsules were placed in an epoxy resin. It was found that the friction coefficient and wear rate of the mating materials decreased by 79.7% and 99.5%, respectively, compared to the material without graphite capsules. Similar results were obtained by [PERSON] et al. [30] by introducing expanded graphite microcapsules into a PA6 composite. They showed a significant reduction in friction coefficient and wear intensity compared to the pure PA6 composite. The addition of graphite also reduced the temperature at the friction node. Expanded graphite is also a material widely used in sealing technology, where it is employed in both resting joints and rotary motion, such as in sealing pump glands, as well as in reciprocating motion, such as in industrial fitting [31; 32]. These seals often operate in aqueous environments, such as in the marine industry [33] and the nuclear power industry [34], where they are required to have a long service life. A proposal for the use of seals made of a steel liner coated with a layer of expanded graphite in nuclear reactors was presented by [PERSON] et al. [35]. Long-term tests showed that these seals provide high seal integrity and durability with respect to seals without expanded graphite. In summary, the use of expanded graphite significantly improves the tribological properties of cooperating materials at friction pairs. Its presence in friction and sliding materials contributes to increased durability and efficiency of technical components operating in demanding conditions. However, there is essentially no research in the literature on the interaction of expanded graphite with metallic materials. Furthermore, the mechanism and the role of the formed sliding layer at the interface of the mating surfaces are not fully explained. There is a lack of research on the behavior of expanded graphite when operating in the presence of water.