## 3 Results and Discussion
### The Comparative Analysis of Dry and Moist Friction on the Sliding Layer Formation
Figure 9: Comparison of the values of the Ra roughness parameter of samples mated with dry and moist rings for a period of 5 min, for loads of 10, 20, and 30 N, at speeds of 25 and 50 mm/s. rings, a significant smoothing of the surface was visible, compared to the surface working with dry rings. In turn, after 30 min of work at a speed of 25 mm/s in humid conditions, the greatest smoothing of the steel surface occurred. The steel surface working in dry conditions had an unevenly applied layer of graphite. Imaging studies confirm the results presented in Figure 10 (blue line). The speed increase to 50 mm/s contributed to a greater smoothing of the steel surface cooperating with the dry ring. Expanded graphite filled the irregularities in the surface. The opposite phenomenon was observed when the steel surface operated with a moist ring. Exposed roughness peaks were visible on the surface of the tested sample. The graphite layer did not cover the surface evenly. This phenomenon was also reflected in the surface roughness results of the samples (yellow line--Figure 10). Figure 11: Photos of the surfaces of steel samples working with dry and moist graphite counter-samples before and after the experiments (speeds of 25 and 50 mm/s, load of 30 N, magnification 250\(\times\) and 500\(\times\)). A preliminary explanation for the increase in the roughness of the mating surfaces of water-soaked samples at 50 mm/s and a load of 30 N is thermal phenomena. The operation of the friction node under the indicated conditions over an extended period of time contributes to an increase in the temperature of the friction node. This is confirmed by studies [51; 52; 53]. However, none of the presented studies concern expanded graphite and stainless steel. The adsorbed water in the graphite samples begins to evaporate with the operating time. Up to now, water molecules have provided protection to the emerging graphite layer. Due to the lack of water, the problem of forming a sliding layer and an increase in roughness occurs. The mechanism of the lubricating action of expanded graphite can be explained by the adsorption theory. According to this theory, graphite has good lubricating properties due to water adsorption. Adsorbed water molecules on the main sliding planes reduce the adhesion between them and thus reduce the coefficient of friction when sliding them. In addition, the adsorbed water functions as a hydrogen and oxygen reservoir for edge passivation of reactive graphene layers on basal planes [54] or in prismatic planes of graphite crystals perpendicular to the sliding direction [55]. However, given the complexity of water activity (density and structure of physisorbed water and chemisorption and reaction of water at the interface under mechanical shear), it is not possible to explain the effect of water sorption by one simple mechanism. Additionally, the previous literature analysis shows that it is important to determine the amount of water necessary to maintain appropriate lubrication conditions. Too much water has an adverse effect on the resulting layer and its smoothness [56]. Therefore, it seems advisable to continue research in the indicated direction. As part of research, the initial wear mechanism was defined. In the first phase of research, graphite particles transferred as a result of adhesion were anchored at the highest ends of the microcavities. Primary wear products were formed. Then, as a result of the movement, the graphite particles transferred to the metal surface were partially ground. The amount of graphite material on the steel surface increased. After 5 min of operation, the visible grooves of the internal surface were filled with graphite. However, in most cases the steel tops were still visible. This phenomenon was practically unnoticeable after 30 min of association of samples cooperating with wet rings. Wear particles not bound to any surface can be re-adsorbed onto the graphite surfaces or removed from the friction zone.