The sliding layer created during operation of the expanded graphite–steel combination has had a huge impact on the effectiveness of the friction process, and thus on the sustainable development of society. Knowledge of the factors determining the properties of the sliding layer will make it possible to reduce friction resistance in the future through the proper design and selection of sliding pairs for given applications. This paper studies the effect of the moisture content of expanded graphite on the formation of a sliding layer on a stainless steel surface. The tests were carried out in static contact for 30 s and dynamic contact for 15 and 30 min, for loads of 10, 20, and 30 N and speeds of 25 and 50 mm/s. To determine the changes in surface geometry due to material transfer, the Ra roughness value of the surface of stainless steel samples was measured. In order to realize the purpose of the work and evaluate the effect of moist rings on the resulting sliding layer, the results of the surface roughness of stainless steel samples working with dry and moist graphite rings were compiled. The obtained results show that the presence of water in the stainless steel-expanded graphite friction node affects the formation of a sliding layer. The resulting layer reduces the surface roughness of the cooperating materials and prevents their accelerated wear. After 5 min of work with the water-soaked graphite counter-sample, depending on the applied friction conditions, a reduction in the surface roughness of the stainless steel sample was achieved in the range of 11–18% compared to the initial value. After 30 min of operation, the surface roughness decreased by 3 to 25%. Pilot studies have shown that operating conditions influence the formation of a sliding layer in the stainless steel-expanded graphite tribological contact. This confirms the validity of conducting further research in this area.

The aim of the research was to evaluate the influence of the initial roughness of a steel pin cooperating with a graphite ring—dry and wet—on the mechanism of sliding layer formation. A ring–pin friction pair was used for the study, where the rings were made of expanded graphite, while the pins were made of acid-resistant steel. In the first case, the steel pin interacted with a dry graphite ring, and in the second case, the graphite rings were moist. To determine the effect of initial surface roughness, the pins were divided into three roughness groups. To determine changes in surface geometry due to material transfer, the Ra and Rz parameters were measured. This project investigated how the initial roughness value of the steel surface pin cooperating with expanded graphite influences the formation of the sliding layer. Increasing the initial roughness of the steel surface interacting with the graphite contributes to faster layer formation and reduced roughness. The state of the expanded graphite—dry and wet—influences the formation of the sliding layer of graphite—a wet graphite component causes a faster smoothing of the steel surface. The running time of the wear apparatus has an effect on the resulting layer. The highest roughness group is the most favorable from the viewpoint of sliding layer formation.

The paper presents the results of static friction coefficient tests of selected material sets used in sealing nodes. Four graphite materials were tested - pure expanded graphite, expanded graphite with the addition of PTFE plastic of different densities (1.4 and 1.8 [g/cm3]) and expanded graphite laminated with austenitic steel foil - matching the steel surface. The combinations operated in a reciprocal movement. The measurements were made at different unit pressures (0.5, 2 and 5 MPa) and for different work cycles (0, 500, 1500 and 3000). On the basis of the results obtained, it was found that both the load and the number of work cycles have a significant influence on the value of the static friction coefficient of the tested materials. Not without significance is also the density of selected graphite materials. It was found that expanded graphite with PTFE and density of 1.8 g/cm3 was characterized by the lowest static friction coefficient in comparison with other materials. It is also worth noting the fact of a layer on the steel surface of combination which promotes the formation of stick-slip phenomenon.

Expanded graphite significantly improves the tribological properties of materials in friction pairs, but there is a lack of research in the literature on its cooperation with metals and the effect of water on friction and wear mechanisms. It is particularly important to understand the phenomenon of graphite layer formation on the steel surface and its effect on tribological properties. The aim of this study is to evaluate the tribological properties of an expanded graphite–alloy steel combination operating under selected loads in both dry and humid conditions. The tests were carried out on a block-on-ring tribological tester (where the blocks were made of expanded graphite and the rings were made of AISI 4130 steel) at a rotational speed of 150 rpm, with loads of 200 and 650 N. The frictional behavior was analyzed on the basis of the measured values of the friction torque and the coefficient of friction (COF) calculated from it (and the applied load). In dry conditions, the friction torque was stable, while in humid conditions it showed cyclical changes. An increase in load from 200 to 650 N caused an increase in the average friction torque by 235% in dry conditions and by 209% in humid conditions. The presence of water reduced the friction pair temperature by 12% at 200 N and by 18% at 650 N; however, it simultaneously increased graphite consumption–by 1179% at 200 N and by 100% at 650 N. The amount of graphite deposited on the steel surface depended on the load–in humid conditions, it increased by 114% at 200 N, while it decreased by 250% at 650 N. The conducted research expanded the understanding of the influence of operating conditions on the tribological properties of the expanded graphite–alloy steel pair. It also provided new data on the friction and wear mechanisms of this material combination in humid conditions, which may have significant engineering applications.

The aim of this study was to evaluate the tribological properties of braided packings impregnated with natural graphite (NG) and expanded graphite (EG), operating against AISI 4130 alloy steel under dry and water-lubricated conditions. Tests were performed using an Amsler A135 tester in the “block–ring” configuration at a load of 350 N, speed of 200 rpm, and a duration of 30 minutes. The coefficient of friction, operating temperature, and mass wear were determined, and SEM/EDS analyses were carried out to characterize transfer layers. The results demonstrated a significant influence of graphite type and operating environment on friction behavior. Under dry conditions, EG-based packings exhibited a lower coefficient of friction (0.11 vs. 0.24 for NG), reduced operating temperature (by ~30%), and markedly lower mass wear. In water-lubricated conditions, COF values remained more favorable for EG; however, its mass wear increased by 367%, while for NG the increase was only 15%. SEM and EDS analyses revealed that expanded graphite formed more uniform and adhesive transfer layers under dry conditions, which promoted friction stabilization. In the presence of water, these films became less homogeneous. Natural graphite in the water-lubricated environment exhibited weak adhesion to the substrate and discontinuous coverage of the steel surface.