## 2 Materials and Methods  
For the study, rings made of expanded graphite were used. This graphite material is used in the construction of technical seals of the highest performance, working mainly in power and industrial installations [13; 45]. The graphite rings used in the study were manufactured by a Polish manufacturer of industrial seals and work as stuffing box packing in industrial fittings. The rivet-shaped sample with a tapered head was made of stainless steel AISI 304. This material is used for piston rods operating in valves. Samples characterized by a Ra roughness = 3.21-3.64 \(\upmu\)m were selected for testing. The choice of materials with such roughness was dictated by studies performed by [PERSON] and [PERSON] [34] and [PERSON] et al. [46], according to which surfaces with higher roughness values are more favorable because, as the researchers assessed, they act as microscopic reservoirs of solid lubricants. The friction node analyzed occurs in popular piston valves. In this type of valve, a cylindrical steel piston moves between graphite sealing rings. Information about the tested materials is shown in Table 1. \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Counter-Sample** & **Sample** \\ \hline  & & stainless steel (AISI \\ Material (chemical composition) & expanded graphite (C-98.9\%) & 304—C-0.08, \\  & (C-98.9\%) & Cr-18.2, Ni-10.5, Fe-65.91, \\  & & Mn-1.2, \\  & & P-0.04, Si-0.48) \\ \hline Shape & ring & tapered head rivet \\ \hline Dimensions [mm] & \(\phi 60\times 40\times 10\) & \(\phi 10\times 30\) \\ \hline Density [g/cm\({}^{3}\)] & 1.6 & 7.8 \\ \hline Roughness Ra [\(\upmu\)m] & - & 3.21–3.64 \\ \hline Hardness [HRC] & - & 29–30 \\ \hline Expansion [mL/h] & 200 \(\div 300\) & - \\ \hline Tensile strength [MPa] & - & Rm = 520 \\ \hline Yield strength [MPa] & & Rp = 210 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Information about the sample and counter-sample material. In order to investigate the process of creating the sliding layer, a pin-on-ring station was used (Figure 1). A steel sample in the shape of the tapered head rivet was placed in the upper holder of the mounting plate tribotest. A ring made of expanded graphite was fixedly mounted in the lower handle. The steel sample moved in a reciprocating motion. The solution used enabled the steel sample to move along the graphite ring over a 6-mm-long section with a frequency of 1 to 10 Hz. The drive was transmitted via a belt transmission. The forming layer was identified by measuring the roughness of the steel surfaces of the samples and observing them. Measurements of 2D surface stereometry were made with a ZAISS contact profilometer (ZAISS, Maple Grove, MN, USA) equipped with heads with an induction transducer and SUFORM software by SAJD METROLOGIA (Kielce, Poland), which allows for measurements and analysis of deviations of straightness and surface roughness. The measurements were made by determining the sampling length \(\lambda\)c = 0.8 mm, where the measuring length Ln was adequately 5 mm. In the analysis of the roughness results, cut-off filtration was used. The tests were carried out in accordance with PN-EN ISO 21920-3:2022-06 [48], PN-EN ISO 3274:2011 [49]. The morphology of the selected samples was performed after the test to assess the chemical composition of the sliding layer. The analysis of the chemical composition of the layer carried out showed that the material from which the layer came into being was coal (Figure 2). Figure 1: Scheme of analyzed friction node, based on [47]. Figure 2: Microscopic image of a layer formed on a steel surface; (**a**) microscopic SEM images, (**b**) EDS spectrum of created layer, based on [50]. The friction surfaces of the samples were examined before mating, after static loading at rest for 30 s, and then after 15 and 30 min of mating operation (Table 2). In order to evaluate the effect of moist graphite rings on the layer formed, tests were performed in the first step for the friction node working dry, that is, the steel sample cooperated with the dry graphite ring.