## 2 Materials and Methods  
Tribological tests were conducted using a block-on-ring Amsler model A135 tester (Figure 1). The friction machine enables continuous and static load regulation, changes in the counter-sample speed, and temperature measurement using a thermocouple. In this system, the sample consisted of expanded graphite with an industrial purity of 98%, and dimensions of 10 \(\times\) 10 \(\times\) 15 mm (H \(\times\) W \(\times\) D). The samples were made at the seal manufacturer's plant. The expanded graphite samples were made from natural flake graphite, which was chemically treated and then rapidly heated, causing it to expand and form a light, porous material. In the next step, the expanded graphite is pressed into sheets of a specified thickness and density, creating a graphite foil. The foil is then layered and compressed. The counter-sample was AISI 4130 alloy steel, from which rings with an external diameter of 45 mm and a width of 12 mm were made. The counter-samples were ground to a surface roughness parameter Ra \(\sim\)0.5 \(\upmu\)m, resulting in an anisotropic surface morphology of the alloy steel. The chemical composition, as well as the physical and mechanical properties of the tested materials, are presented in Tables 1 and 2, respectively. In order to identify the tribological phenomena occurring in the expanded graphite-alloy steel combination, tests were carried out under dry and humid operating conditions. The humid operating conditions were achieved by the dropwise application of 0.3 mL of water at 3 min intervals. The choice of test conditions was dictated by the actual operating \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Expanded Graphite** & **Alloy Steel (AISI 4130)** \\ \hline Chemical composition [\%] & C-98, O-0.5-1, S-0.01-0.1, & Fe-96, C-0.28-0.33, Cr-0.8-1.1, Mn-0.4-0.6, \\  & Si,Al,Fe,Ca,Mg-0.5-2 & P-0.035, Si-0.15-0.35, S-0.035 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of cooperating materials. Figure 1: Diagram of the friction node under analysis. conditions of the combination, which often alternates between humid and dry regimes. The humidity in the test room was 41%. The following kinetic conditions were applied: a ring speed of 150 rpm, loads of 200 and 650 N, respectively, and an operating time of 30 min (Table 3). Tribological behavior was analyzed on the basis of the measured values of the friction torque and the coefficient of friction (COF) calculated from it, as well as the operating temperature of the friction pair. The data were collected cyclically, which means that measurements were taken every 10 s. For statistical purposes, every test was repeated three times. In order to assess the mass wear of the graphite samples, they were weighed before and after the friction test. The amount of wear was determined from Equation (1):  
\[wear(wt\%)=\frac{m_{1}-m_{2}}{m_{1}}\times 100\% \tag{1}\]  
where \(m_{1}\)--mass of the sample before the friction test, \(m_{2}\)--mass of the sample after the friction test. Similar measurements were carried out for the rings, and the aim was to assess the presence of the resulting expanded graphite sliding layer on the steel surface. The percentage of the layer mass was calculated from Equation (2):  
\[layer\ mass(wt\%)=\frac{m_{2}-m_{1}}{m_{1}}\times 100\% \tag{2}\]  
where \(m_{1}\)--mass of the steel ring before the friction test, \(m_{2}\)--mass of the steel ring after the friction test. After tribological testing, the graphite-coated alloy steel surfaces were subjected to SEM and elemental composition analyses by EDS.