## 3 Results  
### Analysis of the Friction and Temperature Indices of Expanded Graphite-Alloy Steel Pairs  
The recorded changes in friction torque over time for an expanded graphite-alloy steel pair, operating under dry and humid conditions as a function of load, are shown in Figure 2. With a load of 200 N under dry conditions, the friction torque remains between 0.54 and 0.6 Nm throughout the test. In humid conditions, the value varies between 0.4 and 0.8 Nm. Characteristic cycles of frictional torque were observed during the 30 min test under humid conditions. Immediately after water is applied, a sharp decrease in friction torque occurs, followed by a rapid increase. At a load of 650 N in dry conditions, the friction torque remained at approximately 1.9 Nm, although three minutes before the end of operation, it increased to around 2.35 Nm. The application of water to the tribological contact at a load of 650 N caused a sharp decrease in friction torque to approximately 0.7 Nm, followed by stabilization and a return to around 2 Nm. This phenomenon occurred cyclically throughout the 30 min friction process. The recorded changes in the COF of the tested pair operating under dry and humid conditions, depending on the load, are shown in Figure 3. Similar trends in COF variations to those of friction torque were observed. Under a load of 200 N in dry conditions, the COF remained at approximately 0.13. In humid conditions, a sharp drop in COF to around 0.09 was observed immediately after water application, followed by a rapid increase to approximately 0.17. At a load of 650 N in dry conditions, the COF remained stable at around 0.13 until the 27 th minute, after which a sudden increase to approximately 0.16 was recorded. In humid conditions, the direct application of water to the friction pair caused a sharp decrease in COF to about 0.05, followed by an increase to approximately 0.14. Figure 3: Variation in COF over time, under dry and humid conditions for load: **(a)** 200 N, **(b)** 650 N.  
Figure 2: Variation in friction torque over time under dry and humid conditions for a load of: (a) 200 N, **(b)** 650 N.  
The average variations in friction torque and COF depending on operating conditions and load are presented in Figure 4. Under dry conditions, an increase in load from 200 to 650 N results in a friction torque increase of approximately 235%, while under humid conditions, the increase is around 209%. No significant average changes in COF were observed for the tested operating conditions and loads. The variation in COF and temperature over time as a function of load under dry operating conditions is presented in Figure 5. Up to the 27 th minute, no influence of load on COF was observed. After this point, at a load of 650 N, a significant increase in COF occurred, preceded by a sudden rise in temperature. This behavior can be explained by the fact that at higher loads, elevated temperature may promote water vapor desorption and surface oxidation of graphite, which may affect the friction pair's operating characteristics [40; 41]. At a load of 200 N, the operating temperature remains stable at around 40 \({}^{\circ}\)C, whereas at 650 N, it gradually increases, eventually reaching approximately 120 \({}^{\circ}\)C.  
The time course of COF and temperature change as a function of load for humid operating conditions is shown in Figure 6. For both 200 and 650 N loads, there is a cyclic loss of COF stability. Characteristic decreases in COF occur when water is applied to the contact zone, which may be related to the presence of an adsorbed water layer on the graphite surface. The presence of water reduces friction by decre  
Figure 4: Average variations in (**a**) friction torque and (**b**) COF depending on operating conditions and load. Figure 5: Variation in COF and temperature over time, depending on load under dry operating conditions. facilitates sliding between graphite layers, which freely move due to their lamellar structure and weak interlayer bonds [42].