Tribological Studies of Braided Packings with Natural and Expanded Graphite under Dry and Water-Lubricated Condition

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.