Experimental Tests of Static Friction Coefficient for Selected Material Combinations of Expanded Graphite - Steel Used in Sealing Nodes

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.