Wear Behavior

Sliding Properties
Among other factors, the very low intermolecular forces result in PTFE having the lowest coefficient of friction of all solid materials. With PTFE, the static and dynamic coefficient of friction is virtually identical. This means that there is no »stick-slip effect«.

Even at temperatures below 0°C these favorable sliding properties are retained. Starting at 20°C, PTFE's friction coefficient shows a slight increase. Unfilled PTFE and modified PTFE show roughly the same abrasion behavior. With dynamic seals of the same design made from modified PTFE, the surface compression ( radial force) is often higher than with the same seals made from regular PTFE. The reason is that due to lower cold flow the modified PTFE seal does not back away as much nor drops off over time as much as the regular PTFE seal. While this characteristic may result in higher compound abrasion on one hand, it often guarantees effective sealing over a longer period of time on the other. The addition of fillers tends to increase the coefficient of friction; abrasion, however, is significantly reduced.

Friction coefficients PTFE/cast perlite in no-lube operation(1)
(p = 0.2 N/mm², T = 30°C, R_{z cast perlite} ≤ 1.5 μm)

The wear resistance of pure PTFE is relatively low. The reason is that PTFE molecules, due to their complete enclosure by fluorine atoms, are capable of developing merely minimal intermolecular interaction. In the crystalline regions of the material the molecular layers, similar to graphite, can be pushed off layer by layer under tribological load. In the amorphous areas the polymer composite is more stable due to intermolecular interlooping, however, this accounts for no more than roughly 30 vol % of the polymer. A significant improvement of wear resistance is achieved by fillers such as carbon, graphite, glass and carbon fibers, bronze or organic fillers.

Compared to PTFE with mineral or metallic fillers, the newly developed special compounds, HS 21059, HS 21037 and HS 10300, even in absolutely dry-running operations have clearly improved wear behavior and very low abrasion tendency on the mating surface, even if the mating surface is unhardened. The sliding friction coefficients of the respective mating surfaces are of lesser importance for abrasion behavior. Rather, wear is much more dependent on operating conditions (medium, pressure, speed, temperature, lubrication, surface roughness). Since no PTFE compound is able to meet all requirements, the PTFE compound type best suiting the particular application must be determined.

When performing wear tests the fact that every testing method provides its own set of data must be taken into account. A direct compound comparison therefore is only possible within each particular test method – using identical or similar testing parameters. The objective should always be to test materials under conditions resembling actual field applications as closely as possible. Elring-Klinger's development labs provide such capabilities.



Wear of Unfilled PTFE Compared to Various PTFE-Compounds
Long-term wear in dry-running (oil-free) conditions⁽²⁾
Test conditions:
Test atmosphere: air
T = 100 °C
v = 4 m/s
p = 0,42 N/mm²
Rz = 2 μm
Test period: 100 h


X210 Cr12 GG25 Alu hard-anodized