Reinforced PTFE

PTFE is often used with different fillers (like glass fiber, carbon, bronze, graphite) in order to reinforce his mechanical properties.

Wear

The contact between two sliding surfaces, because of the inevitable friction generated in the contact zone, results in a certain wear whose magnitude depends on load, speed and time of sliding contact. Theoretically, between these parameters and the resulting wear exists a relation proportional to:

R=KPVT

where, expressed in the measuring units of table:
R = wear in mm

P = specific load in N/mm2 (referring to the surface - Ø x l - in case of bushes, nipples, etc.)
V = sliding speed in m/sec
T = time in hrs
K = wear factor in mm3 sec/Nmh.

The value of the factor PV after which the coefficient of wear loses its linear behavior, assuming remarkable values with the system passing from weak to strong wear condition, is known as “PV limit”. This PV limit and the wear factor are, therefore, characteristic parameters of each material. In practice, however, it can be easily perceived, the wear factor and the PV limit of the same filled material can vary also with the nature, the hardness and the surface finish of the other contact “partner” with the presence, or not, of cooling and/or lubricating fluids.

Deformation under load and compressive strength

PTFE, like most other plastic materials, has no “elastic zone” where the ratio load/deformation (Young modulus) has a constant value. This ratio load/deformation depends upon the time of application of the load and the ensuing deformations; this phenomenon is known as “creep”, and at the removal of the load, there is only a partial return of the deformation to the original state (“elastic recovery”), so that we are always in the presence of a “permanent deformation”.
Creep, obviously not being a linear function of time, results after just over 24 hrs in deformations which in most cases are not taken into consideration. With increasing temperature, there is a falling off of the deformation under load properties and consequently of the compressive strength which is already at 100°C equal to 1/2 of that at 23°C and at 200°C about 1/10th. In any case, PTFE and in particular filled PTFE, is one of the plastic materials retaining, at high temperatures, optimum deformation properties under load. To conclude, the elastic recovery in about 50% of the deformations under load, and the permanent deformations are equal to about 50% of the deformations under load. This applies both to filled and unfilled PTFE. The properties of the first are however decidedly superior. In fact, the deformation under load of the more common types of filled PTFE are about 1/4 of that of the unfilled ones, while the compressive strength is about the double.