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Tribology may be defined as the interaction of contacting surfaces under an applied load in relative motion.
If the surface of a material is viewed on a microscopic scale, a seemingly smooth finish is, in fact, a series of asperities.
Therefore, if two materials are then placed in contact and moved relative to one another, the asperities of both surfaces collide.
The removal of asperities may be considered as wear, and resistance to the motion as a frictional force.
PEEK polymer, and compounds based on PEEK polymer, are used to form tribological components due to their outstanding resistance to wear under high pressure (r) and high velocity (u) conditions.
The friction and wear behavior of a material may be evaluated using one of several test geometries.
The data given in this publication were generated using an AMSLER pad on ring test rig.
The rotating disc used in this apparatus was 2.36 in. in diameter with a 0.236 in. depth and was ground to a 0.4 µm Ra surface finish.
 Wear
The useful life of components which function in tribologically demanding environments is governed by the wear.
The performance of a material may be quantified by evaluating either the specific wear rate (u sp),
where V represents the volumetric loss of the sample, F the force applied and D the total sliding distance, or the specific wear factor (k),
where dh/dt represents the rate of height loss measured in the sample.
The lower the wear rate or wear factor, the more resistant a material is to tribological interactions.
Figure 29 shows a comparative wear factor bar chart of some of the materials commonly used in demanding tribological situations.
These data show that 450FC30 has an extremely low wear factor for a thermoplastic material.
Figure 29 Wear Facor at 392°F (200°C), with 600ft.min.-1 and 44lb. Load for Some of the Highest Tribological Performance Materials
Friction
The friction of a sliding tribological contact may be defined as the tangential force (F) required to move a slider over a counterface,
F = µ N
where N represents the normal force and µ is the coefficient of friction. This relationship is commonly referred to as Amontons' law and states the proportionality of frictional force with normal loading. However, polymeric materials cannot be modeled by simplistic rigid body mathematics because they are viscoelastic. Values of µ quoted for polymers vary with the thermal characteristics of the material and experimental conditions. Therefore, the value of µ and F may vary for PEEK polymer components which experience "real-life" tribological contacts. This variable force may be considered in terms of two elements: a deformation term involving the dissipation of energy in a local area of asperity contact, and an adhesion term originating from the contact of the slider and the counterface.
Limiting Pressure and Velocity
Materials used for tribologically sensitive applications are classified by defining the limiting product of pressure x velocity (Lru). Limiting behavior is taken as the ru condition under which the material exhibits excessive wear, interfacial melting or crack growth from ploughing. Materials in critical tribological interactions may undergo either a pressure or a velocity induced failure. A pressure induced failure occurs when the loading of a sample increases to the point at which the sample undergoes fatigue crack growth from an asperity removal. A velocity induced failure occurs at the point when the relative motion between surfaces is such that thermal work at the material interface is sufficient to catastrophically increase the wear rate. Comparative Lru charts of materials commonly used to form bearings are shown in Figures 30 and 31. The experimental conditions were chosen to reflect realistic bearing conditions for in-engine applications.
Figure 30 - Lru for a Range of Bearing Materials at 68°F (20°C), with v = 600 ft. min.-1
The bar chart shown in Figure 31 contains fewer materials than Figure 30 because many of the bearing materials featured fail at temperatures below that of the second test.
Figure 31 - Lru for a Range of Bearing Materials at 392°F (200°C), with v = 600 ft. min.-1
* See definition of A108 in Table 1
Under these specific conditions, PEEK polymer is shown to be among the highest performance materials. However, bearings for many applications are produced in large numbers where production speed and costs are critical. PEEK polymer is the only high performance tribological material which can be injection molded to form finished components without further thermal treatment. Although Lru values are a useful guide to comparative tribological performance, there are no absolute values because identical experimental conditions cannot be reproduced. Comparative data for high performance tribological materials at ambient and elevated temperatures are shown in Table 1.
(a) Catastrophic increase in temperature, wear or friction.
(b) Average of the coefficient of friction at Lru and 50% Lru.
(c) Wear rate at 50% Lru.
(d) One time lubrication with a mineral oil.
* Resin impregnated carbon
** Vespel® is a registered trademark of DuPont.
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