**5. Characteristic mechanisms in the superficial layers of contacts implying polymeric materials**

Initially, PTFE was simply used as bushes, seals, but its low mechanical characteristics make the researchers for materials to use it as matrix in composites [9, 39], adding material in other polymers, and even metallic sintered materials, more rigid and less prone to wear.

Burris and Sawyer studied the blend PEEK + PTFE [49]. PEEK has wear resistance, mechanical strength and a higher working temperature as compared to other polymers, but a high friction coefficient in dry regime μ = 0,4 and low thermal conductivity. PTFE has a high wear rate, and the fact that has the lowest friction coefficient in similar conditions does not recommend it to be used simple, without blending with another polymer or reinforcements. A qualitative model of a polymeric blend could be modeled as in **Figure 31a**.

Many researchers and producers of polymeric materials recommend only 5–20% PTFE [46, 50, 51], experiments done by Burris and Sawyer [49] obtained an optimum for the wear rate using the blend 30% PEEK +70% PTFE and, thus, underlined the necessity of testing new formulated materials for tribological applications.

Under 20% PEEK, wear has a sharp evolution, explained but not enough PEEK for creating a harder matrix for the soft polymer, thus the last one is easy to be deformed, abraded; the wear is supported by PTFE and not by the harder material (which has a higher wear resistance. The transfer process is more intense, and the wear debris have higher volumes. The authors suggest that preferentially lose of PTFE make the tribolayer grows rich in PEEK and the wear is reduced. At higher

#### **Figure 31.**

*Contact surface 6,35 mm x 6,35 mm, F = 250 N and alternating sliding on 25.4 mm, v = 0.05 m/s, dry sliding on stainless steel AISI 304. (a) Model proposed by [49]. (b) Wear rate as a function of PEEK concentration.*

indicate extreme holes or peaks on the surface, as for the worn surfaces of composite PBT + 20% glass beads. For v = 0.50 m/s (**Figure 30**) and v = 0.75 m/s, Ssk < 0, reflecting the micro-plowing process. For Sku > 3, all worn surfaces indicated long and narrow valleys, with high peaks, the valley are dominated as result of tearingoff glass beads and maintaining the shape of the extracted beads. Smaller values of Ssk indicate broader height distributions but these polymeric materials have narrow

Contact type Conformal, non-conformal, volumes of the

oscillations, impact

Material, shape, size and distribution

triboelement

Components with high volume of polymeric material are less heat conductive and prone to have melt/soften contact. The solution given by research and practice: polymeric coatings, thick enough to reduce friction and to bear wear for a specified

During a test, many influencing factors have to be controlled. These can be

• -mechanical and environmental test conditions (such as contact load or

• **-**specimen(s) parameters (such as material composition, microstructure,

pressure, speed, motion type and environment temperature, composition), and

Some of them could be monitored during the test (as friction force), some only at before and after test. For polymers, investigations must be done just after the test

Researchers have to prioritize what factors are kept constant and what factors

A full program of testing under all combinations of these factors would be time-

Tribologists is now using mapping technique when two (or more) factors are changed in a controlled way (normally more coarsely than in parametric studies),

consuming and costly, and may not be required. Often a single factor can be identified as "key" to the material response, and in this case a good approach is to set all the other factors at constant values and vary the chosen factor in a controlled way in a series of tests. Test campaign must promote an objective, to establish variables (materials, working regime parameters, environment) and the most relevant results to be given, non-destructive investigation in order to understand

height distribution as all values are above 3 (**Table 2**).

*List of important parameters that influence the tribological behavior [42].*

**Parameter Information, unit**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

Pair of materials Composition, phases, structures Temperature (environment and in contact) The second is difficult to measure Type or relative motion Sliding, rolling, combined motion, small

Contact dynamics Stiffness, damping, inertial mass

Load (normally applied), constant or variable N Sliding speed m/s

Particularities of tribosystem (if the case)

abrasive/erosive particles

volume, shape and their initial surface finish).

and direction the damage processes during testing.

as the specimens could age and thus, altering the information.

life and reliability.

will vary on ranges of interest.

grouped in

**88**

**Table 2.**

concentration of PEEK, the wear is dominated by fatigue cracks and the microreservoirs of PTFE are in reduced number and the solid lubrication of PTFE is done only on patches. Wear debris made of PEEK generate a more intense abrasive wear, even as third body, care damage the transfer films on both surfaces in contact.

A similar tribological behavior was noticed by Tomescu [9], when a composite copper + PTFE was tested in dry and water lubrication regime.
