Wear Mechanisms of PTFE and PTFE Based Composites: A Summary of the Effects of

Counter Surface Roughness and Properties on Friction and

Wear Life

Michael Peetros

Friction and Wear of Materials

MANE 6960: Final Paper/Project

PTFE/PTFE-based Composites

• Polytetrafluoroethylene • Used frequently in bearing applications over last 50 years. • Very low coefficient of friction when sliding against metallic

engineering surface– Due to creation of transfer film of PTFE on counterface surface

– Crystalline structures of two PTFE surfaces align and shear easily

– Transfer film continually created, worn away, recreated, worn away etc.

• This cycle results in low wear life of pure PTFE• Different fillers have been added to create PTFE composites to

extend the wear life of the part

Effect of Counterface Roughness and Hardness

• Researched published tests evaluating the effect of the condition of counterface on the friction and wear of PTFE/PTFE composites

• Lancaster tested PTFE fiber/glass fiber-phenolic resin against 4 counter surfaces, all varying roughness.

• Results indicated that total life increased significantly depending on roughness

• Only at low stresses can adequate wear life be achieved on the roughest of surfaces he tested

Figure 1. Effect of Counter face (tool steel) roughness on life-

stress relationship of PTFE fiber based composite

Figure 2. Effect of Counter face (tool steel) roughness on the specific wear rate of PTFE fiber

based composite

• Quaglini, Dubini, Ferroni, and Poggi conducted a similar study testing various engineering plastics, including PTFE.

• Categorized with other “low modulus of elasticity” plastics

• Samples of PTFE were run against two different surfaces, one polished and one mirrored.

• PTFE exhibited lower friction on mirrored surface, likely due to the asperities of the smooth mirrored counter face were high enough to allow the formation of an effective PTFE transfer film, while not plowing into the surface of the PTFE enough to increase the overall friction. On the polished surface, the asperities were large enough to penetrate further into the PTFE, resulting in such an increase in friction coefficient.

Figure 3. Effect of Counter face finish (polished or mirrored) on the dynamic coefficient of friction of PTFE

• W. Wieleba tested PTFE composites against a steel counterfaces of varying hardness and roughness in tribometer

• Compared the results of the test to a statistical analysis (derived from regression functions)

• Goal was to determine which characteristics of the counter surface (hardness, statistical roughness parameters) most affected the friction and wear of the PTFE composites

• Determined that the roughness parameters Sm (mean distance between asperity peaks), (average slope of surface profile, and Rk (core roughness depth) had the strongest influence on value and magnitude of the coefficient of friction

• Various studies, have shown that generally smoother surfaces extend the wear life of PTFE based composites, while the secondary parameter of hardness acts as an indirect influence on the friction and wear of PTFE.

• Further testing can still be done, as the interactions are complex and still not fully understood.

• Other counterface materials (Ti, Inconel, Monel, ceramics, chrome plate) as well as other variables (temperature, speed, etc.) could be added to testing.