Transcript MPeetros_FWM_finalpres.ppt

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 lifestress 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.