mae 493n 593t lec8
TRANSCRIPT
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Mesoscale testing
Mesoscale testing
requires
accurate
motion
at
low
speeds Maximum velocity of 50 mm/s
Load range: 01 N (100 g) Insitu coefficient of friction monitoring (monitoring transitions)
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Small scale testing
Apparent contact
area
1
100
mm
2
More than 400 world wide test methods for small scale testing
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Large scale testing
Apparent contact
area
10
410
5
mm
2
Using large scale specimens aid towards reducing the effect of stress
concentrations near the sample edges
Sliding instabilities which are hidden in small scales show more frequently in
largescale
testing Large scale tests are closer to actual working conditions
Normal loading up to 6000 kN
Polymer samples150x150x20 mm 3
Counterface steel410x200x20 mm 3
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Meso scale to small scale correlation
pv value is contact pressure p x sliding velocity v, solid lubricant is PTFE
Meso scale tests are performed with flat on flat geometry at 1.15 N (0.0018 MPa),
3.30 N (0.0052 MPa), 5.16 N (0.0086 MPa) and 0.12, 0.5, 2.0, 8.0 m/s. Small scale tests are perfomed with cylinder on plate geometry at 50 N, 100 N, 150 N, 200 N and 0.3 m/s.
Scratch testing of polymer surfaces
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Scratch testing of polymer surfaces
Rigid tip on PMMA
pm
average contact pressure
pm
= Fn
/a
Y
Yield stress
a true contact area
R radius
of
tip
Indentation of polymers
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Indentation of polymers
In general, as load increases displacement increases due to both elastic and plastic
deformation of testing material When holding the load, viscoelastic (time dependent) effects are observed Elastic recovery is observed during unloading part
Berkovich tip3sided pyramid
a = 65.03 o
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Multidirectional vs unidirectional sliding
Friction induced molecular orientation in the unidirectional sliding of UHMWPE This is not the case during multidirectional sliding UHMWPE is used in artificial joint prosthetics
http://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.php
Polymer on polymer bearing sliding for MEMS
http://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.phphttp://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.phphttp://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.phphttp://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.phphttp://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.phphttp://bmcortho.com/about/education.stryker_brochures.english.mis_hip.2.php -
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Polymer on polymer bearing sliding for MEMS
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PDMS frictionPolydimethylsiloxane
(PDMS) Elastomers
PDMS A has lower initial molecular weight than PBMS B Black lines are free and pendant chains emerging from the PDMS surface PDMS B Chains can be oriented and can reduce macroscopic friction coefficient
PDMS
A
Chains
are
short
and
are
sticking
to
AFM
tip,
increasing
the
friction
coefficient
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