Tribostand Castellón 21 January 2008

Concerning the experimental approach in tribology, the target results are in general the friction (friction force or friction coefficient), the wear amount and the investigation of the wear mechanisms.

Tribological test Primary tribology quantities

Friction force

Wear (mass loss, wear volume, transversal area…)

Design-using parameters

Friction coefficient

Wear factors (k - specific wear rate…)

Experimental approach in tribology

Tribostand Castellón 21 January 2008

Any tribotest include systematic errorsA systematic error is a consistent and repeatable bias or offset in a measurement from the true value. This is typically the result of miscalibration of the test equipment, or problems with the experimental procedure. Error sources

FrictionIntrinsic reason - friction is not measured directly due to the nature of the friction force. In fact the friction force is a reaction force acting on the sliding interface, thus, can’t be measured itself. In most tribology testers the friction is measured as a force that equilibrates the system.

Dynamic effects - An important aspect of friction is its interaction with the dynamics of the system of which it is a part. The mutual influence of friction and the dynamic response of a system on each other might take several forms.

Non dynamic effects on normal load – Offset of the normal load due to internal adhesion forces, unbalanced masses on the loading system, or even unbalance of the measurement system;

Tribostand Castellón 21 January 2008

WearNonequilibrium events – wear is strongly affected by instantaneous nonequilibrium events like galling points

Measurement errors – Errors intrinsic to the evaluation of very small volumes.

Examples of errors as a result of misinterpretation of friction and wear results

(published papers)

0

0.025

0.05

0.075

0.1

0.125

0.15

0 4 8 12 16

Normal force /N

Fri

ctio

n c

oef

fici

ent y = 0.0473x + 0.0977

R2 = 0.9945

0

0.2

0.4

0.6

0.8

1

0 4 8 12 16

Normal force /N

Fri

ctio

n f

orc

e /N

?NF

Tribostand Castellón 21 January 2008

How to solve these problems?

Using a data analysis procedure that allow to identify and correct the systematic errors.

A data analysis approach that allows a stochastic form of the results should be investigated. Design engineers would have a mean to allow to establish the relationship between the tribology parameter values and their reliability.

y = 4,73E -05x - 1,82E -02

y = 6,00E -05x + 1,82E -03

y = 2,11E -04x - 1,45E -02

0

0,2

0,4

0,6

0,8

1

1,2

0 1000 2000 3000 4000 5000 6000

Dista nc e X / m

Wea

r vo

lum

e V

/ m

m3 Normal load (N) K (mm3/Nm)

50 9.46 x 10-7

100 6.00 x 10-7

200 1.06 x 10-6

Average 8.69 x 10-7

y = 1,06E -06x - 3,55E -02R 2 = 9,88E -01

0

0,2

0,4

0,6

0,8

1

1,2

0 200000 400000 600000 800000 1000000 1200000

NX (dis tanc e x normal load) / Nm

Wea

r vo

lum

e V

/ m

m3

K = 10.61 x 10-7

Using separately each one of the results of the 15 experiments

Max= 10.91 x 10-7 Min= 7.27 x 10-7

Average= 9.11 x 10-7

NxKV

Tribostand Castellón 21 January 2008

Scale effects in tribology

Are there a scale tribological response of engineering materials?

Is the scale response apparent because the relative error is proportional to (1/load and 1/stroke)?

When we will have local models in tribology?

Both friction and wear end use parameters are calculated by a global approach. However, the most precise tools are based I n local analysis (Finite element analysis, Dynamic molecular analysis, …). The global quantities can be used in local analysis? What is the magnitude of the error?

Tribostand Castellón 21 January 2008

FCTUC & IPN

Validation of a suitable data analysis procedure for sliding tests in order to:

Correct the experimental systematic errors.

Allow a precise and easy way to establish a relationship between design-using tribological parameters (e.g. friction coefficient, specific wear rate, energetic wear rate…) and the reliability Verify the robustness of the parameter values to allow their use in a wide range of contact conditions