An analytical utra-high vacuum tribometer :
application to tribochemistry in metal rolling
M. Boehm*, C. Grossiord**, T. Le Mogne**, J.-M. Martin**
* Pechiney, Centr’Alp, Centre de Recherches de Voreppe
** Laboratoire de Tribologie et Dynamiques des Systèmes,
École Centrale de Lyon, Ecully
The demand for improvements in surface quality and
increased productivity in industrial cold metal rolling
requires a good understanding of lubrication mecha-
nisms, especially of the interaction of additives present
in lubricants with reactive nascent surfaces. These addi-
tives act in the area submitted to boundary friction which
is controlled by physico-chemistry. To understand the
complex phenomena, we have to use a simplified
approach.
– at very low partial pressure, there are not enough
molecules to lubricate the contact. Friction and wear
are high ;
– at a transient pressure, friction decreases quickly and
wear is lower. This transient pressure is characteristic
of the molecule efficiency : the lower the pressure is,
the faster the molecule reacts ;
– at high pressure (and in liquid), a tribochemical film is
created : friction and wear are low. The minimum fric-
tion value indicates the friction-reducing capability of
the molecule.
Consequently, a new method was developed by using an
UHV tribometer associated with surface analytical tools
to simulate the molecule chemical reactivity toward sur-
faces. This approach involves the combination of in situ
adsorption of low molecular weight molecules, simula-
ting the heavy lubricant components by their chemical
function, with in situ AES/XPS analysis (depth profiling,
micro-XPS, XPS and AES imaging). In this way, we che-
mically characterized both sliding counterfaces and were
able to carry out friction experiments in a very well
controlled environment.
Adsorption is the first step in the formation of the tribo-
chemical film. The interaction between the molecule and
the surface depends on both the chemical state of the
surface and the functional group of the molecule. During
tribological processes, the surfaces are mechanically
renewed, creating nascent metallic surfaces. Such sur-
faces have been shown to present a high chemical acti-
vity which enhances molecular adsorption and decom-
position. Many tribologists have pointed out that due to
this high chemical activity, interactions between nascent
surfaces and lubricant additives may be very important
for the formation of tribochemical films and, therefore, for
the protection of tribological surfaces in boundary lubri-
cation.
Severe contact conditions were applied in order to create
nascent metallic surface, which is believed to play an
important role in the tribofilm formation. We used a
pin-on-flat geometry with a linear reciprocating motion.
The pin consists of AISI 52100 steel, to simulate the roll
in the cold rolling process. The flat is made from a metal-
lic alloy of iron or aluminum. First, a friction experiment
was conducted in the presence of molecules in the liquid
phase. Secondly, friction experiments were run in the
UHV tribometer by introducing small compounds directly
in the chamber to lubricate the contact. The molecule
partial pressure in the chamber ranged from 10-7 Pa to 1
kPa to change the molecule concentration at the sample
surface, which allows the study of the kinetics of tribofilm
formation. Classically, the change of the partial pressure
of gas reveals three distinct behaviours of friction and
wear :
A clear differentiation of the different compounds by their
friction-induced behaviour can be observed. Moreover,
friction and wear scars morphologies under gaseous
feed are very similar to classical lubricated contacts. As
a result, this method appears to accurately model the
action of boundary molecules and is likely to be a power-
ful tool in the prediction of boundary additives behaviour.
The use of in situ surface analyses and the simplification
are particularly helpful for understanding the action
mechanisms of boundary additives.
métallique nouvelle. Cette surface nouvelle existe lors d’une
opération de laminage sous les fortes contraintes et du fait
de l’augmentation de la surface. Des expériences conduites
avec les mêmes molécules en phase liquide ont été réali-
sées pour comparer la lubrification liquide et la lubrification
en phase gazeuse.
sible de les utiliser sous forme vapeur à des pressions signi-
ficatives. Nous avons simulé ces molécules par des molé-
cules analogues possédant la même fonction chimique mais
une masse moléculaire inférieure. Ainsi, les molécules de
n-hexane et de 1-hexène simulent respectivement les com-
posés paraffiniques et oléfiniques présents dans l’huile de
base. L’acide propanoïque et le 1-hexanol sont utilisés pour
modéliser les additifs d’onctuosité tels que les acides gras et
les alcools. Le disulfure de diallyle simule les additifs pour
extrême-pression soufrés et le phosphate de triéthyle les
Compte tenu de leur poids moléculaire, les additifs utilisés
dans les lubrifiants possèdent une très faible pression de
vapeur saturante à température ambiante. Il est donc impos-
150
La Revue de Métallurgie-CIT/Science et Génie des Matériaux
Février 2001