Photon Factory Activity Report 2004 #22 Part B (2005)
Applied Science

9A/2004C006

S K-edge XANES measurements of tribofilms generated on steel blocks frombelt-drive continuously variable transmission fluid
Yoshimu IWANAMI*1, Hisayuki WADA1, Masaharu NOMURA2
1
Nippon Oil Corporation, Chidoricho Naka-ku, 231-0815, Japan
2
KEK-PF, Tsukuba, Ibaraki 305-0801, JapanFriction coefficient

Results and Discussion
The friction coefficient changed as a function of thetest time changed (Fig. 1). This suggests that thechemistry of the tribofilms changed during the friction
test. Sulfur-containing compounds in CVTFs are thought
to react with metal surface easily and play important roles
to form tribofilms. Thus, the S K-edge XANES specta
were examined. The shape of the S K-edge XANES
spectrum
0.15
changed,asthe friction test
0.13
time (Fig.2).
0.11This indicatesthatthe0.09
chemistry of0.07
the S species
0102030 in the bulks ofFriction test time (min)the tribofilms
Fig. 1 Friction test time dependence ofchanged as thefriction coefficient

IF/I00.15

1.00.8

0.13

0.6
0.11
0.4

Ratio of sulfides
except FeS2

0.2

SulfateSulfiteFeS2Alkyl sulfide
FeS
Friction coefficient

0.00.09

Friction coefficient

Experimental
The block-on-ring friction test[2] was employed toprepare tribofilms and measure the friction coefficients ofthe test fluid. The test conditions are described
elsewhere[3]. Tribofilms after various durations of thefriction test were then subjected to XANES measurement.
The concentrations of S, P, Ca and Mg in the test fluid are
1400, 310, 200 and 180ppm, respectively.
The S K-edge XANES measurements were carried out
on BL-9A. The samples were placed under a flowing
helium environment, and the spectra were collected using
a fluorescent ion chamber in the fluorescence yield mode.

test time changed. So, in
Test time (min)order to estimate the30compositions of the S
20
species in the tribofilms,
10
the XANES spectra of
4tribofilms
were2
deconvoluted with those ofsulfur-containingmodel
1
compounds expected to bein the tribofilms.
Asaresultofdeconvolutions, it wasfound that the tribofilms
2460
2480
2500
Photon energy (eV)were mainly composed ofFig.2 S K-edge XANESFeS, FeS2 and alkyl sulfidespectraat the shorter friction test
time points, corresponding to the lower friction
coefficient points(Fig. 3). As the test time increased, orthe friction coefficient became higher, the ratio of FeS2 in
the tribofilms decreased and the ratio of FeS and alkyl
sulfide increased.

Ratio of S species

Introduction
Recently,
belt-drive
continuously
variabletransmissions (CVTs) have attracted attention because
they give better fuel efficiency than conventional
automatic transmissions. Belt-drive CVT fluids (beltdrive CVTFs) are required to have a high friction
coefficient between metal surfaces in order to transmit
forces efficiently[1]. Friction coefficients are governed by
the the tribofilms generated on the metal surface by
lubrication. So, in order to develop high-performancebelt-drive CVTFs, it is necessary to clarify the nature of
the tribofilms.
In this paper, we report change in chemistry of the S
species in the tribofilms generated on steel blocks during
friction tests.

0.07
1241020
Friction test time (min)30Fig. 3 Ratio of S species in bulks of tribofilms
Therefore, the followings were suggested. (1)As theratio of sulfides except iron disulfide becomes higher, thefriction coefficient becomes higher. (2)Iron disulfide is
not effective for increasing the friction coefficient. (3)Thedecrease of the friction coefficient in the early stage of thefriction test is caused by the formation of iron disulfide.
(4)As the friction test time increases, the iron disulfide
disappears and the friction coefficient increases.
References[1] Y. Mabuchi et. al, Trib. Trans. 43, 229 (2000) .
[2] William C. et. al., ’98 TVT Symposium’, 82 (1998).
[3] Y. Iwanami et. al., Physica Scripta T115, 332 (2005).
* yoshimu.iwanami@eneos.co.jp

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