Photon Factory Activity Report 2019 #37 (2020)

BL-7A/2019C211

B K-edge XANES analysis of tribofilms formed on metal surfacesby CVTF containing boron additive
Yoshimu IWANAMI1*, Nobuharu KIMURA1, Yasushi ONUMATA1, Shinji HASEGAWA1,and Kenta AMEMIYA2
1
JXTG Nippon Oil & Energy Corporation,
8 Chidoricho, Naka-ku, Yokohama, 231-0815, Japan2
Photon Factory, Institute of Materials Structure Science, High Energy Research Organization,
1-1 Oho, Tsukuba, 305-0801, JapanFriction coefficient

2 Experiment
A block-on-ring friction tester [2] was used to preparetribofilms and measure the friction coefficients of four testfluids. The test fluids were prepared by adding different
boron additives to the same base oil. The friction
coefficients of the test fluids are shown in Fig. 1. The
tribofilms were then subjected to XANES measurement.
B K-edge XANES measurements were carried out atBL-7A. The samples were placed under vacuum and thespectra were collected in the fluorescence yield mode.
0.15
0.14
0.13
0.12
0.11

Fig. 1: Friction coefficients of test fluids.
3 Results and Discussion
Figure 2 show the B K-edge XANES spectra of thetribofilms obtained with the four test fluids. A sharp main

peak was observed at around 194 eV and two broad peaks
were also observed at around 198 eV and 203 eV in theXANES spectra of all tribofilms. The spectra werenormalized to the sharp main peak at around 194 eV. Thesharp peak at around 194 eV and the broad peak at around203 eV indicate the presence of boron species with trigonal
planar geometry, while the broad peak at around 198 eV isassociated with a tetrahedral geometry. [3]. The spectra
indicate that all tribofilms contain boron species with both
trigonal and tetrahedral geometries.
2.0
Trigonal B
1.5
I /I0, a. u.

1 Introduction
Belt-drive continuously variable transmissions (CVTs)
have attracted attention because they enable greater fuelefficiency than conventional automatic transmissions(ATs). Belt-drive CVT fluids (belt-drive CVTFs) musthave not only the same physical properties as ATs, such as
low temperature fluidity and high oxidation stability, butalso ensure a high coefficient of friction between the belt
and pulleys in the transmission system to convey forces
efficiently [1]. In a previous study, it was found that addingboron additives to CVTFs increased their friction
coefficients. However, the mechanism of this increase has
not been sufficiently clarified. What is understood is thatthe additives in CVTFs react with metal surfaces to form
tribofilms that directly affect the friction coefficient.
In this paper, we discuss the relationship between thechemistry of the boron species in the tribofilms and thefriction coefficient when boron additives are added toCVTFs.

Tetrahedral B
Trigonal B

1.0

0.5

0.0

Fluid-A
Fluid-B
Fluid-C
Fluid-D
185 190 195 200 205 210
Photon energy, eVFig. 2: B K-edge XANES spectra of the tribofilms
The heights of the tetrahedral boron peaks varied
significantly among the test fluids. We thus focused on thetetrahedral boron peak at around 198 eV in order toelucidate the chemistry of the boron species in thetribofilms. The height of the tetrahedral boron peak wastaken to be a measure of the proportion of boron specieswith a tetrahedral geometry in the tribofilm.
The relationship between the friction coefficient and theheight of the tetrahedral boron peak for four different
tribofilms is shown in Fig. 3. Figure 3 shows that the heightof the tetrahedral boron peak decreased as the friction
coefficient of the tribofilm increased.
This suggests that tribofilms with higher friction
coefficients have a lower concentration of tetrahedralboron species, or a higher concentration of trigonal boron
species.

Photon Factory Activity Report 2019 #37 (2020)

Height of tetragonal B peak

0.4

0.3

0.2

0.1

00.120.13
Friction coefficient

0.14

Fig. 3: Relationship between the friction coefficient andthe height of tetrahedral B peak in the B K-edge
XANES of tribofilms
References[1] Y. Mabuchi et. al, Trib. Trans. 43, 229 (2000).
[2] C. William et. al., ’98 TVT Symposium’, 82 (1998).
[3] G. Pereira et. al., Tribology, 1, 48 (2007).
* iwanami.yoshimu@jxtg.com