Purdue University
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International Refrigeration and Air Conditioning
Conference
School of Mechanical Engineering
1998
Friction and Wear Performance of Polyvinylether
(PVE) in Boundary Lubrication Regime as a
Lubricant for an Alternative Refrigerant
Y. Yamamoto
Kyushu University
S. Gondo
Kyushu University
J. Kim
Kyushu University
Follow this and additional works at: http://docs.lib.purdue.edu/iracc
Yamamoto, Y.; Gondo, S.; and Kim, J., "Friction and Wear Performance of Polyvinylether (PVE) in Boundary Lubrication Regime as a
Lubricant for an Alternative Refrigerant" (1998). International Refrigeration and Air Conditioning Conference. Paper 438.
http://docs.lib.purdue.edu/iracc/438
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FRICTION AND WEAR PERFORMANCE OF POL YVINYLETHER
(PVE) IN BOUNDARY LUBRICATION REGIME AS A LUBRICANT
FOR AN ALTERNATIVE REFRIGERANT
Yuji YAMAMOTO, Seigo GONDO, Joonghyun KIM
Department of Mechanical Engineering, Kyushu University, Japan
ABSTRACT
The tribological performances of a synthetic lubricant of polyvinylethers (PVE) for an
alternative refrigerant were studied together with polyolesters (POE) and polyalkylene glycols
(PAG) using three kinds of sliding contact tester.
Although POE showed excellent wear and friction characteristics in a temperature range
below the transition temperature, its friction and wear characteristics became poor compared
with PAG and PVE above the transition temperature. In contrast, PVE with higher viscositypressure coefficient than PAG and POE gave higher coefficient of friction and lower wear
amount than POE. This feature was attributed to a tendency of PVE to be easily solidified
compared with PAG and POE. In addition, although POE with high adsorption ability
prevented the adsorption of TCP to the rubbing surfaces, the addition of TCP significantly
improved the wear performance of PVE because the solidified film did not prevent the
adsorption of TCP.
INTRODUCTION
Polyalkylene glycol PAG has been used as an lubricant for an alternative HFC refrigerant in
airconditioners for automobiles. PAG, however, could not be used with hermetic compressors
due to its low electric resistivity. Hence polyol ester POE of high electric resistivity is
exclusively used for refrigerators and airconditioners although POE exhibits poor wear
performance[!] and low resistance against hydrolysis. Polyvinyl ether PVE with high
hydrolysis resistance and high electric resistivity is considered to be a candidate for an
alternative lubricant for refrigerators. In this paper the friction and wear performance m
boundary lubrication regime of several kinds of base oils for refrigerators were investigated.
EXPERIMENTAL METHOD
Apparatus
Three kinds of sliding contact testers were used, that is, a ball/ plane reciprocating type
tester (Fig. I), a pendulum type tester, and a ball/disk type tester(Fig.2).
In the pendulum testing a roller of bearing steel was swung on balls of bearing steel under a
boundary lubrication condition in an oil bath filled with an oil to be tested. The coefficient of
friction was evaluated from the change in the pendulum amplitude with swings. The applied
load was 4 N (0.41 kgf), the corresponding maximum Hertzian contact pressure pmn. being 1.3
367
GPa.
In the ball/disk sliding test a ball of 4.75mm in diameter was slid at about 300 mm per
second on a rotating disk. The ball and disk were made of bearing steel of about 760 in Vickers
hardness. The sliding tests were conducted in air by increasing the load from 4.9 N (0.5kgt) to
29.4N (3kgf), the corresponding maximum Hertzian contact pressure p...u. changing from 1.3 to
2.3 GPa. The viscosity of the oils to be tested was kept at 44 eSt by adjusting the oil
temperature.
In the ball/plane reciprocating test a ball of 19.05 mm in diameter was oscillated on a
horizontal plane with a sinusoidal motion The frequency was 500 cycles per minute, and the
amplitude was 2.5 mm. The applied load was 100 N (10.2 kgf), the corresponding pmu. being
1.4 GPa. The ball and plane were made of bearing steel. The main part of the apparatus was
closed in sealed chamber, and the environment could thereby be controlled. The three kinds of
dry gasses, HFC134a, oxygen, and nitrogen, were used as the environment.
Lubricants
The lubricants tested were PAG, POE and PVE for HFC alternative refrigerants, and a
hydrogenated coal-tar oil TN [2] and a synthetic traction oil SN which were chani.cterized by
high viscosity-pressure coefficient and low solidification pressure. The properties of the
lubricants are shown in Table 1.
RESULTS AND DISCUSSION
Pendulum .type test
Figure 3 shows the coefficient of friction in the pendulum type testing with oils for an
alternative refrigerant The viscosity of each oil was kept at 55 eSt. The temperatures of the
oils tested ranged from 22 to 44 t . Since the wear scars on the balls after testing were hardly
detected with all the oils, the rubbing surfaces were considered to be separated by the boundary
films formed. The coefficients of friction were POE<PAG<PVE in an ascending order.
The frictional characteristics of films thinner than a few molecular layers such as an
boundary film are influenced by the molecular structure of oils [3]. The coefficient of friction
of the thin film is closed related with a product of a and p, a and p being a pressure-viscosity
coefficient and pressure, respectively [4]. a pis considered to be a measure of the packing state
of the oil molecule [4,5]. Ohno pointed out that in static state an oil behaved according to the
magnitude of a p, that is, a p<13: viscous fluid, 13< a p<25: viscoelastic solid, 25< a p:
elastic-plastic solid [6]. The relation between the coefficient of friction and a p is shown in
Fig. 4. The coefficient of friction increased with an increase in a p or a . For POE with high
absorption ability and low a , the rubbing surfaces was prevented from the direct contact
mainly by the adsorbed film. In contrast. for PVE with less adsorption ability and higher a
the solidified film of PVE was expected to assist the separation between the rubbing surfaces.
BaiVdisk type test
To further investigate the behavior of boundary films, sliding tests were carried out. The
two kinds of oils, TN and SN, which had high a and were easily solidified at a high pressure
compared with PVE, were also tested. The results are shown in Fig. 5. The scale of the
abscissa is bearing modulus 1J UIW, 7J ,U and W being oil viscosity, sliding speed and load,
respectively. The sliding tests were conducted by stepping up load at a constant oil viscosity
368
and constant sliding speed. The oil film thickness in the tests was at largest 0.04 p m. The
lubrication conditions became severe with increasing the load or decreasing the value of TJ
U/W. Except for PVE the coefficients of friction rapidly increased with decreasing the value
of TJ UIW or increasing the load below critical values of _TJ U/W. The electric resistance
between the ball and disks during runs was monitored. Before the coefficient of friction
increased, the direct contact between the ball and disk hardly occurred. Hence the rise in the
coefficient of friction resulted from the break down of the boundary film formed. As
mentioned above, there are two types of the boundary film, that is, an adsorbed film and
POE with a high ability to form an adsorbed film exhibited the lowest
solidified film.
coefficient of friction as far as the adsorbed film was not broken. In contrast, SN and TN,
which were not expected to form an adsorbed film because they are nonpolar substances but
could easily form a solidified film, gave high coefficients of friction equal to or larger than
0.1.
With PVE, for which the separation of the rubbing surfaces seemed to be ascribed mainly
to the solidified film formed, the increase in the coefficient of friction or the breakdown of the
solidified film did not occur although the value of a was smaller than those of SN and TN.
The rubbing surface of the disk with PVE was scarcely subject to wear. This suggests that to
protect the rubbing surfaces by a solidified film the solidified film must have a high strength
against shearing. In fact, the shear strength of the solidified film of TN is considerably low [7].
Ball/plane type reciprocating test in HFC 134a
There are two kinds of boundary film, that is, adsorbed film and solidified film. All the oils
for an alternative refrigerant tested in this paper, POE, PAG and PVE, are polar substances.
Hence they have more or less an ability to form an adsorbed film. The ranks in the order of
adsorption ability are considered to be POB>PAG>PVE. On the other hand, PVE is the highest
of three oils in the ability to form a solidified film. In this section the effects of the difference
in boundary film type on friction and wear performance and the effectiveness in TCP addition
were examined by carrying out reciprocating sliding contact tests in HFC134a.
The results at loads of SON and 150 N are shown in Fig.6. With POE the coefficient of
friction was as low as about 0.07 at temperatures below about 150 t: of the transition
temperature. It was confirmed that the wear amount of the rubbing surfaces was negligible
small below the transition temperature. Therefore the friction and wear characteristics with
Above the transition temperature,
POE were excellent below the transition temperature.
however, the friction and wear characteristics of POE was poor because the adsorbed film
loses the ability to prevent the direct contact between the rubbing surfaces. In contrast, with
PAG the sudden rise in the coefficient of friction was not observed although the coefficient of
friction was high compared with POE. The wear perlormance was the best of the three oils.
With PVE, the friction characteristics was similar to those with PAG. The wear
characteristics was good at 50 N, but it became poor at 150 N. These feature is attributed to a
tendency of PVE to be easily solidified compared with the POE and PAG. The coefficient of
friction or the shear strength of the solidified film appear to be high. The solidified film could
prevent the direct contact between the rubbing surfaces at 50 N, but it was broken at higher
loads than 100 N, leading to an increase in wear.
The adsorption ability of base oils is considered to affect the function of TCP as an additive.
The friction and wear characteristics of oils with and without TCP are shown in Fig.7. The
load applies was 100 N. With POE the addition of TCP did not improve the friction and wear
performance because POE of high adsorption ability prevented the adsorption of TCP to the
369
rubbing surfaces. With PAG less than POE in adsorption ability, the addition of TCP improved
the friction and wear characteristics. With PVE containing TCP, the friction and wear
characteristics were significantly improved. The final coefficient of friction decreased to 0.08
and the wear was negligible small. This suggested that a solidified type boundary film could
enhance not only the wear performance but also the friction performance by adding some
appropriate additives.
CONCLUSION
The friction and wear performance of polyvinylether was investigated together with polyol
ester POE and polyalkylene glycol PAG. The boundary film was classified into two types, an
adsorbed film and a solidified film. POE with high adsorption ability formed an adsorbed film,
which exhibited excellent friction and wear performance at a temperature below the transition
temperature. Above the transition temperature the friction and wear performance of POE was
poor. In addition, with POE the addition of TCP did not improved the friction and wear
characteristics above the transition temperature.
In contrast, PVE which easily fanned a solidified film compared with POE was superior to
POE in friction and wear perfonnance in a high temperature range. Besides, the addition of
TCP considerably improved the friction and wear performance of PVE.
References
1) Yamamoto, Y. and Gondo, S., Trans Jpn Soc. Mech. Engrs., C, 63, 612 (1997) 2845.
2) Sakai, T. Murakami, T. and Yamamoto, Y .• J. Syn. Lubric., 9, 3 (1992) 223.
3) Gee, M.L., McGuiggan, P.M. and Israelachvilli, J.M., J. Chern. Phy., 93, 3 (1990) 1895.
4) Ohno, N., Kuwano, N. and Hirano, F., J. Jpn Soc Tribologist, 38, 10 (1993) 937.
5) Yamamoto, Y. and Hashimoto, M .• J. Jpn Soc Tribologist, 35,7 (1990) 493.
6) Ohno, N., Kuwano, N. and Hirano, F., Dissipative Processes in Tribology, Edited by
Dowson, D. et al., Elsevier Science, (1994) 507.
7) Kaneta, M., Kanzaki, Y., Kameishi, K. and Nishikawa, H., Proc. Int. Tribology Conf.
Nagoya, (1990) 1695.
Tablet
Properties of oils
POE32
POE68
PAG32
PAG46
PVE32
PVE68
TN22
SN22
Viscosity(cSt@ 40'C)
24.52
65.94
32.27
43.94
30.70
63.48
21.44
20.03
Viscosity( eSt@ lOOt)
5.07
12.31
7.58
9.63
4.91
7.62
3.71
3.57
Viscosity Index
Density(g/cm3 , 15'C)
139
188
216
212
67
77
39
17
0.993
0.927
0.991
0.995
0.904
0.925
0.974
0.901
10.6
10.3
11.3
13.2
15.7
15.1
32.5
28.6
Vicosity-Pressure
Coefficient(GPa" 1,40'C)
370
Weight
Lubricating oil
Hermetic chamber
Fig.l
Ball/plane reciprocating type tester
Fig.2
c:: 0.2
1:: 0.2
.2
.....
.2
.....
·c
....... o.15
;so.t5
.......
(.)
(.)
'0
.....
5
......
Ball/ disk type tester
.D
0
.....
[j
0.1
0
0.1
.....
~
i€ 0.05
D PAG32
• PAG46
0 PVE32
+ PVE68
't 0.05
0
u
0
u
0
e POE32
o POE68
•
(.)
(.)
•
0
0
POE32
0
PAG32 PVE32 POE68 PAG46 PVE68
5
15
10
20
ap
Fig.3
--
Fig.4
Frictional characteristics in pendulum
type test
Change in coefficient of friction with a p ,
pendulum test
c::
0
"()
·;:
.,_
t::{:::,.D.
0.15
D.
0
0.~.:
...
c::
4)
()
:;::
0.1
Q)
0
u
0. 05
D.
D.
~--------------- ---~·---·~-----TN68
PVE68
E
• v•
:;L
•
•
0
O.lmm
'--'--'--'--'--'-----''--'--'--'--'--'--~-'--L_L-'--.L-L-'
0
0.001
0.002
0.003
Surface roughness after run
0.004
1J U/P, m- 1
Frictional characteristics
Fig.5 Ball/disk sliding test
e:POE68 .:PAG46 .:PVE68 \7:TN68 6.:SN22
371
5
0.2
-..:u
r-------. -----r--- ---.
SON
PAG32
·;::
:::: 0.15
- ·o
POE32
0
c::
<1J
IE
QJ
8
0.1
PVE32
0.05 .___ _ _._._t.------- -l"------.J
200
150
100
50
Temperature, "C
§
·e
0.2
---.
----r-.-----.1soN
PAG32
-----111~._.- - - - -
·;::
::: 0.15
~
.~
e
.
01
PVE32
s>&oOOCXJDcO o o o ao cP
QJ
8
--=>.~~~¢~?
l
~u~o:f
crq:ce
o
¢¢
POE32
0.05 L - - - - - 1 . - - - - - - - J L - - - - - . . . . . - J
200
150
100
50
lmm
Temperature, "C
Fig.6 Friction and wear characteristics in HFC134a
O:POE32 D:PAG32 0:PVE32
PAG32
---"1"'1"\"'rrl- - - - POE32
~-·
PVE32
'
s
·.o:::
u
~
-
~
0.2 r-'"':""':"---...-------r-----~
with TCP
~
0
u
POE32
0.15
·~ o.1
oo ''''Oct:octJ
°
PAG32
ooCbooo
~ 000~<cioocx::o
~006::1:'1'""0~0" 0 ~ ¢<>¢¢CO<XX>¢
0.05 !-:-----~----.L.----_j
200
150
100
50
~t
PVE32
E
::i,____ __
.....
1mm
Temperature, "C
Fig.? Effect of TCP addition on friction and wear characteristics in HFC134a
O:POE32 D:PAG32 0:PVE32
372