NLGI Paper 2006

#0626
Investigating alternatives to Molybdenum disulfide by means of RVTand DFBT-Tribometer
Josef Pohlen
Setral Chemie GmbH
Seeshaupt, Germany
Presented at the NLGI 73rd Annual Meeting
Lake Buena Vista, Florida
October 29 - 31, 2006
NLGI Preprint: Subject to revision. Permission to publish
this paper, in full or in part, after its presentation and with
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upon request. NLGI assumes no responsibility for the
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NLGI.
Investigating alternatives to Molybdenum disulfide by means of RVT- and DFBTTribometer
Josef Pohlen, Setral Chemie GmbH, Seeshaupt, Germany
Introduction
The recent years showed a dramatic increase of raw material costs, among others for
Molybdenum disulfide. In various publications alternative solid lubricants were presented
by comparing properties on the well known Four-Ball-Tester and the Timken machine.
The results from these machines were not always satisfactory for finding the right
solution in terms of solid lubricants. Very often the alternatives were combination of
Molybdenum disulfide with other solid lubricants, most likely graphite. By means of the
RVT- and DFBT-Tribometers it was possible to distinguish more between the properties
shown by Molybdenum disulfide and its alternatives like e.g. Bismuth sulfide, Tinn
disulfide, Graphite etc. in lubricating greases and pastes. Various test parameters, such
as low and high speeds and/or loads enabled to show the different effects caused by the
solid lubricant used. By doing so it was possible to find alternatives to Molybdenum
disulfide depending on the application like lubricating grease or paste. The results from
the tribometers were also compared to the findings on the Four-Ball-Tester and the
Press-Fit-Test.
Solid lubricants
There is plenty of literature on the different types of solid lubricants. This paper focusses
on Molybdenum disulfide and alternatives to replace it in lubricants. A driving force to do
so now is the extraordinary increase in prize for molybdenum disulfide during the recent
2 years. This is due to the fact, that the steel consumption world wide grew a lot and with
it the demand for molybdenum as an element to produce stainless steel. It is very
unlikely, that these prices will return to the old level as it happened at the begin of the
nineties. That is why it is of economic interest to develop alternatives. Different attempts
have been done in the past which very often ended up with mixtures of Molybdenum
disulfide with other solid lubricants, mostly graphite [01] or as an example in combination
with PTFE and FEF3 [02]. This is surely an advantage in costs compared to pure
Molybdenum disulfide but not the goal to be achieved. One very promising way was
polarized graphite [03, 04, 05] which is nowadays used in some specific applications like
rock drills [06, 07] and anti friction linings. Since this is patented, there is no competition
and the prizes not very attractive. This paper shall show, by means of using the new
tribometers, how to find possible alternatives to Molybdenum disulfide.
Important in this context is to look at the advantages and disadvantages of Molybdenum
disulfide.
Molybdenum disulfide is very polar and therefore has a very good adherence to the
metal surface. It exhibits excellent lubricity and due to ist lamellar structure it has very
good film-forming ability. The disandvantages by using Molybdenum disulfide are that
under certain conditions sulphur is liberated causing corrosion in a humid environment.
When exceeding 300°C this effect ist increased and the sulphur diffuses into the steel
and causes stress corrosion. In some applications this matters and others it doesn´t.
Graphite seemed to be a good alternative, because it also has lamellar structure and it is
black. Unfortunately the polarity is very low and therefore the adherence to the metall
surface poor. By adding other solid lubricants or salts this effect can be improved,
especially as long as there is a carrier (lubricating oil) involved.
Taking this knowledge into consideration the tests were made using lamellar as well as
mixtures of lamellar and non lamellar solid lubricants to give an overview of the
properties and to see how they correlate with the results on other tests such as the four
ball or press-fit test. The following table shows the different solid lubricants in use
particle size d50 particle size d95
[µm]
[µm]
4,0
40,0
name
MP 1
description
mixed powder with 8% MoS2
MP 2
mixed powder
3,0
10,0
MP 3
mixed powder
3,0
10,0
MP 4
MP1 without MoS2
4,0
40,0
MP 5
mixed powder
3,0
15,0
MP 6
mixed powder
4,0
45,0
MP 7
mixed powder
5,0
45,0
MP 8
mixed powder
3,0
20,0
MoS2
> 98,5% MoS2
3,5
5,0
Bi2S3
> 99,0% Bi2S3
2,0
7,0
SnS2
> 98,0% SnS2
2,0
10,0
Graphite
natural > 99,5% C
6,0
15,0
The mixed powders were special blends of lamellar and non-lamellar solid lubricants,
whereas the content of lamellar solids was below 50%. The particle size at d50 was in
the same range, whereas the d95 value varied because of the nature of the mixed
powders. This was done by intention to determine the differences, if any could be found.
As test apparatus the press-fit test, four-ball tester and the new tribometers were used.
The latters shall be described in the following.
Tribometers - Dynamic Four Ball Tester (DFBT) & Friction and Wear Tester (RVT)
The DFBT and RVT are newly developed tribometers which are very valuabel for
developing and investigating lubricants [08].
Both test machines are easy to handle but at the same time very versatile in terms of
test parameters.
The DFBT [Fig. 01,02,03] is an innovative tribometer to investigate the tribilogical
properties of lubricating greases and pastes. The most significant difference of the DFBT
compared to the Shell four ball tester standardized according to DIN 51350 are the base
balls which are not fixed and can freely move within the testing pot. By means of setting
the load and speed the ratio between sliding and rolling friction can be influenced. This
enables to simulate the dynamic conditions as they can be found in a rolling element
bearing. By choosing the right parameters it is possible to determine the friction and
wear in a braod range.
Fig. 1: DFBT
Fig. 2: View into the DFBT test pot
Fig. 3: Specimen after a test
The RVT [Fig. 04,05,06] was developed to test friction and wear of oils and semi fluid
greases as an alternative to the Timken and Reichert tester and as a consequence its
measuring principle is based on a similar geometry.
The testing machine basically consists of a rigidly mounted test roll, which is pressed
against a revolving friction wheel by means of leverage. The friction wheel is immersed
with its lower third in the sample ( ~15ml ) under test, and its rotating speed is of such a
rate, that a sufficient quantity of lubricant will always get to the contact surface of the test
roll and the friction wheel.
As with the DFBT it is possible to determine friction and wear properties under various
conditions. In difference to Timken and Reichert tester the much wider range of
temperature, speed and load settings offer a broader testing variety.
Fig. 4: RVT- Tester (Zoom test head)
Fig. 5: RVT - geometry
Testing
In the paper on the new tribometers DFBT & RVT several examples for oils with and
without small quantities of solids and grease without solids where shown to demonstrate
the value of these machines for the development work. The next consequent step was
now the testing of lubricating greases containing solids and lubricating pastes. This was
even more of interest because of the actual situation with Molybdenum disulfide. Under
this aspect the different tests where choosen.
For the lubricating pastes data were taken from the four ball tester and the press-fit test.
Testing in the DFBT are actually running and the first trials at very low speed and high
loads show similar ranking as in the press-fit test.
For the tests of grease containing solid lubricants, formulations in the typical range of 1,5
to 3,0% were taken and tested on the four-ball tester, RVT and DFBT.
Lubricating Pastes
Molybdenum disulfide pastes are widely used and due to the prize increase extremely
under pressure. In the literature it is described, that the load carrying capacity does not
vary much by increasing the Molybdenum content [09]. By means of SRV-testing MoS2
and SnS2 were compared. These findings could be confirmed when testing pastes,
made of 50% solid lubricant in 50% SN 500 base oil, in the press-fit test.
The press-fit [10] test gives information about behaviour and adhesion of solid lubricants
under extremely high pressure and low sliding speed. In this test the friction coefficient µ
is measured as well as the occurrence of chatter (stick-slip). Both results are important
for the use of solid lubricants in assembly work (e.g. press-fit operation) or with slide
ways and guide ways (e.g. machine tools).
The test procedure (Dow Corning test method CTM 0394 [draft DIN 51833]) is described
following: An oversized pin is pressed into a test bush at ow speed.The specific surface
pressure can be determinated by measuring the outside diameter of the bush before and
after pressing in, and the dynamic friction-coefficient for pressing-n can be established,
together with the test force. The static friction-coefficient is determined at press-out. The
test also establishes whether and when stick-slip occurs.
Test on the DFBT will be performed in the near future to determine the correlation with
SRV and press fit test.
Diameter
max. press.
[bar]
Time
[s]
25
150
40-45
120
20
90
30
0,0901 no chatter
90
107
17
100
30
0,1178 no chatter
MP 4
91
112
21
180
30-40
MP 5
90
110
20
90
30
No.
before
[µm]
after
[µm]
delta
[µm]
MP 1
90
115
MP 2
100
MP 3
µ
comments
0,1202 heavy chatter
0,1716 heavy chatter
0,0901 no chatter
MoS2
90
110
20
90
30
0,0901 no chatter
SnS2
88
110
22
180
34
0,1638 heavy chatter
As expected the MoS2 containing paste performs very well and shows no chatter,
whereas SnS2 as described by Grebe [09] is not useful in this high concentration..
Further tests with pastes containing MoS2 in the range from 5 – 45% in combination with
non-lamellar solid lubricants showed, that at least 10% MoS2 are necessary to pass this
test. This was confirmed with the mixed powder MP 1 which contained 8% MoS2.
Greases containing solid lubricants
For the grease testing a standard Lithium grease based on mineral oil (ISO VG 150,
paraff./ naphth.) was taken. The solid lubricants content was either 1,5 or 3,0%. The
testing conditions on the four-ball-test, RVT and DFBT. RVT and DFBT varied in time,
load and speed to get as much differentiation as possible.
The four-ball test were dived into wear and weld tests, whereas the wear tests were at
400N load / 1h running and 1500N load / 1min running.
As the results show [table XX,diagram XX], MoS2 has little wear at low load and a high
weld load, which should be met by the alternative product. Looking into other lamellar
solid lubricants, it was confirmed that graphite is not the solution. Bi2S3, also reported
by Rohr [11] and SnS2 as reported by Grebe [09] showed a similar behaviour, except at
the low load. Some mixed powders, especially MP 6,7 & MP 8 were getting close. The
tests on RVT and DFBT should show if the differences were significant or just in the
precision range of the four-ball test.
400N/1h
wear [mm²]
1500N/1min
wear [mm²]
weld
[kN]
1
BG
0,65
4,1
< 2,0
2
BG + 1,5 % MP1
0,50
3,9
2,0
3
BG + 1,5 % MP2
0,13
3,0
2,4
4
BG + 1,5 % MP3
0,07
3,1
2,4
5
BG + 1,5 % MP4
0,07
3,8
2,4
6
BG + 1,5 % MP5
0,50
3,5
2,2
7
BG + 3,0 % MP6
0,4
3,2
2,8
8
BG + 3,0 % MP7
0,3
2,8
3,2
9
BG + 3,0 % MP8
0,3
3,3
2,8
10
BG + 1,5 % MoS2
0,20
3,5
2,8
11
BG + 3,0 % MoS2
0,13
3,2
3,0
12
BG + 1,5% Graphite
0,55
4,0
2,2
13
BG + 3% Bi2S3
0,4
2,1
3,4
14
BG + 3% SnS2
0,3
2,8
2,8
Four Ball Tests
5,00
4,00
3,00
2,00
1,00
0,00
1
2
3
4
5
400N/1h
wear [mm²]
6
7
8
1500N/1min
wear [mm²]
9
10 11 12 13 14
weld load
[kN]
load
load [N]
Wear [mm²]
3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi2S3 3% SnS2
500
5,3
5,5
4,8
5,9
4,8
4,6
1000
9,6
8,3
10,2
10,7
8,3
11,5
1500
14,3
14,3
15,9
15,1
10,8
15,5
2000
18,6
16,2
19,7
18,1
13,8
18,3
load [N]
Friction
3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi2S3 3% SnS2
500
0,71
0,17
0,40
0,36
0,69
0,60
1000
0,40
0,13
0,11
0,15
0,47
0,47
1500
0,36
0,16
0,18
0,16
0,34
0,34
2000
0,33
0,18
0,19
0,16
0,28
0,32
The RVT-test gave a somewhat different picture than the four-ball test. For the SnS2
they were now in line with the findings on the SRV-test by Grebe [09]. It also shows, that
there must not be a correlation between friction coefficient and wear. MP 6 showed very
good results and was choosen for the following DFBT tests. Again the results were close
to those of MoS2, whereas under to more severe conditions the MP6 performed
somewhat better.
RVT - load stage test wear 1500rpm
20,0
[mm²]
16,0
12,0
8,0
4,0
200
600
1000
1400
1800
[N]
3 % MoS2
3 % MP 8
3 % MP 6
3% Bi2S3
3 % MP 7
3% SnS2
2200
RVT - load stage test friction
0,80
0,70
friction
0,60
0,50
0,40
0,30
0,20
0,10
0,00
200
600
1000
1400
1800
load [N]
3 % MoS2
3 % MP 8
3 % MP 6
3% Bi2S3
3 % MP 7
3% SnS2
RVT - long term
3
2,5
2
1,5
1
0,5
0
3%
3%
3%
3
3%
3%
MoS2 GPP 5 GPP 7 %GPP Bi2S3 SnS2
125
wear [mm²]
friction [µ]
2200
DFBT - friction [µ]
0,12
0,08
0,04
0
BG
BG + 3% MoS2
3000rpm
4000rpm
BG + 3% MP6
Discussion
There is not one solution as a replacement to Molybdenum disulfide. This as such is not
new, but the work with tribometers enables to look more into detail and find better
alternatives. Most of the data from the literature are based on the well known four ball
machine or timken tester. The results confirm, that the precision of these test methods is
not god enough to make a clear differentiation between the formulations. With the new
tribometers RVT and DFBT this is possible and enables to find tailor made solutions for
various applications. The mixed powders presented in this papers represent some
possible solutions, being as effective in use and more economical. Further investigations
are ongoing to find the best possible treatment rates.
Acknowledgement
Rachel Kling, Laboratory Manager, Setral S.à.r.l., Romanswiller France, by selecting
and preparing the data.
ECCO Gleittechnik, Germany, for providing the MIPO samples for testing
References
[01] Albert V. Tamashausky; Surface Enhanced Flake Graphite and its Utility as a
Functional Extender for Molybdenum Disulfide; Paper presented at the NLGI AGM 2005,
San Antonio, Texas
[02] Pranesh Aswath; Development of a High Performance Low Molybdenum
Disulfide Grease; Paper presented at the NLGI AGM 2005, San Antonio, Texas
[03] Manfred Jungk ;New Solid Lubricants as Additive for Greases - "Polarised
Graphite"; Paper presented at the ELGI AGM 1999, Oslo, Norway
[04] Rudiger Holinski, Solid lubricant composition; US Patent No. 5,445,748; August
29, 1995
[05] Nornert Aurin, Lubricant and use thereof, US Patent 6,177,386; January 23, 2001
[06] Robert M. Denton; Rock bit with grease composition utilizing polarized graphite;
US Patent application US 2005/0133265, June 23, June 2005
[07] Robert M. Denton; Self-lubricating elastomeric seal with polarized graphite; US
Patent No. 6,789,634; September 14, 2004
[08] Josef Pohlen; New tribometers for reliable screening tests in the lubricant
development; Paper presented at the NLGI AGM 2005, San Antonio, Texas
[09] M. Grebe, P. Feinle und M. Weiß (Goldschmidt TIB);MoS2 und SnS2 als
Festschmierstoffe im Vergleich;Tribologie und Schmierungstechnik, 49. Jahrgang,
Ausgabe 2 / 2002, Expert-Verlag, Renningen-Malmsheim, S.5 -9
[10]
Press-fit test, Molykote-Book (engl. version) 1991, p. 348
[11] Otto Rohr; Investigation on the Effect of Bismuth Sulfide on load carrying capacity
in greases as a solid lubricant; Paper presented at the NLGI AGM 2004, Dana Point,
California