Investigating alternatives to Molybdenum disulfide by means of RVTand DFBT-Tribometer

Transcription

1 #0626 Investigating alternatives to Molybdenum disulfide by means of RVTand DFBT-Tribometer Josef Pohlen Setral Chemie GmbH Seeshaupt, Germany Presented at the NLGI 73 rd 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 credit to the author and the Institute, may be obtained upon request. NLGI assumes no responsibility for the statements and opinions advanced by contributions to its publications. Views expressed are those of the authors and do not necessarily represent the official position of the NLGI.

2 Investigating alternatives to Molybdenum disulfide by means of RVT- and DFBT- Tribometer 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

3 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 d 50 particle size d 95 name description [µm] [µm] MP 1 mixed powder with 8% MoS 2 4,0 40,0 MP 2 mixed powder 3,0 10,0 MP 3 mixed powder 3,0 10,0 MP 4 MP1 without MoS 2 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 MoS 2 > 98,5% MoS 2 3,5 5,0 Bi 2 S 3 > 99,0% Bi 2 S 3 2,0 7,0 SnS 2 > 98,0% SnS 2 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 d 50 was in the same range, whereas the d 95 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.

4 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 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.

5 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 MoS 2 and SnS 2 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).

6 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. No. before [µm] Diameter after [µm] delta [µm] max. press. [bar] Time [s] µ comments MP ,1202 heavy chatter MP ,0901 no chatter MP ,1178 no chatter MP ,1716 heavy chatter MP ,0901 no chatter MoS ,0901 no chatter SnS ,1638 heavy chatter As expected the MoS 2 containing paste performs very well and shows no chatter, whereas SnS 2 as described by Grebe [09] is not useful in this high concentration.. Further tests with pastes containing MoS 2 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% MoS 2. 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.

7 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 % MoS 2 0,20 3,5 2,8 11 BG + 3,0 % MoS 2 0,13 3,2 3,0 12 BG + 1,5% Graphite 0,55 4,0 2,2 13 BG + 3% Bi 2 S 3 0,4 2,1 3,4 14 BG + 3% SnS 2 0,3 2,8 2,8 load Four Ball Tests 5,00 4,00 3,00 2,00 1,00 0, N/1h wear [mm²] 1500N/1min wear [mm²] weld load [kn]

8 Wear [mm²] load [N] 3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi 2 S 3 3% SnS ,3 5,5 4,8 5,9 4,8 4, ,6 8,3 10,2 10,7 8,3 11, ,3 14,3 15,9 15,1 10,8 15, ,6 16,2 19,7 18,1 13,8 18,3 Friction load [N] 3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi 2 S 3 3% SnS ,71 0,17 0,40 0,36 0,69 0, ,40 0,13 0,11 0,15 0,47 0, ,36 0,16 0,18 0,16 0,34 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 SnS 2 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 MoS 2, whereas under to more severe conditions the MP6 performed somewhat better. RVT - load stage test wear 1500rpm 20,0 16,0 [mm²] 12,0 8,0 4, [N] 3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi2S3 3% SnS2

9 RVT - load stage test friction friction 0,80 0,70 0,60 0,50 0,40 0,30 0,20 0,10 0, load [N] 3 % MoS2 3 % MP 6 3 % MP 7 3 % MP 8 3% Bi2S3 3% SnS2 RVT - long term 3 2,5 2 1,5 1 0,5 0 3 % MoS2 3 % GPP 5 3 % GPP 7 3 %GPP % Bi2S3 3 % SnS2 wear [mm²] friction [µ]

10 DFBT - friction [µ] 0,12 0,08 0,04 0 BG BG + 3% MoS2 BG + 3% MP6 3000rpm 4000rpm

11 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/ , 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

12 [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