- Email: [email protected]
sliding contacts
The Third Body Concept / D. Dowson et al. (Editors) (D 1996 Elsevier Science B.V. All rights reserved.
141
Behaviour of PTFE suspensions in rollinghliding contacts S Palios, P M Cann and H A Spikes Tribology Section, Department of Mechanical Engineering, Imperial College of Science Technology and Medicine, London SW7 2BX, United Kingdom
PTFE (polytetrafluoroethylene)powders are quite widely used as concentrated dispersions in liquid lubricants where they form pastes or greases. It has also been suggested that they can be used to reduce friction and wear when suspended at quite low concentrations in oils. Thus a number of PTFE-containing products are currently marketed which, it is claimed, improve crankcase engine performance when added to conventional motor oils. The way that such PTFE particles might function in lubricated contacts is disputed. Some workers suggest that they adhere to rubbing surfaces and are smeared out to form a protective, low friction coating. Other authors propose that the particles pass individually through contacts without adhering, serving to bodily separate the two opposing surfaces. Another suggestion is that PTFE particles have no practical beneficial effects whatsoever on either friction or wear. This paper measures both the film-forming properties of lubricants containing suspended PTFE particles and also their friction and wear properties in order to investigate the effectivenessand mechanism of behaviour of these panicles.
1. INTRODUCTION
The last two decades have seen a rapidly developing interest in the production and use of low fiiction. highly efficient, liquid lubricants. The initial impetus for this was fuel shortages resulting from conflicts in the Middle East in the 1970s. More recently. the main driving force has been the need to reduce global fossil fuel consumption and thus to limit carbon dioxide emissions. Most attention has focused on crankcase engine lubricants and modem engine oil specificationsnow include engine tests, such as the ASTM Sequence VI, to quantify fuel efficiency. However there is also considerable interest in reducing friction and thus increasing the efficiency of transmission lubricants. Lubricant manufacturers have adopted two main approaches to producing high efficiency lubricants. One has been to lower the dynamic viscosity of the lubricant and thus the hydrodynamic friction. This has resulted in a steady reduction in the viscosity of multigrade engine oils over the last fifteen years. A
second, complementary, approach has been to employ soluble, friction modifying additives to reduce friction in the boundary lubrication regime. Typical examples are molybdenum compounds and long chain, unsaturated organic acid salts. A third approach, which is the subject of this paper, is far more contentious. This is to add to the liquid lubricant tiny insoluble particles of polytetrafluoroethylene (PTFE). This material, when used in solid-coating form on the surfaces of components such as dry bearings and non-stick frying pans, is known to show very low friction and it has been suggested that a similar benefit is conferred by dispersed particles in an oil. At present, PTFE dispersions are provided primarily by specialist lubricant suppliers as concentrates, to be added, by the user, to conventional engine, transmission or other lubricants. Their efficacy is controversial. Some studies suggest that they significantly reduce friction and also wear in engines and other systems (1). Other studies have found very little, if any benefit (2).
142
This paper describes an investigation into the friction and film-forming properties of PTFE suspensions in lubricating oils. The aim of the work was to investigate the conditions, if any, under which such PTFE particles can reduce friction and wear and to explore the mechanisms by which such benefits may be produced. 2. BACKGROUND
Dispersed, solid particles have been used as dilute solutions in liquid lubricants for many years (3). The most widely employed materials are graphite and molybdenum disulphide (MoS,) although many other substances such as metal salts and even glass particles have also been used (4)(5). Both graphite and MoS, are lamellar solids which reduce friction when supplied as dry, solid coatings. It is generally presumed that their low friction results from preferential slip parallel to the solids’ low shear strength basal plane. The effectiveness of suspensions of graphite and MoS2 in oils is disputed. Some workers have found that they reduce friction and wear whilst others have found an increase (6). One reason for these observed differences may lie in the fact that, depending upon how graphite and MoS, are manufactured, both of these solids can exist in different forms with differing particle shapes and surface adhesive properties (7). A second possible origin of contradictory results is that the behaviour of suspended solid particles may depend critically upon the geometry, kinematics and surface roughness of the rubbing system used. Thus a number of workers have studied the behaviour of both graphite and MoS, in and around contacts visually, by making one of the rubbing bodies Lransparent; usually of glass (8)(11). This has shown that both types of particle are canied into the contact in pure rolliig conditions and adhere to the surfaces to form a concentrated layer of solid lubricant in the contact track. In high slide-roll ratio contacts, however, particles tend to accumulate in the inlet, causing Starvation and thus loss of lubricant film. Studies of the effects of temperature, lubricant viscosity and of the influence of other additives present support the principle that, to be effective,
graphite and M0S2 particles must adhere to the rubbing surfaces and thus form, due to the mechanical effects of sliding, a solid lubricant coating (12)-(15). By comparison with graphite and MoS,, the behaviour and effectiveness of dispersed PTFE is far from clear. Although PTFE has been used at high concentration in pastes and greases for many years, its use as a low concentration dispersion in oils is quite recent. The concept was first patented by Reick in 1976 (16) and has beem promoted as a means of reducing friction and wear in a number of applications, including crankcase engines, chain WWS,Wire drawing and penetrant lubricants (17)(21).
There has been considerable conmversy concerning the effectiveness of PTFE in this form. Thus reference (22) cites a note from Du Pont, one of the main manufacturers of PTFE who circulated a letter to all news media and customers in 1980 “E$ective February 1, 1980, ‘Teflon’fluorocarbon resins or untrademarked DLX-6000 fluorocarbon micropowder will not be supplied for use as ingredients of oils or oil additives for the lubrication of internal combustion engines. This decision is based on our conviction that these polymers are not useful ingredients in such products. ”
Just over ten years later, another main manufacturer of PTFE powders, ICI states (23) “One of the more recent applications for PTFE lubricant powders has been their inclusion in internal combustion engine lubricating oils. A considerable amount of literature has already been published on this application which has highlighted the role that PTFE particles may play in filling the irregularities in the metal counterface and providing a smooth, low piction sugace between the moving parts. Once the lubricant is in place, the oil base provides a bawierjllm which bonds the PTFE to the sugace to give a very low boundary piction coeflcient which reduces the total running piction of the engine. Further developments are awaited in this expanding automotive applications area. ’’
143 Despite the above statement, surprisingly little scientific work has been carried out on the behaviour of suspended PTFE particles in oils. The most detailed study is due to Reick (22). This work examined the influence of fine, dispersed PTFE particles, of diameter 0.05 to 0.5 tun, on the heat generated in a sliding steelkeel contact. The presence of PTFE particles resulted in significantly lower temperature rises than found with PTFE-free oils, implying lower friction, and also postponed the onset of scuffing. Reick noted that when the steel surfaces were pretreated with a comsion inhibitor. the effectiveness of the PTFE was reduced. Reick also canied out Auger and ESCA analysis of the rubbed steel surfaces. When these were vigorously degreased. no fluorine was found to be present, but after only mild degreasing there was evidence of individual particles of fluorine-rich material in and around the wear track. From this study, Reick concluded that FTFE particles do not adhere strongly to the solid surfaces, nor get smeared out, as is found with graphite and MoS2. but rather pass individually through the contact; “floating particles rolling and sliding between the surfaces”. Reick’s work was carried out in a low pressure, conforming contact. Reick also mentions, without further details. that PTFE particles are ineffective in high pressure contacts such as in the four ball or Fales machine. This was confiied in a study by Cusano et a1 who examined the effect of surface roughness on dispersed solid lubricant behaviour (6). They found that the addition of dispersed PTFE to a mineral oil had negligible effect on friction and wear in a point contact pin on disc machine. Similar findings have also been described by Horsmans who found large wear and friction reduction by dispersed PTFE in a pin of disc machine but very little effect in a four ball tester (24). Li and coworkers used X-ray photoelectron spectroscopy and Auger to analyse the surface films present on specimens from a Falex test lubricated with a PTFE dispersion (25). They confirmed observation of distributed microparticles of PTFE in the contact region but also found that PTFE formed a very thin, structured surface layer under boundary lubrication conditions. This consisted of
M y fluorinated carbon chains in the topmost layer
with partiauy hydrogenated chains mixed with FeF, below this. One major practical problem in using dispersed PTFE particles which may explain some of the differing views as to their ef€icacy is that of dispersion stability. Clearly, to be effective, the PTFE particles must remain in suspension during storage and use. This is achieved in part by using very small particle diameters, so that Brownian motion becomes significant and also by using dispersants. It has also been claimed that electron bombardment of the PTFE can provide a permanent negative charge, which enhances interparticle repulsion as well as promoting particldmetal surface bonding (24)(26). 3. TESTMATERIALS
One problem in studying the behaviour of PTFE particles used in commercial oils is that the particles are very small, typically less than 0.1 pm diameter, which makes them very difficult to observe visually. Most commercial samples also contain additives which are incorjwmted to help disperse the PTFE particles and it may be difficult to distinguish the effects of these additives on friction and wear from the contribution of the PTFE particles themselves. To help tackle these problems, in the current study tests were made on two M e r e n t types of suspended PTFE system. One consisted of a fully-formulated, commercial oil, with and without PTFE particles. The other was a set of dispersions of well-characterised, quite large PTFE particles in very simple base fluids. 3.1 Wellcbaracterised blends Table 1 lists the two types of PTFE particle employed in simple blend).
t
Particle size. um d a c e area. m‘/g I
.
I FL1700 I VydaxHD 1=1
I
3.1
I 51
I 1.0
I
Table 1. PTFE powders employed One. FL1700, Erom ICI Fluorochemicals, had quite fine particle size of about 1 pm, and, it was
144
claimed, could be broken down to submicron-sized particles in high shear conditions and was suitable for use in oil dispersions. The other, Vydax HD from Du Pont Chemicals had somewhat coarser particle size. These PTFE particles were dispersed in additive-free, polyalphaolefm, synthetic hydroc h o n base fluids of differing viscosities, as listed in table 2.
I Viscosity at I SHF41 SHF401
4OoC (cSt) 14.9 440
Viscosity at 100°C (cSt) 3.12 49.5
I
Figure l(a)
1% wt. Vydax in SHF41
Table 2. Characteristics of base oils employed The PTFE particles were dispersed at 1% and
5% wt. in both fluids. The blends were placed in a
beaker on a magnetic stirrer or in an ultrasonic bath for 5 minutes to uniformly disperse the particles. It was more difficult to achieve a uniform mixture with the more viscous oil (SHF401) where the fluid was preheated with a magnetic stirrer and hot plate. In some cases, a Silverson heavy duty laboratory mixer/emulsifier was used in order to ensure an evenly-dispersed solution. The dispersions obtained using these simple systems were not fully stable and settled slowly with time; especially for the larger particle size system. Therefore they were well-dispersed prior to each test and care was taken to periodically agitate the blends during a test to ensure that particles did not settle. Figures la and lb show optical images (x170) of suspended particles of 1% wt. dispersions of the two PTFE powders in SHF41, taken using a differential interference contrast microscope. The PTFE particles, which clearly have a range of sizes, appear white against a dark background 3.2 Commercial materials A fully-formulated 10W/30 engine oil was also tested, with and without the addition of 1% wt. of commercial PTFE particles. The particle size of these particles was not known but was estimated to lie in the range 0.05 to 0.1 pm. The engine oil had a viscosity of 10.6 cSt at 100OC. These dispersions appeared to be fully stable over time.
Figure 1@) 1% wt. FL1700 in SHF41 4. TEST METHODS
A number of different experimental test methods were used in this study. These divide into two groups;
(i) Boundary friction and wear (ii) EHD film thickness, traction and imaging
4.1 Boundary Friction and Wear Tests A reciprocating test rig was used. In this, a 6.0 mm diameter steel ball is held in a chuck and loaded downwards on the flat face of a 10.0 mm diameter steel disc. The disc is held in a bath which is two thirds filled with test lubricant so that the contact between the ball and flat is fully immersed. The bath temperature is controlled to f 0.5OC. During a test, an electrical vibrator is employed to oscillate the ball backwards and forwards in contact with the flat at a stroke length and frequency that can be set by the user. A control
. 145
a precision o f f 1 nm. A highly polished steel ball (AISI 52100, 19.05 mm diameter) is loaded against the underside of a glass disc which is coated with a chromium semi-reflecting layer and a silica spacer layer. In the w e n t study the thickness of the spacer layer was approximately 510 om. The surface of the glass disc was optically smooth and the composite roughness of the undeformed surfaces was 11 nm. The disc is driven by an accurate motor via a series of speed-reduction gears, which provides controlled speeds over a m g e from 0.0002 m/s to 5 d s . The rotating glass disc drives the steel ball in nominal pure rolling. The test rig is shown schematically in figure 2. In these tests, the ball was half-immersed in lubricant Which ens~reedfully-flooded conditions. Test temperature was maintained using thermocouples around the chamber and measured near the contact inlet using a digital thermometer.
circuit maintains constant stroke length regardless of the friction value. Friction between the ball and flat is measured using a load cell attached to the lower specimen holder. The friction coefficient was logged continuously throughout a test and at the end of some tests, the wear scar size on the ball was determined using a microscope. This was taken as the average of the major and minor axes of the elliptical wear scar. Friction tests were carried out according to the following conditions:
1 Strokelength
Stroke frequency
Temperature staees Ball properties Disc properties Time interval
I
1000 wn 20 Hz,50 Hz
I
4OoC, 6OoC, 8OoC, 100OC. 12OOC AISI 52100,800 VPN AISI 52 100,200 VPN 10 min
Table 3. Friction test conditions used in this study
The temperature was raised in stages, with 10 minutes at each stage. Friction coefficient was averaged over the 10 minutes of the test. Wear tests were canied out according to the following conditions:
1000 pn Stroke length Stroke frequency 20 Hz Load 400 g 8OoCor 12OOC Temperature AISI 52100,800 VPN Ball properties Disc ~rotmties AISI 52 100.200 VPN 1Time interval 175min
Table 4. Wear test conditions for wear testing New specimens were used for each test and the temperature was held constant for the duration of the test. 4.2
EHD Film Thickness, Traction and Imaging Methods
EHD film thickness meaSUfementS were made using ultrathin film interferometry (26). This can measure central film thickness as low as 2 nm with
Figure 2. Diagram of EHD Film Thickness Rig
I
The whole test rig operates under microcomputer control. Before the start of the test, prior to the addition of lubricant, the silica spacer layer thickness is measured at a number of Merent positions around the glass disc. Lubricant is then added, temperature stabilized and motion started. During the test, a disc position encoder is used in conjunction with the microcomputer to enable film thicknesses to be measured at the locations on the glass disc where the spacer layer thickness has been praiously detennined. In the current study, film thickness tests were Carried out at temperatures of 40°C, 8OoC and
146 120OC. at a load of 20 N, corresponding to a maximum contact pressure of 0.52 GPa and in nominally pure rolling. The rig used for EHD traction measurements was the same as the one used for the film thickness measurements, except that the glass disc was replaced by a smooth hardened, polished steel disc, with a similar surface finish to that of the ball and both the ball and the disc wete driven by separate DC motors, enabling the sliddroll ratio to be controlled. Traction measurements were made between a 19.0 mm diameter, AISI 5200 steel ball in sliding contact with a steel disc. The Young's moduli of the two surfaces were 2 10 GPa for both. In order to observe the bebaviour of individual particles of PTFE in contacts, a spacer layer imaging method was used. This is fully described in (28). The optical test rig used is essentially the same as in figure 2. However a solid state, colour camdframe grabber is used to capture interference images of the EHD contact. A high speed electronic shutter enables images to be captured within 0.25 ms. From the images thus obtained it is also possible to produce maps of film thickness over the contact using a colour analysis technique (28). 5.
RESULTS
SHF41 SHF41+5% FL 1700 Commercial Oil corn. oil +
4OoC 334 265 164 165
12OOC 419 336 215 19 1
1
Table 6. Wear scar results from reciprocating rig 5.2 EHD Film Thickness Figure 3 contains EHD film thickness plots for a 1% wt. dispersion of FL1700 in SHF41 at two temperatures, 40 and 8OOC. The figure also shows some results for SHF41 without PTFEat 4OOC.
0 1% FL17OO,4O0C
5.1 Boundary Friction and Wear Table 5 summarises friction coefficient results for the lubricants tested.
_FL1700 _
X 1% FL700,80°C
0.09 0.07
0.10 0.07
0.12 0.07
0.12 0.08
0.13 0.09
o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, mls
0.12 0.12
0.12 0.13
0.13 0.13
0.14 0.14
0.14 0.14
Figure 3. EHD Film Thickness of SHF4 1 with and without 1% wt. PTFE
~
SHF401 SHF401+ 5% FL1700 Comm.Oil Comm.Oi1
Friction is seen to rise with temperature, presumably because the contact operates more fully in the boundary as opposed to the mixed regime as the viscosity decreases with increasing temperawe. Table 6 shows wear results at two test temperatures. It can be seen that the FL1700 PTFE dispersion reduces both friction coefficient and wear but the V ~ I Y fine PTFE dispersion in the commercial oil has negligible effect on performance.
Table 5. Friction coefficients in reciprocatiug rig
10
It can be seen that at high speeds, log(film thickness) vetrms log(speed) is linear, in accord with EHD theory. Comparison with the PTFE-free fluid shows that there is no measurable contribution
147
from the PTFE at high speeds. At slow speeds, however. the PTFE-containing oils showed evidence that PTFE was passing through the contact to momentarily increase the film thickness. The fdm thickness measurements were irregular, and quite random, spanning the range from a lower bound corresponding to the PTFE fluid film thickness up to an upper bound of approximately 70
EHD Traction Figure 6 compares the traction behaviour of a 1% wt. and a 5% wt. dispersion of FL1700 in SHF41 with the corresponding PTFE-free base fluid. All measurements were taken at a fied slide-roll ratio of 50%. 5.3
nm.
Figures 4 and 5 compare results for the commercial oil with and without dispersed PTFE at 80 and 120OC. The PTFE-cOntaining oil appears to form a slightly thicker film than the PTFE-free one at very slow speeds, especially at 120OC.
I
0.1
E
aa
'E 0.06 8 8 0.04
H
I
E No PTFE, 80°C
0.08
0.02
O l 0.001
I
I
I
0.01
0.1
1
Entrainment Speed, m/s
10
Figure 6. Tractiodentrainment Speed Plots for SHF41 with FL 1700 PTFE at 80°C o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, m/s
10
Figure 4. EHD Film Thickness Results at 80°C for Commercial Oil with and without PTFE No PTFE, 120°C
E 100
d
!i
F
10
E
E l o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, m/s
Figure 5. EHD Film thickness results at 12OOC for Commercial Oil with and without PTFE
10
The results illustrate how traction coefficient varies with entrainment speed. For the base fluid, this m e is effectively a Stribeck-type plot, showing how traction coefficient varies with EHD film thickness. At high speeds, the traction corresponds to an EHD limiting traction value. As the speed and thus the film thickness is reduced, however, the traction rises progressively towards the b o u n d a ~friction ~ value as the film thickness decreases. With the PTFE-containing fluids, the EHD traction coefficient at high entxainment speed is somewhat higher than the PTFE-free value. possibly because the PTFE particles are blocking the inlet slightly to cause starvation and thus reducing film thickness. In the slow speed,thin film regime however, the traction coefficient of the PTFE-containing oils falls. This collapse is irregular, with the traction rising and falling apparently randomly with time. Figures 7 and 8 compare the traction behaviour of the commercial oil with and without PTFE.
148 Negligible contribution by PTFE can be seen at either temperature. No PTFE, 80°C '5 0.1
and Vydax. No evidence was seen of the accumulation of particles in the contact track, suggesting strongly that PTFE particles do not adhere to or become smeared out on the rolling track.
1% PTFE, 80°C
i= .T-
E5 0*08 0.06
..tj 0.04 c! b- 0.02
01 0.001
I
I
I
0.01 0.1 1 Entrainment Speed, m/s
10
Figure 7. Tractiodentrainment Speed Plots for Commercial Oil with and without PTFE 0.14 1
E 0.12
I
a, 'J 0.1
1
I
I a No PTFE, 120°C
Figure 9. Spacer Layer Image of Contact with 1% wt. of Vydax HD in SHF41 at 0.012m/s
1% PTFE, 120°C
E
3c 0.06 0.08
0
10.04
c 0.02 0.001
0.01
0.1
1
Entrainment Speed, m/s
10
Figure 8. Tractiodentrainment Speed Plots for Commercial Oil with and without PTFE 5.4 Spacer Layer Imaging of EHD Contact
Spacer layer imaging showed that FL1700 and Vydax HD particles (1% wt. in SHF41) pass through slow speed rolling contacts when tested at room temperature. At very slow speeds (less than LO &second) large quantities of particles pass through. As the speed is raised however, the numbers entering the contact diminish sharply and at high speed (greater than 0.1 d s ) very few, if any, particles are entrained. Figures 9 and 10 show interference images taken from the contact, the inlet is on the right. The local colour variations within the contact are PTFE particles of El700
Figure 10. Spacer Layer Image of Contact with 1% wt. of FL1700 in SHF41 at 0.012 m/s It is possible to map the film thickness across these contacts using interference colour analysis. Figures 11 and 12 show the film thickness profiles, taken along the centre line, in the direction of rolling. The corresponding profiles for the base oil are also shown. Figure 1 1 shows a profile for 1% Vydax at slow speeds. The presence of a particle can be clearly seen. It is possible to estimate the volume of this indentation which is about 120 pm3, corresponding to a 6 pm diameter spherical particle. It can be seen that the particles produce a
149 local elastic impression in the two surfaces but that the surrounding contact region remains quite flat. 0.1 m/s
180 :
200 h
I
I
160:
8 120
8
100
40 . 20 : 0 -200
SHF 41 1
-100
'
1
0
.
1
.
-200
'
100
200
Contact position (pm)
Figures 12 to 14 show similar profiles for FL1700 at three speeds. At very slow speeds many particles pass through the contact and individual ones cannot be easily distinguished. As the speed is raised. fewer particles pass through so that at 0.2 m/s the film thickness is similar to the base fluid.
1
200
Figure 13. Fdm Thickness Profile with 1% wt. of FL17OO in SHF41 at 0.1d s .
Figure 11. Film Thickness Profile with 1% wt. of Vydas HD in SHF41 at 0.012m / s .
300
-100 0 100 Contact position (pm)
200 180
--
3- 1601 140 v1
8 120-
4
100: 80: 60: iz 40 : 20 0 : -200
3
-
4
I
-
I
I
-100
0
.
1
100
.
1
200
Contact position (pm) Figure 14. Film Thickness Profile With 1% wt. of FL1700 in SHF41 at 0.2 d s .
-200
-100 0 100 Contact position (pm)
200
Figure 12. Film Thickness Profile with 1% wt. of FL1700 in SHF41 at 0.012ds.
With the commercial oil the particles are much smaller and difficult to distinguish. The spacer layer images suggested that particles were passing through at low speeds where some perturbation of the EHD N m shape was observed.
150 6. DISCUSSION
This study shows quite clearly that PTFE particles of about 1 p diameter are able pass through slow speed, rolling, elastohydrodynamic contacts. However they appear to be largely rejected from high speed contacts. These PTFE particles also appear to penetrate slow speed,mixed rolling/sliding contacts where they help reduce friction in the thin film regime. They may, however, promote higher friction at higher rolling speeds, perhaps by promoting starvation. The behaviour of the much finer particles supplied for use in commercial engine lubricants is far less clear. Contact imaging suggests that some particles may pass through low speed rolling contacts. However traction, boundary friction and wear tests all show no contribution from the dispersed PTFE. If, as appears the case, PTFE particles are unable to adhere and smear on the surfaces to form a reasonably coherent layer on the solid surfaces, then two possible reasons for the lack of effectiveness of tiny, dispersed PTFE particles can be considered. One is that very small PTFE particles are rejected from the contact even down to very low speeds. below those attainable in the current study. If this were the case then the particles will simply not pass through EHD contacts. It has been suggested previously that the mechanism by which particles are drawn into contacts involves their being trapped by friction forces between the converging solid surfaces (11). Very small PTFE particles would both have low friction and would, by virtue of their small size, have to penetrate the inlet a long way before becoming trapped. Thus their almost complete rejection Erom the contact could easily be envisaged. On the other hand, EHD film thickness measurements do show a small enhancement of film thickness due to the PTFE at very slow speeds, suggesting that particles may pass through the contact under these conditions. An alternative explanation for the inability of very small PTFE particles to reduce friction in this study is that the particles may pass through the contact but not support a signifkant propoxtion of the applied load in the process. It should be noted that the friction in a contact derives from the integral, over the whole contact area,of the ratio of
local shear stress to local supported load. In the EHD imaging work it was seen for the large PTFE particles, that the solid surfaces deformed around the particles to fom localised elastic indentations of about 50 nm in depth. The total Hertzian flattening under these ConditioILS is about 2 p. Thus the proportion of the load supported by even these large FTFE particles will be quite small. If tiny, 75 nm diameter, particles pass through the contact, the amount of elastic deformation they will produce, and thus the proportion of the load supported will be very small. Thus they would be expected to cause only a tiny decrease in overall friction. In summary, it is no use having an easily sheared region of the contact if this does not support any of the load. If this is the origin of the lack of effectiveness of small PTFE particles in this study, then it supports the contention made by previous workers that PTFE particles may be more effective in “area contacts” (22). Such contacts will have much lower contact pressures than those in the current study and in such systems, the proportion of the load supported by deforming PTFE particles may become much more significant. In consequence the particles may contribute to reduced friction and wear in such systems. 7. CONCLUSIONS A study has been made of the behaviour of dispersed PTFE particles in high pressure, boundary and EHD contact conditions. It has been found that large, micron-sized PTFE particles appear to reduce friction and wear in reciprocating tests. These particles also reduce friction in slow speed, thin film, mixed sliding/rolling conditions. Optical studies show that these large particles are able to pass through slow speed, rolling contacts where they cause significant elastic deformation of the solid surfaces. The reduction in friction and wear can be ascribed to this behaviour. There is no evidence of the particles adhering m n g l y to the rubbing surfaces to form a pennanent coating. Very small PTFE particles in fully-formulated oil, appear to make no measurable con~butionto friction and wear reduction. This may be because
151 the particles are unable to enter the contact. Alternatively, they may be so small and weak that in passing through the contact they are unable to bear a significant fraction of the applied load. In such a case they would not be expected to reduce friction or wear to any useful extent.
WFERENCES 1.
2. 3. 4.
5.
16.
7.
8.
Saunders, J. “Slick 50 Breaks Boundaries”, Lubricants World, pp. 27-31, May 1995. Winfield, B., “Oil Additives: the Pitch is Slick but do they Work?”, letter to Car and Driver, p. 23, March 1994. Smith E.A. “Colloidal Graphite in Assembly Lubrication”, Engineering 165, pp. 505-507, (1948). Middleton, K. “A Comparative Examination of Some Potential Inorganic Lubricants on the Shell Four-Ball Tester and on a CrossedCyliider Wear Machine”, Paper 7, I. Mech. E. Lubr. and Wear Convention, May 1964. Arizmendi, L., Palacios, J.M. and de la Cruz M., “A Very Ememe-Pressure non-Lamellar Additive for Special Mechanical Designs, Proc.1.Mech.E. C2%/73,pp. 307-311, (1973). Cusano, C. and Goglia, P.R. “Surface Roughness Effects with Solid Lubricants Dispersed in Mineral Oils”, ASLE Trans. 27. pp. 227-236, (1984). Groszek, A.J. and Witheridge, R.E. “Surface Properties and Lubricating Action of Graphite and MoSP, ASLE Trans.14.pp. 254- 266, (1971). C.Cusano and H. E. Sliney “Dynamics of Solid Dispersions in Oil During the Lubrication of Point Contacts, Part I Graphite.” ASLE Trans. 25.pp.183-189, (1981). Cusano,C. and Sliney, H. E. “Dynamicsof Solid Dispersions in Oil During the Lubrication of Point Contacts,Part II Molybdenum Disulphide.” ASLE Trans.25. pp. 190-197, (1981).
-
9.
-
10. Wan, G.T.Y. and Spikes, H. A., “Two Phase Lubricants in Elastohydrodynamic Contacts Graphite in Oil, Dispersions”, Proc. 12th Leeds-Lyon Symposium on Tribology, Lyon Sept. 1985, “Mechauismsand Surface Distress”, ed. D Dowson et al. ,publ. Butterworths, 1986. 11. Wan,G.T.Y. and Spikes, H.A. “The Behaviour of Suspended Solid Particles in Rolling Sliding Elastohydrodynamic Contacts” STLE Trans. pp.12-24, (1987). 12. H O W ,R. “Lubrication Mechanisms of Solid Lubricants in Oils”, ASLE Trans. 18. pp. 263-269, (1975). 13. Bartz, W.J. and Muller, K. “Investigationsof the Lubricating Effectiveness of Molybdenum Disulphide”, Wear-02 pp. 377-379, (1972). 14. Scott, D. and Jamieson, D. “Molybdenum Disulphide as a Lubricant Additive in Rolling Contact a Study of Compatibility with Other Additives, J. Inst. Pet. 48.pp. 91-97, (1962). 15. Rolek, R.J., Cusano, C.and Sliiey, “The Influence of Temperature on the Lubricating Effectiveness Of MoS2Dispersed In M i n d Oils” ASLE Tran~.28.pp. 493502, (1984). 16. Reick, F., “Lubricating Oil with Fluorocarbon Additives”, US Patent 3933656, Jan., (1976). 17. Gutman, M. and Stotter, A. “The Influence of Oil Additives on Engine Friction and Fuel Consumption” Lubr. Eng. -4l pp. 150-154, (1985). 18. Wilson, B. ”PTFEas a Friction Modifier in Engine Oil” Ind. Lubr. and Trib. 44,pp. 35, (1992) 19. Shaub, H., Pandosh, J., Searle, A. and Sprague, S. “Engine Durability, Emissions and Fuel Economy Studies with Special Boundary Lubricant Chemistry”,SAE 941983, (1994). 20. “(Polytctraflwmethylene)Lubricant Saves Energy” Wire Tech, 10.p.20, (1982). 21. Dehlsen, J. and Ferguson, J., “Multipurpose -Lubricant“, Tribolog~Int. p ~282-285, . (1979).
a
-
152 22. Reick, F.G.“Energy-savingLubricants Containing Colloidal FTFE”,Lub. Eng. pp. 635-646.(1982). 23. “Fluoroglide Fluompolymer Lubricants, a Guide to: Applications, Properties and Processing”, 1C1 Fluoropolymers, Blackpool, Lancashire, UK, 1992. 24. Horsmans, J. “NewDevelopments in FTFE Additives for Lubricating Oils”, Proc. 5th Int. Colloq., Esslingen, Jan. 1986,pp 6.5.14.5.8, ed. W.J. Bartz. 25. Li L.C., Yang, M.S. and Ling, Q.Z. “Chemical Structure Characterisationof the Boundary Lubrication Film Using X-ray Photoelectron Spectroscopy and Scanning Auger Microscope Probes”. Wear 140.345357. (1990).
a,
26. Stewart, C.W. Letter in response to Mr B Winfield’s article in Car and Driver, March 1994,Du Pont Central Research, P.O.Box 80323,Wilmington, DE,Feb. 18th 1994. 27. Johnston, G.J, Wayte, R.and Spikes, H.A. “The Measurement and Study of Very Thin Films In Concentrated Contacts” Trib. Trans., 34,pp.187-194,(1991). 28. & , I P.M., Hutchinson, J. aud Spikes,H.A. “The Development of a Spacer Layer Imaging Method (SLIM) for Mapping Elastohydmdynamic Contacts”. To be presented at STLWASME Joint Meeting, (1995).
141
Behaviour of PTFE suspensions in rollinghliding contacts S Palios, P M Cann and H A Spikes Tribology Section, Department of Mechanical Engineering, Imperial College of Science Technology and Medicine, London SW7 2BX, United Kingdom
PTFE (polytetrafluoroethylene)powders are quite widely used as concentrated dispersions in liquid lubricants where they form pastes or greases. It has also been suggested that they can be used to reduce friction and wear when suspended at quite low concentrations in oils. Thus a number of PTFE-containing products are currently marketed which, it is claimed, improve crankcase engine performance when added to conventional motor oils. The way that such PTFE particles might function in lubricated contacts is disputed. Some workers suggest that they adhere to rubbing surfaces and are smeared out to form a protective, low friction coating. Other authors propose that the particles pass individually through contacts without adhering, serving to bodily separate the two opposing surfaces. Another suggestion is that PTFE particles have no practical beneficial effects whatsoever on either friction or wear. This paper measures both the film-forming properties of lubricants containing suspended PTFE particles and also their friction and wear properties in order to investigate the effectivenessand mechanism of behaviour of these panicles.
1. INTRODUCTION
The last two decades have seen a rapidly developing interest in the production and use of low fiiction. highly efficient, liquid lubricants. The initial impetus for this was fuel shortages resulting from conflicts in the Middle East in the 1970s. More recently. the main driving force has been the need to reduce global fossil fuel consumption and thus to limit carbon dioxide emissions. Most attention has focused on crankcase engine lubricants and modem engine oil specificationsnow include engine tests, such as the ASTM Sequence VI, to quantify fuel efficiency. However there is also considerable interest in reducing friction and thus increasing the efficiency of transmission lubricants. Lubricant manufacturers have adopted two main approaches to producing high efficiency lubricants. One has been to lower the dynamic viscosity of the lubricant and thus the hydrodynamic friction. This has resulted in a steady reduction in the viscosity of multigrade engine oils over the last fifteen years. A
second, complementary, approach has been to employ soluble, friction modifying additives to reduce friction in the boundary lubrication regime. Typical examples are molybdenum compounds and long chain, unsaturated organic acid salts. A third approach, which is the subject of this paper, is far more contentious. This is to add to the liquid lubricant tiny insoluble particles of polytetrafluoroethylene (PTFE). This material, when used in solid-coating form on the surfaces of components such as dry bearings and non-stick frying pans, is known to show very low friction and it has been suggested that a similar benefit is conferred by dispersed particles in an oil. At present, PTFE dispersions are provided primarily by specialist lubricant suppliers as concentrates, to be added, by the user, to conventional engine, transmission or other lubricants. Their efficacy is controversial. Some studies suggest that they significantly reduce friction and also wear in engines and other systems (1). Other studies have found very little, if any benefit (2).
142
This paper describes an investigation into the friction and film-forming properties of PTFE suspensions in lubricating oils. The aim of the work was to investigate the conditions, if any, under which such PTFE particles can reduce friction and wear and to explore the mechanisms by which such benefits may be produced. 2. BACKGROUND
Dispersed, solid particles have been used as dilute solutions in liquid lubricants for many years (3). The most widely employed materials are graphite and molybdenum disulphide (MoS,) although many other substances such as metal salts and even glass particles have also been used (4)(5). Both graphite and MoS, are lamellar solids which reduce friction when supplied as dry, solid coatings. It is generally presumed that their low friction results from preferential slip parallel to the solids’ low shear strength basal plane. The effectiveness of suspensions of graphite and MoS2 in oils is disputed. Some workers have found that they reduce friction and wear whilst others have found an increase (6). One reason for these observed differences may lie in the fact that, depending upon how graphite and MoS, are manufactured, both of these solids can exist in different forms with differing particle shapes and surface adhesive properties (7). A second possible origin of contradictory results is that the behaviour of suspended solid particles may depend critically upon the geometry, kinematics and surface roughness of the rubbing system used. Thus a number of workers have studied the behaviour of both graphite and MoS, in and around contacts visually, by making one of the rubbing bodies Lransparent; usually of glass (8)(11). This has shown that both types of particle are canied into the contact in pure rolliig conditions and adhere to the surfaces to form a concentrated layer of solid lubricant in the contact track. In high slide-roll ratio contacts, however, particles tend to accumulate in the inlet, causing Starvation and thus loss of lubricant film. Studies of the effects of temperature, lubricant viscosity and of the influence of other additives present support the principle that, to be effective,
graphite and M0S2 particles must adhere to the rubbing surfaces and thus form, due to the mechanical effects of sliding, a solid lubricant coating (12)-(15). By comparison with graphite and MoS,, the behaviour and effectiveness of dispersed PTFE is far from clear. Although PTFE has been used at high concentration in pastes and greases for many years, its use as a low concentration dispersion in oils is quite recent. The concept was first patented by Reick in 1976 (16) and has beem promoted as a means of reducing friction and wear in a number of applications, including crankcase engines, chain WWS,Wire drawing and penetrant lubricants (17)(21).
There has been considerable conmversy concerning the effectiveness of PTFE in this form. Thus reference (22) cites a note from Du Pont, one of the main manufacturers of PTFE who circulated a letter to all news media and customers in 1980 “E$ective February 1, 1980, ‘Teflon’fluorocarbon resins or untrademarked DLX-6000 fluorocarbon micropowder will not be supplied for use as ingredients of oils or oil additives for the lubrication of internal combustion engines. This decision is based on our conviction that these polymers are not useful ingredients in such products. ”
Just over ten years later, another main manufacturer of PTFE powders, ICI states (23) “One of the more recent applications for PTFE lubricant powders has been their inclusion in internal combustion engine lubricating oils. A considerable amount of literature has already been published on this application which has highlighted the role that PTFE particles may play in filling the irregularities in the metal counterface and providing a smooth, low piction sugace between the moving parts. Once the lubricant is in place, the oil base provides a bawierjllm which bonds the PTFE to the sugace to give a very low boundary piction coeflcient which reduces the total running piction of the engine. Further developments are awaited in this expanding automotive applications area. ’’
143 Despite the above statement, surprisingly little scientific work has been carried out on the behaviour of suspended PTFE particles in oils. The most detailed study is due to Reick (22). This work examined the influence of fine, dispersed PTFE particles, of diameter 0.05 to 0.5 tun, on the heat generated in a sliding steelkeel contact. The presence of PTFE particles resulted in significantly lower temperature rises than found with PTFE-free oils, implying lower friction, and also postponed the onset of scuffing. Reick noted that when the steel surfaces were pretreated with a comsion inhibitor. the effectiveness of the PTFE was reduced. Reick also canied out Auger and ESCA analysis of the rubbed steel surfaces. When these were vigorously degreased. no fluorine was found to be present, but after only mild degreasing there was evidence of individual particles of fluorine-rich material in and around the wear track. From this study, Reick concluded that FTFE particles do not adhere strongly to the solid surfaces, nor get smeared out, as is found with graphite and MoS2. but rather pass individually through the contact; “floating particles rolling and sliding between the surfaces”. Reick’s work was carried out in a low pressure, conforming contact. Reick also mentions, without further details. that PTFE particles are ineffective in high pressure contacts such as in the four ball or Fales machine. This was confiied in a study by Cusano et a1 who examined the effect of surface roughness on dispersed solid lubricant behaviour (6). They found that the addition of dispersed PTFE to a mineral oil had negligible effect on friction and wear in a point contact pin on disc machine. Similar findings have also been described by Horsmans who found large wear and friction reduction by dispersed PTFE in a pin of disc machine but very little effect in a four ball tester (24). Li and coworkers used X-ray photoelectron spectroscopy and Auger to analyse the surface films present on specimens from a Falex test lubricated with a PTFE dispersion (25). They confirmed observation of distributed microparticles of PTFE in the contact region but also found that PTFE formed a very thin, structured surface layer under boundary lubrication conditions. This consisted of
M y fluorinated carbon chains in the topmost layer
with partiauy hydrogenated chains mixed with FeF, below this. One major practical problem in using dispersed PTFE particles which may explain some of the differing views as to their ef€icacy is that of dispersion stability. Clearly, to be effective, the PTFE particles must remain in suspension during storage and use. This is achieved in part by using very small particle diameters, so that Brownian motion becomes significant and also by using dispersants. It has also been claimed that electron bombardment of the PTFE can provide a permanent negative charge, which enhances interparticle repulsion as well as promoting particldmetal surface bonding (24)(26). 3. TESTMATERIALS
One problem in studying the behaviour of PTFE particles used in commercial oils is that the particles are very small, typically less than 0.1 pm diameter, which makes them very difficult to observe visually. Most commercial samples also contain additives which are incorjwmted to help disperse the PTFE particles and it may be difficult to distinguish the effects of these additives on friction and wear from the contribution of the PTFE particles themselves. To help tackle these problems, in the current study tests were made on two M e r e n t types of suspended PTFE system. One consisted of a fully-formulated, commercial oil, with and without PTFE particles. The other was a set of dispersions of well-characterised, quite large PTFE particles in very simple base fluids. 3.1 Wellcbaracterised blends Table 1 lists the two types of PTFE particle employed in simple blend).
t
Particle size. um d a c e area. m‘/g I
.
I FL1700 I VydaxHD 1=1
I
3.1
I 51
I 1.0
I
Table 1. PTFE powders employed One. FL1700, Erom ICI Fluorochemicals, had quite fine particle size of about 1 pm, and, it was
144
claimed, could be broken down to submicron-sized particles in high shear conditions and was suitable for use in oil dispersions. The other, Vydax HD from Du Pont Chemicals had somewhat coarser particle size. These PTFE particles were dispersed in additive-free, polyalphaolefm, synthetic hydroc h o n base fluids of differing viscosities, as listed in table 2.
I Viscosity at I SHF41 SHF401
4OoC (cSt) 14.9 440
Viscosity at 100°C (cSt) 3.12 49.5
I
Figure l(a)
1% wt. Vydax in SHF41
Table 2. Characteristics of base oils employed The PTFE particles were dispersed at 1% and
5% wt. in both fluids. The blends were placed in a
beaker on a magnetic stirrer or in an ultrasonic bath for 5 minutes to uniformly disperse the particles. It was more difficult to achieve a uniform mixture with the more viscous oil (SHF401) where the fluid was preheated with a magnetic stirrer and hot plate. In some cases, a Silverson heavy duty laboratory mixer/emulsifier was used in order to ensure an evenly-dispersed solution. The dispersions obtained using these simple systems were not fully stable and settled slowly with time; especially for the larger particle size system. Therefore they were well-dispersed prior to each test and care was taken to periodically agitate the blends during a test to ensure that particles did not settle. Figures la and lb show optical images (x170) of suspended particles of 1% wt. dispersions of the two PTFE powders in SHF41, taken using a differential interference contrast microscope. The PTFE particles, which clearly have a range of sizes, appear white against a dark background 3.2 Commercial materials A fully-formulated 10W/30 engine oil was also tested, with and without the addition of 1% wt. of commercial PTFE particles. The particle size of these particles was not known but was estimated to lie in the range 0.05 to 0.1 pm. The engine oil had a viscosity of 10.6 cSt at 100OC. These dispersions appeared to be fully stable over time.
Figure 1@) 1% wt. FL1700 in SHF41 4. TEST METHODS
A number of different experimental test methods were used in this study. These divide into two groups;
(i) Boundary friction and wear (ii) EHD film thickness, traction and imaging
4.1 Boundary Friction and Wear Tests A reciprocating test rig was used. In this, a 6.0 mm diameter steel ball is held in a chuck and loaded downwards on the flat face of a 10.0 mm diameter steel disc. The disc is held in a bath which is two thirds filled with test lubricant so that the contact between the ball and flat is fully immersed. The bath temperature is controlled to f 0.5OC. During a test, an electrical vibrator is employed to oscillate the ball backwards and forwards in contact with the flat at a stroke length and frequency that can be set by the user. A control
. 145
a precision o f f 1 nm. A highly polished steel ball (AISI 52100, 19.05 mm diameter) is loaded against the underside of a glass disc which is coated with a chromium semi-reflecting layer and a silica spacer layer. In the w e n t study the thickness of the spacer layer was approximately 510 om. The surface of the glass disc was optically smooth and the composite roughness of the undeformed surfaces was 11 nm. The disc is driven by an accurate motor via a series of speed-reduction gears, which provides controlled speeds over a m g e from 0.0002 m/s to 5 d s . The rotating glass disc drives the steel ball in nominal pure rolling. The test rig is shown schematically in figure 2. In these tests, the ball was half-immersed in lubricant Which ens~reedfully-flooded conditions. Test temperature was maintained using thermocouples around the chamber and measured near the contact inlet using a digital thermometer.
circuit maintains constant stroke length regardless of the friction value. Friction between the ball and flat is measured using a load cell attached to the lower specimen holder. The friction coefficient was logged continuously throughout a test and at the end of some tests, the wear scar size on the ball was determined using a microscope. This was taken as the average of the major and minor axes of the elliptical wear scar. Friction tests were carried out according to the following conditions:
1 Strokelength
Stroke frequency
Temperature staees Ball properties Disc properties Time interval
I
1000 wn 20 Hz,50 Hz
I
4OoC, 6OoC, 8OoC, 100OC. 12OOC AISI 52100,800 VPN AISI 52 100,200 VPN 10 min
Table 3. Friction test conditions used in this study
The temperature was raised in stages, with 10 minutes at each stage. Friction coefficient was averaged over the 10 minutes of the test. Wear tests were canied out according to the following conditions:
1000 pn Stroke length Stroke frequency 20 Hz Load 400 g 8OoCor 12OOC Temperature AISI 52100,800 VPN Ball properties Disc ~rotmties AISI 52 100.200 VPN 1Time interval 175min
Table 4. Wear test conditions for wear testing New specimens were used for each test and the temperature was held constant for the duration of the test. 4.2
EHD Film Thickness, Traction and Imaging Methods
EHD film thickness meaSUfementS were made using ultrathin film interferometry (26). This can measure central film thickness as low as 2 nm with
Figure 2. Diagram of EHD Film Thickness Rig
I
The whole test rig operates under microcomputer control. Before the start of the test, prior to the addition of lubricant, the silica spacer layer thickness is measured at a number of Merent positions around the glass disc. Lubricant is then added, temperature stabilized and motion started. During the test, a disc position encoder is used in conjunction with the microcomputer to enable film thicknesses to be measured at the locations on the glass disc where the spacer layer thickness has been praiously detennined. In the current study, film thickness tests were Carried out at temperatures of 40°C, 8OoC and
146 120OC. at a load of 20 N, corresponding to a maximum contact pressure of 0.52 GPa and in nominally pure rolling. The rig used for EHD traction measurements was the same as the one used for the film thickness measurements, except that the glass disc was replaced by a smooth hardened, polished steel disc, with a similar surface finish to that of the ball and both the ball and the disc wete driven by separate DC motors, enabling the sliddroll ratio to be controlled. Traction measurements were made between a 19.0 mm diameter, AISI 5200 steel ball in sliding contact with a steel disc. The Young's moduli of the two surfaces were 2 10 GPa for both. In order to observe the bebaviour of individual particles of PTFE in contacts, a spacer layer imaging method was used. This is fully described in (28). The optical test rig used is essentially the same as in figure 2. However a solid state, colour camdframe grabber is used to capture interference images of the EHD contact. A high speed electronic shutter enables images to be captured within 0.25 ms. From the images thus obtained it is also possible to produce maps of film thickness over the contact using a colour analysis technique (28). 5.
RESULTS
SHF41 SHF41+5% FL 1700 Commercial Oil corn. oil +
4OoC 334 265 164 165
12OOC 419 336 215 19 1
1
Table 6. Wear scar results from reciprocating rig 5.2 EHD Film Thickness Figure 3 contains EHD film thickness plots for a 1% wt. dispersion of FL1700 in SHF41 at two temperatures, 40 and 8OOC. The figure also shows some results for SHF41 without PTFEat 4OOC.
0 1% FL17OO,4O0C
5.1 Boundary Friction and Wear Table 5 summarises friction coefficient results for the lubricants tested.
_FL1700 _
X 1% FL700,80°C
0.09 0.07
0.10 0.07
0.12 0.07
0.12 0.08
0.13 0.09
o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, mls
0.12 0.12
0.12 0.13
0.13 0.13
0.14 0.14
0.14 0.14
Figure 3. EHD Film Thickness of SHF4 1 with and without 1% wt. PTFE
~
SHF401 SHF401+ 5% FL1700 Comm.Oil Comm.Oi1
Friction is seen to rise with temperature, presumably because the contact operates more fully in the boundary as opposed to the mixed regime as the viscosity decreases with increasing temperawe. Table 6 shows wear results at two test temperatures. It can be seen that the FL1700 PTFE dispersion reduces both friction coefficient and wear but the V ~ I Y fine PTFE dispersion in the commercial oil has negligible effect on performance.
Table 5. Friction coefficients in reciprocatiug rig
10
It can be seen that at high speeds, log(film thickness) vetrms log(speed) is linear, in accord with EHD theory. Comparison with the PTFE-free fluid shows that there is no measurable contribution
147
from the PTFE at high speeds. At slow speeds, however. the PTFE-containing oils showed evidence that PTFE was passing through the contact to momentarily increase the film thickness. The fdm thickness measurements were irregular, and quite random, spanning the range from a lower bound corresponding to the PTFE fluid film thickness up to an upper bound of approximately 70
EHD Traction Figure 6 compares the traction behaviour of a 1% wt. and a 5% wt. dispersion of FL1700 in SHF41 with the corresponding PTFE-free base fluid. All measurements were taken at a fied slide-roll ratio of 50%. 5.3
nm.
Figures 4 and 5 compare results for the commercial oil with and without dispersed PTFE at 80 and 120OC. The PTFE-cOntaining oil appears to form a slightly thicker film than the PTFE-free one at very slow speeds, especially at 120OC.
I
0.1
E
aa
'E 0.06 8 8 0.04
H
I
E No PTFE, 80°C
0.08
0.02
O l 0.001
I
I
I
0.01
0.1
1
Entrainment Speed, m/s
10
Figure 6. Tractiodentrainment Speed Plots for SHF41 with FL 1700 PTFE at 80°C o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, m/s
10
Figure 4. EHD Film Thickness Results at 80°C for Commercial Oil with and without PTFE No PTFE, 120°C
E 100
d
!i
F
10
E
E l o.Ooo1 0.001 0.01 0.1 1 Entrainment speed, m/s
Figure 5. EHD Film thickness results at 12OOC for Commercial Oil with and without PTFE
10
The results illustrate how traction coefficient varies with entrainment speed. For the base fluid, this m e is effectively a Stribeck-type plot, showing how traction coefficient varies with EHD film thickness. At high speeds, the traction corresponds to an EHD limiting traction value. As the speed and thus the film thickness is reduced, however, the traction rises progressively towards the b o u n d a ~friction ~ value as the film thickness decreases. With the PTFE-containing fluids, the EHD traction coefficient at high entxainment speed is somewhat higher than the PTFE-free value. possibly because the PTFE particles are blocking the inlet slightly to cause starvation and thus reducing film thickness. In the slow speed,thin film regime however, the traction coefficient of the PTFE-containing oils falls. This collapse is irregular, with the traction rising and falling apparently randomly with time. Figures 7 and 8 compare the traction behaviour of the commercial oil with and without PTFE.
148 Negligible contribution by PTFE can be seen at either temperature. No PTFE, 80°C '5 0.1
and Vydax. No evidence was seen of the accumulation of particles in the contact track, suggesting strongly that PTFE particles do not adhere to or become smeared out on the rolling track.
1% PTFE, 80°C
i= .T-
E5 0*08 0.06
..tj 0.04 c! b- 0.02
01 0.001
I
I
I
0.01 0.1 1 Entrainment Speed, m/s
10
Figure 7. Tractiodentrainment Speed Plots for Commercial Oil with and without PTFE 0.14 1
E 0.12
I
a, 'J 0.1
1
I
I a No PTFE, 120°C
Figure 9. Spacer Layer Image of Contact with 1% wt. of Vydax HD in SHF41 at 0.012m/s
1% PTFE, 120°C
E
3c 0.06 0.08
0
10.04
c 0.02 0.001
0.01
0.1
1
Entrainment Speed, m/s
10
Figure 8. Tractiodentrainment Speed Plots for Commercial Oil with and without PTFE 5.4 Spacer Layer Imaging of EHD Contact
Spacer layer imaging showed that FL1700 and Vydax HD particles (1% wt. in SHF41) pass through slow speed rolling contacts when tested at room temperature. At very slow speeds (less than LO &second) large quantities of particles pass through. As the speed is raised however, the numbers entering the contact diminish sharply and at high speed (greater than 0.1 d s ) very few, if any, particles are entrained. Figures 9 and 10 show interference images taken from the contact, the inlet is on the right. The local colour variations within the contact are PTFE particles of El700
Figure 10. Spacer Layer Image of Contact with 1% wt. of FL1700 in SHF41 at 0.012 m/s It is possible to map the film thickness across these contacts using interference colour analysis. Figures 11 and 12 show the film thickness profiles, taken along the centre line, in the direction of rolling. The corresponding profiles for the base oil are also shown. Figure 1 1 shows a profile for 1% Vydax at slow speeds. The presence of a particle can be clearly seen. It is possible to estimate the volume of this indentation which is about 120 pm3, corresponding to a 6 pm diameter spherical particle. It can be seen that the particles produce a
149 local elastic impression in the two surfaces but that the surrounding contact region remains quite flat. 0.1 m/s
180 :
200 h
I
I
160:
8 120
8
100
40 . 20 : 0 -200
SHF 41 1
-100
'
1
0
.
1
.
-200
'
100
200
Contact position (pm)
Figures 12 to 14 show similar profiles for FL1700 at three speeds. At very slow speeds many particles pass through the contact and individual ones cannot be easily distinguished. As the speed is raised. fewer particles pass through so that at 0.2 m/s the film thickness is similar to the base fluid.
1
200
Figure 13. Fdm Thickness Profile with 1% wt. of FL17OO in SHF41 at 0.1d s .
Figure 11. Film Thickness Profile with 1% wt. of Vydas HD in SHF41 at 0.012m / s .
300
-100 0 100 Contact position (pm)
200 180
--
3- 1601 140 v1
8 120-
4
100: 80: 60: iz 40 : 20 0 : -200
3
-
4
I
-
I
I
-100
0
.
1
100
.
1
200
Contact position (pm) Figure 14. Film Thickness Profile With 1% wt. of FL1700 in SHF41 at 0.2 d s .
-200
-100 0 100 Contact position (pm)
200
Figure 12. Film Thickness Profile with 1% wt. of FL1700 in SHF41 at 0.012ds.
With the commercial oil the particles are much smaller and difficult to distinguish. The spacer layer images suggested that particles were passing through at low speeds where some perturbation of the EHD N m shape was observed.
150 6. DISCUSSION
This study shows quite clearly that PTFE particles of about 1 p diameter are able pass through slow speed, rolling, elastohydrodynamic contacts. However they appear to be largely rejected from high speed contacts. These PTFE particles also appear to penetrate slow speed,mixed rolling/sliding contacts where they help reduce friction in the thin film regime. They may, however, promote higher friction at higher rolling speeds, perhaps by promoting starvation. The behaviour of the much finer particles supplied for use in commercial engine lubricants is far less clear. Contact imaging suggests that some particles may pass through low speed rolling contacts. However traction, boundary friction and wear tests all show no contribution from the dispersed PTFE. If, as appears the case, PTFE particles are unable to adhere and smear on the surfaces to form a reasonably coherent layer on the solid surfaces, then two possible reasons for the lack of effectiveness of tiny, dispersed PTFE particles can be considered. One is that very small PTFE particles are rejected from the contact even down to very low speeds. below those attainable in the current study. If this were the case then the particles will simply not pass through EHD contacts. It has been suggested previously that the mechanism by which particles are drawn into contacts involves their being trapped by friction forces between the converging solid surfaces (11). Very small PTFE particles would both have low friction and would, by virtue of their small size, have to penetrate the inlet a long way before becoming trapped. Thus their almost complete rejection Erom the contact could easily be envisaged. On the other hand, EHD film thickness measurements do show a small enhancement of film thickness due to the PTFE at very slow speeds, suggesting that particles may pass through the contact under these conditions. An alternative explanation for the inability of very small PTFE particles to reduce friction in this study is that the particles may pass through the contact but not support a signifkant propoxtion of the applied load in the process. It should be noted that the friction in a contact derives from the integral, over the whole contact area,of the ratio of
local shear stress to local supported load. In the EHD imaging work it was seen for the large PTFE particles, that the solid surfaces deformed around the particles to fom localised elastic indentations of about 50 nm in depth. The total Hertzian flattening under these ConditioILS is about 2 p. Thus the proportion of the load supported by even these large FTFE particles will be quite small. If tiny, 75 nm diameter, particles pass through the contact, the amount of elastic deformation they will produce, and thus the proportion of the load supported will be very small. Thus they would be expected to cause only a tiny decrease in overall friction. In summary, it is no use having an easily sheared region of the contact if this does not support any of the load. If this is the origin of the lack of effectiveness of small PTFE particles in this study, then it supports the contention made by previous workers that PTFE particles may be more effective in “area contacts” (22). Such contacts will have much lower contact pressures than those in the current study and in such systems, the proportion of the load supported by deforming PTFE particles may become much more significant. In consequence the particles may contribute to reduced friction and wear in such systems. 7. CONCLUSIONS A study has been made of the behaviour of dispersed PTFE particles in high pressure, boundary and EHD contact conditions. It has been found that large, micron-sized PTFE particles appear to reduce friction and wear in reciprocating tests. These particles also reduce friction in slow speed, thin film, mixed sliding/rolling conditions. Optical studies show that these large particles are able to pass through slow speed, rolling contacts where they cause significant elastic deformation of the solid surfaces. The reduction in friction and wear can be ascribed to this behaviour. There is no evidence of the particles adhering m n g l y to the rubbing surfaces to form a pennanent coating. Very small PTFE particles in fully-formulated oil, appear to make no measurable con~butionto friction and wear reduction. This may be because
151 the particles are unable to enter the contact. Alternatively, they may be so small and weak that in passing through the contact they are unable to bear a significant fraction of the applied load. In such a case they would not be expected to reduce friction or wear to any useful extent.
WFERENCES 1.
2. 3. 4.
5.
16.
7.
8.
Saunders, J. “Slick 50 Breaks Boundaries”, Lubricants World, pp. 27-31, May 1995. Winfield, B., “Oil Additives: the Pitch is Slick but do they Work?”, letter to Car and Driver, p. 23, March 1994. Smith E.A. “Colloidal Graphite in Assembly Lubrication”, Engineering 165, pp. 505-507, (1948). Middleton, K. “A Comparative Examination of Some Potential Inorganic Lubricants on the Shell Four-Ball Tester and on a CrossedCyliider Wear Machine”, Paper 7, I. Mech. E. Lubr. and Wear Convention, May 1964. Arizmendi, L., Palacios, J.M. and de la Cruz M., “A Very Ememe-Pressure non-Lamellar Additive for Special Mechanical Designs, Proc.1.Mech.E. C2%/73,pp. 307-311, (1973). Cusano, C. and Goglia, P.R. “Surface Roughness Effects with Solid Lubricants Dispersed in Mineral Oils”, ASLE Trans. 27. pp. 227-236, (1984). Groszek, A.J. and Witheridge, R.E. “Surface Properties and Lubricating Action of Graphite and MoSP, ASLE Trans.14.pp. 254- 266, (1971). C.Cusano and H. E. Sliney “Dynamics of Solid Dispersions in Oil During the Lubrication of Point Contacts, Part I Graphite.” ASLE Trans. 25.pp.183-189, (1981). Cusano,C. and Sliney, H. E. “Dynamicsof Solid Dispersions in Oil During the Lubrication of Point Contacts,Part II Molybdenum Disulphide.” ASLE Trans.25. pp. 190-197, (1981).
-
9.
-
10. Wan, G.T.Y. and Spikes, H. A., “Two Phase Lubricants in Elastohydrodynamic Contacts Graphite in Oil, Dispersions”, Proc. 12th Leeds-Lyon Symposium on Tribology, Lyon Sept. 1985, “Mechauismsand Surface Distress”, ed. D Dowson et al. ,publ. Butterworths, 1986. 11. Wan,G.T.Y. and Spikes, H.A. “The Behaviour of Suspended Solid Particles in Rolling Sliding Elastohydrodynamic Contacts” STLE Trans. pp.12-24, (1987). 12. H O W ,R. “Lubrication Mechanisms of Solid Lubricants in Oils”, ASLE Trans. 18. pp. 263-269, (1975). 13. Bartz, W.J. and Muller, K. “Investigationsof the Lubricating Effectiveness of Molybdenum Disulphide”, Wear-02 pp. 377-379, (1972). 14. Scott, D. and Jamieson, D. “Molybdenum Disulphide as a Lubricant Additive in Rolling Contact a Study of Compatibility with Other Additives, J. Inst. Pet. 48.pp. 91-97, (1962). 15. Rolek, R.J., Cusano, C.and Sliiey, “The Influence of Temperature on the Lubricating Effectiveness Of MoS2Dispersed In M i n d Oils” ASLE Tran~.28.pp. 493502, (1984). 16. Reick, F., “Lubricating Oil with Fluorocarbon Additives”, US Patent 3933656, Jan., (1976). 17. Gutman, M. and Stotter, A. “The Influence of Oil Additives on Engine Friction and Fuel Consumption” Lubr. Eng. -4l pp. 150-154, (1985). 18. Wilson, B. ”PTFEas a Friction Modifier in Engine Oil” Ind. Lubr. and Trib. 44,pp. 35, (1992) 19. Shaub, H., Pandosh, J., Searle, A. and Sprague, S. “Engine Durability, Emissions and Fuel Economy Studies with Special Boundary Lubricant Chemistry”,SAE 941983, (1994). 20. “(Polytctraflwmethylene)Lubricant Saves Energy” Wire Tech, 10.p.20, (1982). 21. Dehlsen, J. and Ferguson, J., “Multipurpose -Lubricant“, Tribolog~Int. p ~282-285, . (1979).
a
-
152 22. Reick, F.G.“Energy-savingLubricants Containing Colloidal FTFE”,Lub. Eng. pp. 635-646.(1982). 23. “Fluoroglide Fluompolymer Lubricants, a Guide to: Applications, Properties and Processing”, 1C1 Fluoropolymers, Blackpool, Lancashire, UK, 1992. 24. Horsmans, J. “NewDevelopments in FTFE Additives for Lubricating Oils”, Proc. 5th Int. Colloq., Esslingen, Jan. 1986,pp 6.5.14.5.8, ed. W.J. Bartz. 25. Li L.C., Yang, M.S. and Ling, Q.Z. “Chemical Structure Characterisationof the Boundary Lubrication Film Using X-ray Photoelectron Spectroscopy and Scanning Auger Microscope Probes”. Wear 140.345357. (1990).
a,
26. Stewart, C.W. Letter in response to Mr B Winfield’s article in Car and Driver, March 1994,Du Pont Central Research, P.O.Box 80323,Wilmington, DE,Feb. 18th 1994. 27. Johnston, G.J, Wayte, R.and Spikes, H.A. “The Measurement and Study of Very Thin Films In Concentrated Contacts” Trib. Trans., 34,pp.187-194,(1991). 28. & , I P.M., Hutchinson, J. aud Spikes,H.A. “The Development of a Spacer Layer Imaging Method (SLIM) for Mapping Elastohydmdynamic Contacts”. To be presented at STLWASME Joint Meeting, (1995).