compositional changes in lubricated sliding metal surfaces related to seizure

Compositional Changes in Lubricated Sliding Metal Surfaces Related to Seizure

Erik Johansson and Sture Hogmark Materials Science, Institute of Technol- ogy, Uppsala University, Uppsala, Sweden, and Helen Nilsson and Per Redelius Nynas Industry AB, Nyniishamn, Sweden

This paper presents an investigation into the mechanisms of failure in lubricated sliding metal contacts. Reciprocated sliding with cylinder-on-disc geometry was performed with three types of lubricants based on polyalphaolefin (PAO) oils and three sets of additives. The normal force and sliding speed were chosen to give partial scuffing or seizure within a few hours. The chemical surface films which form through reactions between additives in the lubricants and the metal surfaces were analysed by scanning electron microscopy and Auger electron spectroscopy, before and after the onset of seizure.

I t is concluded that all three oils formed a rigid surface film as a result of a combination of chemical and mechanical actions in the contact surface. Seizure was initiated by mechanical fatigue and disruption of the film which exposed the metal surfaces to severe scuffing. I t was also noticed that different additives gave diferent friction and wear properties to the contact system.

KEYWORDS: surfaces, sliding contact, polyalphaolefin, additives, friction, wear, jilm, AES, W D P , hot spots, oxidalion, seizure

INTRODUCTION

Seizure of tribological contacts is often associated with an unforeseen and costly failure of the whole tribological system. It is often preceded by a relatively smooth sliding with negligible wear. The onset of seizure will suddenly increase both friction and wear and the tribological system will soon be out of action, which may cause a costly loss of production and serious damage to ex- pensive machinery. This change in sliding behaviour is often described as a transition from a partial elastohydrodynamic regime to a regime of boundary lubrication and localised seizure (zones I and I1 respectively, of Figure l l ) .

The present study has concentrated on the formation, composition, and action of surface layers in sliding metal contacts in the presence of lubricants

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266 Johansson and Hogmark JSL 8-4

Figure 1 Representation of different sliding regimes in a normal force (F,) vs sliding speed (v) diagram'

A1 A2

A3

"+

containing antiwear additives. For some additives it is relatively well under- stood from previous investigations how these layers are formed by chemical reactions between the metal surface and constituents in the l~bricant.2~ It is also well documented that certain additives have a remarkable effect in reduc- ing wear and the risk of seizure in sliding contacts during conditions of mixed or boundary lubrication.3 However, little work has been done to evaluate the detailed mechanisms of protective action or break down of these films in con- nection with sudden failure in the sliding contact.

EXPERIMENTAL

Tribo testing

Testing was performed using the reciprocating sliding motion of a Plint and Partner High Frequency Friction Machine (HFFM TE 77). A cylinder-on-disc test geometry was used and the contact area was submerged in oil. The friction force, and temperature of the oil bath, were continuously recorded.

A constant amplitude and frequency of sliding, 2.74 mm and 40 Hz respectively, were used in all tests. A normal force of 450 N was gradually ap- plied in steps of 20 N every sccond minute to achieve a gentle running-in of

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JSL 8-1 Changes in sliding metal surfaces 267

the contact surfaces. The temperature of the oil bath was kept at 90°C except for one lubricant, which was also tested at 5OoC and 1 10°C. With these parameters, seizure occurred within a reasonable test duration.

Each test combination was intempted twice, at different stages represent- ing mild sliding and onset of seizure. The latter was accomplished by automat- ic test intemption at a preset value of the friction force. Two tests of each combination were run to seizure.

The wear volume and contact pressure of each test cylinder were obtained from the width of the wear scars.

Test materials

The test cylinders (0 6 mm) and discs were conventional roller bearing rolls of ball bearing steel AISI 52100. The discs were used as received (quenched and tempered to Vickers hardness (HV 820) whereas the cylinders were soft annealed (7OO0C, 1 h in vacuum to HV 240).

Three types of synthetic polyalphaolefin (PAO) based oils, all with a viscosity of 8.1 cSt at 100"C, were used in the test to represent two idealised oils with well characterised additives and one commercial lubricant:

Oil A contained 0.7% zinc dialkyl dithio phosphate. (ZDDP), a well-known and widely used additive for improved antiwear and antioxidative properties. This additive is not described by a unique formula. It is rather a mixture of n- and iso-short chain alcohols (C4-C5).

Oil B contained 2% di tert-nonyl pentasulphide (TNPS) which represents a group of so-called extreme pressure (EP) additives. The important element in TNPS is active sulphur.

Oil C was a fully formulated transmission oil containing many different additives, for example, antioxidants, corrosion inhibitors, EP additives, seal swell additives, etc. the antiwear and EP additives were in contrast to the additives in A and B, based on phosphorous chemistry.

Surface analysis

The test surfaces were investigated by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to obtain information about the topography and elemental composition, respectively. Depth profiling was performed by successive ion beam etching and semi-quandtative Auger analysis.

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268 Johansson and Hogmark JSL 8 4

Table 1 Numerical values of test parameters and test results

Oil Temp. I"C1

A 50 A 90

A 110

B 90

C 90

525 525 0.08 0.15 105 177 0.08 0.10

86 0.1 6 105 120 0.08 0.10

147 0.15 43 75 0.03 0.06

100 0.05 110 110 0.07 0.08

Con tact Wear pressure

0.022 0.019 147 0.012 0.023 180

0.021 0.017 0.018 161

0.01 5 0.153 0.346 77

0.485 0.079 0.068 96

(2)

154 144 150 156 167 59 53

101

RESULTS AND COMMENTS Tribological properties

Table 1 summarises the test parameters and experimental results. One set of experiments was performed with interruption before seizure (indicated by (1) in the table) and two sets with interruption after seizure (2).

As expected, the time to seizure varies substantially, which makes it dif- ficult to quantify the antiwear and antiseizure properties of the oils investigated. This matter was not the main topic of this investigation. One clear observation is, however, that oil A at 50°C gave the best performance as to the time to seizure, most likely due to a relatively high viscosity at this low temperature. It performed less well at 90°C and 1 10°C. It is also obvious that oil A gave a better protection against wear of the cylinder than did oils B and C though it had the highest friction coefficient. This is probably owing to the wear protection mechanism of the ZDDP additive.

For oil A it is seen that the volume of the wear scar is almost independent of the time to test intenuption. The explanation is that after a relatively short running in phase, during which most of the wear occurs, a strong surface film forms by chemical reactions between the additives and the metaL4 With oils B and C the wear rate before seizure is more continuous.

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Changes in sliding metal surfaces

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Figure 2 Friction recording for tests with oil A at 9 O O C . Normal force at seizure 450N

“1

. 60

. 50

- 40

- 30

The sulphur based additive in oil B decomposes to a highly reactive form in the sliding contact, and the wear mechanism is a combination of chemical and mechanical wear. The resulting surface is much smoother than with the ZDDP additive. This is the reason for a high wear rate but a low friction rate for oil B. The chemical reaction mechanism of oil C, a highly complex commercial oil, was not examined. Figure 2 shows a representative friction recording from a test of oil A which was interrupted just after seizure commenced. The positions for interruption before and after seizure during this investigation are indicated by (1) and (2) in the diagram.

Figure 3 SEM micrographs of surface films formed on the cylindrical test piece: (a) oil A, 50°C, before seizure, (b) oil C after seizure. Sliding direction: horizontal

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Figure 4 Oil A, 5O"C, before seizure, cylindrical test piece

Surface film

It is evident that all three oils form some kind of reaction surface film, see Fig- ure 3, which shows two examples from the cylindrical test piece. The film ap- pears to be most pronounced for oil A where it even extends outside the nominal contact area. Oil B gave a surface film which in topography resem- bles that of oil C.

Figure 5 AES depth profile from the cylindrical test piece tested with oil A (P and Zn based additive, ZDDP) at 50°C. This profile corresponds

to the micrograph shown in Figures 3(a) and 4

V ._ E 9 a

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JSL 8-4 Changes in sliding metal surfaces

Figures 6-9 Oil B, 90°C, after seizure, cylindrical test piece

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Film thickness

Thickness measurements were taken in a Cambridge Stereoscan 360 scanning electron microscope. Together with X-ray microanalysis (EDS) and AES depth profiles it was possible to show that these films exist for both additives

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Figure 10 EDS spectrum no. 1 from the cylindrical test piece, tested with oil B (sulphur based additive, TNPS) at 90°C. This corresponds to the micrographs

shown in Figures 8 and 9, taken from within the track Counts (% VFS)

(i.e. oils A and B) and that the thickness was in the range of 0.2-0.3 p.m. From Figures 3(a) and 4 (an overview of the wear surface) the existence

of a surface film formed on the cylindrical test piece under the test with oil A, 50°C can be confirmed. The corresponding AES depth profile (Figure 5) shows that this film contains elements from the additive, in this case, Zn, P

Figure 11 EDS spectrum no. 2 from the cylindrical test piece, tested with oil B (sulphur based additive, TNPS) at 9OOC. This corresponds to beside the track

shown in the micrographs, Figures 8 and 9

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JSL 8 4 Changes in sliding metal surfaces 2 73

and S. Figures 6-9 show the existence of such a film even for the oil containing the sulphur-based additive, TNPS (oil B). Figures 8 and 9 show the film after a scratch was made in it by a razor blade. EDS spectrum no. 1 (Figure 10) was taken from the film just beside the scratch.

The presence of sulphur in the film is apparent. EDS spectrum no. 2 (Fig- ure 11) is taken from inside the scratch and shows no sulphur. Thus, a surface film containing additive elements is present. The corresponding AES profiles can be seen in Figure 12. Figure 13 is a representative measurement of the film thickness, at the edge of the scratch. This film is formed with oil B. It is clear that the thickness is in the range of 0.2-0.3 pn, which corresponds with the reference literat~re.3~~

The EHD film thickness in the contact was calculated, together with h value of our test, as follows:5

The EHD film thickness h,, is calculated from:

where

a is the viscosity-pressure coefficient E' is the elastic modulus of the test materials q, is the dynamic viscosity u is the maximum sliding velocity R is the radius of the test cylinder P is the Herzian pressure

The relevant values of the above parameters give h,, = 0.09 pm, which gives a h value of

hmin h = = 0.3

where Rpl pieces.

EHD lubrication are not fulfilled.

R , = 0.2 pn are the surface roughness values of the two test

The low h-value clearly demonstrates that the necessary conditions for

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Figure 12 AES depth profiles from wear surfaces (disc (a), cylinder (b)) after tests with oil B interrupted after seizure

Sputtering time (min)

Composition of surface film


The semi-quantitative depth distribution of the elements P, S, C, 0, Fe, and Zn, was obtained from both cylinder and disc wear surfaces by AES. No sen- ous attempt has been made to deconvolute the depth scale, but a very rough estimate is that a surface layer of 100 8, is removed each minute. Figure 14 shows typical AES profiles of oil A, 90°C. The following observations are significant:

The surface film is thicker on the wear surface of the cylinder than on the disc.

The elements 0, S, P, and Zn are present within the whole film thickness with decreasing concentrations for the elements in the listed order. The Fe signal is complementary to the sum of the others.

P and Zn have similar depth distributions. No significant differences in composition are observed between the profiles

of the layers before and after seizure.

A chemical shift of the LVV signal for P indicates that this element is present in the form of zinc phosphate, which concurs with the observations of Spedding and Watkins.z (LVV define the type of (Auger) electrons analysed.

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JSL 8 4 Changes in sliding metal surfaces

Figure 13 Oil B, 90°C, after seizure, cylindrical test piece

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In the case of PLVV the phosphorous atoms are ionised by the creation of a vacancy in the L shell. The atom returns to the ground state by filling this vacancy with a valens (V) electron and subsequently emitting another V-electron by the Auger process.) The shift of the Fe MVV signal indicates the presence of iron oxide. Iron sulphide has also been reported to occur in this type of chemical layer.z6

Tests with oil A at 50°C and 110°C gave the same characteristic film composition as the tests at 90°C.

The significant observations from AES profiles of tests with oil B (see Figure 12) are listed below:

S dominates the content of the layer on the cylinder as is to be expected giv- en the additive chemistry.

S and 0 together dominate the layer on the disc, while the 0 signal for the cylinder layer drops rapidly with depth.

The layers appear to be of equivalent thickness on both the cylinder and the disc.

No remarkable differences were seen when comparing profiles obtained from samples interrupted before and after seizure.

Finally, two AES diagrams from experiments with oil C are shown in Figure 15:

The thickness of the layer is about the same on cylinder and disc 0 is the dominating element in the chemical layer, and P is the active element in the additives and this is reflected in its depth profile.

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Figure 14 AES depth profiles from wear surfaces after tests with oil A, 90°C. Disc surface, before seizure (a); cylindrical surface before seizure (b); disc sur-

face, after seizure (c); and cylindrical surface, after seizure (d)

80

70

....'" 1.._ .._._....._......../" - I- ...

Sputtering time (min) Sputtering time (min)

A general observation from all the AES profiles is that carbon is present in high concentration close to the external surface. This is a result of the ad- sorption of hydrocarbons during sample handing after the test. The carbon signal from the interior of the layer as in Figures 14(d), 12 (b), and 15 (a) is probably due to the presence of residual oil, which may be entrapped in cavi- ties in the chemical film.

DISCUSSION

All three oils cause the formation of surface films in the contact area. The mechanism of formation is a combination of chemical and mechanical reactions where the hot spot temperature at the contacting asperities plays a deci- sive role.

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Figure 15 AES depth profiles from wear surfaces (disc (a), cylinder (b)) after tests with oil C interrupted after seizure


Seizure is associated with mechanical fatigue and accelerated break up of the layer on the cylinder surface. It is initiated in local areas where film adhe- sion and mechanical strength are insufficient, see Figure 3(b).

In oil A, ZDDP decomposes to form zinc phosphate as a result of the high hot spot temperatures.’ At the same time, oxidation and sulphidation of iron occurs in the hot spot areas. During the reciprocated rubbing between cylinder and disc these reaction products are frequently loosened and subsequently joined and squeezed into protective layers. The fact that the layer was thicker on the cylinder may be a result of its being in constant frictional contact and thereby reaching a higher average surface temperature. The cylinder surface would thus be more prone to chemical reactions and film formation than the disc surface. Another hypothesis is that for geometrical reasons the cylinder surface is more inclined to pick up reaction products than the disc.

Protection of the wear surfaces by this reaction film is most effective with oil A yet at the same time this oil gave the highest friction. The latter is not primarily determined by its higher viscosity, since the oil temperature did not influence the friction of oil A.

The sulphides of oil B decomposed to form very reactive constituents owing to the thermo-mechanical action of the colliding asperities. Reactions between these products and iron form iron sulphide, see Figure 12. Oxidation of iron also occurs at the hot spot areas. The fact that the layers on the cylinder

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and the disc are of comparable thickness supports the mechanical development of the layers described for oil A. If the layer growth was determined by time and temperature only, there would be much thicker layers on the cylinder which naturally reaches a higher average surface temperature and is exposed to mechanical contact roughly 10 times longer than the disc.

Two interesting conclusions can be drawn from the 0 and S profiles of Figure 12.

S is more reactive on the cylinder than 0, as can be seen from its higher concentration and deeper distribution.

The reactive layer on the disc is formed by a transfer of fragments from the top part of the cylinder layer which has roughly the same composition as the entire disc layer.

The relatively large wear scar on the cylinder shows that the chemical products on the cylinder are fairly easy to remove, which indicates a high 'cor- rosive' wear rate with oil B. Note that the coefficient of friction is still very low with this oil. Thus, iron sulphide acts as a good solid lubricant but, at the same time, the sulphidation of iron yields a high wear rate.

Oil C is too complex to allow an explanation of its chemical reaction mechanisms. A mechanically aided film formation mechanism is likely to prevail also with this oil, as is the fatigue mechanism for film disruption, see Figure 3(b).

CONCLUSIONS

There is a sudden transition from sliding in a mild wear regime to a state of localised seizure, which typically occurs after about one hour of testing at constant load. Chemical interaction between the oil additives and the metal surface creates reaction products which by mechanical action on the contact surface create a solid film. The onset of seizure is correlated to the fatigue of this chemical film rather than to gradual wear or changes in its chemical composition.

ZDDP has the advantage of forming relatively stable surface films which result in low wear and seizure ability but relatively high friction, whereas the TNPS additive proved to have the opposite effect. The fully formulated oil displayed friction, wear, and seizure properties in between the two oils with isolated additives.

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JSL 8-4 Changes in sliding metal surfaces 279

ACKNOWLEDGEMENT

The Swedish Board for Technical Development is gratefully acknowledged for its financial support of this study.

REFERENCES

1.

2.

3.

4.

5. 6.

7.

Solomon, G., 'Failure criteria in thin film lubrication - the IRG program', Wear, 36, 1-6 (1976). Spedding, H.. and Watkins, R.C., 'Antiwear mechanisms of ZDDPs', Tribology Int. , Feb.

Kapsa, Ph., and Martin, J.M., 'Boundary lubricant films: a review', Tribology Int. , Feb. 1982,3742. Alliston-Greiner, A.F.. Greenwood, J.A., and Cameron, A.. The Theology of reaction films formed by ZDDP, Proc. Leeds-Lyon Symp. 1987,161-87. Klamann, D., Lubricants and relatedproducts, Verlag Chemie, Weinheim (1984). Debies, T.P., and Martin, J.M., 'Surface chemistry of some antiwear additives as determined by electron spectroscopy', ASLE Trans., 23.3.289-97 (1979). Jones, R.B., and Coy, R.C., The chemistry of the thermal degradation of ZDDP additives', ASLETrans., 24, 1,91-7 (1980).

1982,9-15.

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compositional changes in lubricated sliding metal surfaces related to seizure

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