ACTAUNIVERSITATISUPSALIENSISUPPSALA2006

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 247

On the Formation of Low-FrictionTribofilms in Me-DLC – SteelSliding Contacts

NILS STAVLID

ISSN 1651-6214ISBN 91-554-6743-1urn:nbn:se:uu:diva-7369

Non, je ne regrette rien

List of papers

I Combination of DLC Coatings and EP Additives for Improved Tri-bological Behaviour of Boundary Lubricated Surfaces, B. Podgornik, D. Hren, J. Vižintin, S. Jacobson, N. Stavlid and S. Hogmark, Wear 261 (2006) 32. (Reprinted with permission from El-sevier)

II Tribolytic phenomena in boundary lubricated WC/C – steel contact,N. Stavlid and U. Wiklund, submitted to Tribology International.

III Extreme pressure tribofilm formation –Compatibility between Me-DLC coatings and lubricant containing sulphur additive, N. Stavlid, E. Coronel and U. Wiklund, submitted to Tribology Letters.

IV Influnce of the metal in Me-DLC materials on the formation of low-friction sulfide tribofilms, N. Stavlid, B. André, F. Svahn, M. Boman and U. Wiklund, in manuscript.

V On the potential of different tungsten substances to yield low friction between lubricated steel surfaces, N. Stavlid, F. Svahn, M. Boman and U. Wiklund, in manuscript.

Contributions

Paper Planning of experiments

Experimentalwork

Evaluation & analysis

Manuscript & writing

I 1 1 2 1

II 3 3 3 3

III 3 3 2 3

IV 3 2 2 3

V 3 2 2 3

1 = minor part, 3 = major part

List of published papers not includedin this thesis

I Adhesion of selected PM tools and PVD/CVD coated tool steels in sliding contact with stainless steel I. Heikkila, N. Stavlid and S. Hog-mark, Advances in Powder Metallurgy Particulate Materials (2003) 963.

II Evaporated vanadium nitride as a friction material in dry sliding against stainless steel, U. Wiklund, B. Casas and N. Stavlid, Wear 261(1) (2006) 2.

III The influence of alkali-degreasing on the chemical composition of hot-dip galvanized steel surfaces, R. Berger, U. Bexell, N. Stavlid, T. M. Grekh, Surface and Interface Analysis 38(7) (2006) 1130.

Contents

Part I - Background to the field.......................................................................9Boundary lubricated sliding contacts .......................................................12

Tribological test methods ....................................................................13Reactive lubricant additives .....................................................................15

Sulfur containing extreme pressure additives......................................17Low friction carbon based PVD coatings.................................................18

Thin film processes..............................................................................18Electron Spectroscopy for Chemical Analysis – ESCA...........................20

ESCA on tribofilms .............................................................................24

Part II - Contributions to the field.................................................................27The discovery of a new class of low-friction tribofilms...........................27Pinpointing slipperiness ...........................................................................29Exotic candidates......................................................................................35The driving force of low friction..............................................................41

Summary and conclusions ............................................................................45

Sammanfattning på svenska..........................................................................48

Acknowledgements.......................................................................................49

References.....................................................................................................50

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Part I - Background to the field

To begin with I would like to have some discourses about the origin and nature of tribofilms. The concept of tribofilm stems from the word tribology, which in turn originates from the Greek word tribos meaning rubbing. Hav-ing two surfaces in contact and in relative motion cause a rubbing effect on the surfaces. In this process we often can detect a thin film the surfaces, that is composed of a different chemistry than the original surfaces, hence the concept tribofilm.

The extreme local stresses and increased local temperatures in tribological contacts facilitate the chemical reaction routes for tribofilm formation. We recognise the force acting against the line of movement as the friction force. The friction force is dependent on the shearing forces required to shear the weakest material in the contact. Since the shearing occurs in the outermost part of the surface the friction force is largely dependent on the properties of the thin tribofilms.

Tribofilms are often called wear transfer films, reaction layers etc. and are studied within materials science and tribology. When the two surfaces in contact are similar in composition the contact is said to be homogenous and if they are composed differently it is called heterogeneous. For the evalua-tion of the resulting tribofilm composition the heterogeneous contacts are somewhat more difficult to study due to the increasing reaction possibilities. Generally, the greater number of compounds in the surfaces and in the envi-ronment, the harder it is to foresee the possible tribofilms.

Traditionally mechanical components such as gears, rolling bearings, camshafts and others have mainly been manufactured in steel. This is of course since the achievable mechanical properties contra the cost of material and manufacturing were in favour for iron-based materials. For the lubricant industry this was good since the lubricant additives could be focused to func-tion well for “only one” material, not forgetting bronzes and other Cu-based materials. In the lubricant oil industry additives serves to improve a vast variety of features. The list of different types of additives is long, including among others: Antioxidants, Detergents, Dispersants, Friction Modifiers, Anti-Wear and Extreme Pressure additives. The additives that are intended to operate when the lubrication is poor, i.e. when component surfaces remain in sliding contact, is the Anti-Wear (AW) and Extreme Pressure (EP) addi-tives.

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As understood from their respective names, the AW additives are aimed to lower the wear rate by forming a tribofilm that is though enough to last in the contact, protecting the underlying component material. Not necessarily should it facilitate low friction. EP additives are activated in situations when the local pressure is high enough to cause galling, i.e. when micro welding causes seizure during the motion. It is obvious that such cases are the least desired in mechanical components. Thus the EP additives form tribofilms possessing easy shearing properties inducing a lowering in the friction force. However, as they are very reactive they might also increase the wear rate of the components.

With increasing demands on environmental and mechanical properties, as well as tough constraints on weight and cost, engineers are nowadays de-signing heterogeneous contacts more frequently than before. Examples in-clude the introduction of aluminium based materials in cylinder linings, hard chrome plated piston rings and carbon-based coatings on cam followers. The two latter are examples of surface treatments in the form of thin coatings. The improved tribological properties of such coatings can reduce the amount of lubricant needed and hence lower viscous losses. Increased power output and lowered fuel consumption are among the other benefits.

Thin film coatings are nowadays utilised in many different types of appli-cations ranging from nanometer-scale diffusion barriers in semi-conductors to tough wear resistant coatings made for cutting tools.

Carbon-based coatings are among the most promising surface treatments to reduce friction and wear in sliding contacts. Without going into depth I will here mention some important different types of carbon coatings, the first being the pure carbon coatings that are free from other elements. Such coat-ings can be produced in a variety of ways where the two most common are Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD). The CVD method is often a high temperature process, above 300˚C.The high temperature associated with the CVD method is unfavourable for steel components as they become soft annealed at elevated temperatures. The PVD process I will treat in this thesis is mainly sputtering. Sputtering is a low temperature process, less than 250˚C, and is therefore appropriate for coating of steel components.

Carbon can be found in two major forms in nature, namely graphite and diamond. CVD carbon coatings can be made to almost entirely consist of diamond bonds. The PVD carbon coatings is often a mixture of both graphite and diamond bonds and hence, more or less justified, such coatings are often referred to as Diamond-Like-Carbon or DLC.

Such carbon coatings can function very well in dry sliding or when lubri-cation is minimal. Due to the extreme high hardness of pure carbon coatings they often reduce the friction forces by decreasing the ploughing component of the friction. The drawback of pure carbon coatings is that they are more

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prone to cracking and spalling. That is because the high pressure localized to individual surface peaks is causing cyclic fatigue.

One remedy for this type of cyclic fatigue is running-in. Running–in of component surfaces mainly involves a flattening and smoothening of the surface topography. After the running-in the surfaces are smoother and thereby lubrication is enhanced and wear is decreased. Remembering the advice to drive your new car gently for the first couple of thousand miles, that is what running-in time is. A quick and smooth, yet predictable, run-ning-in procedure is highly sought for. The extreme material properties ex-hibited by diamond and pure DLC are generally not in line with such de-mands. However, the addition of metal atoms into the carbon matrix cause great changes in chemical and tribological properties of DLC coatings; such metal alloyed coatings are often denoted Me-DLC ‘s. The hardness is for instance affected when incorporating metals into the carbon matrix and the ability to react with the environment is improved. Both of these effects make the running-in of the coated components easier [1].

Today Me-DLC coatings are used in many situations requiring good wear resistance as well as low friction. In engines we have mentioned the cam follower as one example. In some respect the application of the Me-DLC on the cam follower has reduced the need for lubricants. However, the lubricant serves also other purposes in the engine, such as cooling. Further, the auto-motive industry will not have lubricants removed all together. It is therefore important to remember that the lubricant was mainly developed to function for the steel-steel contacts. EP additives are by far the most reactive and frequently used additive type in the areas where also Me-DLC coatings are finding their usage. The simultaneous use of highly reactive lubricant addi-tives and different alloying metals in the carbon coatings is hence a question of compatibility. This is the subject for my scientific investigations in this thesis.

As will become obvious from reading this thesis there are both drawbacks and promising features of this new type of heterogeneous contact situations. The type of tribology research behind this thesis is complex by nature, neighbouring mechanical engineering, materials chemistry, physical chemis-try and nanotechnology among others.

All in all one could claim that modern tribology research is the decathlon of materials science!

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Boundary lubricated sliding contactsAs mentioned, this thesis deals with the tribology, or mainly tribochemistry, of certain coatings and lubricant additives. This first part of the thesis is aimed at the tribology of mechanical components seen with more traditional eyes. The different types of lubricant additives are dealt with in the next chapter.

The overall development in friction and wear properties of mechanical components has mainly been focused towards improved lubrication and the most sought for lubrication is often the full film lubrication. That is when the solid surfaces are completely separated by a lubricant film, minimizing wear and governing low friction coefficient, µ. The definition of the coefficient of friction, µ, is defined as the ratio between the friction force, also called the transverse force FT, and the normal load, FN.

µ = FT / FN

Many studies are undergone in the subject of lubrication and the first to classify the friction coefficient in different lubrication regimes was Stribeck [2]. His work on plain- and roller bearings resulted in the well-known Stribeck curve shown in Fig. 1. This typical curve relates the coefficient of friction as a function of viscosity times sliding speed divided by the normal load for sliding contacts.

Fric

tion

coef

ficie

nt

Bo ndu ary-Lubrication

Mixed

lubrication

Full film lubrication

Viscosity * speed / loadFigure 1.The typical Stribeck curves relating coefficient of friction to lubricant vis-cosity, sliding speed and normal load. The upper curve represents a rougher surface and the lower smoother surfaces.

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We see that in the case of decreased sliding speed and/or increased load the friction increases as we go from full film lubrication towards boundary lu-brication. In boundary lubrication the surface asperities are in contact and possibly being sheared, leading to increased friction coefficient as well as increased wear. The friction level in the boundary-lubricated regime is very little influenced by lubricant viscosity but more on the shearing properties of the solid surfaces and tribofilms in contact. The intermediate mixed lubrica-tion regime possesses a low coefficient of friction since both the viscous loss and the amount of asperity shearing is limited.

Most machine elements are designed to operate in full film lubrication. However, each time such equipment is started or stopped the surfaces will travel the Stribeck curve from zero speed to operational speed. Hence, the friction and wear properties in the boundary-lubricated regime are of great industrial interest.

Finally it should be mentioned that the tribology of mechanical compo-nents is largely affected by the surface topography. In general the smoother the surface is the less wear will occur and the easier it is to reach full film lubrication. There are of course practical limitations as smoothness costs in achieving for example through fine turning, grinding and polishing. A rougher and cheaper initial surface that is rapidly worn down to a smoother finish followed by a steady state in low friction and wear is in some cases optimal. The creation of solid low friction tribofilms onto the surfaces is one other possibility of running-in. The upper curve in Fig. 1 can thus represent a new component and the underlying darker curve, presenting lower friction forces in boundary-lubricated regime, is represent a system that has devel-oped a tribofilm and a smoother surface after running-in.

Tribological test methods There exist many different methods to evaluate sliding contacts. The three I have used are the load scanner, the ball-on-flat and the fretting-rig.

The load scanner device is a crossed cylinder set-up where the normal load is varied so that each point on the contact track corresponds to a certain load. The load scanner is schematically shown in Fig. 2. In this set-up both mating surfaces are experience similar conditions, i.e. each spot on the specimen has a corresponding spot on the counter surface. Also, each spe-cific position corresponds to a specific load. The sliding speed can be con-trolled up to a maximum of 65 mm/s. Generated wear debris can escape the contact zone rather easily.

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Figure 2. The reciprocating load scanner device. Two cylinders are sliding against each other with increasing load.

The perhaps most widely used tribometer is the pin-on-disc or ball-on-disc configuration shown in Fig. 3. In this set-up the ball is in constant contact whereas any point on the disc is in contact only a fraction of the time. Thus ball-on-disc can be used as model test for mechanical component surfaces that are subject to large differences in sliding contact. Wear debris can be pushed in front of the upper specimen and remain in the contact for long times.

Figure 3. Ball-on-disc tribometer. The disc is rotating under a stationary ball nor-mally with a constant load.

A useful tribometer is the fretting rig, see Fig. 4, which operates with an oscillation frequency that can be selected over a large interval. The move-ment of the lower specimen is sinusoidal back and forth and the stationary upper specimen is attached to a friction force detector. Furthermore this test could be rather more severe than the two already mentioned, as the sliding speed is zero at the turning points. Therefore this tribometer could be used to simulate wear and friction for a cylinder liner – piston ring contact like situa-tion.

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Figure 4. Central part of the fretting rig, here represented by a crossed cylinder con-figuration, but e.g. ball-on-flat is also possible.

The common denominator of the three tests is that they can be set to operate in the boundary-lubrication regime, where tribofilms are decisive for the friction coefficient. That is, they all allow high pressures and low sliding speeds to result in the conditions on the left hand side of the Stribeck curve in Fig. 1.

Reactive lubricant additives Reactive lubricant additives are used to alter the surface properties of the components. Either such additive molecules are aimed for protecting the steel surface from wear, so called Anti-Wear (AW) agents, or they are em-ployed to ensure a low friction so that no seizure might occur, referred to as Extreme-Pressure (EP) additives. Reactive lubricant additives often incorpo-rate one or more of the following elements phosphor, sulfur and chlorine. These elements are by nature very reactive. According to the Pauling Scale [3] of electro negativities of the elements they are ranked as follows:

P (2.1): S (2.5): Cl (3.0)

A higher number indicates a stronger power of an atom to attract elec-trons, thus it could be claimed to be more reactive.

The typical AW-additives are of phosphite type, where phosphor bonds to three oxygen atoms, and phosphate type, where phosphor bonds to four oxy-gen atoms. The by far most used AW-additive is the zinc dithiophosphate (ZDDP), which includes zinc, phosphate and sulfur in one molecule. The role of Zn is to react with oxygen from potentially abrasive iron oxides to form ZnO, which is softer than Fe2O3, and thereby reduce the wear from those particles. The iron that has reacted with this type of molecule becomes part of the Fe-Zn-phosphate tribofilm on the component surface, which fur-ther protects the surface [4]. A chemical study of ZDDP formed tribofilms may look like those in Fig. 5 showing ESCA spectra of two tribofilms. These particular tribofilms were formed on the cam surface of a large truck diesel

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engine. We can readily see all the expected components, with Fe from the cam, Zn, P and S from the AW/EP additive and also incorporation of Ca. The occurrence of Ca inside these tribofilms is due to that automotive lubri-cants often contain basic detergents, comprising metals such as Na and Ca, needed to neutralize acidic products created in the combustion process.

The typical EP-additives are sulfurised and chlorinated hydrocarbons. Their function is to very rapidly form iron-sulfide or iron-chloride respec-tively and thereby prevent galling and seizure. Thus reaction kinetics is of importance, which is why the more reactive sulfur and chlorine rather than phosphor are employed. The reaction products should be easy to shear. The iron sulfide, FeS, is soft and of a hexagonal close packed type where sliding and shearing along basal-planes promotes rather low friction. The iron chlo-ride, FeCl2, is of a layered structure that promotes low friction by shearing along weak-bonded layer-layer interfaces.

020040060080010001200Binding Energy [eV]

2000 minutes

200 minutes

Ca 2p3/2

at 347 eV

C 1s

O 1s

Fe 2p

Zn 2p

P 2pS 2p

OKLL

Figure 5. An example of ESCA survey scans of tribofilms formed at low contact pressure areas of cam surface run for 200 and 2000 minutes in controlled environ-ment and commercial lubricant. Un-sputtered surfaces.

There are growing environmental concerns about the usage of typical reac-tive compounds such as P, S and Cl. The development of new lubricant addi-

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tives is then aimed to decrease the amount of reactive elements without loos-ing in AW/EP-efficiency. As the highest pressure regarding regulations is put onto chlorine there will still be a future for additives that carries sulfur and phosphor as no other substitute is easily found. Furthermore, the phos-phor levels are reduced in engine oils as increasing demands are put on lower friction levels and increased fuel efficiency. This opens up more ap-plication areas for sulfur-based additives [5].

Up-to-date no special lubricant additives have been developed for contact situations that involve Me-DLC coatings and steel. This is an important fu-ture niche for lubricant or additive manufacturers as the numbers of coated components are increasing annually.

Sulfur containing extreme pressure additives The activity of sulfur containing additives is often measured using a stan-dardized test, ASTM D-1662, where copper powder and the molecules to be tested are heated to 149˚C for 1 hour. The amount of formed copper sulfide determines the activity. The activity resulting from such measurements does not say how well the additive will work in an actual application. The total performance is influenced by solubility and polarity of the molecule as well as competing and maybe synergistic function together with other additives. The pure EP-function is only one part of the additive functionality. When such adsorbed molecules are working in mild conditions their chain-structure determines the lubricity. So sulphurised additives having long-chained and branched molecules performs better than short-chained and straight mole-cules when the contact pressure is low.

As polar groups are added to a molecule it will attach to metal surfaces more readily. But on the other hand the increased polarity affects the solubil-ity negatively.

Long and straight sulfurised olefins thus have very good solubility but low polarity and thereby poor lubricity.

The simplest of all commercial sulfurised additives is the sulfurised iso-butylene, SIB, shown in Fig. 6. This product is the most used industrial and automotive gear oil EP-additive up to date. As seen from Fig. 6 the sulfur content is high, however, the molecule has only two consecutive sulfur at-oms in the chain-structure and therefore the activity is rather low. Sulfurised olefins can have 3 – 6 consecutive sulfur atoms and can thereby behave more active, i.e. weak sulfur-sulfur bonds add to the activity.

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Figure 6. Sulfurised isobutylene, SIB, EP-additive molecule.

A high activity means that the sulfur is reacting at lower temperatures, or one could say that the additive is reacting with iron at lower pressures. Therefore large amounts of active sulfur are undesirable for AW conditions as there will be too much loss of steel as the iron sulfide is formed and worn at a high rate.

A well-balanced additive can be tailor-made for many applications. How-ever, the SIB-additive is used as the model EP-additive for all the studies presented in this thesis.

Low friction carbon based PVD coatings Thin film processes

PVD – Physical Vapor Deposition A vapor of a material can be produced either by thermal heating or by physi-cally “knocking” atoms out from the surface of the material. The latter refers to as sputtering. Metal thin films can often be produced through some of the thermal heating processes. The heating can be achieved by for example in-ductive, resistive or direct heating by an electron beam or some other radia-tion.

Carbon, in the form of graphite, is a difficult material to melt and vaporise because of its exceptionally high melting point, 3700˚C, and its high heat of vaporisation. Compared to most metals, graphite requires 5-8 times more energy to be vaporised [3,6]. Pointed graphite rods pressed together causing a resistive contact can be used to create a carbon vapour. However is sputter-ing a more used PVD technique for carbon deposition at lower temperatures.

Sputtering is often performed using ionised argon atoms in a plasma. Those argon ions are attracted and accelerated towards a target due to elec-trostatic potential differences. When Ar-ions impact on the target they pos-sess enough energy to cause collision effects that force target surface atoms to escape out into the plasma. A magnetron that creates a closed magnetic field just above the target surface is also often applied. This closed magnetic field entraps electrons so that the plasma intensity will be increased and hence the sputtering yield. Magnetron sputtering can be also be used to sput-

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ter non-conducting materials if they are used in a pulsed or RF (Radio Fre-quency) mode.

The commercial W-DLC coatings (Balinit C made by Oerlikon Balzers) studied in papers I-III are produced in a reactive manner. This means that a reactive gas (acetylene) is present in the argon plasma at the same time as sputtering of a WC-target is employed. The sputtered species are then ion-ised and reacted with pure carbon from the acetylene, which is also ionised in the plasma. The hydrogen content of such coatings is rather high due to the incorporated hydrogen from the acetylene gas. The compositional changes of that coating is shown in the ESCA depth profile in Fig. 7. The content of tungsten is about a level of 23 atomic-% inside the coating. A double layer of pure sputtered WC followed by a pure chromium layer was used as interlayer, also called binder-layer, to improve the adhesion to the substrate.

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400

CarbonChromiumTungstenIron

Sputter time (min)

Figure 7. The commercial W-DLC coating’s chemical composition revealed by an ESCA depth profile. N.B. the chromium interlayer closest to steel substrate.

The chromium doped commercial coating (Graphit-iC made by Teer Coat-ings Ltd.) studied in papers II-III was sputtered non-reactively. Thereby no acetylene is used and no hydrogen is present in such coatings. Instead a solid carbon target and a pure chromium target were sputtered at the same time. The distribution of the doping metal was increased towards the outermost

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layer to a maximum of 8 atom-%. A chromium interlayer was used also for this coating, as seen in Fig. 8.

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400Sputter time (min)

Figure 8. The commercial Cr-DLC coating’s chemical composition revealed by ESCA depth profile. N.B. the chromium interlayer closest to steel substrate.

The in-house produced PVD coatings that are evaluated in paper IV were all produced in a non-reactive sputtering mode. A pure metal target and a pure carbon target were operated simultaneously. For more detailed information about PVD processes please see [6].

Electron Spectroscopy for Chemical Analysis – ESCA The ESCA-instrument is a suitable analysis tool for thin films. In ESCA photoelectrons that are emitted from the sample due to interaction with pho-ton radiation are detected. This phenomenon is called the photoelectric effect and its discovery is attributed to A. Einstein who received the Nobel Prize in Physics for this in 1921.

The analysis apparatus for studying photoelectrons and their kinetic en-ergy was invented by a research group at Uppsala University in the 1960´s [7]. The leader of that group, K. Siegbahn, also received the Nobel Prize in Physics (1981).

The photoelectric effect and analyzing equipment for detection of their kinetic energies are schematically shown in Fig. 9. The ESCA technique is also called XPS (x-ray photoelectron spectroscopy).

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If the incoming x-ray radiation has an energy h and the emitted photo-electron has a kinetic energy of EK the difference between those energies should be the binding energy, EB, if the process was completely elastic. However, a spectrometer impose a certain potential wall, called the spec-trometer work function s, which has to be overcome for the electrons to be detected. Hence the following relationship can be used to express the bind-ing energy:

EB = h – EK – s

The binding energies of electrons depend on the atom’s chemical sur-roundings. If electrons are participating in a bond together with another atom they are somewhat relocated from the original atom. However, the original nucleus still has the same positive charge so the resulting effect on the other electrons, those that not participate in bonding, will be a slightly tighter binding towards the nucleus, see Fig. 10. That is, by using ESCA it is possi-ble to observe chemical shifts in binding energies and thereby receive both elemental and chemical information.

The emission of photoelectrons and their transport to an analyser requires ultra high vacuum, at the order of 10-9 torr, to minimise the inelastic losses. Large inelastic losses would otherwise increase the observed peak widths as well as the peak positions.

Due to very strong absorption of photoelectrons inside materials, only those electrons that are emitted very close to the surface are likely to escape into vacuum. The characteristic maximum depth from which photoelectrons can be emitted is in the range 3-10 nm. Thus the ESCA technique is ex-tremely surface sensitive and therefore well suited for the study of thin films and tribofilms in particular.

To quantify elements obtained from a photoelectron spectroscopy it is necessary to adjust the observed signal, i.e. number of counts inside a peak, with a certain atomic sensitivity factor (ASF). The ASF is needed since ele-ments can participate in photoelectric processes more or less readily. For instance a large ASF means that many photoelectrons a likely to be emitted when the atom is hit by x-rays. Many of the metals have large ASF: Fe (2.7), W (3.0) and Zn (3.3). While lighter elements have smaller ASF: C (0.3), O (0.7), P (0.4) and S (0.6).

The detection limit is at the level of some atomic percent when using ESCA. Therefore ESCA is not suitable to quantify elements appearing in low concentrations. For instance it would be impossible to quantify the amount of carbon in ball bearing steels.

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+ + ++ + ++ ++ +

Photon

Photo

electr

on

+-

Detector

Figure 9. The photoelectric effect used for ESCA– an incoming photon causes the emission of a (core) photoelectron. The kinetic energy of the photoelectron is ana-lyzed using a spherical deflection analyzer (SDA).

Fe

S

Figure 10. Schematic picture of how bonding between Fe and S results in a slightly higher binding energies for those electrons that do not participate in the bonding.

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A modern energy analyzer, as the SDA in Fig. 9, can often be operated either in a “high-resolution”-mode or in a “quantifying”-mode, see Fig. 11. The difference between those modes is the potential between the spherical sur-faces as depicted in Fig. 9. If there is a low potential, only electrons with low kinetic energies will follow the dotted line and be detected. This energy is called the pass energy (P.E.). As electrons with low energies also have nar-row energy distributions, the observed peaks are smaller and thus suitable for detection of chemical shifts, see Fig. 11a. The low energy of the elec-trons makes the signal inside the detector small since it is a charge-coupled device. Long time measurements might then be used to improve the peak-to-noise ratio.

a)

0

200

400

600

800

1000

28303234363840W4f Binding Energy (eV)

11.75 eV Pass Energy

Inte

nsity

24

b)

0

2000

4000

6000

8000

1 104

1.2 104

2530354045

Inte

nsity

W4f Binding Energy (eV)

187.85 eV Pass Energy

Figure 11. Pass energy influence on signal strength and energy resolution in a) “high-resolution”-mode with P.E. 11.75 eV and b) “quantifying”-mode with P.E. 187.75 eV. The spectra are obtained from a tribotested metallic tungsten surface.

If the potential is higher between the analyzer walls, the electrons that pass through will have higher energies and thereby a broader distribution. Hence the resulting wide peaks and high signal suitable for quantification see Fig. 11b. These types of measurements are often quick and the peaks do not im-prove by a longer analysis time.

Ion guns are often attached to ESCA instruments. These guns, often oper-ated with argon ions, are used to sputter clean the sample surface to reach higher peak-to-noise ratio. The ion-guns can also be used to perform a sput-ter depth profile; such as those presented in Figs. 7-8.

ESCA on tribofilms For studying tribofilms, which often have a thickness well below 1 µm, ESCA is very suitable due to it surface sensitivity. Furthermore, tribofilms are often reaction products of the two contacting surfaces and the surround-ing environment. Hence the chemical information obtained by ESCA is needed.

Early work on tribofilms using ESCA was performed by Bird and Galvin [8], and their results and conclusions are still valid. The importance of how to clean the samples before inserting them inside the ultra high vacuum sys-

25

tem is obvious. However, it is also essential not to remove too much of the surface or of the tribofilm of interest. Ethanol flushing and subsequent dry air blowing is often enough. When having polymeric samples one should not use acetone or similar aggressive solvents. Furthermore, when analyzing polymers or samples that are composed of polymerized matter one should be aware of the breakdown of the polymeric structure that can occur when illu-minated by an x-ray. The sputtering equipment, i.e. the ion gun, should also be used with care. There is a risk that the ions cause surface damage and alter the chemistry of the surface. There is also a risk that the material is prone to selective sputtering. That is when one component is much more easily removed by the etching ions than the others. Such a difference occurs when sputtering WS2, as shown in Fig. 12.

28303234363840

1 min. sputtering10 min. sputtering

W 4f Binding Energy [eV]

metallicsulfide

160161162163164165

1 min. sputtering10 min. sputtering

S 2p binding Energy [eV]

(a) (b)

Figure 12. WS2 powder sputtered for 10 minutes exhibits a stronger metallic W peak than it did after 1 minute sputtering, (a). On their own these two spectra could be indicating either selective removal of sulfur or a chemical change of sulfide to metal and sulfur. However, the remaining sulfur is still in sulfide state, (b), thus proving selective sputtering.

Angle dependent profiling is a non-destructive depth sensitive method that can be used when the outermost part of the surface is to be analyzed. By tilting the sample surface into a grazing angle away from the analyzer one can increase the relative surface-to-bulk ratio even further. Consequently, if the sample is tilted so that the surface is facing the analyzer, the bulk atoms are dominating the spectra. The angle between the sample surface and the

26

analyzer is called the take-off-angle (TOA). An example of how such angle dependent measurements are used for surface studies is shown in Fig. 13.

Figure 13. “Bulk” (TOA 90) and “surface” (TOA 20) states for zinc in Hot-Dipped Galvanized steel being degreased with organic solvents. A mixed surface oxide is clearly seen that is diminishing inside the bulk [9].

To be able to draw firm conclusions from ESCA measurements it is advised to perform as many reference measurements on controlled materials as pos-sible. Thereby one can build up a library of peak positions that is valid for the specific instrument and a collection of calibrated atomic sensitivity fac-tors can be built up. When using published reference data one must have in mind that it could be a difference in used pass energies. A high P.E. moves the peak position to higher binding energies due to tail effects from the broader energy distribution as seen in Fig. 11. Furthermore, if possible it is desirable to use a higher resolution setting instead of performing convolution of obtained peaks. Convolution, or curve fitting, is a mathematical tool to split up a measured peak into several smaller contributions, as shown in Fig. 13.

Zn/Al-oxide Zn-metallic

27

Part II - Contributions to the field

The discovery of a new class of low-friction tribofilms In paper I, a tribological study of W-DLC – steel contacts in boundary-lubrication were performed using a lubricant comprising EP-additive. One phenomenon was worth special attention. This was the unusually low fric-tion coefficient that developed with certain amounts of EP-additive in the oil. The load scanner device and the ball-on-flat apparatus were used to evaluate the effect of EP-additive and the friction results are shown in Figs. 14 – 15.

Figure 14. Friction coefficient curves for the W-DLC - steel combination, recorded in load-scanner device [paper I]. Note the drastic friction reduction when adding 0.5 and 2.25% EP to the oil.

28

Figure 15. Typical friction coefficients for W-DLC – steel contacts using different EP-additive concentrations. Tested in a ball-on-flat configuration [paper I].

It was made clear from an ESCA analysis performed after 10000 cycles that tungsten atoms had transferred from the W-DLC coating to the worn steel surface as seen in Fig. 16.

Figure 16. ESCA depth profile of tribofilm formed on the steel ball after 10000 cycles in a lubricant containing 0.5% EP-additive.

29

The tribofilm on the worn steel surface was comprised mainly by iron, sulfur and tungsten. It was further discovered that there were only very small amounts carbon and oxygen inside the tribofilm. These findings of selective transfer of tungsten from the W-DLC coating and sulfur from the EP addi-tive to the un-coated steel counter surface were presented in paper I together with some thoughts on possible WS2 formation.

Pinpointing slipperinessTo follow up the results of paper I, the chemical states and the relative thick-ness of the tribofilms from similar load-scanner testing were evaluated in paper II. Here, the influence of number of cycles and the load on the tri-bofilm formation was studied. The studied coating was again the commercial W-DLC as in paper I.

High-resolution ESCA measurements on the tribofilms that yielded low friction were performed and compared to reference materials, see Fig. 17. It was obvious from those measurements that the solid tribofilm formed onto the uncoated steel surface was indeed composed of WS2 as well as FeS. It was concluded that a high contact pressure was required for this to happen, see Fig. 18, and it was suggested that oxidation of the carbide plays a crucial role in making tungsten react with sulfur.

WS2 is a well-known low-friction solid lubricant that exhibits low shear strength due to weak van der Waals bonding between sulfur-sulfur sheets inside the material [10].

We had thus detected a new class of tribofilm formation. The tribologi-cally induced formation of a low-friction material (WS2) on a steel surface, formed by a selective transfer of W from the W-containing carbon coating on the counter body that reacts with the sulfur in a sulfur-containing EP ad-ditive, to form a crystalline WS2 film. Note that no film forms on the coated part and that virtually no carbon (that dominates both the coating and the EP additive) is present in the film.

The WS2 films were shown to continuously reform and wear off the sur-face, until the Me-DLC coating was worn away, i.e. the W-source was emp-tied.

30

41 40 39 38 37 36 35 34 33 32 31 30

Tungsten binding energy [eV]

WS2 powdertribofilm WC/C coating

W 4f7/2

bonded to S W 4f7/2 metallic or carbidic

W 4f5/2

metallic or carbidic

W 4f5/2

bonded to S

W 5p3/2

(a)

169 168 167 166 165 164 163 162 161 160 159 158

Sulphur binding energy [eV]

S 2p3/2

bonded to WS 2p1/2

bonded to W

WS2 powder

Tribofilm

(b)

Figure 17. Individual ESCA spectra of W 4f (a) and S 2p (b) from a tribofilm pro-viding low friction. Both graphs give evidence for the formation of WS2 in the con-tact track. All spectra are taken after 1 min. of sputtering [paper II].

31

To further investigate commercial coatings (primarily low-friction compo-nent coatings), a Cr-DLC coating was included in paper [III]. Again the load-scanner device was used but in this paper also imaging of the tribofilms was aimed for. A large difference in friction coefficient between the W-DLC and the Cr-DLC coatings sliding against steel was observed, see Fig. 19.

80

85

90

95

100400N 800N 1200N

µ afte

r/ µ

befo

re(%

)

W-doped - SIBCr-doped - SIB

Figure 19. Friction reduction after 5000 cycles in the load-scanner for the W-DLC and Cr-DLC coatings against steel. Boundary lubricated conditions with PAO oil and 2% SIB additive [paper III].

0

0.1

0.2

0 2000 4000 6000Number of sliding cycles

µ

200N 550N

1100N

Figure 18. The effect of different normal loads on the friction coefficient development. Load-scanner device at a load of 200, 550 and 1100N, using PAO with 2% SIB [paper II].

32

It was not until the whole W-DLC coating was consumed at the highest load (Fig. 19) that the W-DLC and the Cr-DLC showed similar values. Then the steel substrate influences the tribofilm formation rather than the tungsten containing coating. The Cr-doped coating was almost unaffected and only a polishing type of wear could be detected. The morphologies of the tribofilms differed in that the tungsten containing one was smoother and more com-pletely covering the steel surface than the chromium containing one, as seen in Fig. 20.

a) b)

Figure 20. SEM micrographs of the tribofilms covering the steel surface run against W-DLC coating (a) and against Cr-DLC coating (b) in the load-scanner rig at 800N load. Arrows indicate the sliding direction [paper III].

The tribofilm created when Cr-DLC slid against steel, Fig. 20bappears to have suffered from many contact cycles, as the directional appearance is obvious. ESCA analysis showed that the tribofilm in Fig. 20b was composed solely of FeS and that the W-DLC coating in addition created a WS2 compo-nent in the tribofilm (Fig. 20a). The easily sheared tungsten disulfide could explain the somewhat smeared appearance in Fig. 20a. It could also be due to that this tribofilm was being regenerated more often.

Transmission electron micrographs of both types of tribofilms can be seen in Fig. 21. The thickness of the films was measured to be about 200 nm. One can also see in Fig. 21a that the tribofilm has a smoothening effect. The roughness of the steel surface has become flattened by the easily sheared mixed sulfide (WS2-FeS).

33

a)

b)

Figure 21. TEM micrograph of the tribofilm formed on the steel surface when worn against the W-DLC coating (a) and the Cr-DLC coating (b). Scale bars equal 500 nm [paper III].

HR-TEM revealed that the tribofilms are composed of nanometer-sized crys-tallites see Fig. 22. Furthermore, basal plane distances for the tribofilm mate-rial in Fig. 22b correspond well to FeS crystalline data. The tribofilm in Fig. 22a however, exhibits different basal plane distances. This implies that the tribofilm generated by the W-DLC coating is comprised of mixed sulfides.

Tribofilm

Tribofilm

34

a)

b)

Figure 22. HR-TEM micrographs of W-DLC generated (a) and Cr-DLC generated (b) tribofilms [paper III].

35

Exotic candidates The two types of coatings examined in papers I-III differ quite a lot. The only common things are that they both contain carbon and both are commer-cial coatings. A large difference is that the W-DLC coating is reactively sputtered from a WC-target in acetylene/argon plasma while the Cr-DLC is purely sputtered from both a Cr-target and a C-target. Therefore the W-DLC coating contains hydrogen and the Cr-DLC does not. Furthermore are the metal distributions inside the coatings very different as was shown in Figs. 7-8.

Having examined the tungsten and chromium as alloying elements in car-bon coatings, the question arose about which other metals could function together with sulfur. Here function could be either the case of providing a low friction tribofilm or forming a coating that is chemically strong against the reactive sulfur.

The desire to examine other alloying elements and also to produce them in a similar manner was then the onset for producing the coatings at the Ång-ström Laboratory. In the Balzers 640R PVD equipment sputtering is possible from two targets. Also e-beam evaporation is available. The coatings used in paper IV were produced using sputtering from the two targets in a non-reactive mode, i.e. only argon gas was used. The following interesting metals were chosen for alloying: Ti, Cr, Mo, W and Sn.

The Cr and W were of course chosen for reference and to connect to ear-lier research. Chromium does not form any layered sulfide structure and its most stable sulfide is CrS. Molybdenum is from the same row in the periodic table as tungsten and is the best known material when it comes to forming low friction sulfides, i.e. MoS2. According to literature [11] more exotic layered disulfides can be produced, e.g. TiS2. Thus also titanium was justi-fied in the study. The four transition metals are all prone to react with carbon to form carbides. Hence, a hard carbide phase embedded in a DLC matrix could be expected from such material combinations. The transition metals thus represent carbide formers, which are also strong or weak sulfide form-ers.

Also Sn, the soft metal situated further away to the right in the periodic table, was selected as an outsider because it is truly an interesting alloying element in this context. First of all this metal is a poor carbide former; tin does not react with carbon no matter what you do. Secondly, it is prone to react with sulfur and more interestingly it can form SnS2, which possesses a layered structure similar to MoS2 [12].

Conclusively, both strong and weak carbide formers and strong and weak sulfide formers were included in the work presented in paper IV.

Tribological testing, using ball-on-disc, of the different Me-C coated discs against steel balls, lubricated with an oil comprising 5 mass% SIB, are sum-marized in Fig. 23. It was found that only Mo-C and W-C facilitated a low

36

friction coefficient, µ 0.05. All the tested coatings were subject to flaking and crushing during sliding which was due to a too soft substrate, 265 HV, and high surface roughness, Rz = 3.1 µm. Thereby small coating fragments are present in the contact which could possibly cause abrasive wear. One example of coating particles in wear track and the corresponding ball surface is shown in Fig. 24. Despite the seemingly unfavorable sliding surface, the resulting friction coefficient levels for the W-C and Mo-C cases are without doubt in the lower end of reported boundary-lubricated friction coefficient values.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 5000 10000 15000 20000

Number of Revolutions

Fric

tion

coef

ficie

nt

W-CMo-C

Sn-C steelCr-C

Ti-C

Figure 23. The five Me-C materials sliding against steel in a ball-on-disc configura-tion. Steel –steel reference gives a friction coefficient of about 0.09. Only Mo-C and W-C were capable of reducing the friction compared to the steel reference [paper IV].

As obvious from Fig. 23, none of the Sn, Ti or Cr carbon coatings improved the friction coefficient compared to reference steel-steel contact. Hence, no or very limited formation of easily sheared disulfides occurred in the corre-sponding tribofilms.

To evaluate the influence of oxygen, a run with W-C – steel contact was performed in an environment chamber flushed with argon gas. The resulting development of friction coefficient, visualized in Fig. 25, is very similar to the one observed in normal air, as in Fig. 23. This indicates that the forma-tion of WS2 is not dependent on oxygen being present.

37

a) b)

Figure 24. SEM pictures showing a) part of the contact track formed at the W-C sample with coating fragments covering the track and b) the corresponding contact area on the steel ball [paper IV].

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0 10000 20000 30000 40000 50000 60000 70000 80000

Fric

tion

coef

ficie

nt

No. of revolutions

Figure 25. Friction curve of W-C – steel boundary-lubricated contact in an inert environment. Compare with the parallel run in normal air shown in Fig. 23 [paper IV].

In the cases of W-C and Mo-C, and to very limited extent in the case of Ti-C, one could detect a thick, black, tribofilm, which was patchy and unevenly distributed across the wear track. A similar type of tribofilm was observed in the inert case but then being completely covering the track. It was suggested that this thick layer was composed of polymerized molecules from the SIB-additive.

38

Transmission electron micrographs and subsequent EDS line-scans showed that indeed such tribofilms were composed of Me-C particles em-bedded in a matrix composed of carbon and FeS, see Fig. 26a. Further, a layer was detected closest to the steel surface composed of iron, tungsten and sulfur see Figs. 26b-c. This is likely a mixed WS2 – FeS layer similar to those observed in papers I-III. Further investigation using EDS line-scans on embedded particles showed that the sulfur content was higher on the surface of a particle see Fig. 27. A transition from FeS in the matrix to, in this case, WS2 could be the cause to this.

a) b)

c)

0

20

40

60

80

100

0 0.05 0.1 0.15 0.2 0.25 0.3

SWFe

Rel

ativ

e co

mpo

sitio

n at

-%

Position (µm)

Figure 26. STEM micrograph of a particle including tribofilm on a steel ball surface (a) and HR-TEM micrograph obtained at an area closest to the steel surface (b) and a EDS-line scan (c) obtained for marked path in (b) [paper IV].

Pt

steel

39

a) b)

0

20

40

60

80

100

0 0.05 0.1 0.15 0.2

SWFe

Rea

ltive

com

posi

tion

at%

Position (µm)

Figure 27. TEM (a) and EDS line-scan (b) for area near large embedded W-C parti-cle inside tribofilm [paper IV].

It was also shown (according to valence band ESCA) in paper IV that the polymerized material was polyethylene-like. When comparing the tribofilms formed in inert environment and in air it was obvious that oxygen was break-ing down the structure of the crossed-linked PE-like material, see Fig. 28.

-50510152025

Contact track on disc (inert)Wear mark on ball (inert)Outside wear mark on ball (inert)Contact track on disc (air)

Inte

nsity

/ ar

b. u

nit

Binding energy / eV

PE-ref

Figure 28. ESCA valence band spectra for tribofilms formed on the W-C discs and steel balls in ball-on-disc experiments run in Ar and air. A polyethylene reference is also shown [13] [paper IV].

steel

100 nm

Pt

40

A visualization of probable composition of the polymerized tribofilm is shown in Fig. 29.

Compacted and mixed sulphides

Figure 29. Schematic cross-section of the thick low-friction tribofilm formed on the steel surface sliding against W-C and Mo-C coatings. W/Mo particles bind to sulfur at their surface and reinforce the thick polymerized tribofilm. Wear and removal of carbon through oxidation further on creates a compacted layer of mixed FeS and WS2/MoS2 adjacent to the steel substrate.

The success of the tribologically induced formation of WS2 films inspired further work to investigate alternative routes to the same type of tribofilm. To investigate if other tungsten containing particles could promote the for-mation of WS2, work presented in paper V was performed. Here powders of WC, WSi2, and WS2 were blended into a lubricant. Also coatings of W-C and pure W were tested. Once again the flaked and crushed W-C coating yielded very low friction coefficient, as did WSi2 after a somewhat longer time, see Fig. 30. The delayed formation of the low-friction tribofilm in the case of WSi2 was suggested to be due to that milling of the powder to finer grains is required before efficient sulfide formation can commence.

41

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 5000 10000 15000 20000

Number of cycles

Fric

tion

coef

ficie

nt

W-DLCWSi2W

steel WS2WC

Figure 30. Friction curves for tested material combinations in reciprocating ball-on-flat sliding. WC, WS2 and WSi2 are powders blended into the lubricant in steel against steel contact. W and W-DLC are coatings run against steel. The steel-steel combination is added for reference [paper V].

The reason why not WC powders react and form a low-friction tribofilms is the high hardness of that material. The hard WC particles in our study are being squeezed between two softer steel bodies. As the pressure in a contact equals the hardness of the softest material the amount of shearing and de-formation of WC particles are not sufficient to achieve milling of those par-ticles and the subsequent reaction with sulfur.

The driving force of low frictionThe discussion about why not all tungsten-containing materials are capable to form WS2 in a tribological contact leads on to the theories about emission of triboelectrons, as presented in [14-16]. As presented by R. Pearson in [17] there exists a theory to explain the relationship between electronic structures and physical hardness as well as the reactivity of materials.

However, as plasticity occurs at rather low loads it is believed that the pressure acting in discrete contact spots equals the hardness of the softest material in the contact. So it is likely to assume that the WC material in our

42

study, paper V, that is squeezed between two softer steel bodies does not experience high enough shearing and deformation to emit triboelectrons at a level that WS2 might be formed. The WC material studied by Zaitschev and Pleskachevsky [18] was in the form of hard metal rods with their face ends sliding against each other. In that case both mating surfaces were of equally high hardness, which would have produced a much higher contact pressure than in our study, so the formation of WS2 in that case could thus be ex-plained.

In addition to the tribological testing in papers IV and V, thermodynami-cal calculations were performed using EkviCalc, originally developed by B. Noläng [19]. This commercial software calculates possible reaction products based on the minimization of Gibbs free energy for a system as a function of temperature and pressure. The reaction products are based on the elements in the reactants. As many element combinations (substances) as possible were included. It is important to remember that thermodynamic calculations only tell the equilibrium composition, which may take a long time to reach. Thus, an equilibrium state is seldom reached in a tribological contact. On the other hand, the tribological condition in terms of extreme normal and shear stresses, act to increase the reactivity of the species present in the contact. This makes chemical reactions likely to occur at much lower temperatures than predicted in the calculations. Nevertheless, free energy thermodynamic calculations can often tell trends and will always tell weather a phase can be formed or not.

In paper IV EkviCalc was used to simulate how certain metal alloying elements in carbon coatings would behave in an assumed tribological con-tact. The respective amounts of reactants used in the calculations in paper IV can be summarized as follows: metal from Me-C coating - 1 mole, carbon from both coating and lubricant - 3 mole, iron from steel counter surface - 3 mole. Further, CS2 in liquid form was used as a substitute molecule for the SIB additive molecule since the thermodynamical data for the latter is not known. CS2 was present at a level of 0.1 mole. Oxygen was also present at a level of 0.1 mole. The pressure was set to 1 atm. An example of the theo-retical calculations is seen in Fig. 31, where Cr-C and W-C coatings are simulated in our assumed environment. The diagrams show the amounts of thermodynamically stable elements and solid compounds in the 100 - 1000˚C temperature interval.

43

a)

0

1

2

100 200 300 400 500 600 700 800 900 1000Temperature / degree C

Stac

ked

amou

nts

/ mol

CrS-

CrS-

Cr2 O3

Cr2 C3

Fe-

Fe-

b)

0

1

2

100 200 300 400 500 600 700 800 900 1000Temperature / degree C

Sta

cked

am

ount

s / m

ol

WS2WC

FeWO4

FeS

FeOFe-

Figure 31. Stable compounds at equilibrium as a function of temperature in the case of (a) Cr-C and (b) W-C [paper IV].

44

The simulation for the Cr-C coating in a sulfur-containing environment pre-dicts that the Cr will react with all accessible sulfur and oxygen and thereby leaving the iron surface unreacted. These results implicates that a Cr-C coat-ing could promote low tribochemical wear on the uncoated surface in the environment assumed. It can also be deduced from the calculations in Fig. 31b that W-C is indeed predicted to form a low-temperature stable WS2component in coexistence with FeS.

The breakdown of WSi2 to form WS2 as detected in paper V, see Fig. 30, was also backed-up by a thermodynamical calculation, see Fig. 32.

0 500 1000 1500 2000

0.00

0.05

0.10

0.150 500 1000 1500 2000

0.00

0.05

0.10

0.15

W2C

W5Si

3(s)

W2C

SiO2(l)

SiS2(l)

SiS2

WS2SiC

SiO2(s)SiO

2(s)

Yie

ld /

mol

Temperature / oC

Figure 32. Amounts of stable compounds at different temperatures when having WSi2 together with O2(g) and CS2(l) in small amounts at a pressure of 1 atm. [paper V].

45

Summary and conclusions

The present thesis thoroughly treats a special friction reduction phenomenon that may appear in boundary lubricated tribological contacts, of the type encountered in numerous mechanical components made of steel. The phe-nomenon involves the formation of a special type of tribofilm that offers very low coefficients of friction. Typically the friction level becomes halved when the film is formed, compared to when it is not formed. Since boundary lubricated mechanical components are so common in all sorts of machinery, the technical and economical potential of this phenomenon is gigantic.

The phenomenon We have revealed a new class of tribofilm formation. The formation is tri-bologically induced and involves the formation of a low-friction disulfide film on uncoated steel surfaces. The systems where the formation has been revealed involves

two hard bodies (in our case steel) sliding under high pressure (non-conformal) contact one of the bodies coated with a Me-DLC type film including tungsten (or molybdenum) a lubricant including a sulfur based additive (in our case often PAO with SIB additive)a few thousand contact cycles to form the film.

The tribofilm is then produced on the steel surface, resulting in friction coefficient reduction from typically 0.08–0.1 to 0.04–0.06. The tribofilm is formed from the metal in the carbon coating and sulfur in the oil additive. The main film studied was WS2, which is a well-known low-friction mate-rial. It includes easy shearing atomic planes, in the same fashion as the solid lubricants MoS2 and graphite. Virtually no carbon is present in the tribofilm, despite carbon being the main constituent of both the coating and the addi-tive. No films form on the Me-DLC coated part.

Analytical techniques The composition, structure and appearance of the tribofilms were analyzed using ESCA, TEM and SEM. The chemical driving forces for formation of the tribofilms were analyzed using EkviCalc, a commercial software for thermodynamical calculations based on minimization of Gibbs free energy

46

for a system as a function of temperature and pressure. The simulations con-firmed that WS2 should be expected to be a stable compound, coexisting with FeS, in the studied environment. This is indeed in agreement with the experimental results.

Alternative routes to achieve similar tribofilms The very beneficial properties of the WS2 based tribofilm and the interesting formation mechanism inspired new investigations. Is it possible to form al-ternative low-friction films from elements other than W? Is it possible to achieve the WS2 based tribofilm without providing the tungsten as a con-stituent in a coating?

To find answer to the first question, five different carbon coatings were produced, each containing a different metal: W (as a reference now), Mo, Cr, Ti and Sn.

It was found that only in the case of the Mo-containing carbon coating, a tribofilm of low-friction characteristics similar to that of the W-containing coating was formed. The other coatings never offered low-friction behavior. This result was also supported by the thermodynamical calculations.

As a spin-off result, the thermodynamical calculations indicated that coat-ings of the Cr-C type should impose very little tribochemical wear of the uncoated steel surface, and even reduce the formation of FeS (the “tradi-tional” tribofilm) on the steel surface in S-containing environments.

As a final spin-off, the thermodynamical calculations indicate that the Ti-C coating should be very resistant to tribochemical wear in the S-containing environment.

To answer the question regarding alternative ways to provide the W to the tribofilm, an experiment was designed that offered five options. W was pro-vided to the tribological interface in the form of:

a W-DLC coating on one of the surfaces and sulfur containing lubricant additive (again as a reference), a metallic W coating on one of the surfaces and sulfur containing lubri-cant additive, WS2-particles in the non-additivated lubricant, WC-particles in the sulfur additivated lubricant. WSi2-particles in the sulfur additivated lubricant.

It was found that the WSi2-particles could result in the formation of WS2-containing tribofilms. It was concluded that they, just as the W-DLC film, were sufficiently weak to mill down to very small particles, and chemically reactive in the prevailing tribological conditions. The WC particles were too stable, both mechanically and chemically, to result in any film formation. Instead, they acted abrasively and increased both the friction and the wear rate.

47

Interestingly, the addition of WS2-particles does not result in low friction. The metallic W coating gave a very beneficial combination of low friction

and extremely low wear. However, no WS2 was formed and the friction level was about 50% higher than the lowest levels achieved for the cases of W-DLC coating and WSi2-particles.

Remarks on the life time of the tribofilm The WS2 films formed are continuously worn, and must thus be regenerated to keep a continuous film. In the case of W provided from a carbon film on the counter body, this implies that the carbon film must be gradually con-sumed. To facilitate a long life of the coating and hence the tribofilm

the tribofilm should wear as slow as possible, and the coating should be made as thick as possible.

Obviously, this requires further optimization work regarding finding the most suitable sulfur containing additive, a coating structure that causes low wear of the tribofilm, a process and coating composition that allows deposi-tion of thick coatings, etc.

The polymerized thick tribofilm observed in some cases could be thought to function as a binder of W and Mo supplying materials. So tribologically induced milling and further disulfide formation could then sustain as long as this type of thick tribofilm is present.

48

Sammanfattning på svenska

Metalldopade kolskikt, Me-DLC, är välkända beläggningar som genererar låg friktion och gott nötningsmotstånd i både torra och smorda glidande kon-takter mot stål. Sådana metalliska dopämnen kan dock reagera med en del smörjmedelsadditiv. Ett smörjmedel innehåller oftast reaktiva tillsatster som kan bilda skyddsfilmer eller lättskjuvade filmer, generellt kallas dessa för tribofilmer, tillsammans med järn på stålytan. Dessa additiv är utvecklade enkom för att fungera i stål-stål kontakter, Genom att tillföra en ytterligare metall, i form av legeringsmetallen från kolskiktet, så kan en del nya kemis-ka reaktioner uppstå. Framförallt så kan den kemiska sammansättningen och de tribologiska egenskaperna hos de bildade tribofilmerna ändras drastiskt.

Det har visats sig att svavelet i en del additiv kan reagera med de metal-liska dopämnena och bilda nya typer av sulfider. Till exempel kan det bildas WS2 och MoS2 som är mycket intressanta material då de har en lagrad och extremt lättskjuvad struktur. Med detta menas att de ger upphov till låga friktionsnivåer i glidande kontakter. I traditionella gränskiktssmorda stål-stål kontakter med sådana additiv så bildas järnsulfid, FeS, som oftast fram-bringar en friktionskoefficient på 0.1. Dett ändras betydligt om man har W eller Mo dopade kolskikt på en av ytorna, då friktionskoefficienten sjunker till omkring 0.05. Med andra ord kan friktionskraften halveras om tribofil-men får inslag av lättskjuvade WS2 / MoS2 partiklar i tribofilmen. Det är visat att även andra wolfram-innehållande substanser än W-dopade kolskikt kan frambringa låga friktionsnivåer. T.ex. så kan pulver av WSi2 i en tribo-logisk kontakt ombildas till WS2. Dock visade det sig att hårda partiklar i form av wolframkarbid (WC) inte kunde ombildas till lättskjuvad sulfid i en tribologisk kontakt. Detta i samband med den triboinducerade sänknigen av aktiveringenergin för de möjliga kemiska reaktionerna ej uppnås då WC är både för mekaniskt och kemiskt stabilt. Temodynamiska beräkningar visade sig vidare ha en viss funktion i att simulera tribologiska kontakter. Genom-förda beräkningar stödde bildandet av WS2 och MoS2 i samexistens med FeS. Vidare indikerar simuleringar att Ti-C baserade material är triboke-miskt inerta mot svavel och att Cr-C baserade material genererar begränsad tribokemisk nötning av den obelagd stålytan.

49

Acknowledgements

This work was performed within the HiMeC (High performance Mechanical Components) research school and the MS2E program. The Swedish Founda-tion for Strategic Research (SFF) and member companies financially sup-porting the HiMeC program are gratefully acknowledged.

Erasteel Kloster, Sandvik SMT, Seco Tools, Ovako Steel, Agrol, Statoil Lubricants, Rhein Chemie, IonBond Sweden, Oerlikon Balzers Sandvik, Nomo Kullager and Svenska Tanso are all examples of helpful companies that supported me with hardware.

I would like to thank my supervisor Urban Wiklund and the “seniors” prof. Staffan Jacobson and prof. Sture Hogmark for all help during the mak-ing of this thesis.

Carin Palm - Caroline Olofsson – Rein Kalm – Janne Gustafsson: you are the lubricant of our department! Thanks for all help ranging from tickets to PVD.

My “old” friend Dr. Fredrik Svahn and my new “friend” Benny André are gratefully acknowledged for the substantial amount of panic reduction dur-ing the six last weeks! Erik Edqvist is acknowledged for the great artistic work on the beautiful front page.

Then … all the rest of my colleagues – thank you all for a hellarious time! I honestly believe that the laughs (sometimes) were mutual.

Further … all my dear friends inside and outside the university – I löv U!

My family – you are the ever-glowing sunlight in my life.

Uppsala 2006-10-06 Nils Stavlid

50

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49(2) (2002) 5. 13. A.A. Galuska and D.E. Halverson, Surface and Interface Analysis 26

(1998) 425. 14. Czeslaw K. Kajdas, Tribology International 38 (2005) 337. 15. K. Nakayama, N. Suzuki and H. Hashimoto, J. Phys. D: Appl. Phys 25

(1992) 303. 16. John J. Gilman, Materials Research Innovations 1 (1997) 71. 17. Ralph G. Pearson, Journal of Chemical Education 76(2) (1999) 267. 18. A.L. Zaitsev and Y.M Pleskachevsky, Wear 181-183 (1995) 495. 19. B. Noläng, PhD-thesis, Acta Universitatis Upsaliensis (1983) No. 691.

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