Engineering Tribology 2011 Part 10 pps

Two basic mechanisms involved in high temperature lubrication at medium loads havebeen found: chain matching and formation of thick films of soapy or amorphous material.Chain matching is

which offer a much wider freedom of chemical specification, systematic optimization of theheat of adsorption may eventually become practicable.

There has always been much interest in oil based lubrication mechanisms which wereeffective at high temperatures

The primary difficulty associated with lubrication is temperature, whether this is the result ofprocess heat, e.g a piston ring, or due to frictional energy dissipation, e.g a high speed gear.Once the temperature limitations of adsorption lubrication were recognized the search beganfor ‘high temperature mechanisms' Although these mechanisms have remained elusivesome interesting phenomena have been discovered

Two basic mechanisms involved in high temperature lubrication at medium loads havebeen found: chain matching and formation of thick films of soapy or amorphous material.Chain matching is the modification of liquid properties close to a sliding surface in a mannersimilar to the ‘low temperature - low load’ mechanism but effective at far highertemperatures and contact pressures, and dependent on the type of additive used The thickcolloidal or greasy films are deposits of material formed in the sliding contact by chemicalreaction They separate the opposing surfaces by a combination of very high viscosity andentrapment in the contact

Chain Matching

Chain matching refers to the improvement of lubricant properties which occurs when thechain lengths of the solute fatty acid and the solvent hydrocarbon are equal This is a conceptwhich is not modelled in detail but which has periodically been invoked to explain someunusual properties of oil-based lubricants

In a series of ‘four-ball’ tests the scuffing load was found to increase considerably when thedissolved fatty acid had the same chain length as the carrier fluid lubricant [43] An example

of scuffing load data versus chain length of various fatty acids is shown in Figure 8.29 Threecarrier fluids (solvents) were used in the experiments, hexadecane, tetradecane and decane of

chain lengths of 16, 14 and 10 respectively.

The maximum in scuffing load occurred at a fatty acid chain length of 10 for decane, 14 for tetradecane and 16 for hexadecane To explain this effect, it was hypothesized that a coherent

viscous layer forms on the surface when chain matching occurred This is similar to the ‘lowtemperature - low load’ mechanism discussed previously except that much higher contactstresses, > 1 [GPa], and higher temperatures, > 100°C, are involved and furthermore themechanism is dependent on the type of additive used It was suggested that when chainmatching occurs, a thin layer with an ordered structure forms on the metallic surface Theadditive, since it usually contains polar groups, may even act by bonding this layer to thesurface If the chain lengths do not match then a coherent surface structure cannot form andthe properties of the surface-proximal liquid remain similar to those of the disordered state

of bulk fluid as shown in Figure 8.30

To support this argument, the near surface viscosity under hydrodynamic squeeze conditionswas measured and a large viscosity was found when chain matching was present [43] Therelationship between the viscosity calculated from squeeze rates versus distance from thesurface for pure hexadecane and hexadecane plus fatty acids of varying chain length is shown

in Figure 8.31

Although chain matching has been confirmed in other studies [59,60] many researchers havefailed to detect this effect and still remain sceptical [33] Recently, however, an influence offatty acids on EHL film thickness was also detected [61] Film thickness or separation distance

versus rolling speed under EHL lubrication by pure hexadecane and hexadecane with stearicacid present as a saturated solution is shown in Figure 8.32.

FIGURE 8.29 Scuffing loads as a function of fatty acid chain length for various aliphatic

hydrocarbon carrier oils [43]

Ordered layer

Ordered layer

Bonding to surface to anchor viscous layer

= Carrier or solvent oil

= Additive (fatty acid)

FIGURE 8.30 Model of chain matching

It can be seen from Figure 8.32 that EHL film thicknesses for pure hexadecane and ahexadecane solution of stearic acid diverge significantly At very low speeds hexadecane gives

no residual film on the surface while the stearic acid/hexadecane solution gives separation ofabout 2 [nm] This effect can be attributed to an adsorbed layer of stearic acid As speedincreases and an EHL film is generated the film thickness for both lubricating liquidsbecomes the same and the effect of stearic acid is diminished

0 0.1

FIGURE 8.31 Viscosity versus distance between squeezing surfaces for pure hexadecane and

hexadecane with dissolved fatty acids of chain lengths 14, 16 and 18 [43]

0.2

0.5

1 2 5 10 20

Pure hexadecane

FIGURE 8.32 Effect of dissolved fatty acid on EHL film [61]

The effects of various fatty acids on friction, i.e lauric, palmitic and stearic acid added tohexadecane, were tested under heavily loaded conditions between sliding steel surfaces [62]

At low friction a layer of adsorbate, thicker than a monolayer, was detected by contactresistance measurements After the friction transition temperature was exceeded and thefriction coefficient rose, this layer seemed to decline to negligible values However, thehighest friction transition temperature of about 240°C was recorded when the chain length of

the fatty acid matched that of the hexadecane, i.e at 16 which corresponds to palmitic acid.

For the other acids, the friction transition temperature was much lower, between 120°C and160°C

Thick Films of Soapy or Amorphous Material

Almost all additives used to control friction and wear can react chemically with the wornmetallic surface This means that in addition to adsorbate films and viscous surface layers, alayer of reaction product can also form on the sliding contact surface It is virtuallyimpossible to control this process once the additive is present in the oil Until recently thisaspect of additive interaction was hardly considered since the reaction products were usuallyassumed to be extraneous debris having little effect on film thicknesses, friction and wear.Recently, however, the idea of films thicker than a mono-molecular adsorbate layer butthinner than the typical EHL film thickness has been developed [62,63,67-69] The thickness of

this film is estimated to be in the range of 100 - 1000 [nm] and the limitations of desorption at

high frictional temperatures have been avoided The consistency or rheology of these filmsvaries from soapy, which implies a quasi-liquid, to a powder or amorphous solid

· Soap Layers

Soap layers are formed by the reaction between a metal hydroxide and a fatty acid whichresults in soap plus water If reaction conditions are favourable, there is also a possibility ofsoap formation between the iron oxide of a steel surface and the stearic acid which isroutinely added to lubricating oils The iron oxide is less reactive than alkali hydroxides but,

on the other hand, the quantity of ‘soap’ required to form a lubricating film is very small.Soap formation promoted by the heat and mechanical agitation of sliding contact wasproposed to model the frictional characteristics of stearic acid [62,63] In the theory ofadsorption lubrication, it was assumed that only a monolayer of soap would form bychemisorption between the fatty acid and underlying metal oxide, e.g copper oxide and lauricacid to form copper laurate No fundamental reason was given as to why the reaction would

be limited to a monolayer

The soap formed by the reaction between a fatty acid and metal is believed to lubricate byproviding a surface layer much more viscous than the carrier oil as shown schematically inFigure 8.33 [62]

Steel

Fe + Fatty acid ⇒ Fe based soap e.g Ferrous stearate

Oil layer Fatty acid

Heat

FIGURE 8.33 Formation of a viscous soap layer on steel by a reaction between iron and a fatty

acid in lubricating oil

The presence of a viscous layer functioning by the mechanism of hydrodynamic lubricationwas deduced from electrical contact resistance measurements [62] When there was ameasurable and significant contact resistance, the thick viscous layer was assumed to bepresent Dependence on hydrodynamic lubrication was tested by applying the Stribeck law.According to the Stribeck law, the following relationship applies at the limit ofhydrodynamic lubrication:

U is the sliding velocity [m/s];

υ is the kinematic viscosity [m2/s];

W is the load [N]

The apparatus used to measure friction was a reciprocating steel ball on a steel plate,oscillating at short amplitude and high frequency as shown schematically in Figure 8.34 Thevalue of the constant in equation (8.4) was found by measuring the loads and velocitieswhere oil film collapse, manifested by a sharp increase in temperature (Figure 8.34), occurredduring lubrication by plain mineral oil Assuming that the constant is only a function of filmgeometry and independent of the lubricant it is possible to calculate the viscosity of the soapfilm An example of the experimental results obtained with 0.3% stearic acid in hexadecane isshown in Figure 8.35 [62]

Test steel plate

Controlled electric heating

Steel ball

Load

µ0.5

0.1

Temperature

Friction transition temperature

Leaf springs

FIGURE 8.34 Experimental principles involved in detecting viscous soap layers during

reciprocating sliding, a) schematic diagram of the test apparatus, b) sharpincrease in friction temperature indicating collapse of lubricating film (adaptedfrom [62])

It can be seen from Figure 8.35 that the calculated viscosity is in the range between 200 - 2000

[cS] which is similar to the viscosity of a soap under the same temperature

The limitation associated with this mode of lubrication is that like chemisorption, reactionwith an oxidized metallic substrate is a pre-requisite Steels and other active metals such ascopper or zinc would probably form soap layers whereas noble metals and non-oxideceramics are unlikely to do so

· Amorphous Layers

It is known from common experience that the process of sliding involves grinding whichcan reduce the thickness of any interposed object Lumps of solid can be ground into finepowders and, at the extreme, a crystal lattice can be dismantled into an amorphous assembly

of atoms and molecules This process is particularly effective for brittle or friable substances

As discussed already, many lubricant additives function by reacting with a substrate to form adeposit or film of reacted material which is inevitably subjected to the process ofcomminution imposed by sliding This material, finely divided (i.e as very fine particles) orwith an amorphous molecular structure, can have some useful load carrying properties andcan also act as a lubricant

0.01

0.005

0.02 0.05 0.1

5000 υ

FIGURE 8.35 Relation between temperature, sliding speed and viscosity of the soap layer

formed in sliding contact during lubrication by stearic acid in hexadecane [62].The process of amorphization of interposed material can be illustrated by a bubble raftanalogue of a crystal lattice Each bubble is analogous to an atom and, when closely packed,the bubbles resemble a crystal lattice if regular and an amorphous distribution if irregularlyarranged [64] An example of a bubble raft model of a sliding interface is shown in Figure 8.36.Material close to the sliding surfaces tends to be crystalline because of the tendency to alignwith a plane surface The bulk of the material, however, is amorphous because the shearingcaused by sliding does not follow exact planes parallel to the sliding direction Instead,transient ripples of shear waves completely disrupt any pre-existing crystal structure asshown in Figure 8.37

Amorphous layers of phosphates containing iron and zinc have been found in steel slidingcontacts when zinc dialkyldithiophosphate (ZnDDP) was used as a lubricant additive [65,66].The formation of these amorphous layers is associated with anti-wear action by the ZnDDPfor reasons still unclear

Finely divided matter as small as the colloidal range of particles has been shown to be capable

of exerting a large pressure of separation between metallic surfaces [67] Very little pressure isrequired to compress a spherical powder particle to a lozenge shape, but when this lozengeshaped particle is further deformed to a lamina, the contact pressure rises almostexponentially This can be visualized by considering the indentation of a layer of powdersupported by a hard surface using a hemispherical punch Initial indentation requires littleforce but it is very hard to penetrate the powder completely The deformation process of a softspherical powder particle is illustrated schematically in Figure 8.38

When the compression force is sufficiently large, the soft material is entrapped within theharder surface as illustrated in Figure 8.38 The resultant strain in the hard material maycause permanent deformation which could be manifested by scratching and gouging [67] Thecompression tests reported were performed without simultaneous sliding The filmsdeposited by ZnDDP presumably have the ability to roll and shear within the sliding contactwhile individual ‘lumps’ of material are not further divided into smaller pieces

FIGURE 8.37 Rippling shear fronts under sliding using the bubble raft analogy as a

mechanism of destruction of the crystal lattice [64]

Light load

Hard metal

Hard metal

Soft powder

Debris entrapment

FIGURE 8.38 Deformation process of a soft material between two harder surfaces

These deposited films of solid powder or amorphous material on metallic surfaces would notsuffer from the drawback of desorption at a limiting temperature or viscosity loss withincreasing temperature as adsorbed films do The mechanism which is involved information of these films evidently forms the basis of the high temperature lubricatingproperties of ZnDDP as compared to fatty acids This research, however, is very recent andthe current models will most probably be revised in the future

A further intriguing aspect of phosphorus-based additives is the spontaneous formation ofdeposits under a full elastohydrodynamic film [68,69] Both pure phosphonate esters andsolutions of phosphonate esters in paraffin tested by optical interferometry showed anincrease in the EHL film thickness from about 200 [nm] to 400 [nm] over 2 [hours] of testing at100°C An example of this increase in EHL film thickness versus rolling time for a 3%solution of didodecyl phosphonate in purified mineral oil is shown in Figure 8.39

Surface analysis of the rolling track of the EHL contact revealed that the deposited layer wascomposed of a polymerized network of iron phosphate with some organic groups included.This indicated that the iron had reacted with the phosphorous additive to form a thick layer

on the surface The layer was extremely viscous and waxy in consistency and almostinsoluble in organic solvents Polymerization by cross-linkage between phosphate and ironatoms was also detected It therefore seemed possible that an irregular network of repeatingphosphate and organic groups and iron atoms formed to create an amorphous structure This

particular process may be the only confirmed observation of a so called ‘friction polymer’ [70].

Friction polymer in general terms refers to the polymerization of hydrocarbon lubricants incontact with metal whose oxide film has been removed by friction A clean metallic surface,particularly of steel, is believed to have strong powers of catalysis which can induce theformation of hydrocarbon polymer films on the worn surface These films are believed toreduce friction and wear

A lubrication mechanism acting at high temperature and high load is generally known as

lubrication by sacrificial reaction films or ‘Extreme Pressure lubrication’ (often abbreviated to

E.P lubrication) This mechanism takes place in lubricated contacts in which loads andspeeds are high enough to result in high transient friction temperatures sufficient to causedesorption of available adsorption lubricants When desorption of adsorbed lubricantsoccurs, another lubrication mechanism based on sacrificial films is usually the most effectivemeans available of preventing seizure or scuffing The significance of temperature has led tothe suggestion that this mode of lubrication be termed ‘Extreme Temperature Lubrication’ [6]but this term has never gained wide acceptance It seems that the term ‘ExtremeTemperature’ is too ambiguous for practical use since oils are never used at extremetemperatures and furthermore the contact stresses under which E.P lubricants are effective

and commonly used considerably exceed the limiting contact stresses of many hightemperature lubricants For example, sulphur-based E.P additives used in pin-on-disc slidingtests ensure moderate wear rates up to 2 [GPa] while methyl laurate (an adsorption lubricant)fails at about 1.3 [GPa] and allows scuffing Plain mineral oil shows an even lower ability tooperate under high contact stress, resulting in excessive wear rates at contact pressures below

1 [GPa] [71]

200 300 400

Time [minutes]

FIGURE 8.39 Increase in EHL film thickness due to chemical reaction between the iron

substrate and the phosphonate additive [68]

Model of Lubrication by Sacrificial Films

Current understanding of E.P lubrication is based on the concept of a sacrificial film Theexistence of this film is difficult to demonstrate as it is thought to be continuously destroyedand reformed during the wear process However, a great deal of indirect evidence has beencompiled to support the existence of such sacrificial films The model of lubrication by asacrificial film between two discs is illustrated in Figure 8.40

The main effect of severe contact loads is to remove the oxide film from asperity peaksduring contact with opposing surfaces As already mentioned, the oxide-free surface of mostmetals is extremely reactive If a lubricant additive containing sulphur, chlorine orphosphorus is present then a sulphide, chloride, phosphide or phosphate film rapidly forms

on the exposed or ‘nascent’ surface The adhesion between opposing asperities covered withthese films is much less than for nascent metallic surfaces and this forms the basis of thelubricating effect The asperities are able to slide past each other with the minimum ofdamage and wear while the film material is destroyed by the shearing that inevitably occurs

If this mechanism fails, asperity adhesion and severe wear occurs as described in the section

on ‘Mixed Lubrication and Scuffing' In general terms the lubrication mechanism bysacrificial films depends on rapid film formation by a reactive E.P additive and on sufficienttime and temperature for the reaction films to form

The evidence for the formation of sacrificial films has gradually been gathered over time Itwas originally observed that when wear tracks and contacts were lubricated by oils containingsulphur, the sulphur accumulated in the heavily loaded regions [72-74] The concept of aniron sulphide film was then proposed [75] and later confirmed when surface analysis was

Region of

Fully grown film

Film produced by quiescent corrosion

Reactive sulphur

or other element from lubricant

Bare unprotected surface

Film destruction

FIGURE 8.40 Model of lubrication by a sacrificial film

sufficiently advanced to detect traces of iron sulphide on the steel surface [76] Films of ironsulphide were then produced on steel surfaces to test their lubricating effect and it was foundthat their survival time in a sliding contact was very short [77]

When the temperature of rubbing steel specimens was deliberately lowered below the ‘E.P.start temperature’ (i.e the minimum temperature for which the E.P reaction becomeseffective, producing a lubricating effect), any lubricating effect rapidly disappeared eventhough it has been well developed at a higher temperature [78] In a more elaborate test, thefriction characteristics of a carbon steel pin sliding against a stainless steel ball were comparedwith those of a stainless steel pin sliding against a carbon steel ball while lubricated by an E.P.oil The load capacity of the stainless steel pin sliding against a carbon steel ball was higherthan when the friction pair materials were exchanged Static corrosion tests of the sulphuradditive outside of the sliding contact showed that stainless steel did not react or corrode asrapidly as plain carbon steel [78] Since the pin is subjected to a very intense wearing contact asacrificial film is unlikely to form on its surface whatever the material It is thereforepreferable for the ball to be made of reactive material, i.e carbon steel, to allow a sacrificialfilm to form on the surface outside the sliding contact giving higher load capacity and betterwear resistance A model of this film formation is schematically illustrated in Figure 8.41

Additive Reactivity and Its Effect on Lubrication

In order for an E.P additive to effectively form sacrificial films it must be chemically active

and react with worn metallic surfaces [75,79] An ‘active’ E.P additive gives a higher seizure load than a ‘mild’ E.P additive [76] The seizure load is the load sufficient to cause seizure of

the balls in a ‘four-ball’ test In this test one ball is rotated under load against three stationaryballs until seizure occurs This test is commonly used in characterizing lubricating oils Exactcomparisons of additive chemical reactivity and E.P performance are rather rare in theliterature because of the limited practical need for them and experimental difficultiesinvolved One test measuring corrosion by E.P additives and load carrying capacity was

conducted on a ‘hot wire’ and ‘four-ball’ test rigs simultaneously [80] The operating principle

of a hot wire corrosion apparatus is shown in Figure 8.42 A wire submerged in a bath of thetest oil is heated by electric current to induce corrosion of the wire Since the corrosionproduct, e.g iron sulphide, usually has a much higher resistivity than the metal wire, theincrease in resistance provides a measure of the depth of the corrosion

The temperature of the wire, which also affects its resistance, is held constant during the test.There is a short period of time required for the wire to reach a steady temperature Thecorrosion rates are usually sufficiently slow so that it can be assumed that no corrosion occurs

Vigorous film

Time Low friction

metal

Reactive metal

Inert metal

Reactive metal

Sacrificial action

of film

FIGURE 8.41 Favourable and unfavourable conditions for sacrificial film formation

V I

Voltmeter Ammeter Power

supply

Hydraulic load

Test oil

Dissipation of heat High pressure to suppress oil boiling r 1

r 2

Wire cross-section Piston

FIGURE 8.42 Schematic diagram of a ‘hot wire’ corrosion apparatus

during this period The ‘hot wire’ method has certain drawbacks since it is possible for uniform corrosion of the wire to occur which may corrupt the experimental data Unless ahigh pressure is maintained in a test chamber a vapour jacket will surround the wire Thevapour has a different chemical characteristic from the liquid oil and additive reactivitybecomes a function of hydraulic pressure [81] The ‘hot wire’ method is an adequate andeffective method for demonstrating a general relationship between chemical activity and

non-lubricating effect for a wide range of additives However, an exact comparison betweensimilar additives also requires an independent confirmation by other methods.

To specifically evaluate both reactivity and lubricating effect of oils two parameters ‘K’ and

‘K 1’ are introduced, i.e.:

and

where:

K is the relative load capacity of the E.P lubricant;

K 1 is the relative corrosivity of the E.P lubricant;

W test oil is the mean Hertzian load where gross wear begins in a four-ball test with test

oil The mean of several measurements is usually taken [N];

W white oil is the mean Hertzian load where gross wear begins in a four-ball test with

white oil The mean of several measurements is usually taken [N];

κtest oil is the corrosion constant of the test oil [m2/s];

κwhite oil is the corrosion constant of the white oil [m2/s]

The corrosion constant ‘κ’ can be deduced from the Wagner parabolic law of hightemperature corrosion,i.e.:

where:

d is the average depth of corrosion [m];

t is the corrosion time [s]

An example of the plot of ‘K’ versus ‘K 1’ for various sulphur, chlorine and phosphorus basedE.P lubricants is shown in Figure 8.43

It is evident from Figure 8.43 that the performance of a lubricant is proportional to itscorrosivity or film formation rate It is also clear that there are fundamental differencesamong the additives depending on the active element Sulphur is the most effectiveelement, i.e it provides the greatest lubricating effect for the least corrosion, followed byphosphorus and chlorine The reasons for this variation are still unknown

A large range of compounds have been tested as E.P additives and it was found that effectiveadditives have a weakly bonded active element A frequently studied additive isdibenzyldisulphide (DBDS), the molecular structure of which is shown in Chapter 3, Figure3.18 In this molecule, two sulphur atoms are arranged in a chain linking two benzyl groups.This molecular structure is relatively weak and the sulphur is easily released to react withcontacting metal It is also possible to increase the sulphur chain length, e.g have foursulphur atoms in one molecule, and this further increases the instability or reactivity of thecompound Pure elemental sulphur without any inhibiting organic radical is highly reactivewith steel and other metals and hence can be very effective as an E.P additive for severeconditions More information on the formulation of the molecular structure of E.P additivescan be found in [82]

1 2 3 4

Relative chemical reactivity

is discussed further in the chapter on ‘Corrosive and Oxidative Wear’

Nascent Metallic Surfaces and Accelerated Film Formation

Sliding between metals under high contact pressures is generally believed to disrupt

naturally occurring oxide layers and expose a ‘nascent surface’ As discussed earlier, this

phenomenon may impede adsorption lubrication For sacrificial film lubrication, theexposure of the nascent surface can accelerate the formation of these films and promote alubricating effect It is very difficult to demonstrate experimentally the transient removal ofoxide films during sliding contact but some evidence of this was provided by applying thetechnique of ellipsometry [83] Ellipsometry is an optical technique that detects oxide films bythe interference between fractions of light reflected from the base and outer surface of anoxide film Enhancement of the experimental method with a laser and light polarizerrevealed that very small patches of surface, typically 10 - 30 [µm] in diameter, with nodetectable oxide film formed on the wearing surface of hard martensitic steel during highstress (0.1 [GPa]) sliding contact [83]

A nascent surface is far more reactive than an oxidized surface because: (i) there is no oxidebarrier between the metal and reactants, (ii) the surface atoms release electrons known as

‘Kramer electrons’ to initiate reactions [84], (iii) the nascent surface formed by sliding contains

numerous defects which provide catalytic sites for reactions [85] These characteristics ofnascent surfaces are schematically illustrated in Figure 8.44 and are discussed in detail in [86].The release of electrons is critical to the initiation of a reaction between the E.P additive andthe metal Low energy electrons emitted by the surface ionize molecules of the additive andthen these ionic radicals (transformed additive molecules) adsorb onto positive points on thesurface [87] The electron emission is associated with initial oxidation of the surface by

Contaminants impede adsorption of oxygen Primary barrier to oxygen: solid state diffusion required for the reaction 4Fe + 3O2⇒ 2Fe2O3 to occur

Adsorbed water and contaminant layer

Oxide

Metal

Normal oxidized surface

Kramer electrons released after inital oxidation to provide catalytic effect

Very weakly bonded

highly active atom

* = preferred reaction sites

FIGURE 8.44 Characteristics of nascent surfaces

atmospheric oxygen [84,88] or possiblyby initialsulphidization Thepositive points orelectron vacancies have only a minute life-time On the surface of a typically conductive

metal this is only about 10 -13 [s] but this is sufficient to initiate chemical reactions Surfaceadsorption reactions are therefore activated by the low energy electrons and may progressvery rapidly The model of an ionic reaction mechanism between E.P additives and ametallic surface is shown in Figure 8.45

R

FrictionS

RS

RS

RSFe

Fe

FIGURE 8.45 Ionic model of reaction between an additive and a worn surface [87]

Measurements of reaction rates between sulphur and a nascent steel surface [89] show that

the rate of sulphidization by elemental sulphur is about 1000 times more rapid for a nascent

surface than for an oxide surface This speed of reaction ensures that a thin film of

sulphidized material, perhaps 5 [nm] thick, would form in a few milliseconds instead of over

several seconds A sacrificial film based on this thin film could be sustained even at highcontact rates, i.e when the angular speed is several thousand revolutions per minute [25] Itwas also found that the presence of a nascent steel surface lowered the temperature at which

a measurable reaction rate occurred between steel and E.P additives [90]

To summarize, the nascent surfaces play a significant role in lubrication by sacrificial filmsthrough: (i) raising the film formation rates to a level sufficient to sustain lubrication by thesacrificial film mechanism in practical high speed contacts, (ii) confining the corrosive attack

by the E.P additive, particularly at low temperatures, to asperity peaks, i.e to locations where

a nascent surface is most often found These characteristics are illustrated schematically inFigure 8.46

Worn asperity peaks:

nascent surface fast reaction

Quiescent areas only slow corrosion

Asperity Asperity

Influence of Oxygen and Water on the Lubrication Mechanism by Sacrificial Films

Oxygen and water or atmospheric moisture exert a strong influence on E.P lubrication aswell as on adsorption lubrication Oxygen is a competing reagent to the E.P active element,e.g sulphur, for the nascent steel surface The chemistry of oxidation is fundamentallysimilar to sulphidization, chloridization and even phosphidization Oxygen is present in airand is not excluded from most sliding contacts Pure films of sulphide or of other E.P activeelements are rarely found in practical systems It is found, for example, that when E.P.lubrication is effective the wear debris is mostly iron oxides [91] High concentrations ofsulphur on worn surfaces are found to coincide with scuffed regions while smooth surfacesare covered with oxygen rich layers [92] More recently, however, it was found that, providedscuffing did not occur, oxygen is more densely concentrated on worn surfaces than sulphurwhen lubricated by E.P additives [93] Films rich in sulphur were found only if oxygen wasdeliberately excluded from the surrounding environment by imposing an atmosphere ofpure nitrogen [93] The requirements of thermodynamic equilibrium ensure that sulphidesare eventually oxidized to sulphates and later oxides in the presence of oxygen In a study ofoxygen and sulphur interactions with clean metallic surfaces under high vacuum, it wasfound that a monoatomic layer of sulphur, probably bonded as sulphide to the iron surface,was gradually converted to iron oxide when oxygen was admitted to the vacuum [94]

These characteristics of an E.P lubricated system can be interpreted in terms of a chemicalsystem subject to mechanical intervention At the asperity peaks where a nascent surface isrepeatedly formed, sulphidization occurs because this is the far more rapid process Anysulphide debris from the sacrificial films collects in the oil where it is oxidized by dissolvedoxygen In between the asperity peaks, a slower form of corrosion or chemical attack occurswhich causes the compounds closest to thermodynamic equilibrium to form, i.e oxides oroxidation products of sulphides This duplex structure and formation of the E.P film isillustrated schematically in Figure 8.47

The lubrication mechanisms under E.P conditions, i.e high temperature and high load, areonly indirectly controlled by the presence of an additive The lubrication mechanismtherefore functions largely according to the Le Chatelier principle When conditions aremoderate the system responds by producing the compounds which are closest tothermodynamic equilibrium On the other hand, under severe conditions, i.e onset ofscuffing, chemical reactions respond to the most immediate disturbance, which is a largequantity of unreacted metallic surface In that case, the most rapid means of neutralizing themetallic surface becomes the primary response

A further complication in the elucidation of the role of oxygen is the difficulty in preciselydefining the structure of the E.P film As discussed above, chemical reactions and film

O2Slow diffusion of oxygen in oil

Rapid sulphidization

Sulfide/oxide mixture Non-sacrificial

corrosion product film

Short lifetime sacrificial film Mostly oxide

S S S S S S S

S S S S S S S

S S

Slow formation of mostly

Fe2O3 (equilibrium product)

FIGURE 8.47 Formation and structure of the E.P film in the presence of atmospheric oxygen.formation at asperity peaks may be very different from events occurring in the grooves andspaces between asperities Surface analysis is most effective in providing an average chemicalcomposition of a surface However, the establishment of a surface ‘map’ of, for example,sulphides is a very laborious if not impossible task requiring the application of modernsurface analysis techniques In other words it is relatively simple to determine thecomposition of the bulk of the surface while overlooking the small but critical areas of

asperity peaks Most work has shown that E.P films are at least 1 [µm] thick simply because

that thickness of sulphide films has been observed on worn surfaces [76,95] Although much

thinner films between 10 - 50 [nm] have also been suggested [78,96], they are still too thick to

be sacrificial films

The presence of these ‘tough’ thick films was originally used as evidence for E.P lubrication.The rapid destruction of thick sulphide films by sliding contact precludes their survival inE.P systems apart from the relatively unworn spaces between asperities In a study ofsulphide films on steel surfaces, it was found that the product of several hours corrosion bysulphur on steel, i.e a thick sulphide film, was destroyed by less than 20 passes of a steelslider [97] which in a high speed sliding contacts would last for about one second or less AnE.P film where there is an accumulation of corrosion product between asperities with short-lifetime or transient films present on asperity peaks is suggested as the most probable [25] and

FIGURE 8.48 Probable structure of the E.P film

Oxygen is not only a strong chemical reagent which actively modifies the chemistry of asacrificial film but it has also been found to improve the lubricating effect The heat of

adsorption of fatty acids on iron sulphide is raised considerably by a small amount ofoxidation [97] The heat of adsorption of unoxidized iron sulphide is similar to iron oxides.This means that the role of oxygen may be to extend the temperature limits of adsorptionlubrication when surfactants and E.P additives are simultaneously employed as additives.Oxygen may also suppress excessive corrosion by E.P additives since it was found thatremoval of oxygen from a lubricated system caused a more severe sulphidization withoutany increase in maximum load capacity [98] A mixed oxide/sulphide film was also found tohave a much higher load capacity than a pure sulphide film under certain conditions of loadand sliding [99] A graph of critical load versus measured ratios of surface sulphur to oxygenconcentrations from the wear scars of a four-ball test is shown in Figure 8.49 The critical load

is defined as the maximum load which permits smooth sliding up to 200°C

0 1 2 3 4

Differences between E.P additives in terms of their chemical composition are also revealed

by the differences in the oxide, sulphide and sulphate composition of wear scar films [100].Films on metallic surfaces lubricated by sulphur-based E.P additives consisted of a mixture ofoxides, sulphides and occasionally sulphates Each additive shows a characteristiccomposition of sulphides and oxides for a given set of sliding conditions The relativeoxide/sulphide/sulphate composition may depend on the rates of sulphidization andoxidation during film formation [25] When elemental sulphur is present, sulphide filmsform very rapidly and remain as almost pure sulphide until their removal by wearingcontact When a milder additive is used, the sulphide forms slowly, if at all, and is usuallyheavily contaminated by oxide and sulphate

In contrast to oxygen, the influence of water or moisture on sacrificial film lubrication hasscarcely been studied One investigation found that water can interfere with the functioning

of E.P additives but the reason for this effect has not been suggested [28]

Mechanism of Lubrication by Milder E.P Additives

Measurements of sulphidization rates on nascent steel surfaces revealed that not all E.P.additives show rapid reaction rates [89] The sacrificial film mechanism of E.P lubrication isprobably not the only mechanism that can occur and there are differing views on this subject.Most of the existing theories suggest that E.P additives also function by a modified form ofadsorption lubrication The sulphidization reaction is not necessarily considered to bespontaneous and E.P additives are thought to be initially adsorbed onto the surface[76,101,102] This adsorption provides a useful lubricating effect or wear reducing effect at

moderate loads and is called the ‘anti-wear’ effect which is often abbreviated to ‘AW’ An

increase in loads, sliding speeds or operating temperatures, causes the adsorbed additive todecompose on the worn surface leaving the sulphur atom (or any other active element) toreact with the iron of the worn metal This mechanism is illustrated in Figure 8.50

Fe

R

S

RS

Fe

RS

RS

Fe

Compound released (e.g alkane, olefin etc.)

FIGURE 8.50 Reaction mechanism of milder E.P additives [101]

When the additive finally decomposes to produce a sulphide film, organic residue moleculessuch as alkanes and olefins are released Identification of these molecules in order to confirmthis model has proved difficult because of their extreme dilution caused by the lubricatingoil It has also been suggested that the conversion of an adsorbed film of additive to asulphide film occurs mainly in the wearing contact where the temperatures are highest.Outside of the wearing contact, adsorption of the additive is the predominant process Thismechanism appears to be more appropriate for milder additives such as DBDS, than forelemental sulphur, since these additives are not observed to form sulphide films rapidly at

the operating temperatures of lubricating oil, i.e 100 - 180°C [89] The model of this

mechanism is illustrated in Figure 8.51

The unresolved aspect of this model is how the adsorbed film simultaneously protectsagainst strong adhesion between opposing asperities and decomposes to a sulphide film Aswill be discussed in the chapter on ‘Adhesion and Adhesive Wear’, more than a monolayer

of film material is required to prevent adhesion between asperities

Function of Active Elements Other than Sulphur

Phosphorus and chlorine are generally believed to provide a lubricating effect similar tosulphur When a phosphorous compound containing a phosphate radical is reactive to ametallic surface, a metallic phosphate film is formed on the worn surface increasing the loadcapacity characteristic of E.P lubrication [30,103] Tricresylphosphate (TCP) and zinc

dialkyldithiophosphate (ZnDDP) are usually used for this purpose although, as discussed inChapter 3, the zinc and sulphur present in ZnDDP considerably complicate the filmformation by this additive [104] The lubricating effect of phosphorous additives depends onthe presence of oxygen but a clear pattern of interaction has not yet been established.Although in early tests, oxygen was found to promote the lubricating effect of TCP [30,103], inother studies it was observed that ZnDDP, but not other phosphorous compounds, gaveenhanced lubrication in the presence of oxygen [105].

Region of EHL contact:

FIGURE 8.51 Sulphide film formation in high contact temperatures as a model for mild E.P

additives

Chlorine is useful as an active element in E.P additives A chlorinated fatty acid ester wasfound to give a higher load capacity in a four-ball test than either sulphur or phosphorus

compounds [106] The mechanism of lubrication by chlorine based additives is believed to

involve sacrificial films in the same manner as sulphur [75] One limitation of chlorineadditives is the low melting temperature of iron chlorides When the transient temperature

in a sliding contact reaches 680°C, melting of the iron chloride causes failure of the sacrificialfilm mechanism resulting in seizure [107] Most chlorine compounds used as additives aretoxic, e.g chlorinated paraffins, or else decompose to release hydrochloric acid in the presence

of water Interest in these additives is therefore limited

E.P lubrication is not only restricted to sulphur, phosphorus and chlorine Any elementcapable of reacting with a metallic surface to form a sacrificial film can be suitable Additivesbased on tin which reacts with iron oxides to form an organometallic complex containingboth iron and tin on the surface have also been proposed [108] Research on these complexes

is only in its initial stage

Lubrication With Two Active Elements

Practical experience has revealed that the combination of two active elements, e.g sulphurand phosphorus or sulphur and chlorine, gives a much stronger lubricating effect than oneelement alone The sulphur-phosphorus system is most widely used because of theinstability of chlorine compounds

The effect of combining additives is demonstrated in Figure 8.52 which shows the data fromTimken tests The mineral oils used in these tests were enriched with the additives dibenzyl

disulphide and dilauryl hydrogen phosphate By using these additives separately andtogether the effect of phosphorus, sulphur and phosphorus-sulphur on seizure load wasfound [109].

0 100

of practical machinery The IAE (Institute of Automotive Engineers) and IP (Institute ofPetroleum) 166 gear tests conducted with the same additives revealed that the phosphorus-based additive allowed the same seizure or failure loads for a much smaller concentrationthan the sulphur-based additive [109] The critical difference between the Timken test and thegear tests is the slide/roll ratio The Timken test involves pure sliding while the gear testsonly impose sliding combined with rolling It appears that the sulphur originated surfacefilms are more resistant to the shearing of pure sliding than films formed from phosphorousadditives

The chemistry of steel surfaces after lubrication by sulphur-phosphorus oils was also studied[109,110] Films found on wear scars formed under severe conditions, e.g the Timken test,consisted mostly of sulphur However, under milder load and lower slide/roll ratios, whichare characteristic for general machinery, it was found that phosphorus predominates in thewear scar films This pattern of film chemistry versus sliding severity is illustratedschematically in Figure 8.53

It can be seen from Figure 8.53 that a sulphur-phosphorus based lubricant providesconsiderable versatility in lubricating performance The sulphur is essential to preventseizure under abnormally high loads and speeds while phosphorus maintains low frictionand wear rates under normal operating conditions

Low friction

property

critical

Anti-seziure property critical Phosphorus

Sulphur

Low

Phosphorous-sulphur composite films

Moderate High (pure sliding) Slide/roll ratio:

Load severity: Moderate

Temperature Distress

Temperature distress is a term used to describe high friction occurring over a relativelynarrow band of intermediate temperature in lubrication by an oil An example of this effect isshown in Figure 8.54 which illustrates the friction coefficient versus temperature resultsfrom a four-ball test where the lubricant tested is white oil with tributylphosphate [111].The tests were conducted at a relatively high contact stress of approximately 2 [GPa] and, toensure negligible frictional transient temperatures, at a very low sliding speed of 0.2 [mm/s].Friction, initially moderate at room temperature, rises to a peak between 100 - 150°C followed

by a sharp decline at higher temperatures This phenomenon is the result of a significantdifference between the desorption temperature of surfactants from the steel surface and thelowest temperature where rapid sacrificial film formation can occur In this test, thesurfactants were relatively scarce consisting only of impurities or oxidation products in thewhite oil In practical oil formulations, however, surfactants are carefully chosen so that thedesorption temperature is higher than the ‘start temperature’ of sacrificial film lubrication.The concept of wide temperature range lubrication which is achieved by employing intandem adsorption and sacrificial film lubrication is illustrated in Figure 8.55

It can be seen that when only the fatty acid is applied, the coefficient of friction is quite lowbelow a critical temperature and then sharply rises Conversely when the E.P additive (in anE.P lubricant) is acting alone, the coefficient of friction remains high below a critical

0 0.1 0.2 0.3 0.4

0.5µ

Temperature

T r

EP lubricant Mixture of EP lubricant and fatty acid

EP lubricant reacts with the surfaces at temperatureT r

FIGURE 8.55 Co-application of adsorption and sacrificial film lubrication to ensure a wide

temperature range of lubrication function [6]