Engineering Tribology Episode 2 Part 7 potx

Lubricant's critical temperature CLubricant's critical temperature B Lubricant's critical temperature A EHL film pressure profile Momentary desorption Some scuffing risk difficult to def

resistance of a sliding contact occasionally giving an overestimate of permissible loads andsliding speeds [41].

Stage 1

Stage 2

Large frictional heat

Thermally induced film collapse (desorption, viscosity loss, catalytic degradation, etc.)

First signs

of scuffing

Critical event

Large heat release

Propagation to next asperity

FIGURE 8.23 Sequence of events leading to scuffing

For these reasons, one reliable means of avoiding scuffing is to apply relatively conservativegear design standards Also, as will be discussed later, E.P lubricants can prevent scuffingwhen the limit of adsorption lubrication is reached There is, however, no quantitativemeasure available for estimating, for example, how much extra load can be applied if specialanti-scuffing lubricants are used

Interpretation of scuffing in terms of friction transition temperatures has been the object ofextensive studies [e.g 42-44] Initially a series of tests on a model friction apparatus with low

Lubricant's critical temperature C

Lubricant's critical temperature B

Lubricant's critical temperature A

EHL film pressure profile

Momentary desorption

Some scuffing risk (difficult to define)

Sustained desorption

High scuffing risk Temperature profile

C B A

Asperity collision transients

Small scuffing risk

where:

C is the concentration of the additive in the solvent base stock [%wt];

E a is the adsorption heat of the additive on the metallic surface [kJ/kmol];

R is the universal gas constant [kJ/kmolK];

T t is the friction transition temperature [K]

According to this model, when the transition temperature is exceeded, damage to theadsorbate film is more rapid than film repair so that the adsorption film is progressivelyremoved High friction and wear are then inevitable

For a narrow range of experimental conditions agreement between the model (8.3) andexperimental data was obtained In Figure 8.25 the relationship between the concentration offatty acids of varying chain length dissolved in purified inert mineral oil and transitiontemperature is shown [42]

The data provides a linear plot which is in agreement with theory The gradient of the graphwhich is a measure of the heat of adsorption is also approximately the same as the heat ofadsorption determined by more exact tests On the other hand, it has been found that a 1%solution of oleic acid improved the lubrication capabilities of white oil even though thetransition temperature for oleic acid was clearly exceeded [45] In other words, this indicatesthat fatty acids do not function merely by adsorption lubrication and some new theories arenecessary

An attempt to estimate the critical temperature in EHL contacts where scuffing or film failure

is likely to occur based on heat of adsorption, sliding speed and melting point of lubricant hasbeen made [35,113,114] The practical applications of the expression found are extremely

limited because the data referring to one of the variables, i.e melting temperature of thelubricant, is only available for pure compounds, not for mixtures, which commercial oils are.

0.1 1 10 100

Stearic acid

FIGURE 8.25 Relationship between friction transition temperature and concentration of

some adsorption additives [42]

It should be realized that an EHL pressure field also affects both the concentration ofadditives within the EHL contact and the mechanism of adsorption lubrication by raising thecritical temperature for desorption [115,125] It has been found that under EHL pressure theconcentration of additives in plain mineral oil (but not certain synthetic oils) tends to decline

to less than half the bulk oil concentration [125] Although the causes of this effect remainunclear, the implications for additive function within the EHL contact and its consequences

on scuffing appear very significant According to Langmuir's theory of adsorption, elevatedpressure increases the fraction of the surface covered by adsorbate for any given temperature.The critical temperature for scuffing to occur is modelled as the temperature where thefractional surface coverage by adsorbate declines to less than half of the available atomic sites

on the surface [116] The critical temperature is approximately 150°C in slow speed slidingexperiments but is between 300°C and 400°C in full scale scuffing tests where a substantialEHL pressure field is present [115] Thus the equations which attempt to predict the criticaltemperature of scuffing according to Blok's theory but which do not allow for the pressuredependence might give incorrect results

The increase in critical desorption temperature with pressure suggests a mechanism ofcombined instability in a lubricated contact Consider an experiment conducted on a ‘twodisc’ apparatus where the discs are subjected to a progressive increase in load until scuffingoccurs Assume that initially the EHL pressure is low and the desorption temperature is close

to that obtained in low speed tests As the load is increased the contact temperatures,frequency of asperity contact and hydrodynamic pressure increase The critical desorptiontemperature also increases so that effective adsorption lubrication is maintained At somelevel of load, however, a limiting EHL pressure is reached The limiting EHL pressure mayeither be due to lubricant characteristics or may be determined by the hardness of the discmaterials Further increases in load merely tend to increase the contact area or shift a greaterproportion of the load onto asperity contacts At this stage, two events may occur; either

there is direct desorption of the adsorbate lubricating films caused by excessive contacttemperature or there is a progressive collapse in EHL pressure caused by the asperityinterference When pressure declines the adsorbed films become unstable The collapse inpressure can be limited to asperity contacts only while the bulk pressure field remainsunaffected If there is a localized reduction in hydrodynamic pressure then the criticaldesorption temperature will precipitately decline to allow local desorption of the adsorbatefilm Scuffing will then be initiated from localized adhesive contacts between asperity peaksdenuded of adsorbate film This mechanism of combined instability is illustrated in Figure8.26.

Stable operation

Load increasing

Scuffing

Uncontrolled scuffing

Critical temperature

Contact temperature

EHL pressure

Direct mechanism of adsorbate

film instability and removal

Stable operation

Load increasing

Scuffing

Uncontrolled scuffing

Critical temperature

Contact temperature

Indirect mechanism of adsorbate

film instability and removal

No macroscopic change in contact temperature during initiation of scuffing

EHL pressure

Localised decline in EHL pressure and critical temperature

FIGURE 8.26 Combined instability in EHL pressure field and adsorbate lubricating films as

cause of scuffing

As may be deduced from Figure 8.26, it is still unclear whether any collapse in EHL or EHL induces desorption and scuffing or whether desorption occurs first and the resultantsurface damage causes the cessation of EHL

micro-A phenomenon of catalytic oil decomposition is believed to contribute to scuffing too micro-Athigh temperatures found in a heavily loaded EHL film, whenever asperity contact occurs, i.e.when the EHL film thickness becomes comparable to the combined surface roughness of thecontacting surfaces, the exposure of nascent surfaces worn by asperity interaction may directlyaffect the base oil of the lubricant which may have severe consequences for the lubricantfilm It is known that a major feature of nascent surface is its elevated catalytic activitycompared to quiescent, oxidized metal [17,119-121] Nascent surface typically catalyzesdecomposition reactions of organic compounds found in oil to release low molecular weightproducts that are often gaseous [119] Such catalysis can have a destructive effect on the

lubricating capacity of an oil It has been suggested that scuffing occurs when there issufficient nascent surface exposed by mechanical wear to cause the rate of chemicaldegradation of oil inside the contact to exceed the rate at which it can be replenished [121].When a critical rate of degradation is reached, contact closure by partial failure of lubricationfurther reduces the supply rate of oil This causes a sharp transition to unstable lubricationand scuffing from stable lubrication below a critical load The catalysed decompositionproducts are unlikely to facilitate lubrication since they do not possess the physical capacity tosustain the extremely high shear rates prevailing in the EHL contacts These products tend toaccumulate between asperity contacts and when their concentration becomes high enough,lack of lubrication occurs leading to scuffing [115] In addition, the decomposition productsmost probably surround the contact excluding fresh lubricating oil or else under theinfluence of extreme frictional heating they react and chemically bind both sliding surfaces.This last effect could cause a catastrophic rise in friction levels The schematic illustration of

this ‘catalytic model of scuffing’ is shown in Figure 8.27.

An important method of controlling scuffing would appear to be the prevention of nascentmetal surface by covering the sliding surfaces with coatings of non-metallic materials such asceramics Steel gears coated with titanium nitride and carbide have been found to offer goodscuffing resistance in gearbox tests compared to uncoated steel gears [122] Selection of a stablelubricant is also important since the decomposition of perfluoroalkyether lubricating oils hasbeen found to initiate scuffing and wear of metal surfaces [123,124]

Oil film collapses when sufficient degradation of oil occurs

Collision of asperities and formation of nascent surface

Figure 8.27 Schematic illustration of catalytic model of scuffing of metal surfaces lubricated

by an oil

In other more mechanically orientated studies, it was also found that operational parameterssuch as loading history and run-in procedure have a strong influence on scuffing andmeasured critical temperatures [46-48] The critical temperature appears therefore to be afunction of many parameters not just pressure, heat of adsorption and sliding speed Despitethe poor understanding of scuffing, research in this area has become scant in recent years.This may be due to the fact that scuffing belongs to ‘industrial tribology’ which has arelatively low priority compared to other aspects of tribology [49]

· Metallurgical Effects

The effect of alloying and heat treatment to produce a specific microstructure also exerts amajor influence on whether a low coefficient of friction can be obtained by oil-basedlubrication Frictional characteristics of steel-on-steel contacts versus temperature for twosteels, a martensitic plain carbon steel and an austenitic stainless steel, are shown in Figure8.28 [50] Both steels are lubricated by mineral oil

It can be seen from Figure 8.28 that the coefficient of friction of the austenitic stainless steelrises sharply at 160°C reaching values greater than unity by 200°C In favourable contrast, the

0 0.5 1.0

FIGURE 8.28 Frictional characteristics of plain carbon and stainless steels versus temperature

under mineral oil lubrication [50]

coefficient of friction of the plain carbon martensitic steel remains moderate in the range of0.2 - 0.3 The difference between these two steels can be explained in terms of reactivity sincethe austenitic steel is considered to be less reactive than the martensitic steel because of thelatter's greater lattice strain The greater reactivity causes more rapid formation or repair ofoxide films and re-adsorption of surfactant films under conditions of repeated sliding contact

In another study [52] it was found that austenitic steels have lower friction transitiontemperatures than martensitic steels

Similar tests conducted with additive enriched oils revealed that low alloy steels exhibitlower coefficients of friction than high alloy steels [51] It appears that both the phase of thesteel and the alloying content are the controlling factors in lubricant performance Forexample, chromium was found to raise the scuffing load for austenitic steels [53] while thecontrary effect was found in other cases [54-56] where martensitic and ferritic steels weretested In a comprehensive study where the effect of different alloying elements on scuffingresistance was tested, it was found that irrespective of the alloying elements themicrostructure has controlling effect on scuffing load For example, ferrite gives the highestscuffing loads and since martensite and cementite are less ‘reactive’ they lower the scuffingload [57]

Austenite is the most unsuitable phase and gives very low scuffing loads The failure load foraustenites is less than one tenth of the failure load for ferritic steels [57] Thus hardening ofsteels does not provide increased protection against scuffing since this induces martensitewith a corresponding reduction in ferrite

· Interaction Between Surfactant and Carrier Fluid

In the model of adsorption lubrication discussed so far, the fatty acid or surfactant was eitherapplied neat to the test surface or as a solution in an inert fluid In practice, the ‘carrier fluid’

or ‘base stock’ can also influence the lubrication mechanism It was found that the heat ofadsorption of stearic and palmitic acid on iron powder were up to 50% greater withhexadecane as the carrier fluid than with heptane [58] The heat of adsorption dictates thefriction transition temperature For example, if hexadecane is used as a carrier fluid inpreference to heptane, a higher friction transition temperature can be expected

This aspect of adsorption lubrication has also been relatively neglected, partly because of thedifficulty in manipulating mineral oil as a carrier fluid With the adoption of synthetic oils

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

8.4 HIGH TEMPERATURE - MEDIUM LOAD LUBRICATION MECHANISMS

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