Surface Engineering of Metals - Principles, Equipment and Technologies Part 14 ppsx

5.8.2.5 The role of surface in the friction process Friction is a very complex process, difficult to present in one simple theory.The force of friction depends on the loading force which

Friction in these bearings is accomplished only in the presence of a cant Besides exceptional cases in which lubrication is by solid lubricants

lubri-(e.g., graphite, molybdenum disulfide), two types of friction occur: fluid and

mixed The first occurs when an elastohydrodynamic film of the lubricant

totally separates the mating surfaces while the second type constitutes acombination of fluid, dry and boundary friction The thickness of lubricantfilms in rolling bearings is 0.1 to 1 µm and is comparable with the roughnessvalue [81]

The coefficient of rolling friction is directly and proportionally dependent

on forces of resistance to movement, determined empirically for different tion conditions and different geometry of the rubbing pair It is also inverselyproportional to the loading force; for technical pairs it varies within the range

of elastic energy in different zones of superficial layers of rolling bodieswhich are alternately loaded and relaxed during motion [9], as well as byinternal friction in the lubricant Rolling resistance depends on the value

of the normally oriented force, the geometry of contact and the rigidity ofmaterials of the rubbing pair [38] Sliding resistance is caused by slips andmicroslips due to deformations, geometry of contact and relative move-ment of the rolling elements [82]

Rolling friction with slip is a case of mixed kinetic friction of bodies, the

relative movement of which causes the simultaneous occurrence of rollingand sliding friction in the contact zone [79] The amount of slip depends onthe velocity of friction

5.8.2.5 The role of surface in the friction process

Friction is a very complex process, difficult to present in one simple theory.The force of friction depends on the loading force which presses the rub-bing surfaces together, on the type of friction and on the coefficient offriction, which all depend, in turn, on the type of superficial layers of therubbing pair (potential properties of the superficial layers) and on thetype and properties of the substance present between the mating surfaces.Moreover, it depends on velocity, temperature and duration of the frictionprocess

Metal surfaces exposed to air are always covered by thin layers ofoxides or adsorbed gases which to a significant extent affect adhesion andfriction between mating surfaces As long as these layers are present atthe metal surface, the coefficient of friction is low and only very seldomattains the value of 1 to 1.5, characteristic of friction of pure metallic surfaces,obtained in high vacuum after heating the metals With pure metallic sur-

faces adhesion is so strong that the mutual bonding of both surfaces occurs

at sites of contact of asperities by forces of metallic bonds which, in turn,causes the creation of adhesive microfusion The fast development of adhe-sive bonding of surfaces of the rubbing pair causes leaps in the rise of frictionresistance forces and in intensity of wear, conducive to deep destruction ofthe surface and often to a stoppage of the relative motion of the bodies Such

a process is known as seizure (galling) [9].

Similar effects have been observed in other materials For example,diamond has a very low coefficient of static friction (0.05) in air, but in vacuumthis coefficient reaches the value of 0.5 Graphite has a low coefficient offriction not only on account of its laminar structure, but also on account ofadsorbed superficial layers such as water and gases In vacuum, graphiteelements are subject to seizure and degreasing, but with access of air, and inparticular moisture, its coefficient of friction and wear drop significantly.Depending on the moisture in air, the coefficient of friction of graphite ongraphite may vary from 0.06 to 1.0

The coefficient of friction on snow or ice is only 0.03 because due tolocal very high pressure the temperature of water-ice phase transforma-tion is lowered and a layer of water is created At low temperatures(-40ºC and lower) the layer of water is not formed and the coefficient offriction rises to a value normal for two sliding solid surfaces, i.e., 0.7 to1.2 [9]

5.8.2.6 Thermal effects of friction

During relative motion of sliding surfaces, a significant amount of heat isdissipated, causing a rise of temperature even when loads and slidingvelocities are relatively low

Heat created as the result of friction is uniformly distributed in thezone of rubbing materials but is localized at peaks of asperities This leads

to rises in temperature all the way up to the melting point of the materialand, consequently, to local structural transformations and changes in re-sidual stresses of the material or to the formation of local microwelds Thesurface temperature depends on the loading force and sliding velocity, butalso on thermal conductivity and coefficient of friction [9] The dissipa-tion of heat is a self-accelerating process since the rise of friction andadhesion at local hot sites of microwelds causes a rise in the rate of heatdissipation This, in turn, contributes to an increase in the number of suchmicrowelds and leads to galling or even to bonding of the two matingsurfaces (friction welding) [84]

The phenomenon of heat dissipation during friction can play a able role in the obtaining of very smooth surfaces in such processes aspolishing with abrasive medium This process consists of smoothing ofunevenness by wearing down material from peaks of asperities and trans-ferring it to grooves As the result of friction between tiny particles of theabrasive powder and the polished surface, hot spots are formed withinthe contact zone at which temperature rises to melting point Melted or soft-

favor-ened metal is smeared on the surface and upon rapid resolidification forms a

characteristic amorphic layer, known as the Beilby layer This layer is built up

of exceptionally small crystallites, formed as the result of a sudden lization of the metal [9]

recrystal-5.8.2.7 Lubrication

Lubrication consists of the introduction of a lubricant between surfaces inrelative motion, in order to reduce friction (by reducing the coefficient offriction) through the elimination of dry friction and its replacement by othertypes friction Lubrication allows a reduction of wear rate and of damage tothe rubbing surfaces, enhancement of vibration damping, intensification ofheat conduction from the rubbing surfaces, removal of wear products, coun-teraction to corrosion and, in some instances, the formation of a usable su-perficial layer

From the point of view of reduction of friction, the optimum type oflubrication is such which allows the formation, between the rubbing sur-faces, of a stable, continuous (unbroken) layer of the lubricant in the form

of a cushion (exclusive to hydrostatic lubrication) or a wedge (in cases ofhydrodynamic lubrication and appropriate design of rubbing surfaces)[12, 83]

This is not possible with every design of the friction node and in allservice conditions For this reason, in practice there occur different types

of friction, described by different laws, on account of the function of thelubricant

Extreme types of friction, on account of the nature of lubrication, isliquid and boundary friction The main discriminant is the relative thick-

ness of the lubricant layer R, connecting the absolute thickness of cant layer to the R a roughness parameter of the rubbing surfaces

neously boundary, fluid and elastohydrodynamic friction The last one takesinto account elastic deformations of the rubbing surfaces and changes in theviscosity of the lubricant with rising pressure The rubbing surfaces are par-tially in direct contact, but partially separated by a layer of the lubricantwhich is usually of a thick composition (1≤R≤5) This type of lubricationoccurs most often in the majority of machine components [8].

Lubrication with boundary friction takes place in the presence of tively small amounts of polar organic compounds such as fatty acids inthe lubricating oil The reaction of polar groups, e.g., carboxyl, with the me-tallic surface causes the formation of a monomolecular layer, strongly adher-ent to the surface

rela-This layer attracts particles which are more distant and, in effect, asubsequent layer is formed, etc As the result of the presence of theselayers the number and surface of metallic connections are reduced The crush-ing strength of such layers is high but shear strength is low, with the netresult that friction wear is significantly reduced The coefficient of friction isreduced with a rise in the thickness of the boundary layer, but below 100 nm

it becomes unstable and friction again rises

A boundary film, formed as the result of sorption processes, exhibits highmechanical strength It is, however, dependent on temperature For example,

at a temperature close to that of melting of soaps (organic acid - metal) thereoccurs a desorption of organic particles connected with the substrate Theordered state undergoes a transition to the disordered state which signifi-cantly lowers the strength of the boundary film, manifest by its resistance topressure and other forces Only the chemisorbed particles, e.g., those of MoS2solid lubricant, are not desorbed

At high pressures and high velocities sometimes additional frictionheat is dissipated As a result, at high surface temperatures a breakdown

of the lubricating film occurs In such cases, special agents are addedwhich raise the stability of the film on the metallic surface These agentsare able to withstand higher temperatures and exhibit satisfactory shearstrength, besides being able to sustain higher loads Among such additivesare organic compounds containing groups of active radicals and sulfur,chlorine or phosphorus These compounds react with the surface of themetal, forming unlimited layers of sulfides, chlorides or phosphates Alubricant with such additives ensures the reproducibility of the thin su-perficial layer if it is destroyed in the course of friction

Lubrication with boundary friction may be attained with the tion of solid lubricants such as graphite, molybdenum disulfide (MoS2) ortungsten disulfide (WS2) Solid lubricants may be present in a colloidalsolution of oil or resin or the self-contained form These materials form thinlayers at the metal surface and ensure high shear strength, as well as hightemperature [9]

applica-All of the above-mentioned lubricants have a laminar structure Atomslocated in a flat layer are bound by covalent bonds which are strong.Between the layers much weaker Van der Waals intermolecular bonds

Fig 5.53 Schematic representation of layers of molybdenum disulfide during friction:

1 - sulfur; 2 - molybdenum (From Janecki, J., and Hebda M [12] With permission.)

exist For that reason they are characterized by clearly defined slip andcleavage planes which run along the layers, giving them good lubricatingproperties For instance, crystalloids of the widely used MoS2 have manyeasy slip planes; practically each plane of elementary crystal connections(molybdenum atom surrounded from both sides by sulfur atoms) offersvery little resistance to deformation, when acted upon by tangential forces(Fig 5.53) [12]

5.8.2.8 Tribological wear and its various versions

The result of interaction of the rubbing elements is tribological wear, derstood as the process of destruction and removal of material from thesurface of solids, due to friction, and manifest by a continuous change ofdimensions and shapes of the rubbing elements [31] The causes of wearare in most cases of a mechanical character, less often mechanical, combinedwith the chemical interaction of the surrounding medium The basic causes

– building-in of fragments of the superficial layer of one rubbing rial into the surface of the second material During sliding wear this causesscratching of the surface and, with extended time (multiple formation of newasperities), destruction of surface,

mate-– adhesive bonds between contacting elements of the surface, cive to transportation of metal from one superficial layer to the otherwhich accelerates wear,

condu-– accumulation of hydrogen in the superficial layer of steel and castiron elements which, depending on service conditions, may accelerate weareven by a factor of 10

Fig 5.54 Classification of tribological wear processes.

Many processes of tribological wear may be distinguished, of which onlythe basic types will be discussed here (Fig 5.54)

In practice, pure wear processes, i.e those existing in only one cal form, are not encountered Usually, combined wear processes takeplace with one of them being the dominant which decides the amount ofwear [21]

classi-National Standards (e.g., Deutch - DIN 50323 [79], Polish - PN-91/M-04301)

usually distinguish ten types of wear, covering both elementary processes

(Table 5.3, Table 5.4) of destructive changes to the surface caused by friction

in microzones of solid surfaces, and technical processes, observed in the

work of machine parts

The most frequently encountered type of wear is abrasive wear, which

is responsible for 80 to 90% of all tribological wear This is a process ciated with bulk properties of the material and for that reason it is the bestknown, purely mechanical wear process It involves the separation of smallparticles of the material of the superficial layer in conditions of friction,usually sliding, caused by the presence in the rubbing zones of elementswhich fulfill the role of an abrasive, harder than the material of the rub-

asso-Fig 5.56 Dependence of abrasive wear on hardness: 1 - for some materials; 2 - for the

majority of materials.

The process of abrasive wear is dominant in conditions of dry friction.During fluid friction abrasive wear occurs only when the lubricant con-tains abrasive particles (usually these are products of wear and contami-nants)

Abrasive wear depends on the type, structure and properties of matingmaterials It is usually assumed that with a rise in hardness, there is a rise

in the abrasive wear resistance of metals and alloys (Fig 5.56) There are,however, exclusions from that rule:

– in some instances the softer metal wears less than the harder metal Thishappens when hard abrasive particles embed themselves in the soft metal in

a permanent way and act as abrasives with respect to the harder metal;– if the surface is excessively hard, it may also be very brittle Conse-quently, the material may crack around points of contact with abrasive grainsand relatively large portions of the material may become detached from thesurface In such cases, wear and damage of the surface may be very signifi-cant

In sliding connections of machines the main source of abrasive wear isconstituted by hard particles of carbide or silicide inclusions at the steelsurface, hardened wear products, very hard oxides, etc [9]

During the process of sliding wear, if the relative velocities are small

and loading pressures high, adhesive wear may occur, due to the creation

of adhesive connections (so-called cold welds) at sites of real surface tact and their subsequent tearing off in motion If this fusion is intensive,the material often exhibits greater strength in the fusion zone than thematerial of one of the rubbing elements In such cases particles of theweaker material are torn out, leaving craters (Fig 5.57)

con-A condition for the occurrence of fusion is the close proximity of ing surfaces, such that the distance between them is less than the range ofaction of intermolecular forces A further condition is the absence ofadsorbed or oxide layers with bonds of a non-metallic character and, forthat reason, not exhibiting tendencies to create adhesive joints (so-calledfusion) [21]

mat-In the event of friction between same materials it is probable that adhesivejoints will have greater strength than the host materials The cause of this phe-nomenon is strain hardening of the joints during friction In such an eventshearing of the joints occurs, as a rule, deeper in the host material and is accom-panied by severe wear of the surface The wear may, however, turn out to bemild because carryover of material occurs in both directions [9].

A modification of adhesive wear (also called fusion of the Ist kind) is mal wear, sometimes termed fusion of the IInd kind It occurs with high rela-tive velocities and high loading forces of mating surfaces in sliding friction

ther-It is caused by insufficient lubrication or the breaking of the lubricant layer,heating of the rubbing surfaces and a change in their properties (a rise ofductility causing smearing of material)

With thermal wear welds are formed (thermo-adhesive macrofusions),followed by their tearing in the zones of contact The rise in intensity ofthe phenomena described here may cause an avalanche process [21] Thismay lead to serious mechanical damage of the mating surfaces and also togalling [8, 86, 87]

Mixed abrasive-adhesive wear (scuffing) occurs as the result of the

joint influence of abrasive and adhesive wear [79] It is a form of tremely rapid wear, caused by breaking the lubricating film under highload Due to mutual interaction of asperities of both rubbing surfaces, itbrings about the fusion and detachment of these asperities in microzones

ex-of contact [8]

Wear by oxidation1 consists of adsorption of oxygen in zones of frictionand its diffusion into plastically and elastically deformed layers of metal,and their mechanical removal due to abrasive wear and spalling of the metalsurface Wear by oxidation, sometimes called normal wear, is viewed as theonly admissible process of wear

During the oxidation process, several layers of oxides of different ness are formed in a predetermined sequence, e.g., for unalloyed steelsthey are Fe2O3, Fe3O4, FeO, core The hardness of oxidized zones on steel ishigher than that of the core material or of the hardened core In the case ofoxidized aluminum, a severalfold rise in hardness occurs This is onereason given in explaining the relatively mild wear by oxidation (Fig.5.58) [21] Wear by oxidation occurs when the rate of formation of theoxide layer is greater than the rate of its wear from the surface It takesplace in sliding and rolling friction In conditions of rolling friction, wear

thick-by oxidation always accompanies fatigue wear In sliding friction, wear

by oxidation is usually dominant when conditions of boundary frictionprevail and, in some cases, in conditions of dry friction Intensive wear byoxidation often takes place in condition of fluid friction It is most

1) According to new opinions, wear by oxidation, as a hydrogen wear and another type of wear with the participation of chemical reactions, is assumed by the gen-

eral term tribochemical wear.

normal stress This may be the root cause of the initiation of microcracks,expanding to macrocracks The end effect is the detachment from the core offragments of the superficial layer Flaws and structural discontinuities of thesuperficial layer and surface itself may also constitute sources of fatigue cracks[21].

Two types of fatigue wear are distinguished:

– by spalling which occurs in conditions of dry friction or insufficient

lubrication It is manifest by local material loss in the form of flakes orscales The intensity of wear by spalling is high and depends on the depth ofplastic deformation of the superficial layer, the value of unit load at the con-tact site, number and frequency of cycles, as well as dimensions and me-chanical properties of the rubbing elements;

– by pitting which occurs in conditions of lubricated wear The

lubri-cant protects the surface from metallic contact, thus from adhesive wear.Besides, it fulfills the role of a shock absorber for contact loads carriedover from one surface to the other In the initial period of friction thepresence of the lubricant inhibits the migration of the point of maximummaterial effort by attenuating unit loads at contact points and causes theretardation of fatigue wear After the appearance of fatigue microcracksthe lubricant plays an unfavorable role (Fig 5.59) It penetrates themicrocrevices and by being forced into them by the mating surface it

Fig 5.59 Mechanism of pitting wear in the case of rolling friction: 1 - zones with

reduced cohesion (extracted); 2 - microcracks; 3 - compressed particles of lubricant; 4

- stretched particles of lubricant, bound to the surface by forces of adsorption or chemisorption.

Fig 5.60 The Rebinder effect - surface-active particles forcing wedge fissures: 1 - polar

particles; 2 - pressure forcing wedge fissure; 3, 4 - mating elements; 5 - direction of pressure of element 4.

creates wedges and increases the size of the cracks (Fig 5.60) When anetwork of cracks and crevices develops in the superficial layer, singleparticles partially lose cohesion with the matrix and are torn out by lubri-cant layers at the surface (adsorbed or chemisorbed by the surface), thelatter subjected to periodic compression and stretching The intensity ofpitting wear is smaller than that of spalling wear by a factor of 2.4 [9, 12,

21, 83]

Spalling and, in particular, pitting are typical processes of fatigue wear ofball bearings and oil-lubricated gear transmissions [21]

Abrasive-corrosive wear (fretting, fatigue-friction wear) is the

phenom-enon of destruction of the superficial layer, consisting of the formation oflocal material losses in elements subjected to the action of vibrations (e.g.,

in fitted joints) or small slip in forward and reverse motion, due to thecyclic interaction of loads and intensive corrosive action of the environ-ment (Fig 5.61)

Fig 5.61 Model of mechanism of abrasive-corrosive wear: 1 - passive layer formed at

surface; 2 - freshly exposed surface; 3 - passive stage in the process of regeneration;

4 - site of destruction; 5 - material deformation; I, II - successive position of instrument.

The direct cause of fretting is mechanical interaction Characteristic ofthis interaction are strong corrosion effects, accompanying all stages ofdestruction [46] Products of abrasive-corrosive wear are usually made up

of metal oxides with relatively high hardness, acting as an abrasive dium Fretting is manifest by the presence of fine brown powder, collect-ing in the vicinity of the friction joint [21, 83, 88]

me-It is worth mentioning that polished and very smooth surfaces corrodeslower than surfaces which are rough The reason for that is the moredeveloped area of the rough surface, able to accommodate more moisturewhich, in turn, accelerates corrosion and oxidation

The concept of erosive wear is understood as phenomena of

destruc-tion of the superficial layer, consisting of the creadestruc-tion of local materiallosses, due to mechanical and corrosive interaction of a flux of particles ofsolids or liquids with high kinetic energy [79] This is encountered pre-dominantly in machines involving flows A flux of particles of gas orliquid with particles of solids suspended in it may cause erosion of some

elements, especially on turbine and compressor blades From the point ofview of type of particles causing erosion, wear may be classified as:

– erosive, in a stream of solid particles,

– erosive, in a stream of liquid (hydroerosion),

– erosive, in a stream of liquid containing solid particles sive)

(hydroabraA specific type of erosive wear is cavitational wear (cavitation erosion)

-the phenomenon of material losses due to mechanical interaction of a liquid

in which cavitation occurs, i.e., the formation within the flowing liquid ofdiscontinuity zones, filled with a heterogeneous mixture of gas and liquid.Table 5.5 shows the dependence of basic types of tribological wear on therelative motion of mating elements

Table 5.5

Elementary processes of change of state and destruction of metals

by friction and wear (From Senatorski, J [80] With permission.)

5.8.2.9 Factors affecting tribological wear

The strongest effect on tribological wear is that exhibited by the sional condition of the rubbing surfaces - both in dry and lubricated friction.Roughness has an especially unfavorable effect With the presence of major