Handbook of Lubrication Episode 1 Part 8 docx

The adhesion theory of wear is based on the assumption that a similar welding action occurs between a limited number of asperities and that the welds are ruptured when the solids slide o

If M is taken as 800 kg the frequency will be 216 Hz Nayak32 has shown that random surface characteristics can excite oscillation at preferred frequencies

Johnson and Gray20 demonstrated that such vibration can cause the interacting surfaces

to develop “corrugations” Some evidence of plastic flow has been found in the crests of corrugations but none in the troughs The position is so serious that a number of railways periodically reprofile rail surface by grinding

Adhesion

When perfectly clean, flat metal surfaces are brought into close proximity (less than 1/5 nm) they unite chemically When separation is wider, they are attracted to each other by van der Waals’ forces At small separations (less than 10–8 m) these are governed by a square law and at greater separations (greater than 10 –7m) by a cube law Practical surfaces are usually covered by oxide films and are so rough on the atomic scale that when bodies are brought together, only a tiny fraction of the contacting area (about 1/1000) at the peaks

of asperities is subjected to powerful adhesive forces Experimental measurements of ad-hesive forces are available with soft materials which conform when pressed together,19with mica which has been cleaved to produce an exceptionally smooth surface,17 and with hard spheres of very small diameters.25,36 Adhesive forces were sometimes two to three orders

of magnitude higher than those applied initially to force the spheres together.25

Buckley8measured the force to rupture junctions made within a vacuum system evacuated

to 10–11 torr Crystals of copper, gold, silver, nickel, platinum, lead, tantalum, aluminum, and cobalt were cleaned by argon ion bombardment before being forced against a clean iron (011) surface by a force of 20 dyn When iron was pressed against iron, a separating force greater than the 400 dyn was required In the case of other metals this force varied from 50

to 250 dyn In every case the strength of the junction was greater than the force used to promote it Even in the case of lead (which is insoluble in iron), Auger analysis indicated transfer of lead to the iron surface Thus the adhesive bonds of lead to iron were stronger than the cohesive bonds within the lead In general, the cohesively weaker metals adhered and transferred to the cohesively stronger

The adhesion theory of wear is based on the assumption that a similar welding action occurs between a limited number of asperities and that the welds are ruptured when the solids slide one relative to the other.54

The actual process of formation of wear particles has been studied by Sasada and Kando43

with a pin and disc machine They concluded that an initial metal-to-metal junction is sheared

by the frictional motion and a small fragment of either surface becomes attached to the other surface As sliding continues this fragment constitutes a new asperity becoming attached once more to the original surface This “transfer element” is repeatedly transferred from one surface to the other continually increasing in size and being flattened by the force between the pin and the disc Once a flattened particle attached to the disc grows to such

a size that it supports the load, it becomes the only contact between pin and disc It then grows quickly to a large size, absorbing many of the transfer elements dotted over the disc surface so as to form a flake-like particle from materials of both rubbing elements Unstable thermal and dynamic conditions brought about by rapid growth of this transfer element finally account for its removal as a wear particle These authors experimented with the following materials: Mo, Fe, Mi, Cu, Ag, Zn, and Al

The combination of AI disc and pins of Mo, Fe, Ni, and Cu produced violent ploughing The following combinations produced smooth sliding: Mo/Mo, Fe/Fe, Ni/Ni, Ni/Fe, Fe/Cu, Cu/Fe, Fe/Mo, Mo/Fe, Ni/Mo, and Mo/Ni Metal transfer was scarcely observed and the wear rate was very low in the case of Mo-Cu, Mo-Ag, Fe-Ag or Ni-Ag where the metals have poor mutual solubility.44

Relative importance of adhesion and plastic flow is covered by Andarelli et al.1 who

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observed the occurrence of dislocations by transmission electron-microscopy Glass fibers were slid against aluminum specimens 10–5m thick, and normal and tangential forces were determined from the shape assumed by the loaded fiber Further tests31 employed a cold-rolled tungsten wire with a hemispherical tip of radius 2.5 10–6 m as the stylus The load ranged from 1 to 100 µN as compared with values of 1.2 to 2.4 µN calculated on the basis

of an interfacial energy of 100 to 200 mJ/m2 This indicates that van der Waals’ forces between metals shielded by absorbed gases were responsible for the adhesion Load had to exceed a critical value before the stylus suddenly penetrated the surface Measurements of friction were consistent with this, nearly zero at low loads as long as deformation remained elastic

These results emphasize that plastic deformation rather than adhesion was the important agency determining friction and wear Comparison of the dislocation density based on tensile tests showed that 90% of the frictional energy was dissipated as heat with only a minor proportion being stored within the material

Fatigue

Sliding Wear

In all machinery there is a periodic variation of stress An element of metal at the surface

of a rotating shaft will be subject to reversal of bending stresses, the race of a rolling contact bearing will experience continual application and release of Hertzian stress, and the surface

of a conformal bearing will experience repeating stresses on a micro scale due to the passage

of asperities on the rotating surface All these repeating stresses can give rise to fatigue action Tsuya et al.57 and Quinn and Sullivan39 have provided evidence of changes in the substrate of a wearing part due to relative motion

Because it provides a more direct account of the formation of a wear particle than the adhesion theory, the fatigue theory of wear warrants close attention Soda et al.51 reported

a series of experiments on the face-centered-cubic metals Ni, Cu, and Au When atmospheric pressure was reduced, wear of Ni and Cu decreased but that of Au remained unchanged This was shown to affect the rate of wear fragment formation in contrast to mechanical factors which affected wear by changing the volume of fragments Mean thickness of the wear fragments was about one fourth of that of the plastically deformed substrate layer Correlation with direct fatigue tests indicated that the number of wear fragments was governed

by the resistance of the materials to fatigue Environmental factors such as atmospheric pressure had similar effect on wear rate as on fatigue strength Kimura23produces additional evidence of a correlation between the thickness of the deformed layer and that of the wear fragments

A particularly comprehensive test program was carried out by Tsuya58who used a variety

of test arrangements and ambient conditions Plastic working of the subsurface regions of materials in contact led to the formation of micronized crystals and cracks which originated

in the boundary region between the micronized crystals and those nearer to the surface which had been simply distorted These cracks tend to develop in the direction of material flow until particles are released

The Delamination Theory of Wear

Koba and Cook24studied the wear of leaded bronze running against steel and demonstrated

by scanning electron electron micrographs that metal flowed freely at the surface, smoothing out hills and valleys Some metal transfer was observed but did not appear to be an essential part of the wear process

Suh52investigated a number of wearing systems and put forward the “ Delamination Theory

of Wear” which can be summarized as follows:

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1 When two sliding surfaces come into contact, asperities on the softer surface are

deformed by repeated loading to generate a relatively smooth surface Eventually asperity-to-asperity contact is replaced by asperity-plane contacts and the softer surface experiences cyclic loading as the asperities of the harder surface plough through it

2 Surface traction by the harder asperities on the softer surface induces plastic shear

deformation

3 As the subsurface deformation continues, cracks are nucleated below the surface

Crack nucleation very near to the surface is inhibited by the triaxial compressive stress existing just below the contact region

4 Further loading causes the cracks to propagate parallel to the surface

5 When these cracks finally intercept the surface, long-thin wear sheets “delaminate”

giving rise to plate-like particles

Figure 4 shows the initiation of subsurface cracks which then spread to release laminae The preponderance of plate-like particles under conditions of lubricated smooth sliding provides powerful evidence for some mechanism of the type proposed in the delamination theory

Pitting

As indicated earlier, counterformal contact between solids leads to shearing stresses which attain their maximum value a short distance within a surface When there is relative motion, either rolling, sliding, or a combination of both, a band of material will be stressed repeatedly and cracks will form at points of stress concentration such as nonmetallic inclusions These subsurface cracks will eventually reach the surface and release a flake of metal to create a pit

A number of investigators have observed spherical particles about 1 µm in diameter which appear to be formed within the growing crack (see Figure 11) These particles are found during the early stages of crack formation and provide an early warning of impending surface pitting

Materials are often evaluated for resistance to pitting using disc machines, but the life of actual gears may be considerably less than would be expected from these results.35 The difference may be due to dynamic loading or the transient nature of the film forming process Berthe6 distinguishes between pitting failure from the action of Hertzian stress and micro-pitting which may be related to the stresses arising from interaction of surface asperities Pitting failure may be minimized by using hard-clean steel When parts are case-hardened, the hardened case must be sufficiently thick to embrace the zone of maximum Hertzian shearing stress

Abrasion

Definition

Abrasive wear may be defined as damage to a surface by a harder material This hard material may be introduced between two interacting surfaces from outside, it may be formed

in situ by oxidation, or it may be the material forming the second surface.

The action of granular abrasive particles has been simulated by Sakamoto and Tsukizoe46

who used cones of mild steel sliding on copper under a normal load of 9.8 N Figure 5 shows front-ridges of displaced material formed by a steel cone having an apex angle of 160° Although the hardness of the steel rider was about twice that of the copper, the depth

of the groove diminishes with distance of sliding This indicates that the harder of two bodies also suffers plastic deformation during the confrontation

Test Methods for Abrasion Resistance

Results of many tests of the resistance of materials to abrasive wear using abrasive paper

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Table 2 RELATIVE WEAR RESISTANCES RECORDED IN THE FIELD AND IN LABORATORY TESTS

Table 3

Material Hardness (MNm–2 )

Silicon carbide 29 420 The laboratory tests using 180 grit paper gave wear resistances for some materials which were much higher than those recorded in soils When larger grit was used (40 or 36) the high resistance was not repeated and there was good correlation with field results A dis-tinction is, therefore, drawn between hard abrasive wear which is little effected by particle size and soft abrasive wear Transition from hard to soft abrasive wear appears to occur when the ratio of the hardness of the metal in the fully work hardened condition to the hardness of the abrasive drops below 0.8 During the soft abrasive wear of heterogenous marterials (these having some phase harder than the abrasive and some softer) particle size

is particularly significant Large carbide particles may obstruct wear whereas features which are small compared with a chip of wear debris are ineffective Suh et al.53conclude that the increasing particle size causes a transition from the cutting type of wear to the sliding mode (see Section on Gouging)

Impact Wear

Percussive Impact

A number of tools, notably rock drills, are used in the percussive mode: they strike the work at right angles to its surface This gives rise to both Hertzian and oscillatory stresses governed by the speed of sound within the material In an investigation using a ballistic impact test machine, Engel11 showed that there was an induction period involving hardly any change followed by roughening and general deterioration of the surface A typical induction period was 10 cycles for air-hardening tool steel

Percussive wear on elements stressed beyond their elastic limit has been studied by Wellinger and Brechel.59They reported good experimental agreement between the logarith-mic slopes of impact wear and impact velocity

Abrasion by Impacting Particles

The limited applicability of erosion abrasion models based on mechanical properties has resulted in recent investigations into the thermal nature of the impact zone It was recognized that only 5% of the expended energy was used in mechanical work, the remaining energy dissipated in heating and melting the target material.49Ascarelli2proposed a thermodynamic parameter “thermal pressure”, the product of coefficient of linear expansion of the metal, bulk modulus and the difference between the target material temperature and its melting point The erosion resistance of metals ranging from tin to tungsten was shown to be proportional to this function Hutchings16suggested that lip formation on the impact crater edge was the main source of erosion damage and concluded that the erosion resistance of

a metal was proportional to the product of specific heat, density, and the difference between the metal temperature and melting point

The parameters above successfully predict the erosion behavior of most ductile metals, notable exceptions are, however, alloy steels Jones and Lewis21have shown for a range of alloy steels considered for use in gun barrels that erosion resistance was inversely proportional

to the linear expansion coefficient, Figure 6

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FIGURE 7. Gouging type wear of grey cast iron (Material extracted from Swansea Tribology Centre, Rep No 76/331; Guide to the Selection of Materials to

Resist Wear, by permission of Swansea Tribology Centre.)

main effect by abrasion It frequently occurs between components which are not intended

to move, press fits, for example While an increase in hardness sometimes reduces fretting, hardening of interacting components does not prevent it

At temperatures above 200°C the fretting of mild steel diminished with temperatures until

a second transition was reached between 500 and 600°C above which the wear rate in-creased.14,15 This variation is attributed to the different nature of oxide formed at different temperatures Above 380°C the proportion of Fe3O4to Fe2O3increased with a corresponding reduction in wear rate FeO appeared to be the most harmful oxide

Environment has an important effect on fretting Wright62 found that dry conditions produced very rapid wear which was reduced as the relative humidity was increased to 30% Further increase in humidity up to 100% resulted in increased wear

There appears to be no complete cure for fretting apart from eliminating relative motion Phosphating the surfaces may be a palliative and ion plating can delay the incidence of fretting.14 Ion-plated chromium, cadmium, and zirconium were effective in preventing fret-ting Best results were obtained from ion-plated boron carbide film which (at small ampli-tudes) prevented fretting up to 5 × 105cycles of movement

Corrosive Wear

Some chemical attack is likely on any exposed surface, and in normal atmospheric con-ditions this likely takes the form of oxidation Quinn and Sullivan39 described a system where oxidation occurs on virgin metal formed by the dislodgement of a wear fragment This oxidation will proceed at an increasing rate until a critical oxide thickness is reached

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FIGURE 8 Fine firecrack pattern typical of a differentially hardened steel (From Dickinson W A and

Porthouse, D., in Tribology 1978 Materials Performance and Conservation, Proc Inst Mech Eng Convention,

1978, 71 With permission.)

Table 4 EFFECT OF METALLURGICAL FEATURES ON WEAR OF ROLL STEELS

Phase Roll properties

Ferrite Resistance to breakage, good grip, good

firecrack resistance, poor wear and low hardness; high toughness

Lamellar Good strength and firecrack resistance;

Pearlite reasonable wear; finer pearlite improves

strength and wear but at the expense of ductility and firecrack resistance; good grip

Spheroidized Combines high toughness with reasonable Pearlite wear resistance; good firecrack resistance;

relatively soft; good grip Bainite High strength and hardness together with

good wear resistance; upper bainite structures have good firecrack resistance Martensite Very high hardness, good surface finish;

very good wear resistance; poor firecrack resistance; low toughness

Carbide Extremely good wear resistance;

high-carbide contents cause embrittlement; this effect can be alleviated by suitable heat treatment with carbon contents of up to 1.4%

Graphite Improves firecrack resistance and spall

resistance; lowers strength due to internal notch effect; this effect can be largely avoided by producing nodular graphite;

improves grip

The film then cracks up due to such factors as differences in thermal expansion of the metal and its oxide

Oxidative wear may occur by spalling of oxide flakes from a substrate which itself shows little evidence of deformation Shivaneth et al.48 investigated the transition from oxidative wear to severe mechanical wear of binary aluminium-silicon alloys

Cavitation Erosion

When material is subjected to a hydrodynamic situation wherein bubbles are formed and then collapse due to violent changes in pressure, the surfaces become damaged by pitting sometimes followed by gross removal of material.55

Lord Rayleigh40 related the instantaneous pressure developed in a liquid due to collapse

of a cavity (bubble) with the compressibility and density of the liquid and the speed of collapse Wilson and Graham60 reported that weight loss of silver surfaces correlated well with this concept and that erosion damage may be related to the product of the density and the speed of sound in a liquid Cavitation erosion has been shown to be characterized by a delay period in which little or no damage occurs followed by a period of wear at a constant rate.30 Duration of the delay period is determined by the initial surface state and is closely related to the endurance limit in mechanical fatigue tests Once cavitation has commenced,

From Dickinson, W A and Porterhouse, D., in Tribology 1978

Materials Performance and Conservation, Proc Inst Mech.

Eng Convention, University College of Swansea, 1978, 71.

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the rate of removal of material is broadly related to its strength as measured by diamond hardness or ultimate tensile strength The effect may not be entirely mechanical because the nature of the liquid, i.e., whether or not an electrolyte, markedly affects test results

Thiruvengadam55has observed the formation of spherical particles of the type illustrated

in Figure 9 Size of the spheroids varied from 0.5 to 30 µm Origin of the particles is attributed to collapse of cavitation bubbles This collapse produces indentations at very high rates of strain causing metal to melt and to splash into the surrounding fluid

Electrical Wear

Electrical switchgear embodies contacting members which function in accordance with the following sequence: (1) to close the circuit, (2) to allow current to flow when required, and (3) to open the circuit and suppress the current Repeated operation results in surface deterioration from electrical effects as well as ordinary mechanical wear

When two charged conductors approach each other, intense electro-static forces are set

up at microscopic protuberances so that conduction can commence even before physical contact is made Once the circuit has been completed, contaminating films or rough surfaces will limit areas of true contact so as to concentrate the current and melt the metal locally

As the contact begins to open, the current becomes concentrated at fewer and fewer points

of contact until finally restricted to a single microscopic area A molten globule of metal is formed and the temperature can reach the boiling point of the metal when it evaporates or even explode Detailed analysis of the rupture of a micro-bridge by Llewellyn Jones29

indicates the following progression

1 A small gap (10–6m) is set up between the two electrodes

2 Each contact spot reaches a high temperature becoming an intense thermionic emitter

3 The gaseous atmosphere becomes mixed with metal vapor Self-inductance of the local circuit can set up a pulse of voltage sufficient to produce ionization of the gas or metal vapor This will generate a micro-arc which may be the primary cause of electrical wear

RESIDUAL STRESS

Stress can become “locked in” to a solid as the result of combinations of plastic and

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FIGURE 9 Particles arising from cavitation erosion test (From

Thi-ruvengadam, A., Trans ASLE, 21, 344, 1978 With permission.)