siandaii ch1 wear course

8/22/2019 SIandAII Ch1 Wear Course

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Surfaces, Interfaces, and their Applications II Introduction to Wear

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1Introduction to Wear1.1 Recap: Definition of TribologyTribology is the science and technology of interacting surfaces in relative motion and related

subjects and processes. Tribology covers the fields of friction, lubrication, and wear.

1.2 Recap: Normal Loads, Contact Areas and Roughness

Figure 1-1: Contacts between real surfaces.

Rubbing surfaces are never perfectly flat, but they have a certain amount of roughness. The

normal load is not borne by the apparent area of contact, but by a much smaller true area ofcontact. This corresponds to the raised areas, or asperities on each surface, coming into

contact with each other. Since these contact areas are relatively small, the correspondingpressures are relatively large, with the result that the asperities become plastically deformed

upon contact with each other. This continues until the contact areas increase sufficiently suchthat the contact pressure drops below the elastic limit of the material.

1.3 Recap: Friction and Lubrication1.3.1 Definition of FrictionFriction can be described as the resistance encountered when one body is moved over another.


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Figure 1-2: The frictional force needs to be overcome, in order to cause a body to slide.

The relation between normal load and frictional force is represented by the friction coefficient,

:

=F/W

-values describe the magnitude of the frictional force. Values beween around 0.001 (e.g. in alubricated bearing under light loads) and around 10 (e.g.when clean metals rub against each

other in vacuum) are encountered, although most -values for sliding surfaces in air fallbetween 0.1 and 1.0.

1.3.2 The Stribeck CurveIt is frequently observed in lubricated systems that the friction coefficient changes as a function

of speed. This is shown in the well-known Stribeck curve, in which the friction coefficient, , isdisplayed as a function of the viscosity multiplied by the tangential velocity, and divided by the

normal load (U/W). The curve can be divided into three regions.

Figure 1-3: Change of the friction coefficient with the quantity U/W for lubricated sliding bearings: TheStribeck curve.


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1.3.3 Boundary LubricantsAt very high loads or low speeds, hydrodynamic forces can no longer maintain a lubricant filmbetween the sliding surfaces and direct contact between the asperities starts to become

important. A boundary lubricant is essential under these conditions, in order to avoidexcessive friction and wear. Boundary lubricants form adsorbed molecular films on the

surfaces. The repulsive forces between the films carry a significant part of the load and shieldthe asperities from unprotected contact.

Figure 1-4: Simplified view of boundary lubrication: polar end-groups of hydrocarbon chains bind to the surfaces

and form a layer of lubricating molecules that reduce direct contact between asperities. There is much evidence

that, in reality, thicker layers are frequently formed, although monolayers have been demonstrated to afford some

degree of protection.

Examples of boundary lubricants are the long-chain carboxylic acids, whose polar end-groupsmake a strong bond with the oxide layer of the metal surfaces. The long chains can orient

themselves and form protective layers, 2-3nm thick, sometimes reacting with the substrate toform thicker layers (metal soaps). The reduction of the friction coefficient is dependent on

the chain length.

Figure 1-5: Variation of friction coefficient as a function of the lubricant chain length for steel surfaces, lubricated

with carboxylic acids and alcohols.


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Figure 1-6: Friction coefficient as a function of chain length for hydrocarbons on tungsten surfaces.

Many plant oils (e.g. rapeseed oil) contain long-chain molecules that are polar, and can function

as boundary lubricants. Only a small concentration (0.1-1wt.%) of such molecules is necessary,in order to afford a certain degree of surface protection under boundary conditions.

A second class of molecules that protects surfaces under extreme conditions is known as

Extreme-pressure additives. These are molecules that only react once extreme conditions(e.g. pressure, temperature) are reached, at which point they react with the surface, producing aprotective layer with low shear strength in exactly the right place. This class of additive

frequently contains sulfur, phosphorus, or chlorine, forming sulfides, phosphates, or chlorideson the surface, which possess low shear strength. Frequently used molecules include zinc

dialkyldithiophosphate (ZnDTP), tricresyl phosphate (TCP) and dibenzyl disulfide (Figure 7-17). The mechanism of action of ZnDTP is extremely complex and has been extensively

investigated, and a model of the structure of the protective film is shown in Figure 7-18.


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Figure 1-7: Structure of three extreme pressure antiwear additives: (a) tricresyl phosphate (TCP) (b) dibenzyl

disulfide (c)zinc dialkyl dithiophosphate (ZDTP).

Figure 1-8 Structure of protective film formed by ZnDTP on steel


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The development of a lubricant requires considerable effort, since the various additives may not

be compatible with each other. In particular, it is possible that competitive adsorption betweenadditives leads to a reduction of the effectiveness of EP additives. On the other hand, excessive

reaction of the EP additives with the surface can lead to corrosive effects. Many EP additives

are ecologically undesirable and alternatives are being urgently sought.

1.4 Wear1.4.1 The Archard Wear EquationWhenever surfaces rub against each other, wear almost always occurs. A simple analysis(Holm and Archard), which was originally developed for metals, yields a wear coefficient, K,

which is frequently used in practice. One of the assumptions for the analysis is that the contactbetween the surfaces occurs at asperities and that the true contact area is the sum of the

individual asperity contacts. This area is proportional to the normal load and the localdeformation is assumed to be plastic.

Figure 1-9: Schematic diagram of the formation of a contact area during the relative motion of two asperities.In Figure 1-9, the contact between two asperities is shown. In (c), the contact area becomesmaximal and the normal load, which is supported by this contact area, is given by:

W=pa2

Flow pressure:p

Radius of the contact area: a

p is the flow pressure, which normally corresponds to the indentation hardness, H.

During sliding [(d) and (e)], the task of supporting the normal load is taken over by other

asperity contacts, such that continuous sliding involves continuous creation and destruction ofsingle asperity-contact areas. Wear is due to the removal of fragments of material from theasperities. The volume of the single fragments depends on the size of the asperity-contact areas,

i.e. the volume of the removed materials is proportional to the cube of the contact dimension, a.

If we assume that this volume consists of a half-sphere of radius a,

V =2

3 a

3


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Not all asperity contacts lead to wear particles. Let us assume that a proportion, forms

particles. The mean wear volume, V, produced by the sliding of an asperity pair over eachother is given, per unit sliding distance, by:

Q = V2 a

= a2

3

The total wear rate, due to all asperity contacts, Q, is the sum of all contributions over the total

contact area:

Q = Q =

3 a

2

The total normal load is given by:

W= W = p a2

And so the wear rate, Q, is

Q =W

3p=

KW

H

In the second part of this equation,/3 has been replaced by K and p by the indentation

hardness, H. This equation, which shows the relation between worn volume per unit sliding

distance and the normal load and hardness of the softer surface, is often called the Archard

Equation. The constant,K, which is usually called the wear coefficient, is dimensionless, andalways


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Figure 1-10: Wear, shown as pin wear volume over the entire sliding distance in an unlubricated pin-on-ring test,sliding on tool steel (when not otherwise mentioned).

The curves in Fig. 1-10 show the characteristics of the materials under steady-state conditions.

Surfaces must often be run-in before this behavior can be observed.

The following table shows values of the dimensionless wear coefficient, K for various materialssliding on tool steel, obtained in an unlubricated pin-on-ring test in air. The values are highly

dependent on test conditions and thus provide little insight into the wear mechanisms involved.

Material Sliding partner Wear coefficients, K

mild steel Mild steel 710

-3

/ Brass Tool steel 610

-4

PTFE Tool steel 2.510-5

Brass Tool steel 1.710-4

PMMA Tool steel 710-6

copper-beryllium Tool steel 3.710-5

hardened tool steel Tool steel 1.310-4

Stellite 1 Tool steel 5.510-5

stainless steel (ferritic) Tool steel 1.710-5

PE Tool steel 1.310-7

The values are all between 10-7

and 10-2

. This is a far greater range than seen with friction

coefficients. There is no apparent correlation between the two quantities.

1.4.2 Mild and Severe WearDuring wear experiments with brass, the following behavior is observed:


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Figure 1-11: Wear and electrical contact resistance for a pin made from /-brass sliding on hardened stellite

(Co-alloy) as a function of the normal load.

These experiments were carried out using hexadecane as a lubricant, but not underhydrodynamic conditions. Similar results were obtained during dry sliding. At low loads, the

Archard equation holds, with a K-value of around 210-6

. At loads between 5 and 10N, the wearrate increases significantly by a factor 100. Above this region, the Archard equation holds once

more, with a new wear coefficient of 10-4

. The wear-rate transition coincides both with a drop in


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electrical resistance, as well as a change in the color and size of wear particles. Below thetransition, the behavior is known as mild wear, and above the transition as severe wear.

Mild wear involves small (0.01 - 1 m) wear particles, which mostly consist of oxides. On theother hand, the characteristic particles during severe wear are much larger (20 - 200m) and

chiefly metallic in composition. Severe wear is unacceptable in most practical applications.

The mechanism of the transition is confirmed by the following results:

Figure 1-12: Variation of wear coefficient with sliding speed for/brass sliding on steel at various temperatures

and environments.

1.5 Measurement of WearA standard result review for wear tests, defined by the ASTM International, should be

expressed as loss of material during wear in terms of volume. The volume loss gives a truerpicture than weight loss, particularly when comparing the wear resistance properties of

materials with large differences in density.

For example, a weight loss of 14 g in a sample of tungsten carbide + cobalt (density =14000 kg/m) and a weight loss of 2.7 g in a similar sample of aluminium alloy (density =

2700 kg/m) both result in the same level of wear (1 cm) when expressed as a volume loss. Theinverse of volume loss can be used as a comparable index of wear resistance.

A common unit for measuring wear is volume/sliding distance/load (mm3/m/N). This makes the

assumption that the Archard equation holds!


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1.6 Types of Wear1.6.1 Adhesive WearAdhesion is the result of the direct contact of bearing metals. When the applied load issufficient to rupture any protective surface film (oxides, etc.), the contacting asperitiesdeform

elastically, then plastically. Welding of these asperities may occur on contact, but occurs morereadily when relative motion takes place. The shearing of these adhesive junctions produces

wear particles; it is through this process also that metal is transferred from one bearing surfaceto the other in sliding or rolling motion. When adhesive-wear damage is severe, it is referred to

asscuffing. If frictional heating causes decomposition and/or desorption of protective filmsfrom the surface, the process can become destructive.

Figure 1-13: Process of adhesive wear (www.machinerylubrication.com)

The factors decreasing adhesive wear (www.substech.com):

Lower load. Harder rubbing materials. Contaminated rubbing surfaces. Presence of solid lubricants. Presence of a lubricating oil. Anti-wear additives in oil.

1.6.2 Abrasive Wear (www.substech.com)Abrasive wear occurs when a harder material is rubbing against a softer material.

If there are only two rubbing parts involved in the friction process the wear is calledtwo-body wear.

In this case the wear of the softer material is caused by the asperities on the harder surface.

If the wear is caused by a hard particle (grit) trapped between the rubbing surfaces it iscalled three-body wear. The particle may be either free or partially embedded into one


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of the mating materials.

Figure 1-14: Process of abrasive wear (www.substech.com)

On the micro-level, abrasive action results in one of the following wear modes:

Ploughing. This wear mechanism is mainlygoverned by plastic deformation. Sliding ofthe constrained abrasive particle occurswithout material being removed from the

wearing surface, as it is shifted to the sides ofthe wear groove. (Picture, EC de Lyon,

Elleuch et al, Industrial Lubrication andTribology, Vol. 55 Iss: 6 pp. 279 286.

Ploughing of Aluminium alloy)

Cutting. Here, also the wear mechanism is alsoin part governed by plastic deformation. In

fact, the abrasive particle acts as a cutting tooland a chip is formed in front of the cutting

edge of the abrasive particle. In this case, lostmaterial from the wearing surface occurs in a

volume equal to the volume of the wear track(groove).


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Picture Substech.com. Cracking (brittle fracture). Always occurs

when highly concentrated stresses areimposed by abrasive particles. Due to a poorplastic deformation ability of the considered

material, large wear fragments are detachedfrom the wearing surface owing to

microcrack formation and propagation. Thevolume of the lost material is higher than the

volume of the wear track. (Photo: R.Crockett, Empa. Worn alumina femoral head

from artificial hip joint)

In conclusion, ploughing and cutting are the dominant wear mechanisms encountered

with ductile materials, while cracking becomes important on brittle materials where the

hardness is considered as a unique answer to oppose the abrasive wear in service.

1.6.3 Surface FatigueFatigue wear of a material is caused by cycling loading during friction. Fatigue occurs if theapplied load is higher than the fatigue strength of the material. Fatigue cracks start at the

material surface and spread to the subsurface regions. The cracks may connect to each other

resulting in separation and delamination of the material pieces. One of the types of fatigue wearis fretting wear caused by cyclic sliding of two surfaces across each other with a smallamplitude (oscillating). The friction force produces alternating compression-tension stresses,

which result in surface fatigue.


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Figure 1-15: Process of fatigue wear (www.substech.com)

1.6.4 Erosive WearErosive wear is caused by impingement of particles (solid, liquid or gaseous), which remove

fragments of materials from the surface due to a momentum effect.

Erosive wear of Engine bearings may be caused by cavitation in the lubrication oil. Thecavitation voids (bubbles) may form when the oil exits from the convergent gap between the

bearing and journal surfaces. The oil pressure rapidly drops, providing conditions for voidsformation (the pressure is lower than the oil vapor pressure). The bubbles (voids) then collapse

producing a shock wave, which removes particles of the bearing material from the bearing.

1.6.5 Corrosive WearWear may be accelerated by corrosion (oxidation) of the rubbing surfaces. Increased

temperature and removal of the protecting oxide films from the surface during the friction

promote the oxidation process. Sliding provides continuous removal of the oxide film followedby continuous formation of new oxide film. Hard oxide particles removed from the surface andtrapped between the sliding/rolling surfaces additionally increase the wear rate by three-body

abrasive wear mechanism.

1.7 Stages of Wear(NASA SP 8063, June 1971)

Wear takes place even in properly lubricated mechanisms (ball bearings, gears, bushings, cams,etc.) according to a definite pattern that is shown generalized in Figure 1-16. The initial wear

rate (run-in) is relatively high because the microscopic surface asperities penetrate the lubricantfilm, particularly during the low speeds of start-up and shut-down. This rate is aggravated by

oscillating motion and by increasing loads. If conditions are not sufficiently severe to causescuffing in this stage, the wear rate is termed mild.

Normal wear begins when the true area of bearing contact (total asperity-contact area) has been

substantially increased by plastic deformation and wear to the extent that the lubricant is fullyable to support the load; there will then be only occasional metal-to-metal contacts. From this

point on, wear occurs at a negligible rate, modified only by such events as lubricant breakdown,sudden rises in temperature (which reduce oil viscosity), shock loads, or a significant reduction

in speed.

Normal wear may proceed indefinitely, depending on the several controlling variables, untilsufficient debris has accumulated to cause one of the following failure modes:

Stress risers in the path of motion, ultimately initiating fatigue

Abrasive wear sufficient to change surface roughness or dimensions appreciably


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Physical blockage (e.g., in gear-teeth roots)

When severe wear or scuffing sets in, the system is so close to failure that the rates of wear areonly of academic interest.

Figure 1-16: Generalized pattern of the wear process (NASA SP-8063, 1971)

siandaii ch1 wear course

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