ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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23. E. Yamamoto, K. Wada, T. Fukyuka, K. Shimogori, K. Fukiwara, and K. Tsuji, “Lubricating Films to

Prevent Galling of Stainless Steel Parts,” 38th ASLE Meeting, April 1983 (Houston), American Society

of Lubrication Engineers

24. D.G. Frick, “Drill Collar Connection Trial,” Report DGF2-85, Carpenter Technology Corporation, July

1985

25. G.W. White, Eliminating Galling of High-Alloy Tubular Threads by High-Energy Ion Deposition

Process, J. Pet. Technol., Aug 1984, p 1345–1351

Sliding Contact Damage Testing

Prevention of Galling

Preventing galling damage is a critical part in applications where parts are sliding against each other under high

loads and low speeds. It becomes a bigger issue when corrosion-resistant alloys, such as stainless steels, are

required under nonlubrication conditions. Despite the best efforts of designers and users, occasions also arise

when close clearances result in the contact and rubbing of components in rotating machinery. Of key

importance in this case is the prevention of galling, because this can cause seizure and severe damage.

Galling can be resisted in several ways. For applications in which galling is of concern, the following

guidelines should be considered:

• Lubricate where possible.

• Keep load, temperature, and speed as low as possible.

• Parts should be dimensionally tight with sufficient clearance.

• Use a surface finish between 0.25 and 1.75 μm (10 and 70 μin.) whenever possible (many stainless parts

are electropolished, which can lead to galling and wear).

• Increase contact area, so that there is less stress on parts and less depth of wear.

• Carefully select alloys in unlubricated systems, or where insufficient lubrication may be present.

Dissimilar-mated couples with high threshold galling stress values can be chosen or high work-

hardening rate austenitic stainless alloys can be selected for improved adhesive and cavitation wear

resistance and galling resistance.

• Use surface treatments, such as nitriding, carburizing, and hardface coating, or solid lubricant coatings

(i.e., molybdenum disulfide or graphite).

Galling resistance can sometimes be aided by heat treating the opposing parts so that they have a hardness

difference of at least 50 HB, which encourages wear of the softer material rather than adhesion and resultant

part-to-part material transfer (Ref 26). Another method of discouraging galling is to machine grooves in one or

both of the close-clearance components, so that as wear takes place the debris can collect somewhere other than

at the close running clearance. This also promotes rapid heat transfer at rubbing interfaces, keeping parts cool

and hard. Local surface temperatures can become very high even with grooving, because of the flash-

temperature effect, but such temperatures decay in short distances and do not result in galling if surface heat

removal is effective (Ref 27). Grooving the surfaces results in a design compromise, however. Although

grooving reduces clearance leakage, it also reduces the beneficial shaft support provided by the Lomakin effect

(Ref 28).

Finally, the test methods described in this article also provide a basis in the evaluation and prevention of

galling. The general comparison of galling potential for different materials can be done by the measurement of

the contact stress required for cold welding and subsequent material pullout for a material mated against itself.

This is called threshold galling stress. A complete listing of threshold galling data for stainless steels can be

found in Ref 12.

References cited in this section

12. J.H. Magee, Wear of Stainless Steels, Friction, Lubrication, and Wear Technology, Vol 18, ASM

Handbook, P. Blau, Ed., ASM International, 1992, p 710–724

26. W. Marscher, A Phenomenological Model of Abradable Wear in High Performance Turbomachinery,

Wear, Vol 59, 1980, p 191–211

27. W. Marscher, “A Critical Evaluation of the Flash Temperature Concept,” Preprint 81-Am-1D-3,

American Society of Lubrication Engineers, 1981

28. W. Marscher, Wear of Pumps, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, P.

Blau, Ed., ASM International, 1992, p 594, 595

Sliding Contact Damage Testing

Fretting Wear

R.B. Waterhouse, University of Nottingham (United Kingdom)

Fretting refers to a special form of surface wear that occurs from small-amplitude tangential oscillations

between two surfaces in contact. The amplitude (or magnitude) of the relative motion in fretting wear is what

distinguishes it from other forms of wear during unidirectional and reciprocating sliding contact. In practical

cases, fretting wear occurs from extremely small repetitive motion, usually less than 25μm peak-to-peak

amplitude.

One immediate consequence of the fretting process in normal atmospheric conditions is the production of oxide

debris; hence the terms “fretting wear” and “fretting corrosion” are used for this phenomenon. Surface damage

from fretting begins with local adhesion between mating surfaces and progresses when adhered particles are

removed from a surface. When adhered particles are removed from the surface, they may react with air or other

corrosive environments. Affected surfaces show pits or grooves with surrounding corrosion products.

The movement is usually the result of external vibration, but movement also occurs from cyclic contact stresses

(fatigue) between mated parts. This fact gives rise to another and usually more damaging aspect of fretting,

namely the early initiation of fatigue cracks. This is termed fretting fatigue or contact fatigue. Fatigue cracks

may also be initiated where the contacting surfaces are under a very heavy normal load or where there is a static

tensile stress in one of the surfaces. There are cases where the movement is not simply tangential, but is

complicated by the normal force also oscillating to the extent that the surfaces lose contact in each cycle. This

leads to a hammering effect, which is termed impact fatigue. In this case, the phase relationship between the

two motions can be an important factor. Fretting fatigue and the associated methods of testing are described in

more detail in the article “Fretting Fatigue” in this Volume.

This section describes the testing and the special problems in the evaluation of fretting wear. For example, one

important feature of fretting is that the debris or wear product remains between the surfaces and can play a role

in the development of the process. This is particularly true where the surfaces are flat or conforming as in, for

example, a hub on an axle. In many experimental investigations, the common type of geometry has been the

sphere or cylinder on a flat.

Another problem in investigating fretting wear in the laboratory has been devising systems to produce

controlled movement of extremely small amplitude and the ability to measure and monitor that amplitude in the

very area of the contact. It follows, of course, that the amount of wear debris produced is also very small, which

creates problems where quantitative measurements are required. This section describes how these problems

have been overcome by investigators in the past.

Sliding Contact Damage Testing

Fretting Mechanism

In general, fretting occurs between two tight-fitting surfaces that are subjected to a cyclic, relative motion of

extremely small amplitude. Although certain aspects of the mechanism of fretting are still not thoroughly

understood, the fretting process is generally divided into the following three parts: initial conditions of surface

adhesion, oscillation accompanied by the generation of debris, and fatigue and/or wear in the region of contact.

Fretting wear occurs from repeated shear stresses that are generated by friction during small amplitude

oscillatory motion or slip between two surfaces pressed together in intimate contact. In fretting, the term slip is

used to denote small amplitude surface displacements, in contrast to sliding, which denotes macroscopic

displacements. In many cases, slip only occurs over part of the contacting surfaces and is therefore referred to

as partial slip. Fretting damage has been detected at amplitudes of less than 1 μm (Ref 29). As the amplitude is

increased, the process resembles unidirectional or reciprocating sliding wear. The upper limit has been

suggested as 75 μm (Ref 30), and Fig. 13 shows the volume of wear damage as a function of slip amplitude.

Fig. 13 Effect of slip amplitude on fretting damage of mild steel. Source: Ref 30

The severity of fretting damage is influenced by several factors including:

• Contact Load. As long as fretting amplitude is not reduced, fretting wear will increase linearly with

increasing load.

• Amplitude. There appears to be no measurable amplitude below which fretting does not occur. However,

if the contact conditions are such that deflection is only elastic, it is not likely that fretting damage will

occur. Fretting wear loss increases with amplitude. The effect of amplitude can be linear, or there can be

a threshold amplitude above which a rapid increase in wear occurs (Ref 30). The transition is not well

established and probably depends on the geometry of the contact.

• Frequency. When fretting is measured in volume of material removed per unit sliding distance, there

does not appear to be a frequency effect.

• Number of Cycles. An incubation period occurs during which fretting wear is negligible. After the

incubation period, a steady-state wear rate is observed, and a more general surface roughening occurs as

fretting continues.

• Relative Humidity. For materials that rust in air, fretting wear is higher in dry air than in saturated air.

• Temperature. The effect of elevated temperature on fretting depends on the oxidation characteristics of

the material.

The effects of these factors are discussed in more detail in Ref 30 and 31. The article “Fretting Fatigue

Testing”in this Volume also provides additional details on the effects of these test variables. The main focus of

this section is on fretting test rigs and wear measurements.

References cited in this section

29. S.R. Brown, Materials Evaluation under Fretting Conditions, STP 780, ASTM, 1981, p 30

30. R.B. Waterhouse, Fretting Corrosion, Pergamon, 1972, p 69, 133

31. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook,

P. Blau, Ed., ASM International, 1992, p 242–256

Sliding Contact Damage Testing

Fretting Rigs

Experimental rigs for investigating fretting wear are either driven mechanically or by an electromagnetic

vibrator. Figure 14 shows a mechanical rig driven by an electric motor with eccentric loading device. Other

methods of producing small amplitude have been the use of rotating out-of-balance weights, but control here is

much more difficult.

Fig. 14 Mechanical fretting wear rig. LVDT, linear variable differential transformer

A key factor in test rigs is the type of contact, because the ease with which debris can escape from the contact

region influences the fretting process itself. For example, the escape of debris in the crossed-cylinder

arrangement (Fig. 15) is greatly influenced by the direction of motion (Ref 32). The arrangement shown in Fig.

15(b) allows the debris to escape by being pushed out by the axial movement of the upper cylinder, leading to

more frequent metal-to-metal contact and a higher wear rate than the arrangement shown in Fig. 15(a).

Fig. 15 Two test directions for determining fretting in a crossed-cylinder arrangement. (a) Parallel to

axis of lower specimen. (b) At right angles to axis of lower specimen. Source: Ref 31

The original machine designed by Tomlinson (Fig. 16) comprised a long horizontal lever connected to an

annular specimen in contact with a horizontal flat specimen with the load applied by a vertical rod through the

center of the specimen passing through a hole in the flat specimen. By applying an oscillating circumferential

motion to the end of the lever of, for example, 5 mm (0.2 in.), this would be reduced at the specimen to 5 μm

(200 μin.) for a lever 1 m (3.28 ft) in length. More recent machines (e.g., Fig. 14) use an adjustable eccentric

producing a horizontal movement that is transmitted via a sleeve bearing to the upper specimen, which can be a

ball bearing, a spherical- ended slider, or a flat in contact with a fixed-flat specimen. The normal load can be

applied by a dead-weight system. It is advisable to have as few junctions as possible between the eccentric and

the specimen since movement will be lost in them. The motor driving the eccentric should have sufficient

power to force the movement, particularly as considerable changes in the coefficient of friction can occur

during fretting. Amplitude is measured with a proximity or capacitance gage. Rotational movement can be

measured with an optical lever.

Fig. 16 Fretting test of Tomlinson involving contact between annular ring and flat. Source: Ref 30

In experimental investigations, the ease and cost of preparing specimens are significant factors, and the crossed-

cylinder arrangement (Fig. 15) is one of the most convenient. However, as previously noted, the ease with

which debris escapes can influence results. In this regard, Tomlinson's original design of an apparatus with

torsional vibration of annulus on flat has much to commend it because no part of the contacting surfaces

becomes exposed, and, for debris to escape, it must move at right angles to the direction of motion. A further

slight advantage is that the amplitude of slip has a small variation from the inner to the outer edge and can

therefore be used to investigate the effect of amplitude in one test. With flat contact surfaces, however, the

initiation and development of areas of wear damage are sporadic no matter how carefully the surfaces are

prepared and the alignment controlled. Contact pressure is usually expressed as the nominal value calculated

from the apparent area of contact and the applied load.

Friction Monitoring. It is usually necessary to monitor the friction during a test. This can be accomplished by

strain gaging the connecting rod to the specimen to measure the tangential force. Such an arrangement can be

made more sensitive by using a tubular member. This arrangement can also provide cooling if the fretting

couple is to be enclosed in a furnace.

Frequency is controlled by driving the device with a variable-speed dc motor. Using electromagnetic vibrators

requires them to be of sufficient power for the purpose and has the same problems in transmitting the

movement to the fretting specimens. For low frequencies of less than 5 Hz, mechanical machines have the

advantage, but for higher frequencies even up to kHz, electromagnetic rigs (Fig. 14) are recommended. For

very high frequencies and small fretting amplitude piezoelectric rigs have been used (Ref 33). Other methods of

producing the small-amplitude movement have been to use rotating out-of-balance weights, but control here is

much more difficult.

The environment is an important feature of fretting testing. Humidity can have a marked effect, particularly

with reactive materials such as aluminum and titanium, and it should be controlled by enclosing the fretting

couple in a suitable container. This can also be used to provide other environments such as argon or carbon

dioxide. More specialized environments such as high temperature, low temperature, seawater or body fluids,

and even vacuum require more complicated equipment, but have all been successfully accomplished and are

documented in the literature. The author's most difficult task was to study fretting of stainless steels in an

atmosphere of carbon dioxide at 4050 kPa (40 atm) pressure and 600 °C (1110 °F). Descriptions of this rig and

others can be found in Ref 29 and 30.

References cited in this section

29. S.R. Brown, Materials Evaluation under Fretting Conditions, STP 780, ASTM, 1981, p 30

30. R.B. Waterhouse, Fretting Corrosion, Pergamon, 1972, p 69, 133

31. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook,

P. Blau, Ed., ASM International, 1992, p 242–256

32. M. Kuno and R.B. Waterhouse, The Effect of Oscillatory Direction on Fretting Wear under Crossed

Cylinder Contact Conditions, Eurotrib 89, Proc. 5th International Congress on Tribology, 12–15 June

1989 (Helsinki), Vol 3, p 30–35

33. P. Rehbein and J. Wallaschek, Wear, Vol 216 (No. 2), 1998, p 97–105

Sliding Contact Damage Testing

Measurement of Fretting Wear

The amount of material removed in fretting wear is extremely small, particularly in the partial slip regime, so

determining it by loss in weight of the specimen is impractical. The minute amount of debris, usually an oxide,

is difficult to remove for the purpose of weighing. The debris is often compacted. On steel, it is the α ferric

oxide, which is red in color but, when compacted, is blackish gray. It is the mineral hematite, which is black in

massive form but red when powdered.

It is much more satisfactory to attempt to measure the volume of material removed. One of the earliest methods

was to measure the area of damage (A) and then to find the depth by carefully machining the surface until the

last trace of damage had just disappeared and to estimate the depth (d) from the weight of material machined

off. The product Ad was thus a measure of the volume of material lost as wear. Nowadays computerized

profilometers are available that can survey the scar and produce a view of it as in Fig. 17. The computer can

also provide information on the volume below the datum of the original surface as well as material above the

datum, since often there is local plastic deformation arising from the fretting action. It is usual to express the

wear as the difference between these two figures.

Fig. 17 Profilometer projections of wear scars on two crossed-cylinder contacting specimens

Obviously if the wear is to be accurately determined, all the debris must be completely removed. This can

usually be achieved by ultrasonic cleaning. There have been chemical methods that involve the use of complex

selenium compounds that remove the debris but do not attack the underlying material. Removal of the debris

assists subsequent examination in the scanning microscope. Fretting wear is usually expressed, as are other

types of wear, as a specific wear rate, that is, the volume of material lost per unit distance of sliding per unit of

applied load.

A more sophisticated method of assessing fretting wear is to use thin-layer activation (TLA). It involves

irradiation of one of the surfaces with protons from the cyclotron, which results in the formation of

radionuclides, which in the case of steel is mainly cobalt-56. Measurement of the γ radiation from the debris or

transferred material is claimed to give accurate results with a minute amount of debris (Ref 34).

Reference cited in this section

34. S.R. Brown, Materials Evaluation under Fretting Conditions, STP 780, 1981, p 7

Sliding Contact Damage Testing

Specimen Preparation

There are two factors that can influence the results in fretting wear tests. These are the surface roughness of the

specimens and the existence of residual stresses in the specimen surface. Generally speaking, the more highly

polished the surface the greater the wear. This is attributed to the fact that the oxide debris is an abrasive

material and participates in the wear process. On a rough surface, contact is via well-defined asperities and the

debris can drop into the adjacent grooves. Residual stress has an effect because one of the basic mechanisms is

surface fatigue as exemplified in Suh's delamination theory (Ref 35). A residual tensile stress increases the

amount of wear, whereas a compressive stress reduces it (Ref 36). This is an argument for the application of

shot peening to prevent fretting wear since it roughens the surface and generates a compressive residual stress

in the surface. In most experimental investigations, it is usual to give the specimens a light polish with 000

emery paper followed by degreasing in acetone or ultrasonic cleaning. The SEM is a very suitable piece of

equipment for examining the surface damage. Figure 18 shows a typical example.

Fig. 18 SEM micrograph of fretting damage on a mild steel specimen showing compacted debris and

delamination

References cited in this section

35. P. de Baets and K. Strikckmans, Tribology Int., Vol 29 (No. 4), 1996, p 307–312

36. J. Labedz, Metal Treatments against Wear, Corrosion, Fretting and Fatigue, A. Niku-Lari and R.B.

Waterhouse, Ed., Pergamon, 1988, p 87–98

Sliding Contact Damage Testing

Reducing Fretting Wear

The first approach to a fretting problem is to consider the basic design of the contacting components. This is

particularly so if the fretting is the result of one of the members of the contact being subjected to a cyclic stress,

that is, fatigued. In this case, it is important if possible to reduce stress concentrations in the region of the

contact. In the case of a hub on a shaft, this is achieved by increasing the shaft diameter at the wheel seat with a

generous fillet radius. A similar effect can be achieved by providing a stress-relieving groove. Such design

changes are those customarily recommended in designing against fatigue. If the problem cannot be tackled in

this way, then recourse has to be made to surface treatments. Increasing the hardness of the surface by work

hardening, for example, by shot peening or surface rolling, or by diffusion treatments such as carburizing or

nitriding in the case of steel can be effective. Beyond that there is now a wide variety of treatments

encompassed in the term “surface engineering.” Hard coatings such as titanium nitride (TiN) can be

recommended if the substrate material is sufficiently strong to support them (Ref 37). However, if breakdown

occurs the result can be disastrous because one then has a very abrasive material in the contact. Ion

implantation has the advantage of surface alloying not possible by other means and also the development of

residual compressive stress. More information on the control of fretting wear is given in Ref 31 and the article

“Fretting Fatigue Testing” in this Volume.

References cited in this section

31. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook,

P. Blau, Ed., ASM International, 1992, p 242–256

37. M. Shima, J. Okado, I.R. McColl, R.B. Waterhouse, T. Hasegawa, and M. Kasaya, Wear, 225–229; Part

I, 1999, p 38–45

Sliding Contact Damage Testing

References

1. P. Heilman, J. Don, T.C. Sun, W.A. Glaeser, and D.A. Rigney, Sliding Wear and Transfer, Proc. Int.

Conf. Wear of Materials, American Society of Mechanical Engineers, 1989, p 1–8

2. W.R. Jones, S. Pepper, et al., The Preliminary Evaluation of Liquid Lubricants for Space Applications

by Vacuum Tribometry, 28th Aerospace Mechanisms Symposium, National Aeronautics and Space

Administration, May 1994

3. Glossary of Terms, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, P. Blau, Ed.,

ASM International, 1992

4. E. Rabinowicz, Wear Coefficient—Metals, Wear Control Handbook, American Society of Mechanical

Engineers, 1980, p 475–506

5. E. Rabinowicz, Wear Coefficient—Metals, Wear Control Handbook, American Society of Mechanical

Engineers, 1980, p 475–484

6. Friction and Wear Testing: Source Book of Selected References from ASTM Standards and ASM

Handbooks, ASM International, 1997

7. “Standard Test Method for Wear Testing with a Crossed-Cylinder Apparatus,” ASTM G 83, Annual

Book of ASTM Standards

8. Friction and Wear Testing: Source Book of Selected References from ASTM Standards and ASM

Handbooks, ASM International, 1997, p 110–114

9. A.W. Ruff, Wear Measurement, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook,

P. Blau, Ed., ASM International, 1992, p 362–369

10. J.H. Magee, Stainless Steels That Resist Wear and Galling, Stainl. Steel World, May 1997