Bayer R.G. Mechanical Wear Fundamentals and Testing, Revised and Expanded

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Figure 4.48 Transition in wear behavior under combined rolling and sliding conditions. The wear

rate is much higher for loads and speeds above the curve. ‘‘A’’ shows wear scar morphology for con-

ditions below the curve; ‘‘B’’ shows conditions above the curve. (From Ref. 98, reprinted with per-

mission from ASME.)

While not limited to these situations, galling tends to be a problem in two types of

situations. One type of situation is when the contact is heavily loaded and the device is

operating infrequently. A valve that tends to be closed most of the time and only occasion-

ally opened would be an example of such a situation. The other type of situ ation is when

Figure 4.49 Examples of wear scars on metal surfaces where galling has occurred. (‘‘A’’ from Ref.

54; ‘‘B’’ and ‘‘C’’ from Ref. 55, reprinted with permission from ASME.)

Figure 4.50 Effect of surface roughness on the galling threshold stress. The ordinate is the galling

threshold stress. The data are for a 440C stainless steel (59HRC) against 1020 steel (90HRB). (From

Ref. 54, reprinted with permission from ASME.)

Table 4.1 Threshold Stresses for Galling as a Function of Cycles for Self-mated Stainless Steels

Material Threshold stress (MPA)

Stainless steel Hardness Single rotation

Triple rotation

S20161 95 HRB > 104 > 104

28 HRC > 104 > 104

S28200 96 HRB 166 7

S21800 92 HRB > 104 48

T440C 55 HRC 124 14

T304 86 HRB 55 < 7

T430 98 HRB 10 < 7

S42010 50 HRC > 104 21

T420 49 HRC 55 14

S24100 23 HRC 97 14

S45500 48 HRC 97 7

S66286 30 HRC 14 < 7

Type 303 85 HRB 138 < 7

ASTM G98 Test Method.

Source: Ref. 73.

Table 4.2 Galling Resistance of Material Pairs

400

Series

(soft)

400

Series

(hard)

300

Series

Soft

Steel

Hard

Steel

Cast

Iron

Ductile

Iron

Hastalloy

A and B

Hast-

alloy

Hast-

alloy

D Stelite Nitride

Plate

Bronze

(leaded)

Bronze

400 Series

SS (soft)

NFFNFS S N FS S F F S F S S

400 Series

SS (hard)

FSFSSSS F SSS S S S S S S

300 Series

FFNNFSS N FSS S S S F S S

Soft steel N S N N S S S N F S S S S S S S S

Hard steel F S F S S S S F S S S S S S S S S

Cast iron S S S S S S S S S S S S S S S S S

Ductile iron S S S S S S S S S S S S S S S S S

Hastalloy

A and B

NFNNFS S N FS S S S S F S S

Hastalloy C F S F F S S S F F S S S S S S S S

Hastelloy D S S S S S S S S S S S S S S S S S

Stellite S S S S S S S S S S S S S S S S S

Nitride F S S S S S S S S S S S S S F S S

Cr plate S S S S S S S S S S S S ? SSSS

Bronze

(leaded)

SSSSSSS S SSS SS S S S S

Bronze A F S F S S S S F S S S F S S F S S

Bronze B S S S S S S S S S S S S S S F S S

Bronze D S S S S S S S S S S S S S S S S S

Level of resistence: S, satisfactory; F, fair, N, little or none.

Source: Ref. 74.

Figure 4.51 Wear scars showing the effect of increased hardness on galling, ‘‘A’’ and ‘‘B’’ are

examples of the galling that took place on lug prior to hardening the surface. ‘‘C’’ shows the wear

on the same surface after the surfaces were nitrided.

Figure 4.52 Examples of fretting wear scars on metal surfaces. (From Refs. 24, 50, 99, and 100,

reprinted with permission from ASME, Elsevier Sequoia S.A., and ASTM.)

there are tight tolerances between almost or nearly conforming surfaces such as a piston in

a cylind er. While nominal contact pressures in this type of situation are often small or 0,

galling tends to occur as a result of high contact stresses caused by misalignment and

errors-in-form. When galling occurs in these and other applications, the concern is gener-

ally not with material loss but binding that results from the size of the protuberances and

debris that galling causes. In addition to the identiﬁcation of galls in these situations, com-

mon manifestations of a galling problem are seizure, increased difﬁculty in operating a

device, and scouring. The minimal criterion for avoiding galling in unlubricated situations

is to design so that the maximum contact pressure is below the threshold galling stress.

A design approach to a galling wear problem is discussed in Sec. 5.16 in EDW 2E.

4.6 FRETTING

When the amplitude of a reciprocating sliding motion is less than a few millimeters, the

motion is generally referred to as fretting or fretting motion. The wear that results from

this type of motion is called fretting wear, or simply fretting. If oxide formation is

involved in the wear process , it is often called fretting corrosion. Fretting wear is the

more general form and may occur with any material; fretting corrosion is generally

limited to non-noble metals. Examples of fretting wear and fretting corrosion wear scars

are shown in Fig. 4.52. As illustrated by the micrographs in the ﬁgure, fretting wear scar

features tend to be ﬁne. In addition, the morphology of fretting scars can be similar to

that resulting from unidirectional and larger amplitude motion. However, as illustrated

in these examples, striations, directionality, and evidence of adhesion tend to be less

pronounced with fretting scars than with gross sliding. Also wear debris in fretting situa-

tions tends to be ﬁner than in many other sliding situations. This modiﬁed appearance,

the presence of ﬁne and often oxidized debris, and the possibility of small amplitude

vibrations are general indicators of fretting wear (58).

Fretting wear can involve the transition from non-abrasive to abrasive wear beha-

vior (58). In the initial stages of fretting wear, it is a non-abrasive, typically resulting from

adhesive, repeated-cycle deformation, or chemical wear mechani sms, or some combination

of these. With the entrapment of work-hardened and oxidized wear debris from these

mechanisms in the contact region abrasive wear behavior can become dominant. When

this occurs, single-cycle deformation mechanisms tend to become signiﬁcant and wear is

generally accelerated by the abrasion. When this does not occur, longer-term behavior

is characteristic of non-abrasive sliding wear. In such cases, repeated-cycle deformation

or chemical wear mech anism likely dominates wear behavior. However, adhesive, thermal

and triboﬁlm mechanisms are also possible as dominating and contributing mechanisms to

the overall wear [(41,59–64) See Secs. 5.6 and 7.3 in EDW 2E].

While the mechanisms are the same for general sliding and fretting, there are some

speciﬁc trends in fretting situations. For fretting amplitudes less than 1 mm, the dimen-

sionless wear rate, O, deﬁned in Eqs. (4.4) and (4.5) tends to dependen on the amplitude

and load (58,60,61,63). Trend s with amplitude and load are shown in Figs. 4.53 and 4.54,

respectively. Analysis of the data suggests the following relationship between O and slip

amplitude, e, in the region between 10 and 100 mm. Above and below that range, it tends

to be independent of amplitude. Wear has been observed with slip amplitudes as low as

0.06 mm (65,66).

O / e

2

ð4:6Þ

Figure 4.54 Example of the effect of load on slip amplitude and wear rate in fretting. (From Ref.

61, reprinted with permission from ASME.)

Figure 4.53 Effect of slip amplitude on wear rate. Each curve is from different investigations.

(From Ref. 58, reprinted with permission from ASM International.)

The data also suggest the following relationship with load, P:

O / P

0:31:5

ð4:7Þ

While the frequency of the motion is generally not a signiﬁcant factor, it can be

in some situations, particularly unlubricated situations (58,61). With unlubricated

metals, wear rates tend to be inversely dependent on frequency below 10–20 Hz,

because of increased time for oxide growth at lower frequencies. This behavior is

shown in Fig. 4.55. Wear rates can also be affected by frequency as a result of the

effect that frequency has on the power (energy per unit time) that is dissipated in

the contact and consequently temperature. Generally, this is only signiﬁcant in situa-

tions where there is the potential to develop high enough temperatures to affect wear

behavior (41,62,63 ). In situati ons where this does not occur, there is no apparent effect

over several orders of magnitude change in frequency. For exampl e, under some con-

ditions, wear rates of unlubricated steel couples have been found to be independent of

frequency in the range between 100 and 210

Hz (67). An additional illustration of the

typical trend with frequency can be found in Fig. 4.58.

While a small temperature increase of 20–30



C is likely in many fretting situations,

larger increases, such as several hundred degrees, are also possible in others (41,62,63).

In fretting situations, temperatures can be obtained by treating the interface as a sta-

tionary heat source. Equation (4.8) is an approximate relat ionship for the temperature

rise, DT, based on a linearized form of the ﬁrst law of heat ﬂow. C is the power dis-

sipated and k

’s are the thermal conductivities. l

’s are the distance from the surface

at which there is no temperature rise in each body. A is the apparent area for the bulk

temperature rise and is the real area for the ﬂa sh temperature.

DT ¼

þ k



ð4:8Þ

Figure 4.55 Example of the effect of frequency on fretting wear. (From Ref. 58, reprinted with

permission from ASM International.)

Because of friction and the compliance of surfaces, small amplitude oscillatory dis-

placements of bodies do not necessarily result in slip at the interface (68,69). There is a

threshold displacement, which is required for slip to occur. Three regions of slip are gen-

erally identiﬁed in fretting situations. These are indicated on the graph in Fig. 4.54.

Below the threshold is the stick region. In the stick region, there is no slip. However,

some damage may occur as a result of plastic ﬂow and repeated deﬂection. Above the

threshold, there are two regions, the stick-slip and gross slip regions. The stick-slip

region occurs at lower displacements than the gross slip region. In the gross slip region,

there is slip across the entire contact and it is equal to the displacement of the bodies. In

the middle region, there are limited and varying amounts of slip across the interface.

Wear takes place in both these regions. Normalized wear rates tend to be lower in

the mixed region as a result of reduced slip and tend to be more sensitive to frequency,

as indicated in Fig. 4.58 (67). Frett ing maps are used to identify combinations of para-

meters, most often displacement and load or load and shear force, which represent

boundaries between the three regions of slip behavior. Examples of fretting maps are

shown in Fig. 4.56. These maps indicate the general progression from one region to

the other as a function of load, fretting displacement amplitude, and shear force. The

progression of slip is illustrated in Fig. 4.57 for a point contact. In the simulation of

a fretting contact between a sphere and a plane, stress analysis shows that there is a

threshold for slip to take place and when it does, it does not occur uniformly over

the entire contact surface (70,71). It starts in an annulus at the edge of the contact

region. The width of this annulus grows with increasing displacement amplitude until

slip occurs over the entire contact region. In the analysis, an applied shear force simu-

lates displacement. The slip region is deﬁned as the region over which the applied shear

stress exceeds the product of the contact pressure and the static coefﬁcient of friction.

Fretting fatigue is another phenomenon that results from fretting motion. It

involves progressive damage to a solid surface and leads to the formation of fatigue

cracks (72). It is a combination of normal structural fatigue, which results from cyclic

strain and stress, and fretting wear. An example of a situation where fretting fatigue

occurs is the contact between a clamp and a ﬂexing beam. The ﬂexing motion of the

beam could result in slip between the clamp surface and the beam surface. In fretting

fatigue, the wear that is caused by the fretting can accelerate the formation of fatigue

cracks, which then propagate through the material, leading to fracture of the compo-

nent. An example of typical fretting fatigue behavior as a function of frequency is shown

in Fig. 4.58, along with the corresponding behavior of fretting wear in both the gross

slip and partial slip regions.

The general methods to control and reduce sliding wear can be used to reduce and

control fretting wear. An additional element to consider with fretting is the possible

transition to abrasive wear. Materials and designs should be chosen to eliminate the

accumulation of debris harder than the surfaces involved. This may be accomplished

by providing paths for the debris to escape or lubrication methods designed to ﬂush

out debris. There is an additional way for resolving fretting problems. Fretting motions

are generally not intended or required. They are either su perimposed on an intended

motion, such as fretting motions generated in an impact situation and at the reversal

of directions or occur in nominally stationary situations, such as clamped joints or

mated electrical contact, as a result of vibrations, actuation cycles, or thermal cycling.

Consequently, the elimination or reduction of these motions offers another possibility

for solving fretting wear problems. One approach would be to eliminate or isolate the

cause or source of these motions. For example, in the case where these are caused by