Wear mode diagram in lubricated sliding friction of carbon steel

Wear mode diagram in lubricated sliding friction of carbon steel

303 Wear, 129 (1989) 303 - 317 WEAR MODE DIAGRAM CARBON STEEL IN LUBRICATED SLIDING FRICTION OF T. AKAGAKI Department (Japan) of Mechanical Eng...

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303

Wear, 129 (1989) 303 - 317

WEAR MODE DIAGRAM CARBON STEEL

IN LUBRICATED

SLIDING FRICTION OF

T. AKAGAKI Department (Japan)

of Mechanical

Engineering,

Tsuruoka

Department of Mechanical Sendai, 980 (Japan)

Engineering,

Faculty

Technical

College, Tsuruoka,

997

K. KATO of Engineering,

Tohoku

University,

(Received June 20, 1988; accepted October 6, 1988)

The predominant wear mechanisms of 0.45 wt.% C steel were studied as a function of contact pressure and sliding velocity in thin-film-lubricated sliding under conditions of concentrated contact with a hard chromium steel ball. Flow wear, abrasive wear (ploughing) and adhesive wear were found. These mechanisms were plotted in a wear mode diagram, representing the predominant wear mechanism as a function of the applied contact pressure and sliding velocity. Wear debris, generated in each region of the wear mode diagram, was observed using a ferrographic technique and scanning electron microscopy. It was found that the shapes and sizes of wear debris depended strongly on the wear mechanism.

1. Introduction In lubricated sliding friction, the frictional force and the resultant wear are important phenomena. The frictional force depends strongly on the mode of lubrication. The modes of lubrication in concentrated sliding friction can be classified into three regimes in a transition diagram [1 - 31, i.e. partial elastohydrodynamic lubrication, boundary lubrication and metallic wear (severe scuffing). Although the transition diagram is very useful for evaluating lubricants [4], additives [5 - 71 and materials of mating surfaces [6], as well as for understanding the lubrication behaviour of a thin oil film, it does not tell which wear mechanism will be predominant. Five types of wear have been reported to occur in lubricated sliding, i.e. adhesive wear [8], plastic flow and fracture of surface asperities (flow wear) [9 - 111, delamination [9,12], abrasive wear (ploughing) [9,13] and 0043-1648/89/$3.50

@ Elsevier Sequoia/Printed in The Netherlands

304

formation and removal of chemical reaction films [14 - 181. However, a wear mode diagram, which corresponds to the transition diagram representing the mode of lubrication, has not yet been established. Further, there are not many reports which have related, in detail, the wear mechanisms to the shapes and sizes of the wear debris. Concerning the wear mode diagram, which shows the predominant wear types or wear mechanisms, its importance has been well discussed and reviewed by Childs 1191. Up to the present, two types of wear mode diagrams have been reported. One type is an abrasive wear mode diagram, which has been reported by Kato and Hokkirigawa [20,21]. It shows the possible region of each type of wear, cutting, wedge and ploughing, with the parameters D, (degree of penetration) and the shearing strength at the contact interface in abrasive wear. The abrasive wear mode diagram is useful for understanding the wear mechanism in abrasive wear [22]. Another type is a wear mechanism map for the unlubricated sliding of steel, which has been reported by Ashby and coworkers [23, 241. It shows the regime of dominance of each of a number of wear mechanisms, i.e. seizure, meltdominated wear, oxidation-dominated wear (mild and severe oxidational wear) and plasticity-dominated wear (including delamination wear). The wear mechanism map is also useful for understanding the wear mechanism in unlubricated sliding of steel. Based on the same idea as the two types of diagrams, in this study, the predominant wear mechanism in lubricated sliding for different combinations of experimental conditions was studied using scanning electron microscopy (SEM) observation. As a result, a wear mode diagram for lubricated sliding was introduced which shows the predominant wear mechanisms of carbon steel. Further, wear debris, generated in each region of the wear mode diagram, was observed and classified using a ferrographic technique [25] and SEM.

2. Experimental

apparatus

and procedure

The type of wear test machine used in this study was of the pin-on-disk type; it has been described in detail elsewhere [lo, 111. A schematic diagram is shown in Fig. 1. As the parts 1 - 5 in Fig. 1 are not disassembled during the measurements of wear losses and the SEM obse~ations, the position of the pin with respect to the disk remains fixed and the wear process can be observed at successive intervals. The shapes and sizes of the specimens are shown in Fig. 2. The pin was a high carbon chromium bearing steel ball (SUJl) of diameter 3.2 mm with a flat surface. Its apparent contact area was about 0.35 mm* and its hardness was 863 HV. The mating surface was a 0.45 wt.% C steel (S45C) disk of outside diameter 28 mm. Its hardness was 222 HV. After grinding and buffing, both specimens were finished to a surface roughness of 0.09 I.tm I%,,.

305

(b)

(a)

Fig. 1. Schematic diagram of experimental apparatus: 1, holder of pin specimen; specimen; 3, guide; 4, jig; 5, shaft; 6, hardware; 7, drive shaft; 8, bearing. Fig. 2. (a) Upper specimen

TABLE

1

Friction

conditions

Sliding velocity Contact

pressure

Sliding distance Lubricant

and (b) lower specimen

sizes (dimensions

(m s-l)

0.25

- 1.0

(MPa)

19.6

- 156.8

(pass, km)

= 8

x

2, disk

in millimetres).

lo4 passes (= 5 km)

Alkylnaphthalene

oil 28

- 35 cSt at 38 “C

The lubricant used was alkylnaphthalene oil (viscosity 28 - 35 cSt at 38 “C) without any additives. The oil was supplied to the disk at a flow rate of 0.5 ml mind’ using a microtube pump. The friction conditions are summarized in Table 1. The contact pressure and the sliding velocity were varied in the ranges 19.6 - 156.8 MPa and 0.25 - 1.0 m s-r respectively. The total number of passes amounted to 8 X 104, which corresponded to a sliding distance of 5 km. Room temperature and relative humidity were kept at 25 “C and 40% respectively. All specimens were cleaned using trichloroethylene in an ultrasonic cleaner. Wear losses were measured only at the disk. The profile of the wear scar on the disk was recorded using a profilometer, perpendicular to the sliding direction. A schematic profile is shown in Fig. 3. The wear volume was calculated from the change of cross-sectional area, measured with a planimeter. The measurements of cross-sectional area were conducted at four different locations in the wear scar, i.e. at 0, 90”, 180” and 270”. The wear volume was the average value of these four measurements. The specific wear

306

Wear Scar Area A =AI-(AzrAx) Wear Volume

AV=y,$At

(D=lSmm)

= 59.69 A (mm31

Fig. 3. Schematic profile of wear sear of disk: A llapparent wear scar area; A2 and A3, the area of both side ridges; A, net wear scar area; A, average value of four measurements of wear scar area; V, wear volume; D, diameter of wear scar.

rate was calculated from the slope of the straight wear-time steady state of the wear process.

curve in the

3. Results 3.1. Classification of wear mechanism Representative wear loss curves are shown in Fig. 4. There are three types of loss curves, indicated as types A, B and C in Fig. 4. Type A is characterized by an extremely small and ahnost constant wear volume. In type B the wear volume is relatively large and increases linearly with increasing number of passes of the pin. In type C the wear volume firstly increases linearly like in type B and increases suddenly after a certain number of passes of the pin. At that moment, heavy vibration occurs and

I

(x109

N , Pass

Fig. 4. Representative wear loss curves: 0.75 m SO’; S45C; 11; 156.8 MPa; l, 137.2 MPa; V, 117.6 MPa; 0,98.0 MPa; 0,78.4 MPa;A, 58.8 MPa.

307

the driving motor (40 W) stops. Therefore wear in type C is catastrophic wear, leading to seizure. Figure 5 and Fig. 6 show the scanning electron micrographs and the surface profiles of the wear scar on the disk for the above three types respectively. In type A, as shown in Figs. 5(a), 5(b) and 6(a), the wear scar is small and smooth. Even if the number of passes of the pin increases, the profile hardly changes. In type B, as shown in Figs. 5(c), 5(d) and 6(b), the wear scar is large and relatively rough. It becomes very large with the increase in the number of passes of the pin. Many ploughing grooves can be observed. In type C, as shown in Figs. 5(e), 5(f) and 6(c), the wear scar is extremely

B

Type

C

Type

Fig. 5. Scanning electron micrographs of wear scars of pin and disk in three types of wear: (a), (b), (c) and (d), 8 x lo4 passes; (e) and (f), 3.21 x lo4 passes. The arrows indicate the relative directions of motion of the counterface. Fig. 6. Surface profiles of wear scar of disk in three types of wear.

308

rough. Large wedge-like agglomerates (fragments), which are more than a few hundred micrometres in size, are observed frequently on the scar. They are also observed on the wear scar, formed on the pin. Table 2 shows the specific wear rates. In type A, the specific wear rates are small and lie in the range 1O-9 - lo-’ mm3 N-’ m-‘. Most of them are of the order of lo-* mm3 N-’ m- ‘. In type B, they are large and lie in the range 2 X lop6 - 5 X lop6 mm3 N-’ m-l. In type C, they are extremely large and lie in the range lop4 - lo-’ mm3 N-’ m-‘. In order to determine the predominant wear mechanism for these three types of wear, the different wear scars were observed using SEM. Figure 7 shows the scanning electron micrographs of the wear scar in type A (specific wear rate of the order of lo-* mm3 N-’ m-l). The wear scar is very smooth as shown in Fig. 7(a). Filmy layers are extruded in a direction, perpendicular to the sliding direction, as shown in Fig. 7(b). In the border region of the wear scar, similar filmy layers are extruded, as shown in Fig. 7(c). These layers will probably separate and become loose filmy wear debris. This kind of wear has been TABLE

2

Specific

wear rates in three types of wear

Type

W, (mm3 NP1 m-l)

A B C

10-s - 10-7 2 x 10-e - 5 x 10-s 10-a - 10-Z

F. D. -

Fig. 7. Scanning electron micrographs of wear scar of disk in flow wear. The scanning electron micrograph of (a) is the enlargement of the wear scar shown in Fig. 5(a). The arrow indicates the relative direction of motion of the counterface.

309

named flow wear [lo, 261. Therefore it is concluded that flow wear is predominant when the specific wear rate is of the order of lo-’ mm3 N-’ m-l. Figure 8 shows the scanning electron micrographs of the wear scar in type B, (specific wear rate of the order of 10P6 mm3 N-l m-l). Many concentric ploughing grooves are found, as shown in Figs. 5(c) and 8(a). These grooves are probably formed by the severe ploughing action of the workhardened surface asperities and wear debris. Many filmy layers and platelike layers are formed in this ploughing process, as shown in Figs. 8(b) 8(d). Failure of the ridges of the grooves is also observed, as shown in Fig. 8(e). In this case, these layers and ridges will probably separate and become loose wear debris. Therefore it is concluded that abrasive wear (ploughing) is the predominant wear mechanism when the specific wear rate is of the order of 10P6 mm3 N-’ m-‘. Figure 9 shows the scanning electron micrographs of the wear scar in type C, in which the specific wear rate is more than 10m4 mm3 N-’ m-l. Large agglomerates, which are more than a few hundred micrometres in size, are observed frequently on the wear scar, as shown in Fig. 9(a). Many plate-like layers are observed in front of them, as shown in Fig. 9(b). These layers are probably formed when large fragments slide on the disk, while it is forming the wedge. Characteristic patterns, clearly formed during stickslip motion, are observed in the rear of the agglomerates, as shown in Fig. 9(c). It is supposed that many cracks are generated in a direction, perpendicular to the sliding motion and that their inlet parts are stripped off

Fig. 8. Scanning electron micrographs of wear scar of disk in abrasive wear (ploughing). The scanning electron micrograph of (a) is the enlargement of the wear scar shown in Fig. 5(c). The arrows indicate the relative directions of motion of the counterface.

b

D\sk Pass

(a)

8x103

(b)

8~10~

Pa55

Fig. 9. Scanning electron micrographs of wear scar of disk in adhesive wear. The scanning electron micrograph of (a) is the enlargement of the wear scar shown in Fig. 5(e). The scanning electron micrographs of (b) and (c) are enlargements of regions indicated by the arrow tips in (a). The scanning electron micrograph of (d) is the enlargement of the region indicated by the arrow tip in (c). The arrow indicates the relative direction of motion of the counterface. Fig. 10. passes.

Surface

profiles

of pin and disk in flow

wear:

(a) 8 x lo3

passes;

(b) 8 x lo4

during sliding. In previous work [27] the same patterns have been observed on the wear scar after seizure. Many shear dimples are observed in the places where the stick-slip motion occurred, as shown in Fig. 9(d). These observations show that severe adhesion and separation are predominant in type C. Therefore it is concluded that adhesive wear is predominant when the specific wear rate is of the order of 10P4 - lop2 mm3 N-i m-i. Summarizing, the predominant wear mechanisms can clearly be classified into three types, i.e. flow wear, abrasive wear (ploughing) and adhesive wear. Figure 10, Fig. 11 and Fig. 12 show the surface profiles of pin and disk specimens in flow wear, abrasive wear (ploughing) and adhesive wear respectively. In flow wear (Fig. lo), the shape of the wear surface on the disk deforms plastically. Thereby it conforms closely to the profile of the pin specimen. Thus it is supposed that the wear loss occurs only in the surface layers. In abrasive wear (ploughing) (Fig. 11) the profiles of the pin and disk initially do not conform. However, after 8 X lo4 passes they are found to conform closely. This fact suggests that, in an early stage, the disk is ploughed by free wear debris, i.e. not by asperities on the pin surface. Later, both the pin and the disk are ploughed by the work-hardened asperities of the mating surfaces. In adhesive wear, the profiles of the wear surfaces of the pin and disk do not conform to each other, as shown in Fig. 12. This is

311

A

(b) BxlO'Pass

Fig. 11. Surface profiles passes; (b) 8 x lo4 passes.

of

pin

and

disk

in abrasive

wear

Fig. 12. Surface profiles of pin and disk in adhesive wear: The profile of (b) is the enlargement of that shown in (a).

(ploughing):

(a) and (b), 3.21

(a)

1.6 x lo4

X lo4 passes.

because, in this case, the wear surface changes by the repetition of severe adhesion and separation. 3.2. Wear mode diagram In Fig. 13 the predominant wear mechanism is shown in a wear mode diagram. This relates the predominant wear mechanism to sliding velocity and contact pressure. The above wear mechanisms clearly fall into three separate regions of the wear mode diagram. At low sliding velocity (less than 0.75 m s-i), with increase in contact pressure, the predominant wear mech-

0

0.25

0.5 0.75 V (m/s)

1.0

Fig. 13. Wear mode diagram A, abrasive wear (ploughing).

in lubricated

sliding friction:

0, flow wear; A, adhesive

wear;

312

anism changes from flow wear to abrasive wear (ploughing) and further to adhesive wear. At high sliding velocity (greater than 0.75 m s-l), it changes directly from flow wear to adhesive wear without passing the abrasive wear (pfoughing) region. 3.3. SEM observation of wear debris Figure 14 shows the scanning electron micrographs of the wear debris, generated in the flow wear region of Fig. 13 (point A). Most of the wear debris generated is filmy debris with a smooth surface and less than 15 pm in size, The thickness of the individual particles is so small that they curl and fold, as shown in Figs. 14(a) and 14(b). These filmy wear particles are generated in the process of side plastic flow of surface layers, as shown in Fig. 8. Figure 15 shows the scanning electron micro~aphs of the wear debris, generated in the abrasive wear region of Fig. 13 (point B). Many large wear

Fig. 14. Scanning electron micrographs of wear debris generated in the flow wear region.

Fig. 15. Scanning electron micrographs of wear debris generated in the abrasive wear (ploughing) region. The scanning electron micrograph of (e) is the enlargement of the edge of wear debris indicated by the arrow tip in (d).

313

particles are generated in this condition, as shown in Fig. 15(a). Most of them are filmy debris, below 40 pm in size, as shown in Fig. 15(b), and long and slender wear debris below 90 pm in size, as shown in Figs. 15(c) - 15(e). Figure 15(e) shows a magnification of the edge of the wear debris, shown in Fig. 15(d). It can be seen that the thickness of the particles is small, i.e. less than 0.1 pm. These wear particles are generated in the process of ploughing action by the work-hardened surface asperities and wear debris, as shown in Fig. 8. In general, abrasive wear can be classified into three types, i.e. cutting, ploughing and wedge type [20,21]. Spiral-like wear debris, which is predominant in the cutting, was not observed in this study. Therefore the assumption that, in this case, abrasive wear is of the ploughing type is confirmed. In adhesive wear, it has been reported that many plate-like and blocklike wear debris, greater than 100 pm in size, are generated [27]. Summarizing, it is shown that the size and shape of the wear debris, generated in each region of the wear mode diagram, differ completely.

4. Discussion The predominant wear mechanisms in lubricated sliding friction are clearly confined to separate regions in the wear mode diagram, i.e. flow wear, abrasive wear (ploughing) and adhesive wear, as shown in Fig. 13. In flow wear the specific wear rate was of the order of lo-* mm3 N-’ m-‘. This is acceptable for many practical applications. Thus the operating conditions in a practical machine have to be selected in such a way that flow wear occurs. The location of the regions in the wear mode diagram will change, depending on materials, lubricants and additives. Thus a wear mode diagram can be used to characterize materials and lubricants with respect to their ability to achieve flow wear conditions. Figures 14 and 15 show that the shape and the size of the wear debris depend strongly on the wear mechanism. In fact the predominant wear mechanism, the specific wear rate and the type of wear debris are closely connected with each other. Therefore it is possible to estimate the specific wear rate and the predominant wear mechanism roughly by the shape and the size of the wear debris. .This is useful in condition monitoring of machines. The wear models based on SEM observations are shown in Figs. 16 - 18. Figure 16 shows the wear model, applicable to the flow wear process, which has been proposed before [lo]. As the contact pressure is low and the microasperities hardly penetrate the mating surface, contact occurs locally and the wear scar consists of wave-like streaks. During the formation of streaks, thin filmy wear layers are extruded in the side direction by plastic flow of surface layers, Fig. 16(a). With the increase in the number of passes of the pin, the

314

(b)

(a) (a)

PI n

Disk

wear Debns

Flimy Layer (c ) ,Wear Debris

I ’\ Workhardened Asperities

0 :C)

Fig. 16. Schematic diagram showing the wear process in flow wear. The arrow indicates the direction of motion of the pin. Fig. 17. Schematic diagram showing the wear process in abrasive wear (ploughing). The frictional direction is from left to right.

Wear Debris (d )

Mbrks of Stick-Slip

Mot,on

Agglomerates

Fig. 18. Schematic diagram showing the wear process in adhesive wear (seizure). The arrow indicates the direction of motion of the pin.

filmy layers grow and new wave-like streaks are also generated. In the process, the filmy layers overlap on the opposite layers (Fig. 16(b)) or both layers collide and then are compressed and new layers are extruded in the side direction again, Fig. 16(c). These filmy layers separate and become loose filmy wear debris, Fig. 16(d). Figure 17 shows the wear models, applicable to the abrasive wear (ploughing) process. As the contact pressure is high, large wear debris is generated, Fig. 17(a). The disk surface is ploughed by the free wear debris and becomes rough, Fig. 17(b). The asperities on the rough surface are work hardened. Loose wear debris is also work hardened. Therefore the ploughing action is promoted still more by the work-hardened asperities and wear debris. As a result, the pin and disk plough each other, Fig. 17(c). In the

315

ploughing action, separation of filmy layers and fracture of ridges occur and wear proceeds at a relatively high rate. Figure 18 shows the wear model, applicable to the adhesive wear process (seizure). As mentioned in Section 3, abrasive wear precedes adhesive wear. In the abrasive wear process, large wear debris is generated. The large wear debris transfers to the pin surface or is trapped between the pin and the disk. As a result, contact occurs between the large wear debris and the disk surface, Fig. 18(a). This contact may be similar to third-body formation, reported by Godet et al. [28,29]. In the sliding process, a wedge is formed, Fig. 18(b). The wedge grows and stick-slip motion occurs, Fig. 18(c). If the pin gets over the wedge or the large wear debris separates from the pin surface, the pin moves up and down and vibration occurs [30,31]. The large wear debris is dislodged as wedge-like agglomerates on the wear scar, Fig. 18(d). This process is repeated and many large plate-like and block-like wear debris are generated. In the process (Fig. 18(c)) when the force, necessary for the growth of a wedge, exceeds the driving torque of the motor, seizure occurs. It is well known that the friction coefficient is greatest (r_l> 2) when the wedge type is predominant in abrasive wear [20,21]. This fact supports the wear model of adhesive wear (seizure) proposed in this study. The mechanisms of the mutual transfer and growth of the wear particles by adhesion was well discussed and reviewed by Rigney [ 32 1. The predominant wear types in abrasive wear, i.e. cutting, wedge and ploughing, fall into three separate regions of the abrasive wear mode diagram [20,21]. When the wear mode diagram in this study is compared with the abrasive wear mode diagram, it is supposed that flow wear and abrasive wear (ploughing) in lubricated sliding correspond to ploughing in the abrasive wear mode diagram and further that adhesive wear corresponds to the wedge type. Thus it may be concluded that the wear mode diagram in lubricated sliding is closely related to the abrasive wear mode diagram. The wear mechanism map shows the regime of dominance of each of a number of wear mechanisms for a wide range of experimental conditions [23,24]. Therefore it will be applicable over the wide operating conditions of practical machine parts. Further, it will become the basis of other wear diagrams which will be constructed for other materials in the future. In the wear mechanism map, the boundary line of seizure does not depend much on the sliding velocity (normalized velocity) and is almost constant. From the fact, it is supposed that seizure results from the wedge formation proposed in this study rather than the increase in the real contact area by frictional heat. It has been reported that delamination wear becomes predominant for a certain range of experimental conditions in lubricated sliding as well as in unlubricated sliding 191. Further, oxidational wear will also become predominant for a certain range of experimental conditions in lubricated sliding. These wear modes were not observed clearly in this study. In order to clarify the ranges of these wear modes in lubricated sliding, more experiments have to be conducted over a wide range of experimental conditions.

316

5. Conclusions (1) A wear mode diagram was introduced. The predominant wear mechanisms, flow wear, abrasive wear (ploughing) and adhesive wear, fall into three separate regions of this mode diagram. (2) The specific wear rates were found to depend strongly on the predominant wear mechanisms. They were 10e9 - lo-’ mm3 N-’ rn-’ (mostly, of the order of lops), 2 X 1O-6 - 5 X lO-‘j mm3 N-’ m-l and lop4 - lo-* mm3 N-’ m-l for flow wear, abrasive wear (ploughing) and adhesive wear respectively. (3) In the region of flow wear, filmy wear debris less than 15 pm in size, was generated. (4) In the region of abrasive wear (ploughing), filmy wear debris less than 40 pm in size as well as long and slender wear debris less than 90 pm in size were generated. (5) Analysis of the wear debris may lead to a rough estimate of the specific wear rate. Thus it can be useful in condition monitoring.

Acknowledgments The authors thank Mr. H. Hasegawa, Mr. T. Sato, Mr. K. Kurata and Mr. T. Hada, who are graduates of Tsuruoka Technical College, for their experimental assistance.

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