Wear, I41 (1990) 1-15
Effects of hardness on the wear mode diagram in lubricated sliding friction of carbon steels T. Alcagaki Tsunwku
Technical College, Tswruoka, Yamagata 997 (Japan)
K. Kato Tohoku University,
(Received November 1, 1989; revised April 2, 1990; accepted May 15, 19903
Abstract The predominant wear mechanisms were studied using discs with five diierent hardnesses 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. Mild flow wear, severe flow wear, adhesive wear and oxidative wear were found. These wear mechanisms were plotted in a wear mode diagram for each hardness, representing the predominant wear mechanism as a function of the applied contact pressure and sliding velocity. It was found that the location of each possible region in the diagram changed, depending strongly on the hardness. It was also found that the shapes and sizes of wear debris particles depended strongly on the predominant wear mechanism as well as on the specific wear rate. This diagram was related to the abrasive wear mode diagram and discussed.
1. Introduction A final goal of wear research is to control wear behaviour. For that reason, the generation of wear debris must be controlled. As the first step, it is very important to study the predominant wear mechanism with the microscopic approach. Further, from the viewpoint of failure diagnosis of practical machinery, information on the shape and size of wear debris is very important [ 11.Therefore it is very useful for failure diagnosis to clarify the relationships between the characteristics of wear debris, the predominant ___^ ^_ -^-L--:-Am-1-----WtxlJ IlLecIla,lusIllaltu weaI _^A.^ LaLe. Five types of wear have been reported to occur in lubricated sliding, i.e. adhesive wear , plastic flow and fracture of surface asperities (flow wear) [ 3, 41, delamination [ 3, 51, abrasive wear (ploughing) [ 61 and formation and removal of chemical reaction films [ 71. Recently, the wear mode diagram, which shows the predominant wear types or wear mechanisms, has been introduced in order to understand the wear mechanisms. Its importance has been discussed and reviewed by Childs [ES].Up to the present, six types of wear mode diagram have been reported. The first type is an abrasive wear mode diagram, which has been reported by Kato and coworkers [9-l 11. The
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second type is a wear mechanism map for ~lubri~ated sliding of steel, which has been reported by Lim and coworkers [ 12, 131. The third type is a fretting map, which has been reported by Vingsbo and Sonderberg [ 141. The fourth type is a wear mode diagram for lubricated sliding of steel, which has been reported by Akagaki and Kato [ 15, 161. The fifth type has been described by Antoniou [ 17 ] for sliding of Al-Si alloys (17). The sixth type of map has been reported by de Gee [ 181. These wear mode diagrams are useful for understanding wear mechanisms in abrasive wear, fretting, unlubricated sliding and lubricated sliding. Based on the same idea as these types of diagram, in this study the predominant wear mechanism in lubricated sliding was studied from the view point of hardness using scanning electron microscopy (SEM) observations. As a result, a wear mode diagram was introduced. Further, the wear debris generated in each region of the wear mode diagram were observed and classified using a ferrographic technique f 191. The wear mode diagram in lubricated sliding was related to the abrasive wear mode diagram  and discussed. 2. Experimental
Experiments were carried out using a pin-on-disc test machine. A schematic diagram of this is shown in FYig.1. The pm and disc were not detached from the apparatus during the measurement of wear volume and SEM observation. Therefore the pm rubbed exactly on the same track of the disc in repeated rotation. The shapes and sizes of the specimens are shown in Fig. 2. The pm was a high chromium bearing steel ball (SUJ 1) of diameter
F’ig. 1. Schematic diagram of experimental apparatus: I, holder of pin specimen; 2, disc specimen; 3, guide; 4, jig; 5, shaft; 6, hardware; 7, driving shaft; 8, bearing. Fig. 2. (a) Upper specimen and (b) lower specimen sizes (dimensions in millimetres).
3.2 mm with a flat surface. Its apparent contact area was 0.35 mm2 and its Vickers hardness was 863 HV. Five kinds of disc with different hardnesses and microstructures were used as the mating surfaces: (a) 133 HV, ferrite; (b) 222 HV, ferrite and pea&e; (c) 248 HV, ferrite and line cementite; (d) 5 10 HV, ferrite and ilne aggregate of pear&e; (e) 73 1 HV, tempered martensite. Of these, discs (c)-(e) were prepared by heat treatment of 0.45 wt.% C steel (S45C). The details of the discs are summarized in Table 1. The composition of the specimens is shown in Table 2. The SEM micrographs of the microstructures are shown in Pig. 3. After grinding and bufllng, both specimens of 0.09 km. The lubricant used were finished to a surface roughness R, was alkylnaphthalene oil (28-35 cSt at 38 “C) without any additives. The oil was used after filtering with a filter of 1 pm size to ensure that no other debris was. The oil was supplied to the disc at a flow rate of 0.5 ml min-’ using a microtube pump. The friction condition is summarized in Table 3. The sliding velocity and contact pressure were varied in the ranges 19.6-156.8 MPa and TABLE 1 Properties of test materials used in this study (R,=O.O9
Hv (4.9N) (W
Pin (SUJ 1)
863 f 32
Disc (pure Fe)
Ferrite and pearl&e
Heated to 810 “C, oil quenched, tempered at 700 “C
Ferrite and 9ne cementite
Heated to 810 “C, oil quenched, tempered at 400 “C
Ferrite and fine aggregate of pea&e
Heated to 810 “C, oil quenched, tempered at 200 “C
TABLE 2 Chemical composition of test materials used in this study Material
SUJ 1 Pure Fe s45c
Amount (wt.%) of following elements C
1.01 0.005 0.45
0.25 0.27 0.23
0.30 0.20 0.73
0.020 0.012 0.026
0.005 0.008 0.014
1.05 0.02 0.10
Fig. 3. SEM micrographs of the microstructures of disc materials: (a) 133 HV, ferrite; (b) 222 IW, ferrite and pearlite; (c) 248 HV, ferrite and fine cementite; (d) 510 HV, ferrite and fine aggregate of peariite; (e) 731 HV, tempered martensite. TABLE 3 Friction conditions Sliding velocity (m s ~-‘) Contact pressure (MPa) Number of passes Sliding distance Lubricant
0.25-1.0 19.6-156.8 8x10” 5 km Alkylnaphthalene oil (28 - 35 cSt at 38 “C)
0.25-I.0 m s-’ respectively. The total number of passes was 8 X lo*, which corresponded to a sliding distance of 5 km. The room temperature and relative humidity were kept at 25 “C and 40% respectively. All specimens were cleaned using trichloroethane in an ultrasonic cleaner. The wear loss was measured only for the disc. The profile of the wear scar of the disc was recorded with a profilometer perpendicular to the sliding direction at each stage of the test. A schematic profile is shown in fig. 4. The wear volume was calculated from the change in 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 angles of 0”,
5 1.5 * -D Q
AV = a
: AV 2 Ai
Hv133 Hv222 Hv246
( D = 19 mm )
4.5 6.0 N (Pass)
Fig. 4. Schematic profile of wear scar of disc: A,, apparent wear scar area; A2 and As, 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. Fii. 5. Representative wear loss curves (0.75 m s-‘;
90”, 180” and 270”. The wear volume was the average value of these four measurements. The specific wear rate was calculated from the slope of the linear plot of wear-time data obtained during steady state wear.
3. Results 3.1. Wear characteristics Representative wear loss curves are shown in Fig. 5. There are four types of loss curve, indicated as types A, B, C and D. Type A is characterized by an extremely small and almost 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 as in type B and then increases suddenly after a certain number of passes of the pin. At that moment, heavy vibration occurs and the driving motor (40 W) stops. Therefore, wear in type C is catastrophic wear, leading to seizure. Type D is characterized by a negligible wear volume and its value is less than several tenths of that in type A. Thus the wear process depends strongly on the hardness of the disc even if the experimental conditions are the same. 3.2. Cl.ussi&ation of wear mechanism In order to clarify the predominant wear mechanism, all the disc and pm specimens were observed by SEM in detail. Wear debris was also observed using SEM and a ferrographic technique. Figure 6 shows surface profiles of wear scars on the discs for the above four types of wear. F’igure 7 shows SEM micrographs of a wear scar on the disc and wear debris generated for type A. The wear scar is small and smooth, as shown in Figs. 6(a), 7(a)
(a) Type A (248HV)
Type C (222HV)
(b) Type B (133HV)
(d) Type D (731HV)
Fig. 6. Surface orofiles of the wear scars of discs in four types of wear (0.75 m s-‘; MPa).
Fig. 7. SEM micrographs (a)-(e) of the wear scar of a disc and (f) typical wear debris generated in mild flow wear. The wear debris was collected using a ferrographic technique. The SEM micrographs in (b)-(e) are enlargements of (a). The arrow indicates the relative direction of motion of the counterface.
and 7(b). Filmy layers are extruded by the mild ploughing action of surface asperities in a direction perpendicular to the sliding direction, as shown in Figs. 7(c) and 7(d). In the border region of the wear scar, similar filmy layers are extruded, as shown in Fig. 7(e). These layers separate and filmy wear debris less than 15 pm in size is generated, as shown in Fig. 7(f). This kind of wear has been called mild flow wear [ 15, 161.
7 Figure 8 shows SEM micrographs of a wear scar on the disc and wear debris generated for type B. The wear scar is large and relatively rough, as shown in Figs. 6(b) and 8(a). Many concentric ploughing grooves, which are probably formed by the severe ploughing action of work-hardened asperities and wear debris, are formed as shown in Figs. 8(a) and 8(b). Many filmy layers and plate-like layers are formed in this ploughing process, as shown in Figs. [email protected]
)-8(d). These layers and the ridges of grooves separate and Illmy and bar-like wear debris are generated, as shown in Fig. 8(e). They are less than 40 pm and 90 pm respectively in size. Thus it is concluded that severe flow wear is predominant in type B. Figure 9 shows SEM micrographs of a wear scar on the disc and the wear debris generated for type C. The wear scar is extremely rough, as shown in Figs. 6(c) and 9(a). Large wedge-like agglomerates (fragments), which are more than a few hundred micrometres in size, are observed frequently on the wear scar, as shown in Fig. 9(a). They are also observed on the wear scar of the pin specimen, formed by transfer of the disc material to the pm. Characteristics patterns, possibly formed during stick-slip motion, are observed behind the agglomerates, as shown in Fig. 9(b). Many shear
Fig. 8. SEM micrographs of (a)-(d) the wear scar of a disc and (e) typical wear debris generated in severe flow wear. The wear debris was collected using a ferrographic technique. The SEM micrographs in (b) and (c) are enlargements of (a). The SEM micrograph in (d) is an enlargement of region indicated by arrow tip in @>. The arrow in (a) indicates the relative direction of motion of the counterface.
Fig. 9. SEM micrographs of (a)-(d) the wear scar of a disc and (e) typical wear debris generated in adhesive wear. The wear debris was collected using a ferrographic technique. The SEM micrographs in (b) and (d) are enlargements of regions indicated by the arrow tips in (a). The SEM micrograph in (c) is an enlargement of region indicated by the arrow tip in (b). The black arrow indicates the relative direction of motion of the counterface.
dimples are observed in places where the stick-slip motion occurred, as shown in Fig. 9(c)_ Many plate-like layers are observed in front of the agglomerates, as shown in Fig. 9(d), and these layers are probably formed when large fragments slide on the disc, while it is forming the wedge. These observations show that severe adhesion and separation are predominant in type C. Therefore it is concluded that adhesive wear is predomin~t in type C. In the process of the formation of the wedge, large plate-like and blocklike wear debris particles more than 100 pm in size are generated, as shown in Fig. 9(e). F’igure 10 shows SEM micrographs of a wear scar on the disc and wear debris for type D. The wear scar is extremely smooth, as shown in Figs. 6(d), 10(a) and 10(b). Many pit-like separations, which are less than a few micrometres in size and extremely shallow, are observed, as shown in FYgs. 10(b) and IO(c). The wear scar was slightly discoloured (light grey) when it was observed with an optical microscope. The discolouration occurs as a result of the oxidation of surface layers [ 41. X-ray diffraction could not identify the structure of the oxide layer, because the layer was very thin. Nevertheless, oxidation is involved, and the term oxidative wear is used to
Fig. 10. SEM micrographs of (a)-(c) the wear scar of a die and (d), (e) typical wear debris generated in oxidative wear. The SEM micrograph in (d) shows wear debris rem~g at the outer region of the wear scar of a disc. The SEM micrograph in (e) shows wear debris precipitated on a ferrogram. The SEM micrograph in (c) is an enlargement of the region indicated by the white arrow tip in (b). The arrow indicates the relative direction of motion of the counterface.
describe type D. Small wear debris particles less than a few micrometres in size are generated, as shown in Figs. 10(d) and 10(e). Summ~g, the predo~~t wear mech~sm can be classified into four types, i.e. mild flow wear, severe flow wear, adhesive wear and oxidative wear. Further, the shapes and sizes of wear debris particles are strongly dependent on the predominant wear mechanisms. 3.3. Wear mode diagram Four types of wear mechanism were plotted in a wear mode diagram for each hardness. Figure 11 shows the wear mode diagram in lubricated sliding. The diagram relates the predom~~t wear mechanism to sliding velocity and contact pressure. At 133 and 222 KV, the predominant wear mechanisms clearly fall into three separate regions of the wear mode diagram, as shown in Figs. 1 l(a) and 1 l(b). At 248 and 530 HV, only mild flow wear was observed in this study, as shown in Figs. 11 (c) and 11 (d). These results show that the boundary curve between mild flow wear and severe flow wear shifts upwards as the hardness increases. Thus the ability to achieve the
& E a
0.5 V (m/s)
0.5 V (m/s)
0.5 V (m/s)
Fig. 11. Wear mode diagram in lubricated HV; (d) 510 Hv; (e) 731 HV.
(a) 133 Hv, (b) 222
Hv; (c) 248
mild flow wear condition increases with increasing hardness. From these results, it is concluded that the location of the regions in the wear mode diagram change, depending strongly on the hardness. At 731 HV, only oxidative wear was observed in this study, as shown in Fig. 1 l(e). Thus the predominant wear mechanism changes with further increase in the hardness.
3.4. Relationship between wear rate, wear mode and hardness Table 4 shows the specific wear rate W, for four types of wear mechanism for various hardnesses (microstructures). The specific wear rate W, in lubricated sliding lies in the range 10e2 to lo-” mm” N-l m-l. The values of W, depend strongly on the predominant wear mechanism. They are of similar order of magnitude, if the predominant wear mechanism is the same. They varied between 6~ lo-’ and 10e7 mm3 N-’ m-l (mostly of the order of IO-* mm3 N-l m-‘) for mild flow wear, between 2 x 10m6 and 10m5 mm3 N- ’ m- ’ (mostly of the order of 10e6 mm3 N- ’ m- ‘) for severe flow wear, between 4 X lop5 and lo-’ mm3 N- ’ m- ’ for adhesive wear and between 3~ lo-” and 3~ lo-’ mm3 N-’ m-l for oxidative wear. Figure 12 shows the relationship between the specibc wear rate W, and the Vickers hardness I&. Roughly speaking, the specific wear rate decreases TABLE
Specific wear rates for four types of wear mechanism Specific wear rate (mm” N-’ m-‘)
133 222 248 510 731
Mild flow wear
Severe flow wear
9 x lo-Q-10-7 5 x 10~s-10-7 6x1O-g-9x1O-8 6x10~Q-5x10~8 -
10-e-10-5 (2-5)x 1O-6 -
4x 10-6-3x 10-4-10-2 -
Oxidative wear 1o-4
3 x lo-‘O-3 x 10-Q
Fig. 12. Relationship between the specific weaf rate W, and the hardness &.
linearly with increasing hardness, i.e. W,=4.9 X 10’ Hv-6.2. From Fig. 12, the following facts are found. (1) The values of IV, are similar for 133-222 HV, and also at 248-530 Hv. (2) Even if the hardness is almost the same (222 and 248 HV), the values of IV, can be completely different. (3) The values of W, depend strongly on the predominant wear mechanism. If the wear mechanisms are the same, the values of W, are similar even if the hardnesses are different. Thus the wear rate is strongly dependent on not only the hardness but also the predominant wear mechanism. Fact (2) suggests that the variation in microst~ct~e affects the wear behaviour and wear mechanism. It is a subject which in future needs to be clarified in detail. 4. Discussion
In Fig. 11, the location of each possible region in the wear mode diagram changes, depending strongly on the hardness. Thus a wear mode diagram can be used to characterize materials with respect to their ability to achieve mild flow wear conditions. From Figs. 11 and 12 and Table 4, it might be said that an increase in the hardness does not always mean a decrease in wear rate but, rather, an increase in the ability to achieve the low wear rate condition. Differences in the values of IV, have been reported frequently in the literature [ 20!. These differences and facts (1) and (2) above can be explained from the viewpoint of the wear mode diagram as well as arising from differences in the mode of lubrication [ 211, If the predominant wear mechanisms are the same, the values of W, are similar even if the operating conditions and the hardnesses are different. On the contrary, if the predominant wear me~h~sms are different, the values of Ws are different even if the operating conditions and the hardnesses are the same. Thus a wear mode diagram is very useful for understanding the wear mechanism in lubricated sliding. An abrasive wear mode diagram [9-l 1 ] shows the possible region of each type of wear, cutting, wedge and ploughing, with the parameter I), (degree of penetration) or attack angle and shearing strengthfat the contact interface in abrasive wear. In order to relate the wear mode diagram in lubricated sliding to the abrasive wear mode diagram, the surface profile of the wear scar on the pm was recorded using a profilometer trace parallel to the sliding direction. The attack angle was measured for three types of wear mechanism as shown in Fig. 1 l(b), i.e. adhesive wear, severe flow wear and mild flow wear. The representative surface profile of a pm surface in adhesive wear is shown in Fig. 13. The values of the attack angle lay in
the ranges 15”-25” for adhesive wear, 0.4”-4.0” for severe flow wear and O.Z”--2.0” for mild flow wear. As the coefficient of friction has not been measured in this study, it was assumed to be OX-l.2 for adhesive wear,
Fig. 13. Surface profile of wear scar of pin in a direction, parallel to the sliding direction: a! is defined as the attack angle. The arrow indicates the direction of motion of pm. - - -, Before seizure; -, after seizure. TABLE 5 Values of the attack angle, coefficient of friction and shear strength at the interface Wear mechanism
Mild flow wear Mild flow wear Severe flow wear Severe flow wear Adhesive wear Adhesive wear
0.2 2.0 0.4 4.0 15 25
o.04b_o.l o.04b-0.1 0.Zb-0.3 0.Zb-9.3 0.8=-1.2 0.8’-1.2
0.2-0.35 0.1-0.26 0.72-0.95 0.5-0.75 0.81-0.9 0.51-0.7
‘%&mated from ref. 23. bEstimated from ref. 22. ‘Estimated from the maximum torque of motor.
0.2-0.3 for severe flow wear and 0.04-0.1 for mild flow wear 1221. Further, the shear strengthf at the contact interface was estimated using the diagram reported by Challen and Oxley . These results are summarized in Table 5. Then the regions of adhesive wear, severe flow wear and mild flow wear were plotted in the abrasive wear mode diagram. The result is given in Fig. 14 which shows that mild flow wear and severe flow wear in lubricated sliding correspond to ploughing type in the abrasive wear mode diagram and, further, that adhesive wear corresponds to wedge type. Thus it is concluded that the wear mode diagram in lubricated sliding is closely related to the abrasive wear mode diagram which has been introduced theoretically and confirmed experimentally.
(1) Mild flow wear, severe flow wear, adhesive wear and oxidative wear were found. In order to show the predominant wear mechanisms and each possible region, the wear mode diagram was introduced.
Region A Region B Region C
: ; ;
Mild Flow Wear Severe FLOW Wear Adhesive Wear
0.2 0.4 0.6 0.8 1I.10 Shear Strength at Contacl Interface f
Fig. 14. Correspondence mode diagram .
the wear mode in lubricated
sliding and the abrasive wear
(2) The location of each possible region in the diagram changed, depending strongly on the hardness. (3) The specific wear rates were strongly dependent on the predominant wear mechanisms. The values of W, varied between 6 X lo-’ and 10e7 mm3 N-’ m-’ for mild flow wear, between 2X 10m6 and low5 mm3 N-’ m-’ for severe flow wear, between 2 x lop5 and 10e2 mm3 N-’ m-’ for adhesive wear and between 3 X IO-” and 3 x lo-’ mm3 N-l m-’ for oxidative wear. (4) The shapes and sizes of wear debris particles depended strongly on the predominant wear mechanism. (5) The mild flow wear and severe flow wear observed in lubricated sliding corresponded to the ploughing type in the abrasive wear mode diagram, and adhesive wear corresponded to the wedge type. (6) The relationship between the specific wear rate W, and the hardness I& could be expressed roughly by the following empirical formula: W s = 4 .9 x 108 IY-6.2. Acknowledgments The authors wish to thank Mr. T. Sato, Mr. H. Hasegawa, Mr. S. Saito and Mr. M. Niizeki, who are graduates of the Tsuruoka Technical College, for their experimental assistance.
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