Wear, If 7 (1987) 179 - 196 179

PLASTIC FLOW PROCESS OF SURFACE LAYERS IN FLOW WEAR UNDER BOUNDARY LUBRICATED CONDITIONS

T. AKAGAKI

Department of Mechanical Engineering, Tsuruoka Technical College, Tsuruoka (Japan)

K. KATO

Deportment of Mechanical En~.nee~~~ Faculty of Enginee~ng, Tohoku Uniuersity, Sendai {Japan)

(Received June 5,1986;accepted October 13,1986)

Summary

The process of filmy wear debris generation in boundary lubrication was observed successively using scanning electron microscopy. On the rough surface, the contact occurred alternatively between higher asperities and filmy wear debris was generated as a result of the accumulation of plastic flow of surface layers of deformed asperities. The following two types of plastic flow were observed: forward plastic flow; side plastic flow.

On the smooth surface, filmy wear debris was generated as a result of side plastic flow accompanying ploughing. In this way, the wear type, which was controlled by the plastic flow of asperities and the generated filmy wear debris, was named “flow wear” in this paper.

The particle sizes of filmy wear debris, which were estimated in flow wear, were less than 15 pm and dependent on the surface roughness.

1. Introduction

Most practical machines are used under lubricated conditions and many various kinds of wear particles are generated [ 1 - 31, Among them, the filmy wear particle is the most popular under boundary lubricated conditions. It is so thin that it looks transparent using transmission electron microscopy [4] and scanning electron microscopy (SEM) [ 51. Most filmy wear particles are less than 15 pm in size and less than 1 pm in thickness [6]. Although the generation of filmy wear particles indicates that the machine is operating under good conditions, this kind of wear is inevitable in the present day. The final state of wear control in lubricated machine parts is to control the generation of this kind of wear particle. For this reason, it is very impo~~t to unde~t~d the mechanism of filmy wear particle generation.

0043-1648/87/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

The delamination theory [ 71, which explains the plate-like wear part]- cle by fatigue theory, cannot be used for this type of thin filmy wear parti- cle. The machining marks deform and flow plastically in boundary lubricated friction and the frictional surface becomes very smooth [ 8 - lo]. The plastic deformation and fracture of machining marks in the smoothing process [ll] and the continual removal of sheared mixed layers [6,12] and surface reaction films [4, 13, 141 have been reported for the understanding of the mechanism of filmy wear particle generation.

Concerning the filmy wear particle, although many investigations, such as those mentioned above, have been reported the process of the generation and growth of filmy wear particles has not been identified in detail.

The purposes of this paper are to observe successively the process of the generation and growth of filmy wear particles, using SE&I and newly developed testing apparatus, and to clarify the mechanism of filmy wear particle generation.

2. Experimental apparatus and procedure

The type of wear test machine used in this study was ball on disk. A schematic diagram is shown in Fig. 1. A disk specimen 2 was fixed on the shaft 5 which was connected to the drive shaft 7 directly and could be rotated at various speeds. The jig 4 and hardware 6, and the shaft 5 and drive shaft 7 were connected by good clearance fit and fixed during the tests by screws and a key respectively. The load was applied at the top of

011 n Load

Fig, 1. Schematic diagram of experimental apparatus: 1, holder of ball specimen; 2, disk specimen; 3, guide; 4, jig; 5, shaft; 6, hardware; 7, drive shaft; 8, bearing.

181

Fig. 2. (a) Upper specimen and (b) lower specimen sizes (dimensions in millimetres)

ball holder 1 by a lever loading system. The ball holder 1 was inserted into the guide 3 using a key and could slide easily without rotating. As the parts of 1 - 5 in Fig. 1 were not taken to pieces in the series of tests, the disk specimen could be rubbed on the same track correctly by the ball specimen. They were incorporated in the scanning electron microscope at each stage of the wear test and the process of filmy wear particle generation was ob- served successively.

The shapes and sizes of specimens are shown in Fig. 2. The upper specimen was a high carbon chromium bearing steel ball of diameter 3 mm with a flat surface. Its apparent contact area was about 0.3 mm2. Its hard- ness was 63 HRC. After grinding with emery paper it was polished by buffing and its surface roughness was 0.09 pm R,,,. The lower specimen was 0.45 wt.% C steel (S45C) of outside diameter 28 mm. Its hardness was 15 HRC. By grinding and further buffing, three kinds of surface roughness were prepared for the tests; 4.0,1.2 and 0.09 pm R,,, .

The lubricant was alkylnaphthalene oil (28 - 35 cSt at 38 “C) without any additives in the hy~oc~bon series. The lubricant system was drop lubrication and the oil was supplied to the disk at a flow rate of 0.5 ml min-’ using a microtube pump.

Experiments were carried out at a sliding velocity of 0.52 m s-l and a contact pressure of 9.8 MPa. The passage number of the ball specimen was between 0 and 8 X lo4 passes. The room temperature and relative humidity were kept at 20 “C and 40% respectively. All specimens were cleaned using trichloroethylene in an ultrasonic cleaner.

3. Results and discussion

3.1. ~~~ o~~~va~ion of filly wear particles Filmy wear particles, which are the most popular under good boundary

lubrication conditions, are shown in Fig. 3. These particles were generated in a sliding wear test carried out using a thrust collars test machine. Both

Fig. 3. SEM micrographs of’ t’ilmy wear particles: (a) overview; (b) enlargement, ol’ the

edge of particle indicated by arrow in Fig. 3(a).

materials of the specimens were 0.1 wt.% C steel (SlOC) and both specimens were finished using emery paper at 1.2 pm R,,, . The lubricant was turbine oil IS0 VG32 (85 cSt at 20 “C). The specimens were submerged in the oil and the experiment was carried out at a sliding velocity of 0.32 m s ’ and a contact pressure of 1.8 MPa. The sliding distance was 10 km. Wear parti- cles were observed using SEM and a ferrographic technique.

Using the above experimental conditions, the wear coefficient and specific wear rate were very small; 8.7 X lo--’ and 3.9 X 10 ~I’ mm3 rnrn~~’ N-~’ respectively. In spite of this, many wear particles were generated as shown in Fig. 3(a). Most of them were filmy wear particles with very smooth surfaces. They had various kinds of shapes and were less than 80 pm in size. In Fig. 3(a) some of the filmy wear particles look transparent using SEM and, therefore, a particle underneath another particle is visible easily through the upper particle. Figure 3(b) shows an enlargement of the edge of a filmy wear particle, indicated by the arrow in Fig. 3(a). It is very thin and is less than 0.1 pm in thickness.

As mentioned above, it is said that many thin filmy wear particles are generated under good boundary lubrication conditions which results in a very low wear rate.

3.2. Process of filmy wear particle generation 3.2.1. Case of a rough surface Figure 4 shows the process of plastic flow of surface asperities ob-

served successively using SEM. The surface roughness of the disk specimen

was 4 I.tm R,,, and the frictional direction was perpendicular to that of the grinding marks. With an increase in the passage number of the ball, the asperities deform and flow forward plastically to cover the surface grooves. Then, the frictional surface becomes very smooth, as shown in Fig. 4(c).

When the frictional direction and that of the grinding marks were the same, the asperities deformed and flowed plastically in a side direction which was perpendicular to the frictional direction and thereafter the

183

8 x 1 O4 Passes

8 xl O3 Passes

Frictional Direction ,30LJm,

I

Fig. 4. Process of plastic flow of asperities on the rough disk surface (4 pm J&s,, 0.52 m s-l, 9.8 MPa).

actions surface became very smooth. In this way, two types of plastic flow were observed in the process of filmy wear particle generation. One type was forward plastic flow which occurred in the frictional direction. The other type was side plastic flow which occurred perpendicular to the frictional direction.

(A) Furward p~a~~i~ flow type The following two modes of filmy wear particle generation were

observed for the forward plastic flow type: (1) generation from the edges of deformed asperities; (2) generation on the flat terraces of flowed asper- ities. Figure 5 shows the process of filmy wear particle generation in the former mode. The SE&l micrographs of Figs. 5(a) - 5(d) are the same regions in the wear scar observed successively using SEM.

5(a) After 1.6 X lo3 passes. The asperity deforms and flows plastically and the frictional sufface

becomes smooth.

1.6 x 10 3 Passes axlO3 Passes

1.6 xl OL Passes 4.8 xl O4 Passes Fig. 5. Process of filmy wear particle generation at the edges of flowed asperities on the rough disk surface (4.0 pm R,,,, 0.52 m s -I, 9.8 MPa).

5(b) After 8 X lo3 passes. A thin filmy layer is extruded from the edge of the flowed asperity.

It is about 8 pm in length. 5(c) After 1.6 X lo4 passes. With the increase in the passage number of the ball the asperity flows

plastically even further and the thin filmy layer is extruded still more. Its length is about 12 pm. It looks transparent and therefore the grinding marks are easily visible through it. New filmy layers are also extruded. Some cracks, which lead to the generation of small particles, are generated.

5(d) After 4.8 X lo4 passes. The thin filmy layers separate from the edge and become loose filmy

wear particles and there is further plastic flow of the flowed asperity. Figure 6 shows the process of filmy wear particle generation on the

flat terrace of a flowed asperity. SEM micrographs of Figs. 6(al) - 6(cl) and 6(a2) - 6(c2) are the same regions observed successively using SEM. SEM micrographs of Fig. 6(a2), Fig. 6(b2) and Fig. 6(c2) are enlargements of the regions indicated by the white arrow tips in the micrographs of Fig. 6(al), Fig. 6(bl) and Fig. 6(cl) respectively.

185

1.6 x 1O”Passes

8 x lo4 Passes

4.8 x 7 04Passes

F.D. -;

8 x 1 O4 Passes

Fig. 6. Process of filmy wear particle generation on the flat terraces of flowed asperities on the rough disk surface (4.0 pm Em,,,, 0.52 m s -I, 9.8 MPa). The SEM micrographs of (a2), (b2) and (~2) are the enlargements of regions indicated by the white arrow tips in (al), (bl) and (cl) respectively. The black arrow indicates the frictional direction.

flow

6(al) and 6(a2) After 1.6 X lo3 passes. Filmy layers are extruded on the flat terrace formed by the plastic of the asperity.

6(bl) and 6(b2) After 4.8 X lo4 passes. With the increase in the passage number of the ball, the filmy layers

grow and the smooth surface becomes porous. Some cracks, which lead to the generation of filmy wear particles, are generated.

6(cl) and 6(c2) After 8.0 X lo4 passes. The filmy layers on the terrace separate, as shown in Figs. 6(c2) and

6(d), and become loose filmy wear particles. As shown in Figs. 5 and 6, the tops of the asperities make contact at

the early stage of friction and thereafter they deform and flow plastically. Therefore the frictional surface becomes smooth. In the process of plastic flow filmy layers are extruded from the edges of asperities and are also formed on the flat terraces of asperities. Filmy wear particles are generated by the separation of these thin filmy layers. The size of a filmy wear par- ticle, as estimated by the extrusive length of a filmy layer, is less than about 15 pm.

(B) Side plastic flow type The process of filmy wear particle generation, which results from the

side plastic flow of asperities, is shown in Fig. 7. Figure 7(a) shows that the filmy layers are extruded from the edges of asperities perpendicular to the frictional direction and that they have some cracks. The size of a filmy wear particle, which was estimated by the length of an extrusive layer, is less than about 10 pm. In this way, filmy wear particles are also generated in the process of side plastic flow of asperities. Figure 7(b) shows that

Fig. 7. Filmy wear particle generation in the process of side plastic flow on the rough disk surface (8 X lo4 passes, 4.0 I_tm Rmax; 0.52 m s-l, 9.8 MPa). The arrow indicates the frictional direction.

187

curl-like filmy layers are extruded below. In this case, long and slender wear particles are generated.

3.2.2. Case of a smooth surface Figure 8 shows the SEM micrographs of a wear scar in the case of a

smooth surface. The surface roughness of the disk specimen was 0.09 pm R m*x* Figure 8(a) shows the wear scar at the initial stage of friction.

In the case of a rough surface, as shown in Section 3.2.1, the higher asperities made contact alternatively at first and thereafter they deformed and flowed plastically. In this process filmy layers were extruded from the edges of the asperities and also formed on the flat terraces of the asperities. Filmy wear particles were generated by the separation of these layers.

On the contrary, in the case of a smooth surface, the wear scar consists of some wave-like streaks formed during the ploughing process, as shown in Fig. 8(a). These streaks result from side plastic flow of the surface layer. With a further increase in the passage number of the ball the wave-like streaks increase and then the wave-like extruded filmy layers combine with each other, as shown in Fig. 8(b). The combination occurs rapidly with an increase in the contact pressure or a decrease in the sliding velocity.

,30um,

~ F. D.

Contact

Width

Fig. 8. SEM micrographs of a wear track on the smooth disk surface (0.09 pm Rmax, 0.52 m s- ‘, 9.8 MPa): (a) 4.8 x lo4 passes; (b) 8 x lo4 passes. The arrow indicates the frictional direction.

Fig. 9. Filmy wear particle generation in the process of side plastic flow on the smooth disk surface (8 x lo4 passes, 0.09 pm R,,,, 0.52 m SC’, 9.8 MPa). The arrow indicates the frictional direction.

Two modes in the combination of extrusive filmy layers were observed. One mode is that the filmy layers are extruded on opposite layers, as shown in Fig. 9(a). The other mode is that, after combining, both layers are com- pressed and they are again extruded in the side direction, as shown in Fig. 9(b). Particularly, wear particles, just before separation and a few microm- etres in size, are observed. Figures 9(c) and 9(d) show the extrusive filmy layers, which consist of a few layers, in the surrounding region of the wear scar. Figure 9(d) shows the loose filmy wear particle which is a few microm- etres in size. Figure 9(e) shows that many filmy layers are extruded by the repetition of friction in the inside region of the wear scar.

As mentioned above, in the case of a smooth surface, filmy wear particles are generated as a result of side plastic flow of surface layers ac- companying ploughing. The plastic flow of the surface layers is always maintained by friction. As a result of this, the filmy layers are extruded in the side direction and they separate and become filmy wear particles. There- fore it is said that wear occurs and proceeds gradually in this way under boundary lubrication conditions.

189

In Sections 3.21 and 3.2.2 it was shown that the generation of filmy wear particles resulted in the repetition of plastic flow of asperities or surface layers. This kind of wear type is different from adhesive, abrasive, fatigue and corrosive wear which, in general, are known widely. Therefore this wear type is named ‘Ylow wear” in this paper.

Many thin filmy layers are accumulated on the wear snrface by the repetition of plastic flow during friction, as shown in Figs. Q(a) and 9(e), and thereafter they separate layer by layer, as shown in Fig. Q(f).

Above 8 X 103 passes, typical patterns were always observed in the wear surface which flowed plastically, as shown in Figs. 6(b) - 6(d), Fig. 7 and Fig. 8. These typical patterns look like white and black patches in the SEM micrographs and brown and blue colours in the optical microscope. These patterns were formed on the wear surface inspite of the roughness of the disk specimen. The typical patterns were analysed using X-ray micro- analysis (XMA) facilities. The results are shown in Fig. 10. Figures 10(a) and 10(b) indicate that the regions of the patterns are rich in oxygen. Alter- natively, concerning carbon, there is no differences between the original surface and the frictional surface for the patterns, as shown in Fig. 10(c).

Fig. 10. XMA photographs of the typical patterns on the frictional surface (9.8 MPa, 0.52 m s-l, 8 x lo4 passes): (a) SEM image and 0 Kc% line analysis; (b) 0 KC% image;(c) SEM image and C KCV line analysis. The arrow indicates the frictional direction.

190

It has been known generally that a reaction film is formed on the frictional surface by the chemical reaction between additives in the oil and the metal surface [ 15, 161 and, as a result, it reduces the degree of wear. The oil used in this study did not contain any additives. Therefore it may be concluded that the patterns are oxidative films which result from the reaction between the flowed metals and the oxygen, and not the additives.

We estimated the mean value of the frictional temperature using Block’s formula [ 171 and it is less than about 40 “C under these experimental con- ditions. Although it is so small to give oxidation of the frictional surface, the flash temperature, which is generated at the reaI contact points, easily reaches above a few hundred degree Celsius [ 181. Fu~hermore, the activated metal surface appears in the process of plastic flow of the surface layers in fIow wear. Therefore it is considered that the oxidative films are formed easily on the frictional surface.

3.3. Model of filmy wear particle generation 3.3.1. Case of a rough surface Figure 11 shows the model of filmy wear particle generation which

results from forward plastic flow at the rough surface. The frictional direc- tion in Fig. 11 is perpendicular to the direction of the grinding marks. This model corresponds to the SEM micrographs in Figs. 5 and 6.

(a) Asperities before friction takes place, (b) At the initial stage of friction the tops of the asperities make con-

tact alternatively and flat terraces are formed.

frictional *directIon

ia) (b)

FIImv Separat t on

(ei

Fig. 11. Schematic diagram showing the generation of a filmy wear particle in the process of forward plastic flow on the rough surface.

191

(c) The asperities deform and flow plastically to cover the grooves. After a certain extent of plastic flow, filmy layers are extruded forward. Two modes of filmy layer extrusion occur. One mode is that the filmy layers are extruded from the edges of the asperities. The other mode is that the filmy layers are formed on the flat terraces of the asperities.

(d) With increasing friction the filmy layers grow further and some cracks are generated.

(e) The filmy layers separate extensively and become large filmy wear particles less than 15 pm in size or they become small particles less than a few micrometres in size by small-scale separation.

In this way, when the surface is rough and the frictional direction is perpendicular to that of the grinding marks, filmy wear particles are gen- erated in the process of forward plastic flow.

Figure 12 shows the model of filmy wear particle generation for the case when the frictional direction is the same as that of the grinding marks. This model corresponds to the SEM micrographs in Fig. 7.

(a) Asperities before friction takes place. (b) The contact regions of asperities become flat in the process of

plastic deformation and flow during the initial stage of friction. Thereafter, filmy layers are extruded from the edges of flowed asperities in the process of side plastic flow. Two modes of extrusion of filmy layers occur. One mode is when filmy layers are extruded to cover the grooves in the side direction, as shown in Fig. 12(b). The other mode is when filmy layers are extruded with curling towards the bottoms of the grooves, as shown in Fig. 12(b’).

(c 1 Cc’) Fig. 12. Schematic diagram showing the generation of a filmy wear particle in the pro- cess of side plastic flow on the rough surface.

(c) In the former mode, filmy layers separate and become filmy wear particles, as shown in Fig. 12(c). In the latter mode, filmy layers separatr and become long and slender wear particles, as shown in Fig. 12fc’).

In this way, when the frictional direction and that of the grinding marks are the same, filmy wear particles are generated in the process of side plastic flow.

3.3.2. Case of a smooth surface Figure 13 shows the model of filmy wear particle generation in the

case of a smooth surface. This model corresponds to the SEM micrographs in Figs. 8 and 9.

(a) At the early stage of friction, as the contact occurs partially, the wear scar consists of some wave-like streaks. The wave-like streaks are formed during the ploughing process; thin filmy layers are extruded par- tially in the side direction by plastic flow.

(b) With increasing friction the filmy layers in the wave-like streaks are extruded still more and new streaks are also generated. As a result, the extrusive filmy layers collide with each other and combine. There are two modes in the process of combination of filmy layers. One mode is when filmy layers overlap on the opposite layers, as shown in Fig. 13(b). The other mode is when, after colliding, both layers are compressed and new layers are extruded in the side direction again, as shown in Fig. 13(b’).

: : Y

: ,q

~

b r, .: $‘.j. Separation

,,’ : +Pr ess

(b) Cd /Filmy m Layer

( b’)

Fig. 13. Schematic diagram showing the generation of a filmy wear particle in the process of side plastic flow on the smooth surface.

193

(c) The filmy layers separate and become loose filmy wear particles less than a few mierometres in size in both modes.

In this way, when the surface is smooth, filmy wear particles are generated during the process of side plastic flow.

3.4. Estimation of particle size distribution for filmy wear particles The extrusive length of a filmy layer, i.e. the possibility of it becoming

a wear particle, was measured in the SEM micrographs and the particle size distribution for filmy wear particles was estimated in the case of surface roughness in the range 0.09 - 4.0 pm R,,, . The result obtained is shown in Fig. 14 in relation to the maximum surface roughness. In Fig. 14, the A and I3 regions mean that the frictional direction and that of the grinding marks are vertical and the same respectively. In other words, the A and B regions correspond to the forward plastic flow and the side plastic flow respectively. As shown in Figs. 9 and 14, most of the filmy layers are less than 6 pm in size in the case of 0.09 E.trn R,,, . With the increase in the surface roughness the ratio of formation of the large filmy layer increases in the A region. Therefore it is said that the size of filmy wear particle generated becomes larger with increasing surface roughness. In the B region there is little clear difference.

Figure 15 shows the relationship between the maximum surface rough- ness R,,, and the maximum length of the extrusive layer obtained in

0 10 20 Length of Extrusive

Layers (urn 1 0 %--c-r-

Rmax(,um)

Fig. 14. Relationships between the distribution of length of an extrusive layer and the surface roughness and frictional direction: 0, 0.09 pm R,,,; 0, 1.2 pm R,, (region A); 0, 1.2 /..krn Rmax (region B); A, 4.0 firn R,, (region A); A, 4.0 pm Rm,,, (region B). (9.8 MPa, 0.5 m s-r, 1.6 X lo4 passes, alloy S45C.)

Fig. 15. Relationship between the maximum length of extrusive layers and the surface roughness: 9.8 MPa; 0.5 m s-l ; 1.6 X lo4 passes; alloy S45C.

194

Fig. 14. The maximum length of the extrusive layer was the mean value of three filmy layers measured in order of length. In the range I?,,, < 1 pm the maximum length of the extrusive layer increases with increasing surface roughness. In the range R,,, > 1 E.tm it is almost constant at about 13 pm. Therefore it is said that the maximum size of filmy wear particles generated in the range 4 > R,,, > 1 pm is little dependent on the surface roughness.

Figure 16 shows the distribution of length of the extrusive filmy layer in relation to the frictional direction and the number of frictional passes. With the increase in the number of frictional passes the ratio of formation of the large filmy layer increases, independent of the frictional direction. At 1.6 X lo4 passes the distribution of length of the filmy layer in region A differs significantly from that in region B. However, with the increase in the number of frictional passes, it becomes almost the same in both regions. After all, it is said that the size of the generated filmy wear particles in- creases with the increasing number of frictional passes in spite of the forward plastic flow and side plastic flow on the rough surface.

In this way, the size of the filmy wear particles estimated by flow wear is less than about 15 pm. This value agrees well with the size of filmy wear particles observed in general [ 1 - 3,6,12]. Therefore it is said that filmy wear particles are generated in flow wear which resulted from the plastic flow of asperities. When the large filmy layers have some cracks small filmy wear particles, which are less than a few micrometres in size, are generated.

In the case of a smooth surface filmy layers are generated in the process of side plastic flow, as shown in Fig. 9. Most of them are less than a few micrometres in length. Therefore it is said that many small filmy wear par- ticles are generated on the smooth surface.

0.3

IO 20 Length of Extrusive

Layers (urn 1

Fig. 16. Relationships between the distribution of length of the extrusive layer and the number of frictional passes and the frictional direction: 0, 1.6 x lo4 passes (region A); l , 1.6 x lo4 passes (region B); A, 8.0 x lo4 passes (region A); A, 8.0 x lo4 passes (region B). (9.8 MPa, 0.5 m s-l, 4.0 pm Rmax. alloy S45C.)

195

It is well known in general that the rough surface becomes smooth in boundary lubricated friction [S - lo]. During the smoothing process, for- ward and side plastic flow occur and large filmy wear particles less than 15 km in size are generated, as shown in Figs. 5 and 6. After smoothing, side plastic flow occurs and small filmy wear particles less than a few mi- crometres in size are generated, as shown in Fig. 9. Thereafter, side plastic flow is repeated and wear proceeds gradually. The wear regimes before and after smoothing of the surface correspond to initial and steady wear respec- tively.

The size of the filmy wear particle, i.e. the length of the extrusive filmy layer which depends on the chara~~ristics of plastic flow of the sur- face layer, will depend not only on the surface roughness but also on other factors, e.g. materials, lubricants, additives, the contact geometry of the specimen, ratio of mating areas etc. This hypothesis is assured by the fol- lowing fact. Filmy wear particles, which are shown in Fig. 3, are generated in sliding wear tests which are carried out using the thrust collars wear machine. Both materials of the specimens were 0.10 wt.% C steel (SlOC) and the lubricant was turbine oil (IS0 VG32). Under these conditions many filmy wear particles were generated and most of them were less than 80 pm in size. Compared with the results obtained in Sections 3.2 and 3.4, these filmy wear particles are much larger than those observed in the ball-on-disk- type wear test. This fact is explained by the differences in the ch~a~te~stics of plastic flow of the surface layer, as mentioned above.

4. Conclusions

The process of filmy wear debris generation in boundary lubricated sliding contact was observed successively with a ball-on-disk wear test machine and using SEM. The following conclusions were obtained in this study.

(1) On the rough surface the contact occurred alternatively between higher asperities and the asperities deformed and flowed plastically. As a result of the accumulation of plastic flow of surface layers of the deformed asperities, many filmy wear particles were generated. The following two modes of plastic flow of the surface layers were observed: forward plastic flow; side plastic flow.

(2) On the smooth surface some ploughing marks were formed on the wear track and filmy wear particles were generated as a result of side plastic flow of the surface layers accompanying the ploughing process. In this way, the wear type, which was controlled by the plastic flow and generated many filmy wear particles, was named “flow wear” in this paper.

(3) The size of filmy wear particles estimated by flow wear was depen- dent on the surface roughness. It was less than a few micrometres at 0.09 pm R raax and less than 15 ,um at 4.0 pm R,,, .

196

Acknowledgments

The authors thank Mr. S. Igarashi and Mr. T. Honma (Tsuruoka Tech- nical College) for their assistance in manufacturing experimental apparatus and Mr. M. Akazawa and Mr. M. Goto (Miyagi Technical College) for their assistance with the XMA.

References

1 W. W. Seifert and V. C. Westcott, A method for the study of wear particles in lubri- cating oil, Wear, 21 (1972) 27.

2 A. W. Ruff, Characterization of debris particles recovered from wearing systems, Wear, 42 (1977) 49.

3 R. Bowen, D. Scott, W. W. Seifert and V. C. Westcott, Ferrography, Tribol. Int., 6 (1976) 109.

4 J. M. Martin, J. L. Mansot, I. Berbezier and H. Dexpert, The nature and origin of wear particles from boundary lubrication with a zinc dialkyldithiophosphate, Wear, 93 (1984) 117.

5 T. Akagaki, K. Kato and T. Kayaba, Flow wear at sliding contact in boundary lubri- cation. In Y. Tamai (ed.), Proc. JSLE ht. Tribology Confi, Tokyo, July 8 - 10, 1985, Elsevier, Amsterdam, 1985, p. 891.

6 A. A. Reda, R. Bowen and V. C. Westcott, Characteristics of particles generated at the interface between sliding steel surfaces, Wear, 34 (1975) 261.

7 N. P. Suh, The delamination theory of wear, Wear, 25 (1973) 111. 8 S. Jahanmir, Wear of AISI 4340 steel under boundary lubrication. In S. K. Rhee,

A. W. Ruff and K. C. Ludema (eds.), Proc. Int. Conf. on Wear of Materials, Sun Francisco, CA, March 30 -April 1, 1981, American Society of Mechanical Engi- neers, New York, 1981, p. 648.

9 T. E. Fisher and M. J. Luton, Effect of simple lubricants on deformation and wear in concentrated sliding contact, ASLE Trans., 26 (1983) 31.

10 G. W. Rowe, H. Kaliszer, G. Trmal and A. Cotter, Running-in of plain bearings, Wear, 34 (1975) 1.

11 S. Jahanmir, Wear mechanisms of boundary-lubricated surfaces, Wear, 73 (1981) 169.

12 D. Scott, W. W. Seifert and V. C. Westcott, Sci. Am., (1974) 88. 13 P. Heilmann, J. Don, T. C. Sun, W. A. Glaeser and D. A. Rigney, Sliding wear and

transfer. In K. C. Ludema (ed.), Proc. Znt. Conf. on Wear of Materials, Reston, VA, April 11 - 14, 1983, American Society of Mechanical Engineers, New York, 1983, p. 414.

14 L. K. Ives, I. S. Harris and M. B. Peterson, Evaluation of a new wear resistant addi- tive - SbSb&. In K. C. Ludema (ed.), Proc. Znt. Conf. on Wear of Materials, Reston, VA, April 11 - 14, 1983, American Society of Mechanical Engineers, New York, 1983, p. 507.

15 B. J. Bird, R. C. Coy and J. F. Hutton, The preparation and nature of surface films from zinc dialkyldithiophosphate, ASLE Trans., 23 (1978) 121.

16 J. M. George, J. M. Martin, T. Mathia, Ph. Kapsa, G. Meille and H. Montes, Mechanism of boundary lubrication with zinc dithiophosphate, Wear, 53 (1979) 9.

17 H. Block, Theoretical study of temperature rise at surfaces of actual contact under oiliness lubricating conditions, Proc. Inst. Meek. Eng., London, 2 (1937) 222.

18 F. P. Bowden and K. E. Ridler, hoc. R. Sot. London, Ser. A, 54 (1936) 640.