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Scanning electron microscopy observations of worn surfaces of metals in tribological contact with loose solid particles
Wear, 73 (1981) 311 - 323 0 Elsevier Sequoia S.A., Lausanne -Printed
311 in The Netherlands
SCANNING ELECTRON MICROSCOPY OBSERVATIONS SURFACES OF METALS IN TRIBOLOGICAL CONTACT SOLID PARTICLES
OF WORN WITH LOOSE
A. D. SARKAR* Santana, 2 Sandforth
Close, Liverpool L12 ILJ (Gt. Britain)
(Received July 14,1981)
Summary Surfaces of a 60-43 brass worn by three-body abrasion with Sic grits were observed under the scanning electron microscope. Similar observations were carried out on targets of steel and commercially pure aluminium which had been impacted with hardened steel balls, The brass develops alternating bands of hills and valleys when undergoing three-body abrasion. The hills wear by direct rubbing against the steel counterface. The valleys, however, are embedded with grits which are later removed and probably carry an amount of brass during their departure. The impact process creates craters which show severe defo~ation. Radial plastic flow of metal also occurs in the target around a crater. It appears that under repeated attack these deformed areas produce wear debris by microspalling. Metal may also be removed by nucleation and propagation of cracks.
1. Introduction
Metal surfaces undergo tribological interaction with solids mainly by three-body abrasion and by impact at various velocities and angles by particles. The three-body abrasion mode is common in bearings where trapped wear debris and adventitious solid particles which have ingressed from the surroundings cause surface damage and wear. A three-body abrasion process also occurs under arduous conditions of load such as in jaw crusher plates which are sizing rocks and ores. A typical example of interaction due to impacting solid particles is seen with helicopter blades undergoing erosive wear by atmospheric grit. Both three-body abrasion and wear by impact with solid particles are technologically important and basic studies have been carried out by several workers. The nature of worn surfaces determined from laboratory studies of three-body abrasion and of single-particle impact as revealed by scanning electron microscopy is reported in the present paper. *Present address: f)epwtment of Mechanical Engineering, Wniversity of Petroleum and Minerals, Dhahran, Saudi Arabia.
312
2. Three-body
abrasion
The most prevalent view held by tribologists is that grits, whether in loose form or held rigid as they are in a grinding wheel, remove metal by a ploughing action. It is important then that the abrasive particle has sharp edges and a model has shown [ 1, 21 that, for a conical grit,
V - = 0.63 cot 0 ; S
(1)
where V is the volume of material removed by ploughing a metal of hardness H, S is the sliding distance, W is the applied normal load and 0 is the semiapex angle of the cone. The role of the cone angle has not been investigated in detail but macromodels of pyramidal tools have been employed to establish the part played by the angle of attack of the abrasives [ 31. When the attack is normal, the motion is jerky but it is not so at a smaller angle of, for example, 50”. At smaller angles of attack, the amount of metal removed by ploughing diminishes. In addition to the angle of attack, the hardness of the abrasive particle relative to the metal is regarded as important. The conclusion agrees with the intuitive thought that a soft metal will wear readily when in tribological contact with a harder abrasive. The hardness of the grit is not included in the wear law (eqn. (1)) but it is implicit in that the softer a metal is the more it will wear. The hardness criterion, however, does not predict wear rates completely [4, 51 so that eqn. (1) cannot be regarded as the global formula for three-body abrasion.
2.1. Laboratory study of three-body abrasion In the present investigation a martensitic steel bush 50 mm in diameter was rotated in a bed of fine Sic grits at 140 rev mir-r. The abrasive particles were held in a cylindrical container coaxial with the bush which had an inside diameter of 62 mm. As the bush rotated, it was covered with a layer of abrasive upon which 60-40 brass pins with an average hardness of 150 HV slid at loads of 5 and 10 N. Figure 1 shows that the wear rate increases with load as it should do from eqn. (1) and that it is very much higher than for. brass pins running dry on the hard steel bush where the interface is free of the Sic abrasive particles (Fig. 2). Scanning electron microscopy observation of worn surfaces shows that one effect of the abrasive particles is to cut grooves in the soft metal. A typical example is shown in Fig. 3 where a single particle has produced a furrow and has come to rest against a wall of metal. The serrated nature of the sides of the furrow suggests a jerky motion and, according to observations from macromodels [ 31, it is suggested that the cutting edge is normal to the metal surface. The groove is not straight: this is possibly because the grit interacts with relatively hard microconstituents in the brass and is thus
313
2.0
-
I.8 . I.6 .
IQ SLIDING
DISTANCE
Km
SLIDING
2.0
3-o
DISTANCE
4.0 Km
Fig. 1. Mass losses of 60-40 and 10 N (A).
brass pins sliding against loose Sic grits under loads of 5 N
Fig. 2. Mass losses of 60-40 5 N (0) and 10 N (A).
brass pins sliding in the absence of abrasives under loads of
(0)
Fig. 3. An abrasive particle ploughing a groove in brass.
deflected. Figure 3 shows other Sic particles which appear to have settled innocuously on the metal surface. Figure 3 shows an example from an early stage of the sliding process and is a positive proof of ploughing. As sliding continues the prior machine marks on the pin surfaces (Fig. 4) soon disappear and a characteristic pattern is attained. The topography of the pin surface shows alternating layers; each has a distinctive character and is arranged parallel to the direction of motion (Fig. 5). Enlargedviews (Figs. 6 and 7) of the worn surface show the undulating nature of the field. The hills are metallic and the valleys are embedded with abrasive particles. Figure 7 shows that the grooves extend to the edge of the pin. The metallic nature of the hills is confirmed in Figs. 8 and 9 where the valleys incorporate abrasives arranged in a definite pattern. The hills are flat and polished whereas the abrasive particles in the valleys are arranged in the form of an arc. Further examination shows the hill surfaces to be saucer shaped as if the counter-face asperity is hemispherical (Fig. 10). A region of
Fig. 4. A machined
60 - 40 brass pin before
Fig. 5. A worn surface ploughing.
sliding.
of a 60 -40 brass showing
grits embedded
Fig, 6. An enlarged
view of the surface
shown
in Fig. 5.
Fig, 7. An enlarged
view of the surface
shown
in Fig. 6.
Fig. 8. A worn surface sides. Fig. 9. An entarged
of 60-40
brass showing
view of the hill shown
a hill surrounded
in Fig, 8: adhesive
in the valleys created
by abrasives
wear can be seen.
on both
by
315
the valley shows a large cavity (Figs. 10 and 11) and is similar in appearance to a pot-hole in roads owing to the washing action of flowing water. Grits are also embedded in the hard steel counter-face, which shows signs of wear. By contrast, the topography of brass which slides on the hard steel counterface without any abrasives at the interface shows typical flow patterns (Fig. 12) and this surface subsequently fails by spalling (Fig. 13).
Fig. 10. A crater in a valley. The curved nature of the hill is evident. Fig. 11. An enlarged view of the crater shown in Fig. 10.
Fig. 12. The topography of a surface of 60-40 abrasives.
brass undergoing wear in the absence of
Fig. 13. The spalling of a brass surface undergoing sliding without abrasives and producing wear debris.
3. Erosive wear The process of erosive wear is mainly simulated by two methods. In one method, particulate matter is caused to impinge on a surface at various velocities. The second approach employs a single projectile such as a hardened steel ball or a cylinder; this projectile strikes a flat target at predetermined velocities. The wear rate is usually expressed as the mass loss per unit mass of abrasive.
316
If an impinging particle of mass m produces V=
+ mu2 ok?
a crater of volume V, (2)
where u is the incident velocity, u is the yield stress of the target material and g is the acceleration due to gravity. It has been shown [ 61 that eqn. (2) holds irrespective of the shape of the projectile. The crater volume is expected to bear some relationship to the loss of metal due to impact. Thus erosive wear should be a function of v2; however, this is not so. Experiments [ 71 with steel balls and cylinders striking mild steel targets show that the wear rate is proportional to v2.‘. Diamond dust has been used for the abrasive particles, and the wear rate has been shown [ 81 to be proportional to v~.~. Other things being equal, the amount of wear increases with the angle of impact until a maximum is reached, the amount of wear then decreases to a low value when particles strike a target normally. Experiments [ 71 with steel or titanium targets using single particles of hard steel show that, at low velocities, a lip forms at the edge of the crater and is removed by subsequent impact. The lip is removed as it forms if the velocity is high; this effect is consistent when the angle of impact is about 30”, which is approximately the angle at which the maximum amount of wear occurs with many metals. If the angle of impingement is increased, a lip continues to form but it is folded over the underlying metal. The topography of impacted surfaces therefore should be observed, and selected results from a scanning electron microscopy study of steel and aluminium targets are reported here. Flat surfaces of the metals were struck by hard steel balls 6 mm in diameter at a low velocity of 10 m s-l with the aid of an air gun. The angle of impact was varied in certain cases. The surfaces produced by repeated impacts of the same region together with those formed after a single shot were examined microscopically. 3.1. Aluminium at normal impact Figure 14 shows the view of a target after 35 impacts. Although the same spot was aimed at, the ball deflected and a large part of the target was covered with overlapping craters. Figure 14 shows a region which received several blows and this multiply impacted area exhibits a deformed band around it. This band is in the adjacent matrix of the target and has a Vickers microhardness of 90 - 95 compared with the prior target hardness of about 45 HV. Another view (Fig. 15) of the region in Fig. 14 shows how difficult it was to strike the same spot each time; it is therefore inevitable that the overall size of the cavity is determined by the joining-up of adjacent craters. The craters themselves are not smooth and certain areas show scabbed metal (Fig. 16). These are either lips which have folded or layers which are beginning to spall. An enlarged view (Fig. 17) of a crater shows the boundary between the crater and the unattacked parent target. The boundary shows deformation patterns and a small region in the crater shows spalling. A fea-
Fig. 14. A crater in an aluminium target 6 mm in diameter (speed, 10 m s-l).
produced
Fig. 15. An aluminium
target
showing
overlapping
Fig. 16. An aluminium
crater
showing
scabbed
by 35 normal craters
strikes with a steel ball
and a deformed
zone.
areas.
Fig. 17. An aluminium target showing the boundary between a crater and the adjacent region of the target. A radial plastic displacement and a spalled area in the crater should be noted.
ture to note is that the deformed layer outside the crater has patches (Fig. 18, region A) which are similar in appearance to metal transfer in sliding wear. One important effect of multiple impact in the vicinity of a region is the folding of metals. Figure 19 shows the centre of a large cavity with a base diameter of about 5 mm produced by striking the region 35 times. Enlarged views of this region show petals of metals with radial cracks (Figs. 20 and 21). Figure 20 shows a small splinter of metal which probably formed as a result of direct impact and was trapped in the surrounding area. 3.2. Aluminium struck at 30” Most materials show a maximum amount of erosive wear at an attack angle in the range 15” - 30”. A single shot produces an elliptical crater with strong evidence of deformation in the region immediately surrounding the parent metal (Fig. 22). Examination of the area outside the crater shows
Fig. 18. A deformed layer outside the crater on an aluminium target giving the appearance of metal transfer A. Fig. 19. Aluminium target after 35 impacts. The photograph was taken normal to the centre of the impacted area.
Fig. 20. An aluminium target showing folds and a particle of wear debris. Fig. 21. Enlargement of a fold in Fig. 20 showing radial cracks.
Fig. 22. A single crater impacted in aluminium at 30”. Fig. 23. Enlargement of the top left-hand region of the crater shown in Fig. 22. The micropetals should be noted.
319
clearly the severe plastic flow of the deformed target (Fig. 23). It is most interesting that the trailing edge of the crater is not smooth but is in the form of micropetals delineated in a manner analogous to that of grains in metals. Each micropetal also shows longitudinal bands of deformation if examined at a yet greater magnification (Fig. 24).
Fig. 24. A micropetal Fig. 25. Aluminium
showing target
deformation
after 40” impact
bands. showing
two overlapping
craters.
3.3. Aluminium struck at 40” In practical situations such as helicopter blades blasted by desert sands, myriads of single craters form, overlap and produce wear. It is therefore instructive to observe two overlapping craters (Fig. 25) produced by impacting a commercially pure aluminium target at an incident angle of 40”. The most significant aspect is the severe deformation at the boundary and in its vicinity (Fig. 26). The craters themselves deform and evidence of this is shown in Fig. 27 which is the trailing edge of a micropetal such as that shown in Fig. 24.
Fig. 26. Aluminium target two craters A and B.
after 40” impact
showing
deformation
at the junction
between
Fig. 27. Aluminium target after impact at 40” showing a deformation band at the trailing edge of the crater. Deformation bands follow the overall direction of impact.
320
3.4. Steel struck at 90” and at 70” Intense deformation adjacent to a crater is also seen in steels {Fig. 28) and folds are observed in the craters (Fig, 29). Figures 28 and 29 show a part of the region of a cavity formed by striking steel with steel 70 times at an impingement angle of 90”. Figure 30 shows the interfacial region of two overlapping craters produced by striking a steel target at 70”. The area shown in Fig. 30 was chosen arbitrarily from a large number of overlapping craters produced by firing the steel ball randomly over the target at an angle of 70”. The region shows deformation and two crumpled zones which meet to create an interface. It is possible that such crumpling is a prelude to cracking and spaliing.
Fig. 28. Radial compression around a crater in a steel target impacted at 90”. Fig. 29. A crater showing folds in mild steel impacted at 90”.
Fig. 30. Surface of mild steel after 70 impacts at 70’. This has created overlapping craters and the crumpled appearance of two adjacent craters is shown.
4. Discussion It can be seen in Figs. 5 - 7 that a three-body abrasive process produced with a pin-bush machine provides a topography such that there are hills surrounded by grooves which are embedded with particles of grit. The hills
321
alternate with valleys and follow a straight path parallel to the overall direction of motion. A single abrasive particle does not follow a true straight line but the valleys are created by many particles ploughing in the general direction of sliding and the topography appears straight. The hills are almost free of abrasives (Fig. 9). In a typical example (see Fig. 3) an Sic particle cuts a groove in the brass. The process results in a topography where the prior matrix forms a hill: the maximum height of the hill may be above the level of the target (as in a ploughed field). The grits cannot settle on the hill and their activity is restricted at the lower valleys created by ploughing. Since a valley is created by the cooperative effort of a number of abrasive particles, its width is greater than that of a single grit of Sic. As sliding proceeds, the pattern of alternate hills and valleys is consolidated. The hills appear to be rubbed directly by the counterface and close examination shows folds of metal (Fig. 9) as for brass rubbing on steel only (Fig. 12). The hills are rubbed flat and there is an indication of a shallow depression; this tempts us to conclude that the asperities of the counterface are hemispherical and remove brass from these high walls mainly by an adhesive mode (Fig. 10). Figures 10 and 11 show that an abrasive-loaded valley develops a crater as if a portion of the surface has collapsed. The appearance of the crater is analogous to a pot-hole in a road which starts as a small cavity and then enlarges with time. It is probable that a particle of Sic then cuts a groove in the metal unlike the simple adhesive situation shown in Fig. 12. The groove extends owing to sliding but the particle may become stuck long before the edge of the pin. Other particles will follow a similar process until a complete valley is ploughed from one end of the pm to the other. Particles become stuck against a wall of metal or against the leading particles which moved before them and were forced to come to a halt. In this way the valleys become embedded with grits. The reason for alternate hills and valleys is probably that in the very early stage of sliding, the abrasive particles lodge preferentially in the valleys of the machined bush, i.e. the counter-face. They then cut grooves in the brass; this produces the observed valleys. On being further subjected to the ploughing action of loose abrasives, one or more embedded particles are detached from the valleys. Once a point of weakness is established, neighbouring grits are plucked readily from the valleys. The effect of this is the removal of an amount of brass; the other regions which wear are the hills and these lose material by sliding directly against the hard steel counterface. The action of the grits is more aggressive than simple adhesive wear, as a comparison of Fig. 1 with Fig. 2 shows. With respect to erosive wear, the first effect of impingement attack by a ball is the formation of a crater. As the ball hits the target a certain amount of slipping and spinning occurs; this can be seen by the small-step-like appearance of a region in the bottom right-hand portion of the crater in Fig. 22. The ball picks up an amount of target material by mere adhesion but this is not noticeable after only a single strike. The crater itself undergoes significant changes. As Fig. 23 shows, the edge of the crater is wavy because impacting forces have caused the target to be extended and deformed in the
32‘2
direction of the attack. The deformation is irregular; this results in the formation of what can be aptly described as micropetals with distinct boundaries between them. These petals in the present investigation have so far been observed at the exit end of a crater and each petal shows further deformation bands at higher ma~ifi~ations. We can speculate that the micropetals will lift off once a critical number of impacts has been exceeded. The regions immediately outside a crater also show severe deformation. This is to be expected because it has been shown that at a velocity greater than about 8 m s-l plastic yielding of steel targets takes place [ 61. Aluminium is also expected to yield readily at that velocity and radial compression of the target medium is obvious in Figs. 14 and 17. Repeated impact of the same area shows scabbed regions which can be recognized even at a low magnification (Fig. 16) and the cause of these regions is possibly the intense work hardening. The scab within a crater is usually located in the vicinity of the interface created by the deformed parent metal and the crater (Fig. 17). An effect of repeated impact is to form folds which probably become brittle with time owing to work hardening. This is shown in Fig. 20 where radial cracks form in these folds and splinters of wear debris escape, These cracks are similar to the ring cracks observed [ 9] in glass at normal angles of incidence which occur as a result of hertzian stress induced in a brittle material which remains elastic as it fractures. In practical situations a target is blasted indiscriminately by loose solid particles. Two overlapping craters show that the deformation band in their vicinity extends and assumes a crumpled appearance as seen, for example, in Fig. 30. We expect these areas to become brittle and to disappear by microspalling or by nucleating radial cracks which extend and join up to produce wear debris. The role of ductility in erosive wear has been reported in the literature [lo]. A ductile metal shows the characteristic increase and decrease in erosive wear with increasing impact angle. With a brittle material, however, erosive wear increases continuously with a maximum at normal impact. Both materials used in the present experiments, i.e. aluminium and steel, were ductile. This may be why both metals show generally similar topographical features under a scanning electron microscope. The evidence supports the view that repeated impact will make a surface brittle in which case the rate of erosive wear should accelerate, particularly when the particles impinge normally. It will be very informative to evaluate the quantitative wear of metals caused by repeated impact with single particles.
5. Conclusions On the basis of the present investigation, the following conclusions have been drawn. (1) In a three-body abrasive situation, the grits create grooves such that the topography consists of hills and valleys orientated in the general direction
323
of sliding. The hills are free of abrasive and they wear by sliding directly against the hard steel counter-face. The embedded abrasive particles are uprooted from the valleys and they are expected to remove an amount of metal during their departure. (2) The impact of a metal by a single particle creates a large amount of radial plastic flow in the target surrounding the crater, These work-hardened regions crumple and small areas spa11 off or cracks nucleate and join up to produce wear debris. The crater formed by impact also exhibits severe plastic flow. The edges show features which can be termed micropetals; these micropetals contain severe deformation bands. It is suggested that with repeated impact the petals spall off the matrix once a critical number of strikes has been exceeded.
References 1 2 3 4 5 6 7 8 9 10
E. Rabinowicz, Friction and Wear of materials, Wiley, New York, 1966. E. Rabinowicz, L. A. Dunn and P. G. Russel, Wear, 4 (1961) 346. A. J. Sedriks and T. 0. Mulhearn, Wear, 6 (1963) 457. R. C. D. Richardson, Wear, 1 I (1968) 245. M. M. Khruschov, in nroc. Cont. on Lubrication and Wear, 1957, Institution of Mechanical Engineers, London, Paper 46. A. D. Sarkar, Friction and Wear, Academic Press, London, 1980. I. M. Hutchings, R. E. Winter and J. E. Field, Proc. R. Sot. London, Ser. A, 348 (1976) 379. G. P. Tilly and W. Sage, Wear, 16 (1970) 447. I. Finnie, Wear, 3 (1960) 87. G. P. Tilly, Wear, 14 (1969) 63.
311 in The Netherlands
SCANNING ELECTRON MICROSCOPY OBSERVATIONS SURFACES OF METALS IN TRIBOLOGICAL CONTACT SOLID PARTICLES
OF WORN WITH LOOSE
A. D. SARKAR* Santana, 2 Sandforth
Close, Liverpool L12 ILJ (Gt. Britain)
(Received July 14,1981)
Summary Surfaces of a 60-43 brass worn by three-body abrasion with Sic grits were observed under the scanning electron microscope. Similar observations were carried out on targets of steel and commercially pure aluminium which had been impacted with hardened steel balls, The brass develops alternating bands of hills and valleys when undergoing three-body abrasion. The hills wear by direct rubbing against the steel counterface. The valleys, however, are embedded with grits which are later removed and probably carry an amount of brass during their departure. The impact process creates craters which show severe defo~ation. Radial plastic flow of metal also occurs in the target around a crater. It appears that under repeated attack these deformed areas produce wear debris by microspalling. Metal may also be removed by nucleation and propagation of cracks.
1. Introduction
Metal surfaces undergo tribological interaction with solids mainly by three-body abrasion and by impact at various velocities and angles by particles. The three-body abrasion mode is common in bearings where trapped wear debris and adventitious solid particles which have ingressed from the surroundings cause surface damage and wear. A three-body abrasion process also occurs under arduous conditions of load such as in jaw crusher plates which are sizing rocks and ores. A typical example of interaction due to impacting solid particles is seen with helicopter blades undergoing erosive wear by atmospheric grit. Both three-body abrasion and wear by impact with solid particles are technologically important and basic studies have been carried out by several workers. The nature of worn surfaces determined from laboratory studies of three-body abrasion and of single-particle impact as revealed by scanning electron microscopy is reported in the present paper. *Present address: f)epwtment of Mechanical Engineering, Wniversity of Petroleum and Minerals, Dhahran, Saudi Arabia.
312
2. Three-body
abrasion
The most prevalent view held by tribologists is that grits, whether in loose form or held rigid as they are in a grinding wheel, remove metal by a ploughing action. It is important then that the abrasive particle has sharp edges and a model has shown [ 1, 21 that, for a conical grit,
V - = 0.63 cot 0 ; S
(1)
where V is the volume of material removed by ploughing a metal of hardness H, S is the sliding distance, W is the applied normal load and 0 is the semiapex angle of the cone. The role of the cone angle has not been investigated in detail but macromodels of pyramidal tools have been employed to establish the part played by the angle of attack of the abrasives [ 31. When the attack is normal, the motion is jerky but it is not so at a smaller angle of, for example, 50”. At smaller angles of attack, the amount of metal removed by ploughing diminishes. In addition to the angle of attack, the hardness of the abrasive particle relative to the metal is regarded as important. The conclusion agrees with the intuitive thought that a soft metal will wear readily when in tribological contact with a harder abrasive. The hardness of the grit is not included in the wear law (eqn. (1)) but it is implicit in that the softer a metal is the more it will wear. The hardness criterion, however, does not predict wear rates completely [4, 51 so that eqn. (1) cannot be regarded as the global formula for three-body abrasion.
2.1. Laboratory study of three-body abrasion In the present investigation a martensitic steel bush 50 mm in diameter was rotated in a bed of fine Sic grits at 140 rev mir-r. The abrasive particles were held in a cylindrical container coaxial with the bush which had an inside diameter of 62 mm. As the bush rotated, it was covered with a layer of abrasive upon which 60-40 brass pins with an average hardness of 150 HV slid at loads of 5 and 10 N. Figure 1 shows that the wear rate increases with load as it should do from eqn. (1) and that it is very much higher than for. brass pins running dry on the hard steel bush where the interface is free of the Sic abrasive particles (Fig. 2). Scanning electron microscopy observation of worn surfaces shows that one effect of the abrasive particles is to cut grooves in the soft metal. A typical example is shown in Fig. 3 where a single particle has produced a furrow and has come to rest against a wall of metal. The serrated nature of the sides of the furrow suggests a jerky motion and, according to observations from macromodels [ 31, it is suggested that the cutting edge is normal to the metal surface. The groove is not straight: this is possibly because the grit interacts with relatively hard microconstituents in the brass and is thus
313
2.0
-
I.8 . I.6 .
IQ SLIDING
DISTANCE
Km
SLIDING
2.0
3-o
DISTANCE
4.0 Km
Fig. 1. Mass losses of 60-40 and 10 N (A).
brass pins sliding against loose Sic grits under loads of 5 N
Fig. 2. Mass losses of 60-40 5 N (0) and 10 N (A).
brass pins sliding in the absence of abrasives under loads of
(0)
Fig. 3. An abrasive particle ploughing a groove in brass.
deflected. Figure 3 shows other Sic particles which appear to have settled innocuously on the metal surface. Figure 3 shows an example from an early stage of the sliding process and is a positive proof of ploughing. As sliding continues the prior machine marks on the pin surfaces (Fig. 4) soon disappear and a characteristic pattern is attained. The topography of the pin surface shows alternating layers; each has a distinctive character and is arranged parallel to the direction of motion (Fig. 5). Enlargedviews (Figs. 6 and 7) of the worn surface show the undulating nature of the field. The hills are metallic and the valleys are embedded with abrasive particles. Figure 7 shows that the grooves extend to the edge of the pin. The metallic nature of the hills is confirmed in Figs. 8 and 9 where the valleys incorporate abrasives arranged in a definite pattern. The hills are flat and polished whereas the abrasive particles in the valleys are arranged in the form of an arc. Further examination shows the hill surfaces to be saucer shaped as if the counter-face asperity is hemispherical (Fig. 10). A region of
Fig. 4. A machined
60 - 40 brass pin before
Fig. 5. A worn surface ploughing.
sliding.
of a 60 -40 brass showing
grits embedded
Fig, 6. An enlarged
view of the surface
shown
in Fig. 5.
Fig, 7. An enlarged
view of the surface
shown
in Fig. 6.
Fig. 8. A worn surface sides. Fig. 9. An entarged
of 60-40
brass showing
view of the hill shown
a hill surrounded
in Fig, 8: adhesive
in the valleys created
by abrasives
wear can be seen.
on both
by
315
the valley shows a large cavity (Figs. 10 and 11) and is similar in appearance to a pot-hole in roads owing to the washing action of flowing water. Grits are also embedded in the hard steel counter-face, which shows signs of wear. By contrast, the topography of brass which slides on the hard steel counterface without any abrasives at the interface shows typical flow patterns (Fig. 12) and this surface subsequently fails by spalling (Fig. 13).
Fig. 10. A crater in a valley. The curved nature of the hill is evident. Fig. 11. An enlarged view of the crater shown in Fig. 10.
Fig. 12. The topography of a surface of 60-40 abrasives.
brass undergoing wear in the absence of
Fig. 13. The spalling of a brass surface undergoing sliding without abrasives and producing wear debris.
3. Erosive wear The process of erosive wear is mainly simulated by two methods. In one method, particulate matter is caused to impinge on a surface at various velocities. The second approach employs a single projectile such as a hardened steel ball or a cylinder; this projectile strikes a flat target at predetermined velocities. The wear rate is usually expressed as the mass loss per unit mass of abrasive.
316
If an impinging particle of mass m produces V=
+ mu2 ok?
a crater of volume V, (2)
where u is the incident velocity, u is the yield stress of the target material and g is the acceleration due to gravity. It has been shown [ 61 that eqn. (2) holds irrespective of the shape of the projectile. The crater volume is expected to bear some relationship to the loss of metal due to impact. Thus erosive wear should be a function of v2; however, this is not so. Experiments [ 71 with steel balls and cylinders striking mild steel targets show that the wear rate is proportional to v2.‘. Diamond dust has been used for the abrasive particles, and the wear rate has been shown [ 81 to be proportional to v~.~. Other things being equal, the amount of wear increases with the angle of impact until a maximum is reached, the amount of wear then decreases to a low value when particles strike a target normally. Experiments [ 71 with steel or titanium targets using single particles of hard steel show that, at low velocities, a lip forms at the edge of the crater and is removed by subsequent impact. The lip is removed as it forms if the velocity is high; this effect is consistent when the angle of impact is about 30”, which is approximately the angle at which the maximum amount of wear occurs with many metals. If the angle of impingement is increased, a lip continues to form but it is folded over the underlying metal. The topography of impacted surfaces therefore should be observed, and selected results from a scanning electron microscopy study of steel and aluminium targets are reported here. Flat surfaces of the metals were struck by hard steel balls 6 mm in diameter at a low velocity of 10 m s-l with the aid of an air gun. The angle of impact was varied in certain cases. The surfaces produced by repeated impacts of the same region together with those formed after a single shot were examined microscopically. 3.1. Aluminium at normal impact Figure 14 shows the view of a target after 35 impacts. Although the same spot was aimed at, the ball deflected and a large part of the target was covered with overlapping craters. Figure 14 shows a region which received several blows and this multiply impacted area exhibits a deformed band around it. This band is in the adjacent matrix of the target and has a Vickers microhardness of 90 - 95 compared with the prior target hardness of about 45 HV. Another view (Fig. 15) of the region in Fig. 14 shows how difficult it was to strike the same spot each time; it is therefore inevitable that the overall size of the cavity is determined by the joining-up of adjacent craters. The craters themselves are not smooth and certain areas show scabbed metal (Fig. 16). These are either lips which have folded or layers which are beginning to spall. An enlarged view (Fig. 17) of a crater shows the boundary between the crater and the unattacked parent target. The boundary shows deformation patterns and a small region in the crater shows spalling. A fea-
Fig. 14. A crater in an aluminium target 6 mm in diameter (speed, 10 m s-l).
produced
Fig. 15. An aluminium
target
showing
overlapping
Fig. 16. An aluminium
crater
showing
scabbed
by 35 normal craters
strikes with a steel ball
and a deformed
zone.
areas.
Fig. 17. An aluminium target showing the boundary between a crater and the adjacent region of the target. A radial plastic displacement and a spalled area in the crater should be noted.
ture to note is that the deformed layer outside the crater has patches (Fig. 18, region A) which are similar in appearance to metal transfer in sliding wear. One important effect of multiple impact in the vicinity of a region is the folding of metals. Figure 19 shows the centre of a large cavity with a base diameter of about 5 mm produced by striking the region 35 times. Enlarged views of this region show petals of metals with radial cracks (Figs. 20 and 21). Figure 20 shows a small splinter of metal which probably formed as a result of direct impact and was trapped in the surrounding area. 3.2. Aluminium struck at 30” Most materials show a maximum amount of erosive wear at an attack angle in the range 15” - 30”. A single shot produces an elliptical crater with strong evidence of deformation in the region immediately surrounding the parent metal (Fig. 22). Examination of the area outside the crater shows
Fig. 18. A deformed layer outside the crater on an aluminium target giving the appearance of metal transfer A. Fig. 19. Aluminium target after 35 impacts. The photograph was taken normal to the centre of the impacted area.
Fig. 20. An aluminium target showing folds and a particle of wear debris. Fig. 21. Enlargement of a fold in Fig. 20 showing radial cracks.
Fig. 22. A single crater impacted in aluminium at 30”. Fig. 23. Enlargement of the top left-hand region of the crater shown in Fig. 22. The micropetals should be noted.
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clearly the severe plastic flow of the deformed target (Fig. 23). It is most interesting that the trailing edge of the crater is not smooth but is in the form of micropetals delineated in a manner analogous to that of grains in metals. Each micropetal also shows longitudinal bands of deformation if examined at a yet greater magnification (Fig. 24).
Fig. 24. A micropetal Fig. 25. Aluminium
showing target
deformation
after 40” impact
bands. showing
two overlapping
craters.
3.3. Aluminium struck at 40” In practical situations such as helicopter blades blasted by desert sands, myriads of single craters form, overlap and produce wear. It is therefore instructive to observe two overlapping craters (Fig. 25) produced by impacting a commercially pure aluminium target at an incident angle of 40”. The most significant aspect is the severe deformation at the boundary and in its vicinity (Fig. 26). The craters themselves deform and evidence of this is shown in Fig. 27 which is the trailing edge of a micropetal such as that shown in Fig. 24.
Fig. 26. Aluminium target two craters A and B.
after 40” impact
showing
deformation
at the junction
between
Fig. 27. Aluminium target after impact at 40” showing a deformation band at the trailing edge of the crater. Deformation bands follow the overall direction of impact.
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3.4. Steel struck at 90” and at 70” Intense deformation adjacent to a crater is also seen in steels {Fig. 28) and folds are observed in the craters (Fig, 29). Figures 28 and 29 show a part of the region of a cavity formed by striking steel with steel 70 times at an impingement angle of 90”. Figure 30 shows the interfacial region of two overlapping craters produced by striking a steel target at 70”. The area shown in Fig. 30 was chosen arbitrarily from a large number of overlapping craters produced by firing the steel ball randomly over the target at an angle of 70”. The region shows deformation and two crumpled zones which meet to create an interface. It is possible that such crumpling is a prelude to cracking and spaliing.
Fig. 28. Radial compression around a crater in a steel target impacted at 90”. Fig. 29. A crater showing folds in mild steel impacted at 90”.
Fig. 30. Surface of mild steel after 70 impacts at 70’. This has created overlapping craters and the crumpled appearance of two adjacent craters is shown.
4. Discussion It can be seen in Figs. 5 - 7 that a three-body abrasive process produced with a pin-bush machine provides a topography such that there are hills surrounded by grooves which are embedded with particles of grit. The hills
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alternate with valleys and follow a straight path parallel to the overall direction of motion. A single abrasive particle does not follow a true straight line but the valleys are created by many particles ploughing in the general direction of sliding and the topography appears straight. The hills are almost free of abrasives (Fig. 9). In a typical example (see Fig. 3) an Sic particle cuts a groove in the brass. The process results in a topography where the prior matrix forms a hill: the maximum height of the hill may be above the level of the target (as in a ploughed field). The grits cannot settle on the hill and their activity is restricted at the lower valleys created by ploughing. Since a valley is created by the cooperative effort of a number of abrasive particles, its width is greater than that of a single grit of Sic. As sliding proceeds, the pattern of alternate hills and valleys is consolidated. The hills appear to be rubbed directly by the counterface and close examination shows folds of metal (Fig. 9) as for brass rubbing on steel only (Fig. 12). The hills are rubbed flat and there is an indication of a shallow depression; this tempts us to conclude that the asperities of the counterface are hemispherical and remove brass from these high walls mainly by an adhesive mode (Fig. 10). Figures 10 and 11 show that an abrasive-loaded valley develops a crater as if a portion of the surface has collapsed. The appearance of the crater is analogous to a pot-hole in a road which starts as a small cavity and then enlarges with time. It is probable that a particle of Sic then cuts a groove in the metal unlike the simple adhesive situation shown in Fig. 12. The groove extends owing to sliding but the particle may become stuck long before the edge of the pin. Other particles will follow a similar process until a complete valley is ploughed from one end of the pm to the other. Particles become stuck against a wall of metal or against the leading particles which moved before them and were forced to come to a halt. In this way the valleys become embedded with grits. The reason for alternate hills and valleys is probably that in the very early stage of sliding, the abrasive particles lodge preferentially in the valleys of the machined bush, i.e. the counter-face. They then cut grooves in the brass; this produces the observed valleys. On being further subjected to the ploughing action of loose abrasives, one or more embedded particles are detached from the valleys. Once a point of weakness is established, neighbouring grits are plucked readily from the valleys. The effect of this is the removal of an amount of brass; the other regions which wear are the hills and these lose material by sliding directly against the hard steel counterface. The action of the grits is more aggressive than simple adhesive wear, as a comparison of Fig. 1 with Fig. 2 shows. With respect to erosive wear, the first effect of impingement attack by a ball is the formation of a crater. As the ball hits the target a certain amount of slipping and spinning occurs; this can be seen by the small-step-like appearance of a region in the bottom right-hand portion of the crater in Fig. 22. The ball picks up an amount of target material by mere adhesion but this is not noticeable after only a single strike. The crater itself undergoes significant changes. As Fig. 23 shows, the edge of the crater is wavy because impacting forces have caused the target to be extended and deformed in the
32‘2
direction of the attack. The deformation is irregular; this results in the formation of what can be aptly described as micropetals with distinct boundaries between them. These petals in the present investigation have so far been observed at the exit end of a crater and each petal shows further deformation bands at higher ma~ifi~ations. We can speculate that the micropetals will lift off once a critical number of impacts has been exceeded. The regions immediately outside a crater also show severe deformation. This is to be expected because it has been shown that at a velocity greater than about 8 m s-l plastic yielding of steel targets takes place [ 61. Aluminium is also expected to yield readily at that velocity and radial compression of the target medium is obvious in Figs. 14 and 17. Repeated impact of the same area shows scabbed regions which can be recognized even at a low magnification (Fig. 16) and the cause of these regions is possibly the intense work hardening. The scab within a crater is usually located in the vicinity of the interface created by the deformed parent metal and the crater (Fig. 17). An effect of repeated impact is to form folds which probably become brittle with time owing to work hardening. This is shown in Fig. 20 where radial cracks form in these folds and splinters of wear debris escape, These cracks are similar to the ring cracks observed [ 9] in glass at normal angles of incidence which occur as a result of hertzian stress induced in a brittle material which remains elastic as it fractures. In practical situations a target is blasted indiscriminately by loose solid particles. Two overlapping craters show that the deformation band in their vicinity extends and assumes a crumpled appearance as seen, for example, in Fig. 30. We expect these areas to become brittle and to disappear by microspalling or by nucleating radial cracks which extend and join up to produce wear debris. The role of ductility in erosive wear has been reported in the literature [lo]. A ductile metal shows the characteristic increase and decrease in erosive wear with increasing impact angle. With a brittle material, however, erosive wear increases continuously with a maximum at normal impact. Both materials used in the present experiments, i.e. aluminium and steel, were ductile. This may be why both metals show generally similar topographical features under a scanning electron microscope. The evidence supports the view that repeated impact will make a surface brittle in which case the rate of erosive wear should accelerate, particularly when the particles impinge normally. It will be very informative to evaluate the quantitative wear of metals caused by repeated impact with single particles.
5. Conclusions On the basis of the present investigation, the following conclusions have been drawn. (1) In a three-body abrasive situation, the grits create grooves such that the topography consists of hills and valleys orientated in the general direction
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of sliding. The hills are free of abrasive and they wear by sliding directly against the hard steel counter-face. The embedded abrasive particles are uprooted from the valleys and they are expected to remove an amount of metal during their departure. (2) The impact of a metal by a single particle creates a large amount of radial plastic flow in the target surrounding the crater, These work-hardened regions crumple and small areas spa11 off or cracks nucleate and join up to produce wear debris. The crater formed by impact also exhibits severe plastic flow. The edges show features which can be termed micropetals; these micropetals contain severe deformation bands. It is suggested that with repeated impact the petals spall off the matrix once a critical number of strikes has been exceeded.
References 1 2 3 4 5 6 7 8 9 10
E. Rabinowicz, Friction and Wear of materials, Wiley, New York, 1966. E. Rabinowicz, L. A. Dunn and P. G. Russel, Wear, 4 (1961) 346. A. J. Sedriks and T. 0. Mulhearn, Wear, 6 (1963) 457. R. C. D. Richardson, Wear, 1 I (1968) 245. M. M. Khruschov, in nroc. Cont. on Lubrication and Wear, 1957, Institution of Mechanical Engineers, London, Paper 46. A. D. Sarkar, Friction and Wear, Academic Press, London, 1980. I. M. Hutchings, R. E. Winter and J. E. Field, Proc. R. Sot. London, Ser. A, 348 (1976) 379. G. P. Tilly and W. Sage, Wear, 16 (1970) 447. I. Finnie, Wear, 3 (1960) 87. G. P. Tilly, Wear, 14 (1969) 63.