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The wear mechanism obtained in copper by repetitive impacts
Wear, 74 (1981-
123
1982) 123 - 129
THE WEAR MECHANISM OBTAINED IN COPPER BY REPETITIVE IMPACTS*
E. B. ITURBE, I. G. GREENFIELD
and T. W. CHOU
Materials and Metallurgy Faculty, College of Engineering, DE 19711 (U.S.A.) (Received April 6,198l;
University of Delaware, Newark,
in revised form April 27,198l)
summary A hardened steel spherical indenter used to impact repetitively the flat surface of single crystals of copper at right angles produced a permanent indentation in the shape of a crater. A study of the crater revealed the following. (1) Material flow consists mainly of the movement of a surface layer 100 grn thick in the direction parallel to the surface towards the center of the crater and a revolving motion of the material under the base of the crater extending to a depth of 400 pm. (2) Cracking and flaking of a surface layer approximately 20 pm thick occurred at the bottom of the crater, with material removal in the form of thin plates. As a result of material flow the damaged surface layer extended to approximately 300 pm below the surface. Material removal appears to be a fatigue process resulting from sliding friction between the indenter and the contact surface.
1. Introduction The damage caused by multiple impacts of a steel ball on the same point and normal to the surface of a copper specimen was investigated, The energy of each impact is equal to the kinetic energy of the impacting ball at the time of contact. For a single impact against an annealed surface, most of the impact energy is expended in plastic and elastic deformation. Other effects, such as the generation of stress waves, the friction between the contact surfaces and the deformation of+the impacting particle, consume a smaller fraction of the total energy. (Calculations based on the work of Hunter [l] and Kirchner and Gruber [2] indicate that in the present experiments the energy dissipated in stress waves is less than 1%) Some of the energy losses from *Paper presented at the In~rnation~ Conference on Wear of Materials 1981, San Francisco, CA, U.S.A., March 30 - April 1,198l. 0043-1648/81/0000-00001$02.50
@ Elsevier Sequoia/Printed
in The Netherlands
124
these processes are in the form of heat dissipation. For repetitive impacts, rapid work hardening of the impacted surface occurs. The size of the indentation increases linearly with the logarithm of the number of impacts and a small fraction of the total energy of each impact is expended in the enlargement of the crater after approximately 20 impacts [ 31. Measurements of the coefficient of restitution indicate that, after the stage of rapid hardening, only approximately one-half of the impact energy is expended in elastic deformation. The fact that a fraction of the total energy cannot be accounted for by elastic or plastic deformation has been reported for work-hardening materials [ 2 - 41. Hunter [I] attributes the energy loss to anelastic effects in the metal. Estimates ]3] indicate that anelastic energy involved in the motion of the high density of dislocations produced during work hardening can account for a large fraction of the energy 10s~. It has been observed that, after a number of impacts of the order of 105, surface wear in the form of flaking and chipping 14, 53 leads to material removal. It is inferred that the mechanism of surface wear must also be responsible for the energy loss. In the present experiments we attempt to correlate surface and subsurface changes produced by repetitive impacts with material properties and wear mechanisms. 2. Experiments Damage by multiple impacts of a single indenter on the surface of single crystals of copper was studied. Figure 1 shows the impact tester. The indenter is a hardened-steel ball supported by a steel rod. The position of the indenter is monitored by a linear variabledifferential transducer. The magnetic field produced by a solenoid raises the indenter to a fixed position. When the magnetic field is removed, the indenter falls and accelerates under the action of gravity to a predetermined velocity at the time of impact. After the impact the electromagnet is activated and raises the indenter to the initial height for the next impact. Low friction bearings guide the indenter and assure an accuracy of 50 pm in the impact position. This accuracy is 2.5% of the diameter of an indentation produced by lo5 impacts. The indenter mass is 100 g, the diameter of the steel ball is 3 mm and the impact velocity is 25 cm s-l. The experiments were performed in air. The impact and rebound velocities were measured from the trajectory curve as the derivatives with respect to time at the start and at the end of contact respectively. Material changes in the vicinity of the impact such as the deformation of the surface, the slip bands surrounding the indentation and the subsurface damage were recorded by optical and scanning electron microscopy. 3. Results The shape of the indention produced by multiple impacts and the configu~tion of the slip lines surrounding it are dependent on crystal orien-
125
Fig. 1. The impact tester. Fig, 2. A photomicrograph
tation
of the base of an indentation produced by lo5 impacts.
and have been described previously [3]. The present results are not dependent on crystal orientation. The experiments were performed on a crystal surface of o~en~tion shown in Fig. 2. For the first 100 impacts the surface of the crater is smooth. Gray and reddish colored regions develop slowly on further impacts. After about lo* contacts the entire surface of the crater is colored. When the number of impacts reaches 105, the bottom of the indentation develops a flaky appearance (Fig. 2) with many plate-like particles up to 30 E.rmin length outside the indentation. The flakes composing the cracked layer have an average dimension of 20 pm. Portions of material of the same size are missing from the crater surface. The damaged surface extends over a circular region approximately 0.5 mm in diameter in the center of an indentation 2 mm in diameter. Electron Auger spectroscopy indicates that the flaky layer and the scattered particles are composed mainly of metallic copper covered by a film of copper oxide and iron particles. Figures 3 and 4 show different regions of a cross section of one indentation cut perpendicular to a surface impacted 10’ times. Halfway inside the crater, Fig. 3 shows subsurface cracks parallel to the surface and surfaceconnected cracks, in a layer a few microns thick. In Fig. 4 the wavy lines close to the base of the crater indicate substantial material flow. Cracks and folding of material could have taken place in these regions. In order to trace material flow the target surface was plated with a layer of nickel approxi-
126
Fig. 3. A photomicrograph of a portion of the cross section of an indentation cut perpendicular to the impacted surface. The surface shown is approximately halfway inside the crater. Subsurface cracks parallel to the surface and surface-connected cracks can be seen a few microns below the surface. The number of impacts is 105. Fig. 4. The same cross section as in Fig. 3, showing the surface of the crater closer to the bottom of the indentation. The dark wavy lines indicate that there is substantial flow of material.
mately 5000 A thick. Figures 5 and 6 show a cross section perpendicular to a nickel-plated surface after impacting 2 X lo5 times. Figure 5 shows the base of the crater. Figure 6 shows the Ni Kar X-ray emission obtained with an energydispersive X-ray analysis of the same area as Fig. 5. The area of high nickel concentration follows the same pattern as the wavy lines of Fig. 5. Figures 7 and 8 show the wavy pattern and the nickel mapping respectively of a cross section of an indentation produced by 1.3 X lo6 impacts. Three sets of patterns of wavy lines are seen in Fig. 7. Figure 8 shows that nickel is found in the left portion of the lowest-lying pattern. This result indicates that the material initially at the surface has moved down approximately 300 pm. Figure 9 shows a portion of another crater formed by 1.3 X lo6 impacts where flow lines were revealed by chemical etching. A region of flow in the shape of a spherical cap occurs under the crater. The position and shape of the wavy patterns of Figs. 7 and 9 indicate that the lines of flow are those represented by the arrows in Fig. 9. Wellinger and Breckel [4] observed the flow of steel particles into the surface of copper due to multiple impacts by a 6 kgf steel indenter. They observed the flow of material under the base of the crater. Steel particles can act as a wedge, modifying to a certain extent the characteristics of the experiment. The low impact energy used in the present work allowed us to observe the actual material flow by following the patterns of wavy lines. Figure 10 shows the right half of the cross section of an indentation produced by 10’ impacts. The flow pattern shown in Fig. 9 can also be seen in Fig. 10, region A, where the density of flow lines is much higher because of the greater number of impacts. In Fig. 10, region B, a layer of material approximately 100 pm thick is flowing over the surface towards the center of the crater.
Fig. 5. The cross section of an indentation cut perpendicular to the impacted surface, showing the bottom of the crater. The number of impacts is 2 X 105. The specimen was plated with 5000 a of nickel before the impacts. Fig. 6. Nickel mapping of the same portion of the crater shown in Fig. 5. The regions of high concentration of nickel under the surface coincide with the wavy pattern of Fig, 5.
Fig. 7. The cross section of an indentation formed by 1.3 X lo6 impacts in a nickel-plated surface. The bottom of the crater is shown at the top of the photograph. Three separate patterns of wavy lines can be seen under the surface. Fig. 8. Nickel mapping of the same cross section shown in Fig. 7. A high concentration of nickel is found in the left portion of the lowest-lying pattern.
Examination of the indenter ball shows some damage due to multiple impacts. The smooth steel surface develops a roughness of approximately 1 pm after 8 X lo6 impacts.
4. Comments Repetitive normal impacts of a steel ball on a copper surface produce a permanent inden~tion in the shape of a crater. A study of the crater shows the following.
Fig. 9. The cross section of an indentation formed by 1.3 x 10’ impacts. A region of flow in the shape of a spherical cap below the center of the crater was revealed by chemical etching. The patterns of wavy lines can be seen inside the flow region. The arrows indicate the direction of material flow. Fig. 10. The right half of the cross Region A under the bottom of the by the high number of impacts. In thick is flowing towards the center
section of an indentation formed by 10’ impacts. crater shows a high density of flow lines produced region B a surface layer of approximately 100 pm of the crater.
(1) Substantial material flow occurs and this consists mainly of (a) movement of the surface layer approximately 100 pm thick in the direction parallel to the surface towards the base of the cater and (b) a revolving motion of the material underneath the base of the crater extending to a depth of 400 pm. The revolving motion takes place in a region of radius approximately equal to a/2 and of depth approximately equal to a/4, with a being the contact radius of the indentation. (2) Cracking and flaking of a layer approximately 20 pm thick takes place at the bottom of the crater with material removal in the form of thin plates. The appearance of the cracked layer and the crack growth pattern (Fig. 3) present similarities, at the onset of the process, to the results obtained in metallic sliding wear [6] and fretting [ 71 experiments. The extent of material flow parallel ,to the surface indicates that there is substantial sliding friction between the indenter and the contact surface. These observations indicate that, during the initiation of the process, a plausible cause of cracking and flaking is the generation of shear cracks after cold working of the sliding surfaces. At the center of the crater the superposition of the two directions of flow, parallel and perpendicular to the surface, produces the wavy pattern of dark lines. The morphology of the dark lines in Figs. 4 and 5 indicates that they may be produced by a folding-over of material. An oxide surface layer, or other surface impurities, can prevent the complete welding of the folded material, causing deformed surfaces to move into the bulk. Continuous pounding on such inhomogeneous material can produce delamination of the top layers. The formation of other subsurface patterns of wavy lines after the nickel layer has moved down the surface (Figs. 7 and 8) indicates that the nickel does not affect the friction at the contact surface.
129
Friction should be treated as occurring between copper oxide and steel surfaces. The three clearly separate wavy patterns seen in Fig. 7 are puzzling. On the assumptions of continuous flaking and flow, the volume under the crater should show a continuous pattern of dark lines. The results indicate that it takes approximately 3 X lo5 impacts to develop each of the wavy line patterns that moves down at the rate of a few Angstroms per impact. The possibility that a recrystallization process is related to the discontinuity of the patterns found under the surface will be investigated in future experiments.
5. Conclusions (1) Substantial flow of material takes place on and below the surface; this flow must be responsible for a fraction of the energy loss. (2) Material removal occurs as a consequence of surface layer cracking and flaking by an apparent fatigue process, resulting from sliding friction between the indenter and the contact surface, and the formation of a highly inhomogeneous region as a consequence of the folding-over and flow of material. (3) The discontinuous nature of the crack pattern under the surface points toward a discontinuous rate of material flow or to a recrystallization process in preferential regions.
Acknowledgment This work was supported DMR-800 7282.
by a National
Science
Foundation
Grant
References 1 S. C. Hunter, Energy absorbed by elastic waves during impact, J. Mech. Phys. Solids, 5 (1957) 162 - 171. 2 H. P. Kirchner and R. M. Gruber, Effect of localized damage on energy losses during impact, Mater. Sci. Eng., 33 (1978) 101 - 106. 3 E. Iturbe, I. G. Greenfield and T. W. Chou, Surface layer hardening of polycrystalline copper by multiple impacts, J, Mater. Sci., 15 (1980) 2331 - 2334. 4 K. Wellinger and H. Breckel, KenngrSssen und Verschleiss Beim Stoss Metallischer Wekstoffe, Wear, 13 (1969) 257 - 281, 5 E. Iturbe, I. G. Greenfield and T. W. Chou, A study of surface layer damage due to impingement fatigue, in Proc. Znt. Conf. on Erosion by Solid and Liquid Impact, Cambridge, 1979, Cavendish Laboratory, Cambridge, Cambs., 1979, Paper 30. 6 N. P. Suh, The delamination theory of wear, Wear, 25 (1972) 111 - 124. 7 D. J. Duquette, The role of cyclic wear in fatigue crack nucleation in steels, in P. Haasen, V. Gerold and G. Kostorz (eds.) Proc. 5th Znt. Conf. on the Strength of Metals and Alloys, Aachen, August 1979, Pergamon, Oxford, 1980, pp. 213 - 218.
123
1982) 123 - 129
THE WEAR MECHANISM OBTAINED IN COPPER BY REPETITIVE IMPACTS*
E. B. ITURBE, I. G. GREENFIELD
and T. W. CHOU
Materials and Metallurgy Faculty, College of Engineering, DE 19711 (U.S.A.) (Received April 6,198l;
University of Delaware, Newark,
in revised form April 27,198l)
summary A hardened steel spherical indenter used to impact repetitively the flat surface of single crystals of copper at right angles produced a permanent indentation in the shape of a crater. A study of the crater revealed the following. (1) Material flow consists mainly of the movement of a surface layer 100 grn thick in the direction parallel to the surface towards the center of the crater and a revolving motion of the material under the base of the crater extending to a depth of 400 pm. (2) Cracking and flaking of a surface layer approximately 20 pm thick occurred at the bottom of the crater, with material removal in the form of thin plates. As a result of material flow the damaged surface layer extended to approximately 300 pm below the surface. Material removal appears to be a fatigue process resulting from sliding friction between the indenter and the contact surface.
1. Introduction The damage caused by multiple impacts of a steel ball on the same point and normal to the surface of a copper specimen was investigated, The energy of each impact is equal to the kinetic energy of the impacting ball at the time of contact. For a single impact against an annealed surface, most of the impact energy is expended in plastic and elastic deformation. Other effects, such as the generation of stress waves, the friction between the contact surfaces and the deformation of+the impacting particle, consume a smaller fraction of the total energy. (Calculations based on the work of Hunter [l] and Kirchner and Gruber [2] indicate that in the present experiments the energy dissipated in stress waves is less than 1%) Some of the energy losses from *Paper presented at the In~rnation~ Conference on Wear of Materials 1981, San Francisco, CA, U.S.A., March 30 - April 1,198l. 0043-1648/81/0000-00001$02.50
@ Elsevier Sequoia/Printed
in The Netherlands
124
these processes are in the form of heat dissipation. For repetitive impacts, rapid work hardening of the impacted surface occurs. The size of the indentation increases linearly with the logarithm of the number of impacts and a small fraction of the total energy of each impact is expended in the enlargement of the crater after approximately 20 impacts [ 31. Measurements of the coefficient of restitution indicate that, after the stage of rapid hardening, only approximately one-half of the impact energy is expended in elastic deformation. The fact that a fraction of the total energy cannot be accounted for by elastic or plastic deformation has been reported for work-hardening materials [ 2 - 41. Hunter [I] attributes the energy loss to anelastic effects in the metal. Estimates ]3] indicate that anelastic energy involved in the motion of the high density of dislocations produced during work hardening can account for a large fraction of the energy 10s~. It has been observed that, after a number of impacts of the order of 105, surface wear in the form of flaking and chipping 14, 53 leads to material removal. It is inferred that the mechanism of surface wear must also be responsible for the energy loss. In the present experiments we attempt to correlate surface and subsurface changes produced by repetitive impacts with material properties and wear mechanisms. 2. Experiments Damage by multiple impacts of a single indenter on the surface of single crystals of copper was studied. Figure 1 shows the impact tester. The indenter is a hardened-steel ball supported by a steel rod. The position of the indenter is monitored by a linear variabledifferential transducer. The magnetic field produced by a solenoid raises the indenter to a fixed position. When the magnetic field is removed, the indenter falls and accelerates under the action of gravity to a predetermined velocity at the time of impact. After the impact the electromagnet is activated and raises the indenter to the initial height for the next impact. Low friction bearings guide the indenter and assure an accuracy of 50 pm in the impact position. This accuracy is 2.5% of the diameter of an indentation produced by lo5 impacts. The indenter mass is 100 g, the diameter of the steel ball is 3 mm and the impact velocity is 25 cm s-l. The experiments were performed in air. The impact and rebound velocities were measured from the trajectory curve as the derivatives with respect to time at the start and at the end of contact respectively. Material changes in the vicinity of the impact such as the deformation of the surface, the slip bands surrounding the indentation and the subsurface damage were recorded by optical and scanning electron microscopy. 3. Results The shape of the indention produced by multiple impacts and the configu~tion of the slip lines surrounding it are dependent on crystal orien-
125
Fig. 1. The impact tester. Fig, 2. A photomicrograph
tation
of the base of an indentation produced by lo5 impacts.
and have been described previously [3]. The present results are not dependent on crystal orientation. The experiments were performed on a crystal surface of o~en~tion shown in Fig. 2. For the first 100 impacts the surface of the crater is smooth. Gray and reddish colored regions develop slowly on further impacts. After about lo* contacts the entire surface of the crater is colored. When the number of impacts reaches 105, the bottom of the indentation develops a flaky appearance (Fig. 2) with many plate-like particles up to 30 E.rmin length outside the indentation. The flakes composing the cracked layer have an average dimension of 20 pm. Portions of material of the same size are missing from the crater surface. The damaged surface extends over a circular region approximately 0.5 mm in diameter in the center of an indentation 2 mm in diameter. Electron Auger spectroscopy indicates that the flaky layer and the scattered particles are composed mainly of metallic copper covered by a film of copper oxide and iron particles. Figures 3 and 4 show different regions of a cross section of one indentation cut perpendicular to a surface impacted 10’ times. Halfway inside the crater, Fig. 3 shows subsurface cracks parallel to the surface and surfaceconnected cracks, in a layer a few microns thick. In Fig. 4 the wavy lines close to the base of the crater indicate substantial material flow. Cracks and folding of material could have taken place in these regions. In order to trace material flow the target surface was plated with a layer of nickel approxi-
126
Fig. 3. A photomicrograph of a portion of the cross section of an indentation cut perpendicular to the impacted surface. The surface shown is approximately halfway inside the crater. Subsurface cracks parallel to the surface and surface-connected cracks can be seen a few microns below the surface. The number of impacts is 105. Fig. 4. The same cross section as in Fig. 3, showing the surface of the crater closer to the bottom of the indentation. The dark wavy lines indicate that there is substantial flow of material.
mately 5000 A thick. Figures 5 and 6 show a cross section perpendicular to a nickel-plated surface after impacting 2 X lo5 times. Figure 5 shows the base of the crater. Figure 6 shows the Ni Kar X-ray emission obtained with an energydispersive X-ray analysis of the same area as Fig. 5. The area of high nickel concentration follows the same pattern as the wavy lines of Fig. 5. Figures 7 and 8 show the wavy pattern and the nickel mapping respectively of a cross section of an indentation produced by 1.3 X lo6 impacts. Three sets of patterns of wavy lines are seen in Fig. 7. Figure 8 shows that nickel is found in the left portion of the lowest-lying pattern. This result indicates that the material initially at the surface has moved down approximately 300 pm. Figure 9 shows a portion of another crater formed by 1.3 X lo6 impacts where flow lines were revealed by chemical etching. A region of flow in the shape of a spherical cap occurs under the crater. The position and shape of the wavy patterns of Figs. 7 and 9 indicate that the lines of flow are those represented by the arrows in Fig. 9. Wellinger and Breckel [4] observed the flow of steel particles into the surface of copper due to multiple impacts by a 6 kgf steel indenter. They observed the flow of material under the base of the crater. Steel particles can act as a wedge, modifying to a certain extent the characteristics of the experiment. The low impact energy used in the present work allowed us to observe the actual material flow by following the patterns of wavy lines. Figure 10 shows the right half of the cross section of an indentation produced by 10’ impacts. The flow pattern shown in Fig. 9 can also be seen in Fig. 10, region A, where the density of flow lines is much higher because of the greater number of impacts. In Fig. 10, region B, a layer of material approximately 100 pm thick is flowing over the surface towards the center of the crater.
Fig. 5. The cross section of an indentation cut perpendicular to the impacted surface, showing the bottom of the crater. The number of impacts is 2 X 105. The specimen was plated with 5000 a of nickel before the impacts. Fig. 6. Nickel mapping of the same portion of the crater shown in Fig. 5. The regions of high concentration of nickel under the surface coincide with the wavy pattern of Fig, 5.
Fig. 7. The cross section of an indentation formed by 1.3 X lo6 impacts in a nickel-plated surface. The bottom of the crater is shown at the top of the photograph. Three separate patterns of wavy lines can be seen under the surface. Fig. 8. Nickel mapping of the same cross section shown in Fig. 7. A high concentration of nickel is found in the left portion of the lowest-lying pattern.
Examination of the indenter ball shows some damage due to multiple impacts. The smooth steel surface develops a roughness of approximately 1 pm after 8 X lo6 impacts.
4. Comments Repetitive normal impacts of a steel ball on a copper surface produce a permanent inden~tion in the shape of a crater. A study of the crater shows the following.
Fig. 9. The cross section of an indentation formed by 1.3 x 10’ impacts. A region of flow in the shape of a spherical cap below the center of the crater was revealed by chemical etching. The patterns of wavy lines can be seen inside the flow region. The arrows indicate the direction of material flow. Fig. 10. The right half of the cross Region A under the bottom of the by the high number of impacts. In thick is flowing towards the center
section of an indentation formed by 10’ impacts. crater shows a high density of flow lines produced region B a surface layer of approximately 100 pm of the crater.
(1) Substantial material flow occurs and this consists mainly of (a) movement of the surface layer approximately 100 pm thick in the direction parallel to the surface towards the base of the cater and (b) a revolving motion of the material underneath the base of the crater extending to a depth of 400 pm. The revolving motion takes place in a region of radius approximately equal to a/2 and of depth approximately equal to a/4, with a being the contact radius of the indentation. (2) Cracking and flaking of a layer approximately 20 pm thick takes place at the bottom of the crater with material removal in the form of thin plates. The appearance of the cracked layer and the crack growth pattern (Fig. 3) present similarities, at the onset of the process, to the results obtained in metallic sliding wear [6] and fretting [ 71 experiments. The extent of material flow parallel ,to the surface indicates that there is substantial sliding friction between the indenter and the contact surface. These observations indicate that, during the initiation of the process, a plausible cause of cracking and flaking is the generation of shear cracks after cold working of the sliding surfaces. At the center of the crater the superposition of the two directions of flow, parallel and perpendicular to the surface, produces the wavy pattern of dark lines. The morphology of the dark lines in Figs. 4 and 5 indicates that they may be produced by a folding-over of material. An oxide surface layer, or other surface impurities, can prevent the complete welding of the folded material, causing deformed surfaces to move into the bulk. Continuous pounding on such inhomogeneous material can produce delamination of the top layers. The formation of other subsurface patterns of wavy lines after the nickel layer has moved down the surface (Figs. 7 and 8) indicates that the nickel does not affect the friction at the contact surface.
129
Friction should be treated as occurring between copper oxide and steel surfaces. The three clearly separate wavy patterns seen in Fig. 7 are puzzling. On the assumptions of continuous flaking and flow, the volume under the crater should show a continuous pattern of dark lines. The results indicate that it takes approximately 3 X lo5 impacts to develop each of the wavy line patterns that moves down at the rate of a few Angstroms per impact. The possibility that a recrystallization process is related to the discontinuity of the patterns found under the surface will be investigated in future experiments.
5. Conclusions (1) Substantial flow of material takes place on and below the surface; this flow must be responsible for a fraction of the energy loss. (2) Material removal occurs as a consequence of surface layer cracking and flaking by an apparent fatigue process, resulting from sliding friction between the indenter and the contact surface, and the formation of a highly inhomogeneous region as a consequence of the folding-over and flow of material. (3) The discontinuous nature of the crack pattern under the surface points toward a discontinuous rate of material flow or to a recrystallization process in preferential regions.
Acknowledgment This work was supported DMR-800 7282.
by a National
Science
Foundation
Grant
References 1 S. C. Hunter, Energy absorbed by elastic waves during impact, J. Mech. Phys. Solids, 5 (1957) 162 - 171. 2 H. P. Kirchner and R. M. Gruber, Effect of localized damage on energy losses during impact, Mater. Sci. Eng., 33 (1978) 101 - 106. 3 E. Iturbe, I. G. Greenfield and T. W. Chou, Surface layer hardening of polycrystalline copper by multiple impacts, J, Mater. Sci., 15 (1980) 2331 - 2334. 4 K. Wellinger and H. Breckel, KenngrSssen und Verschleiss Beim Stoss Metallischer Wekstoffe, Wear, 13 (1969) 257 - 281, 5 E. Iturbe, I. G. Greenfield and T. W. Chou, A study of surface layer damage due to impingement fatigue, in Proc. Znt. Conf. on Erosion by Solid and Liquid Impact, Cambridge, 1979, Cavendish Laboratory, Cambridge, Cambs., 1979, Paper 30. 6 N. P. Suh, The delamination theory of wear, Wear, 25 (1972) 111 - 124. 7 D. J. Duquette, The role of cyclic wear in fatigue crack nucleation in steels, in P. Haasen, V. Gerold and G. Kostorz (eds.) Proc. 5th Znt. Conf. on the Strength of Metals and Alloys, Aachen, August 1979, Pergamon, Oxford, 1980, pp. 213 - 218.