- Email: [email protected]
Structure-based selection of wear-resistant materials
WEAR ELSEVIER
181-183
Wear
selection
Structure-based
(1995)
50-55
of wear-resistant
materials
1.1. Garbar Department
of Mechanical
Engineering, Ben-Gurion Received
26 April
University of the Negev, PO Box 653, Beer-Sheva, 1994; accepted
18 October
84105, Israel
1994
Abstract The selection of wear-resistant materials is usually based on long-time testing. A method of structural investigations of materials can be used for reducing the duration of such tests. This method is based on the results of the transmission electron microscopy and X-ray investigations of the surface layers. Results of these studies show that every material is characterized by a definite state of the surface layer structure, which corresponds to the conditions of friction. Under constant conditions of friction the characteristics of the structure change only during running-in period. Thereafter the structure of surface layer remains unchanged throughout the entire subsequent test period. Consequently, for determination of the structural characteristics that correspond to particular friction conditions, it is necessary to test the material only until the end of the running-in period. Specimens of different materials were investigated before and after testing in order to establish the relationship between the change of the structure of the surface layers of a metal under friction and the comparative wear resistance. These investigations showed that in the absence of phase transitions in the material under friction, the smaller the structural changes, the higher the wear resistance. This criterion can thus be used to reduce the duration of the testing period for wear resistance of materials under nominal friction conditions. It is thus possible to select materials with higher wear resistance for different parts of machines, such as trucks or looms. The results can be also applied in the development of a new wear-resistant materials for different friction conditions. Keywords;
Structure;
Surface
layers;
Fragmentation;
Wear
resistance
1. Introduction
Wear resistance is one of the major factors influencing the choice of materials for friction pairs. According to Fridman [l] and Lubarsky [2], failure, in general, and wear, in particular, are structure-sensitive processes that result from the development of small localities of failure. Production of wear particles takes place in the surface layers of a material. Thus, wear is caused by structures formed in these layers, and their failure rate under friction determines the wear resistance of the material. An important property of the structures formed during friction is their relationship to the test conditions imposed on certain materials. Studies of these structures has made it possible to establish the rules, at least the tendencies, for changes in the structures in response to alterations in the friction conditions. Therefore, a knowledge of the structures formed under friction can be used either to determine the friction conditions for the given materials on the basis of the structure of the surface layer, or the relative wear resistance of different materials under selected friction conditions.
0043-1648/95/$09.50 0 1995 Elsevier SSDI 0043-1648(94)07061-X
Science
S.A. All rights
reserved
It is thus necessary to define the type of structure formed under particular friction conditions and how this structure alters as friction conditions are changed.
2. Experimental
procedure
Copper (99.95%), low-carbon steel (O.OS%C), steel 45 (0.45%C) and steel U8 (0.78%C) were used in the study. Prior to the tests, the specimens of copper and low-carbon steel were annealed at 640 “C and 960 “C respectively. The heat treatment of steel 45 specimens comprised hardening at 840 “C and tempering at 540 “C, while steel US specimens were hardened at 780 “C and tempered at 180 “C. The tests were carried out in a reciprocating friction machine at an average speed of 0.05 m s-l under pressures of 0.8, 1.6 and 3.3 MPa, as is described previously by Garbar [3]. Lubrication was performed by the drip-feed method. Two types of specimen were used: copper foils for transmission electron microscopy (TEM) studies, and cylindrical copper and steels specimens, 20 mm in diameter and 7.5 mm in height, for X-ray studies.
I.I. Garbar I Wear 181-183
For TEM studies of the changes in surface layer dislocation structures, a special test procedure was developed [4], as shown schematically in Fig. 1. A thin copper plate, 4, of thickness 50-150 mm was mounted on the stage of the friction machine. Openings were made in the centre of the plate, the diameter of the openings being equal to the diameter of the specimen holder of the electron microscope. Metal foils were produced by one-sided electropolishing such that one side of the foil was preserved intact, i.e. it remained planar, and the other side had an indentation in its centre. After TEM examination, each foil specimen was mounted in the housings formed by the openings in the plate and the surface of the friction machine stage. The specimen was mounted with the planar side facing upwards; as is shown in Fig. 1. The specimen foils were made with the same thickness as the plate, 4. Since electropolishing reduced the thickness of the specimens, fluoroplastic washers, 3, of suitable thickness were placed under the foil specimen to facilitate contact between the specimen, 1, and the counter body, 2, under friction. Washer elasticity compensated for the difference in the thicknesses of the central part and the periphery of the foil specimen. After a fixed number of running-in cycles, the foils were degreased, washed, and again examined by TEM. This procedure enabled us to follow successive changes in the dislocation structures of the metal during friction after various running-in times. Since all intermediate foil-preparing operations were eliminated, by our method the reliability of the results was enhanced. To prevent structure transformation as a result of heating during TEM investigations, the studies were performed at minimum beam brightness and maximum vacuum in
Fig. 1. Schematic representation of friction tests of foil specimens: 1, specimens; 2, counterbody; 3, washer; 4, plate (P, load, V, velocity).
(1995) 50-55
51
the microscope column, using the object-cooling system. This made it possible to examine the same specimen repeatedly for the study of the sequence of the structural changes of the metal surface layer under friction. It should be noted that electron microscopy studies give detailed information on the structures of small areas. Therefore, the electron microscopy data were treated statistically to provide an average pattern of the state of the material structure. To obtain information on a surface layer structure from the large areas of specimen, X-ray studies were conducted. Cylindrical specimens were studied before and after particular test periods in the friction machine; one of the bases of the cylinder constituted the working surface. The X-ray studies were performed on a diffractometer with monochromatization of the secondary (diffracted) beam with the aid of a pyrolytic graphite monochromator. CuK, radiation was used for the copper specimens, and Co& for the steel specimens. The intensity and structural broadening of Cu (111) and Cu (420) for copper specimens, and of a-Fe (110) and cu-Fe (220) diffraction lines for steel specimens were determined. The findings illustrated microcrystallinity of the specimen structure and the degree of crystal lattice structure imperfections. The sequence of the structural changes during the running-in period determined by X-ray diffraction was correlated with the findings of the TEM investigations. Wear was determined by weighing the cylindrical specimens on a chemical balance after the particular periods of exposure to friction.
3. Results
and discussion
The results of the TEM studies are shown in Fig. 2. The initial structure of copper (Fig. 2(a)) is characterized by single dislocations that are distributed statistically uniformly throughout the volume of the metal. The typical area of the structure formed after 10 cycles (sliding distance 0.5 m) is shown in Fig. 2(b). This structure is characterized by increased dislocation density and dense networks of dislocations. After testing for 100 cycles (sliding distance 5 m), we observed not only areas with statistically uniform dislocation distribution, but also cells oriented along the direction of friction, with single dislocations visible in their walls. One such area is shown in Fig. 2(c). We also observed a large number of uniformly distributed dislocations in the interior of the cells. Obviously, in the process of deformation the formation of elongated cells follows the increase in the production of dislocations. This stage is already evident in the structure shown in Fig. 2(b).
52
1.1. Garbar I Wear 181-183
(1995) 50-55
Cd)
(e) Fig. 2. Dislocation structure of copper: (a) before testing; (b) after testing for 10 cycles (friction distance 0.5 m); (c) after testing for 100 cycles (friction distance 5 m); (d) after testing for 1000 cycles (friction distance 50 m); (e) after testing for 10000 cycles (friction distance 500 m); (f) after testing for 100000 cycles (friction distance 5000 m). The arrows show the direction of sliding.
The typical area of the structure formed after 1000 test cycles (friction distance 50 m) is shown in Fig. 2(d). In this figure the late stages of the formation of a cellular and fragmented structure ’ are visible. A totally fragmented structure oriented along the direction of friction was formed after 10 000 test cycles (Fig. 2(e)). Disorientation between neighbouring fragments in individual parts of the boundaries reached tens of degrees. ’Van Dijck [5] has shown a structure analogous to that shown in Figs. l(e) and l(f), which he termed “fine-grained”. It seems to us that a more appropriate name for such a structure, which is particularly associated with active plastic deformation, would be “fragmented”, as suggested by Rybin [6].
Further running produced no significant changes in the dimensions or disorientation of the fragments. As can be seen from Fig. ,2(f), th.e structure after 100 000 test cycles (sliding distance 5000 m) was similar to that after 10 000 cycles. It is thus evident that the period of structural running-in had come to an end under the given friction conditions by 10 000 cycles. The results of X-ray studies of copper samples tested on the friction machine under different pressures are given in Fig. 3. Specimens of low-carbon steel served as counterfaces. It is clear from Fig. 3 that both the structural broadening and the intensity of the diffraction lines increase during the first periods of testing. After 10 000 cycles, these characteristics stabilize and practically do not change during the subsequent test period.
I.I. Garbar / Wear 181-183
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[email protected] MI% *-. -B-ll.GMPn : ,.......... ...*... .. __,y_ 3.JMPa .._ _
0
20
40 60 N,lOOOrev
60
100
s
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40 60 N,1000ICU
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Fig. 3. Changes in (a) wear, (b) structural broadening of the Cu (420) diffraction line and (c) intensity of the Cu(ll1) diffraction line of copper specimens under different pressures as a function of number of operation cycles.
Fig. 4. Changes in (a) surface roughness, (b) structural broadening of the cx-Fe (220) diffraction line and (c) intensity of the a-Fe (110) diffraction line of different steel specimens at a pressure of 0.8 MPa as a function of number of operation cycles.
The results of similar studies for low carbon steel and steel 45 are shown in Fig. 4. The two materials were tested under equal friction conditions, under a pressure of 0.8 MPa, with lubrication by drip feed. Comparison of the results of the structural investigation with wear data (Fig. 3) and data on roughness changes (Fig. 4) shows that the time of transition to stable wear and equilibrium roughness of the surface coincides with the time of structure stabilization. It can be seen from Figs. 3 and 4 that wear intensity and
surface roughness, as well as structural characteristics, almost did not change after 10 000 cycles during the subsequent test period. The structural state corresponds to given friction conditions. The results shown in Fig. 3 illustrate that under particular friction conditions, the structural state of the surface layer will correspond to a certain load under friction. The results presented in Fig. 4 show that under certain friction conditions, every material has its own definite structural state of surface layers.
I.I. Garbar I Wear 181-183
54
(1995) SO-55
Table 1 Intensity changes of diffraction lines after exposure to friction and relative wear resistance of the specimens and machine parts Specimens or machine part
Low-carbon steel Steel U8 Steel elements of a globe hinge of a steering mechanism of a truck Nickel alloy elements of a globe hinge of a steering mechanism of a truck Chromium coating of loom elements Nickel alloy coating of loom elements
This state is characterized by a definite degree of crystal lattice imperfection and surface layer fragmentation level. Similar results were obtained for different counterface materials (copper specimens were tested vs. molybdenum, copper and nickel under 0.8 MPa). Thus, these data show a that definite structural state of the surface layers corresponds to the nature of the material and the friction conditions. Our results also show that the running-in period is characterized by a change of the structural state of the surface layers. The transition to stable wear and equilibrium surface roughness is accompanied by stabilization of the structural state of the material. As is shown in Figs. 3 and 4, the time of structural change from the initial state to the operating state coincides with the running-in period. Consequently, structural parameters such as structural broadening of diffraction lines or their intensity can be used to determine the running-in period. These correlations can be also used to speed up the estimation of comparative wear resistance under nominal friction conditions. It is clear from Figs. 2-4 that the main structural changes occur solely during the running-in period. Since the structural characteristics that appear to be the criteria for wear resistance can be obtained at the beginning of the stabilization period, it is sufficient to carry out tests only until the end of the running-in period in order to obtain structural estimation of the comparative wear resistance of materials. Previous structural investigations [3] have shown, that under fatigue 2 wear structural changes are proportional to wear intensity. Consequently, as can be seen from the results presented in Fig. 3, under the prevailing fatigue wear, soft friction conditions (in our case the minimal pressure) correspond to the smallest structural changes. Therefore, when estimating the fatigue friction conditions, such as different lubricants, *Under
conditions of abrasive wear as well as in cases of adhesion in friction and scoring, the correlation between the structural changes and wear intensity are different from fatigue wear.
Relative *ear resistance
Diffraction lines
Relative intensity of diffraction lines
(‘W
(11lr,)
1 g-10 1 3.5-5 1 8
a-Fe (110) a-Fe (110) a-Fe (110) Ni (111) Cr(ll0) Ni(ll1)
3 1 2.7 1.2 2.4 1.1
loads, velocities etc., the optimal conditions are those that give a surface layer structure that the closest to the initial structure. An important engineering problem is the selection of wear-resistant materials for the details of friction units. To solve this problem, it is necessary to establish the relationship between structural changes of different materials under equal friction conditions and their wear resistance. The results given in Fig. 4 show that it is indeed possible to define such a relationship; for example, the wear resistance of steel 45 under particular test conditions was higher than the wear resistance of low-carbon steel, whereas the structural changes of low carbon steel were more significant. Thus, smaller structural changes correspond to materials with greater wear resistance. Tests of specimens and machine parts made of different materials were also conducted. The specimens made of low-carbon steel (WIN 90) and steel U8 (VI-IN 720) were tested on a friction machine, and some parts of trucks and looms were tested under the working conditions. The truck parts were manufactured from steel (HR, 56-61) or nickel alloy (HR, 54-60), and the surface layers of the loom parts were made of chromium or nickel alloy. X-ray investigations of the steel specimens and machine parts were conducted both before and after testing. The intensities of the diffraction lines at small reflection angles indicated the extent of fragmentation of the surface layers of the metal. The results of the X-ray study and data on the wear resistance of the specimens and machine parts are shown in Table 1. It can be seen that the materials with higher wear resistance are those for which changes in the intensities of the diffraction lines are minimal 3.
3This rule is applicable only in the absence of phase transition in material surface layers under friction. If such a phase transition does take place, changes in diffraction lines intensities can be due to changes in the quantities of the different phases.
I.I. Garbar / Wear 181483
(1995) 50-55
The corresponds established in this study can be used for selection of optimal materials and/or of the optimal methods for their treatment for given friction conditions.
(3) The structure-based
4. Conclusions
References
selection of wear resistant materials can thus be used to reduce the duration of wear-resistance tests. The tests have to be conducted only till the end of running-in period and they can be performed under nominal operating conditions.
111 Y.B.
(1) Under constant friction conditions, the structural characteristics of the surface layers of a metal change only during the running-in period. Therefore, such characteristics can be serve as a measure of the length of this period. (2) Changes in the structure of the surface layer depend on the friction conditions. Under extreme friction conditions, the structural changes are more significant. When a number of materials are compared under equal friction conditions, the material that exhibits the smallest structural changes of the surface layers has the highest wear resistance.
55
Fridman, Mechanical Properties of Metal, Vol. 1, Mashinostroenie, Moscow, 1974, p. 18. PI I.M. Lubarsky, Hardening and negative hardening under friction, in G.A. Preis (ed.), The Problems of Friction and Wear, Vol. 1, Technique, Kiev, 1971, pp. 27-34. 131 1.1. Garbar, Fragmentation of low-carbon steel and copper surface layers during fatigue and adhesive wear, Frict. Wear, 7 (6) (1986) 1043-1053. 141 1.1. Garbar, Method of electron microscopy investigation of the friction surface structure, Plunt Labomtoty, 3 (1983) 59-60. PI J.A.B. van Dijck, The direct observation of the transmission electron microscope of the heavily deformed surface layer of the copper pin after dry sliding against a steel ring, Wear, 42 (1977) 109-117. Fl V.V. Rybin, The Large Plastic Deformation and Metal Failure, Metallurgia, Moscow, 1986, p. 38.
181-183
Wear
selection
Structure-based
(1995)
50-55
of wear-resistant
materials
1.1. Garbar Department
of Mechanical
Engineering, Ben-Gurion Received
26 April
University of the Negev, PO Box 653, Beer-Sheva, 1994; accepted
18 October
84105, Israel
1994
Abstract The selection of wear-resistant materials is usually based on long-time testing. A method of structural investigations of materials can be used for reducing the duration of such tests. This method is based on the results of the transmission electron microscopy and X-ray investigations of the surface layers. Results of these studies show that every material is characterized by a definite state of the surface layer structure, which corresponds to the conditions of friction. Under constant conditions of friction the characteristics of the structure change only during running-in period. Thereafter the structure of surface layer remains unchanged throughout the entire subsequent test period. Consequently, for determination of the structural characteristics that correspond to particular friction conditions, it is necessary to test the material only until the end of the running-in period. Specimens of different materials were investigated before and after testing in order to establish the relationship between the change of the structure of the surface layers of a metal under friction and the comparative wear resistance. These investigations showed that in the absence of phase transitions in the material under friction, the smaller the structural changes, the higher the wear resistance. This criterion can thus be used to reduce the duration of the testing period for wear resistance of materials under nominal friction conditions. It is thus possible to select materials with higher wear resistance for different parts of machines, such as trucks or looms. The results can be also applied in the development of a new wear-resistant materials for different friction conditions. Keywords;
Structure;
Surface
layers;
Fragmentation;
Wear
resistance
1. Introduction
Wear resistance is one of the major factors influencing the choice of materials for friction pairs. According to Fridman [l] and Lubarsky [2], failure, in general, and wear, in particular, are structure-sensitive processes that result from the development of small localities of failure. Production of wear particles takes place in the surface layers of a material. Thus, wear is caused by structures formed in these layers, and their failure rate under friction determines the wear resistance of the material. An important property of the structures formed during friction is their relationship to the test conditions imposed on certain materials. Studies of these structures has made it possible to establish the rules, at least the tendencies, for changes in the structures in response to alterations in the friction conditions. Therefore, a knowledge of the structures formed under friction can be used either to determine the friction conditions for the given materials on the basis of the structure of the surface layer, or the relative wear resistance of different materials under selected friction conditions.
0043-1648/95/$09.50 0 1995 Elsevier SSDI 0043-1648(94)07061-X
Science
S.A. All rights
reserved
It is thus necessary to define the type of structure formed under particular friction conditions and how this structure alters as friction conditions are changed.
2. Experimental
procedure
Copper (99.95%), low-carbon steel (O.OS%C), steel 45 (0.45%C) and steel U8 (0.78%C) were used in the study. Prior to the tests, the specimens of copper and low-carbon steel were annealed at 640 “C and 960 “C respectively. The heat treatment of steel 45 specimens comprised hardening at 840 “C and tempering at 540 “C, while steel US specimens were hardened at 780 “C and tempered at 180 “C. The tests were carried out in a reciprocating friction machine at an average speed of 0.05 m s-l under pressures of 0.8, 1.6 and 3.3 MPa, as is described previously by Garbar [3]. Lubrication was performed by the drip-feed method. Two types of specimen were used: copper foils for transmission electron microscopy (TEM) studies, and cylindrical copper and steels specimens, 20 mm in diameter and 7.5 mm in height, for X-ray studies.
I.I. Garbar I Wear 181-183
For TEM studies of the changes in surface layer dislocation structures, a special test procedure was developed [4], as shown schematically in Fig. 1. A thin copper plate, 4, of thickness 50-150 mm was mounted on the stage of the friction machine. Openings were made in the centre of the plate, the diameter of the openings being equal to the diameter of the specimen holder of the electron microscope. Metal foils were produced by one-sided electropolishing such that one side of the foil was preserved intact, i.e. it remained planar, and the other side had an indentation in its centre. After TEM examination, each foil specimen was mounted in the housings formed by the openings in the plate and the surface of the friction machine stage. The specimen was mounted with the planar side facing upwards; as is shown in Fig. 1. The specimen foils were made with the same thickness as the plate, 4. Since electropolishing reduced the thickness of the specimens, fluoroplastic washers, 3, of suitable thickness were placed under the foil specimen to facilitate contact between the specimen, 1, and the counter body, 2, under friction. Washer elasticity compensated for the difference in the thicknesses of the central part and the periphery of the foil specimen. After a fixed number of running-in cycles, the foils were degreased, washed, and again examined by TEM. This procedure enabled us to follow successive changes in the dislocation structures of the metal during friction after various running-in times. Since all intermediate foil-preparing operations were eliminated, by our method the reliability of the results was enhanced. To prevent structure transformation as a result of heating during TEM investigations, the studies were performed at minimum beam brightness and maximum vacuum in
Fig. 1. Schematic representation of friction tests of foil specimens: 1, specimens; 2, counterbody; 3, washer; 4, plate (P, load, V, velocity).
(1995) 50-55
51
the microscope column, using the object-cooling system. This made it possible to examine the same specimen repeatedly for the study of the sequence of the structural changes of the metal surface layer under friction. It should be noted that electron microscopy studies give detailed information on the structures of small areas. Therefore, the electron microscopy data were treated statistically to provide an average pattern of the state of the material structure. To obtain information on a surface layer structure from the large areas of specimen, X-ray studies were conducted. Cylindrical specimens were studied before and after particular test periods in the friction machine; one of the bases of the cylinder constituted the working surface. The X-ray studies were performed on a diffractometer with monochromatization of the secondary (diffracted) beam with the aid of a pyrolytic graphite monochromator. CuK, radiation was used for the copper specimens, and Co& for the steel specimens. The intensity and structural broadening of Cu (111) and Cu (420) for copper specimens, and of a-Fe (110) and cu-Fe (220) diffraction lines for steel specimens were determined. The findings illustrated microcrystallinity of the specimen structure and the degree of crystal lattice structure imperfections. The sequence of the structural changes during the running-in period determined by X-ray diffraction was correlated with the findings of the TEM investigations. Wear was determined by weighing the cylindrical specimens on a chemical balance after the particular periods of exposure to friction.
3. Results
and discussion
The results of the TEM studies are shown in Fig. 2. The initial structure of copper (Fig. 2(a)) is characterized by single dislocations that are distributed statistically uniformly throughout the volume of the metal. The typical area of the structure formed after 10 cycles (sliding distance 0.5 m) is shown in Fig. 2(b). This structure is characterized by increased dislocation density and dense networks of dislocations. After testing for 100 cycles (sliding distance 5 m), we observed not only areas with statistically uniform dislocation distribution, but also cells oriented along the direction of friction, with single dislocations visible in their walls. One such area is shown in Fig. 2(c). We also observed a large number of uniformly distributed dislocations in the interior of the cells. Obviously, in the process of deformation the formation of elongated cells follows the increase in the production of dislocations. This stage is already evident in the structure shown in Fig. 2(b).
52
1.1. Garbar I Wear 181-183
(1995) 50-55
Cd)
(e) Fig. 2. Dislocation structure of copper: (a) before testing; (b) after testing for 10 cycles (friction distance 0.5 m); (c) after testing for 100 cycles (friction distance 5 m); (d) after testing for 1000 cycles (friction distance 50 m); (e) after testing for 10000 cycles (friction distance 500 m); (f) after testing for 100000 cycles (friction distance 5000 m). The arrows show the direction of sliding.
The typical area of the structure formed after 1000 test cycles (friction distance 50 m) is shown in Fig. 2(d). In this figure the late stages of the formation of a cellular and fragmented structure ’ are visible. A totally fragmented structure oriented along the direction of friction was formed after 10 000 test cycles (Fig. 2(e)). Disorientation between neighbouring fragments in individual parts of the boundaries reached tens of degrees. ’Van Dijck [5] has shown a structure analogous to that shown in Figs. l(e) and l(f), which he termed “fine-grained”. It seems to us that a more appropriate name for such a structure, which is particularly associated with active plastic deformation, would be “fragmented”, as suggested by Rybin [6].
Further running produced no significant changes in the dimensions or disorientation of the fragments. As can be seen from Fig. ,2(f), th.e structure after 100 000 test cycles (sliding distance 5000 m) was similar to that after 10 000 cycles. It is thus evident that the period of structural running-in had come to an end under the given friction conditions by 10 000 cycles. The results of X-ray studies of copper samples tested on the friction machine under different pressures are given in Fig. 3. Specimens of low-carbon steel served as counterfaces. It is clear from Fig. 3 that both the structural broadening and the intensity of the diffraction lines increase during the first periods of testing. After 10 000 cycles, these characteristics stabilize and practically do not change during the subsequent test period.
I.I. Garbar / Wear 181-183
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[email protected] MI% *-. -B-ll.GMPn : ,.......... ...*... .. __,y_ 3.JMPa .._ _
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40 60 N,lOOOrev
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Fig. 3. Changes in (a) wear, (b) structural broadening of the Cu (420) diffraction line and (c) intensity of the Cu(ll1) diffraction line of copper specimens under different pressures as a function of number of operation cycles.
Fig. 4. Changes in (a) surface roughness, (b) structural broadening of the cx-Fe (220) diffraction line and (c) intensity of the a-Fe (110) diffraction line of different steel specimens at a pressure of 0.8 MPa as a function of number of operation cycles.
The results of similar studies for low carbon steel and steel 45 are shown in Fig. 4. The two materials were tested under equal friction conditions, under a pressure of 0.8 MPa, with lubrication by drip feed. Comparison of the results of the structural investigation with wear data (Fig. 3) and data on roughness changes (Fig. 4) shows that the time of transition to stable wear and equilibrium roughness of the surface coincides with the time of structure stabilization. It can be seen from Figs. 3 and 4 that wear intensity and
surface roughness, as well as structural characteristics, almost did not change after 10 000 cycles during the subsequent test period. The structural state corresponds to given friction conditions. The results shown in Fig. 3 illustrate that under particular friction conditions, the structural state of the surface layer will correspond to a certain load under friction. The results presented in Fig. 4 show that under certain friction conditions, every material has its own definite structural state of surface layers.
I.I. Garbar I Wear 181-183
54
(1995) SO-55
Table 1 Intensity changes of diffraction lines after exposure to friction and relative wear resistance of the specimens and machine parts Specimens or machine part
Low-carbon steel Steel U8 Steel elements of a globe hinge of a steering mechanism of a truck Nickel alloy elements of a globe hinge of a steering mechanism of a truck Chromium coating of loom elements Nickel alloy coating of loom elements
This state is characterized by a definite degree of crystal lattice imperfection and surface layer fragmentation level. Similar results were obtained for different counterface materials (copper specimens were tested vs. molybdenum, copper and nickel under 0.8 MPa). Thus, these data show a that definite structural state of the surface layers corresponds to the nature of the material and the friction conditions. Our results also show that the running-in period is characterized by a change of the structural state of the surface layers. The transition to stable wear and equilibrium surface roughness is accompanied by stabilization of the structural state of the material. As is shown in Figs. 3 and 4, the time of structural change from the initial state to the operating state coincides with the running-in period. Consequently, structural parameters such as structural broadening of diffraction lines or their intensity can be used to determine the running-in period. These correlations can be also used to speed up the estimation of comparative wear resistance under nominal friction conditions. It is clear from Figs. 2-4 that the main structural changes occur solely during the running-in period. Since the structural characteristics that appear to be the criteria for wear resistance can be obtained at the beginning of the stabilization period, it is sufficient to carry out tests only until the end of the running-in period in order to obtain structural estimation of the comparative wear resistance of materials. Previous structural investigations [3] have shown, that under fatigue 2 wear structural changes are proportional to wear intensity. Consequently, as can be seen from the results presented in Fig. 3, under the prevailing fatigue wear, soft friction conditions (in our case the minimal pressure) correspond to the smallest structural changes. Therefore, when estimating the fatigue friction conditions, such as different lubricants, *Under
conditions of abrasive wear as well as in cases of adhesion in friction and scoring, the correlation between the structural changes and wear intensity are different from fatigue wear.
Relative *ear resistance
Diffraction lines
Relative intensity of diffraction lines
(‘W
(11lr,)
1 g-10 1 3.5-5 1 8
a-Fe (110) a-Fe (110) a-Fe (110) Ni (111) Cr(ll0) Ni(ll1)
3 1 2.7 1.2 2.4 1.1
loads, velocities etc., the optimal conditions are those that give a surface layer structure that the closest to the initial structure. An important engineering problem is the selection of wear-resistant materials for the details of friction units. To solve this problem, it is necessary to establish the relationship between structural changes of different materials under equal friction conditions and their wear resistance. The results given in Fig. 4 show that it is indeed possible to define such a relationship; for example, the wear resistance of steel 45 under particular test conditions was higher than the wear resistance of low-carbon steel, whereas the structural changes of low carbon steel were more significant. Thus, smaller structural changes correspond to materials with greater wear resistance. Tests of specimens and machine parts made of different materials were also conducted. The specimens made of low-carbon steel (WIN 90) and steel U8 (VI-IN 720) were tested on a friction machine, and some parts of trucks and looms were tested under the working conditions. The truck parts were manufactured from steel (HR, 56-61) or nickel alloy (HR, 54-60), and the surface layers of the loom parts were made of chromium or nickel alloy. X-ray investigations of the steel specimens and machine parts were conducted both before and after testing. The intensities of the diffraction lines at small reflection angles indicated the extent of fragmentation of the surface layers of the metal. The results of the X-ray study and data on the wear resistance of the specimens and machine parts are shown in Table 1. It can be seen that the materials with higher wear resistance are those for which changes in the intensities of the diffraction lines are minimal 3.
3This rule is applicable only in the absence of phase transition in material surface layers under friction. If such a phase transition does take place, changes in diffraction lines intensities can be due to changes in the quantities of the different phases.
I.I. Garbar / Wear 181483
(1995) 50-55
The corresponds established in this study can be used for selection of optimal materials and/or of the optimal methods for their treatment for given friction conditions.
(3) The structure-based
4. Conclusions
References
selection of wear resistant materials can thus be used to reduce the duration of wear-resistance tests. The tests have to be conducted only till the end of running-in period and they can be performed under nominal operating conditions.
111 Y.B.
(1) Under constant friction conditions, the structural characteristics of the surface layers of a metal change only during the running-in period. Therefore, such characteristics can be serve as a measure of the length of this period. (2) Changes in the structure of the surface layer depend on the friction conditions. Under extreme friction conditions, the structural changes are more significant. When a number of materials are compared under equal friction conditions, the material that exhibits the smallest structural changes of the surface layers has the highest wear resistance.
55
Fridman, Mechanical Properties of Metal, Vol. 1, Mashinostroenie, Moscow, 1974, p. 18. PI I.M. Lubarsky, Hardening and negative hardening under friction, in G.A. Preis (ed.), The Problems of Friction and Wear, Vol. 1, Technique, Kiev, 1971, pp. 27-34. 131 1.1. Garbar, Fragmentation of low-carbon steel and copper surface layers during fatigue and adhesive wear, Frict. Wear, 7 (6) (1986) 1043-1053. 141 1.1. Garbar, Method of electron microscopy investigation of the friction surface structure, Plunt Labomtoty, 3 (1983) 59-60. PI J.A.B. van Dijck, The direct observation of the transmission electron microscope of the heavily deformed surface layer of the copper pin after dry sliding against a steel ring, Wear, 42 (1977) 109-117. Fl V.V. Rybin, The Large Plastic Deformation and Metal Failure, Metallurgia, Moscow, 1986, p. 38.