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Study on reduction in wear due to magnetization
196
Wear, 162-164 (1993)
Study on reduction
196-201
in wear due to magnetization
Kazuo Kumagai Akita University, 1-I Tegata Gakuen-machi, Akita 010 (Japan)
Koshi Suzuki Tohoku Electric Power Co., Inc, 3-7-l Ichiban-cho, Aoba-lac, Sendai 980 (Japan)
Osamu Kamiya* Akita University, 1-I Tegata Gakuen-machi
Akita 010 (Japan)
Abstract Operating the pin-rotor type wear test using the ferromagnetic materials, an Ni pin and steel rotor combination, even a weak magnetization of the pin decreased the wear. In order to probe that phenomenon, the grain size distributions of wear particles have been analysed by means of the computer-aided image analyser. The experimental results indicate the relationship that wear rate reduces with the fining down of the wear particles. It is concluded that magnetization accelerated the oxidation at rubbing surfaces and wear particles. Oxidation prevents the rubbing surfaces from mutual material transfer and the consequent pile up to form larger particles, therefore the wear particles remain fine. The cause of wear reduction was the fining down of wear particles by accelerated oxidation in magnetic effect, and those oxidized fine-wear-particles existed between rubbing surfaces and were attracted by a magnetic force, which acted as a lubricant and reduced the wear. The observation that the magnetization accelerated oxidation was verified by the fact that the oxidation reaction of a magnetized steel piece was more severe than one without magnetization. Moreover, the oxygen density at the surface of ferromagnetic materials in air was a little increased by magnetization, which also contributed to the acceleration of oxidation at a rubbing surface.
1. Introduction The moving parts of electromagnetic apparatus are somewhat exposed and are operated in a magnetic field. Therefore it is very important to understand the wear behaviour of the rubbing interface under the magnetic field. Several studies on the effect of a magnetic field on wear behaviour have been reported as follows. For example, Yamamoto and Gondo [l] reached the conclusion that the magnetization increased the surface activation energy. Muju and Radhakrishna [2] stated that the magnetic field decreased the wear activation energy. Subsequently, Sasada [3] and Hiratsuka et al. [4] reported that the magnetization decreased the wear of the transition metals. On the contrary two of the present authors and a coworker have made repeated-type wear tests on combinations of several kinds of ferromagnetic material. As a result, the wear drastically decreased under extremely weak magnetization such as 20 mT [5]. We also indicated that the activation energy of oxidation was decreased by magnetization and explained the
0043-1648/93/$6.00
decrease in wear due to magnetization as follows. The magnetization promoted oxidation and refinement of the wear particles that were attracted and held at the rubbing interface, and these particles prevent the rubbing materials from wearing. Sasada and coworkers [3, 41 also recognized the refinement of wear particles by magnetization. The relationship, however, between the refinement of wear particles and the wear rate has not as yet been examined. Therefore in this study the relationship between the refinement of wear particles and the wear rate has been realized quantitatively, and some evidence concerning the oxidation-promoting effect due to magnetization is discussed.
2. Reduction in wear due to magnetization 2.1. Test specimens and experimental apparatus Figure 1 shows schematically the pin-rotor type of repeated-wear-test apparatus. Table 1 shows the materials and sizes of the pin and rotor.
0 1993 - Elsevier
Sequoia.
All rights reserved
197
K. Kwnagai et al, / Magnetization effects on wear
Fig. 1. Sketch of wear-testing
apparatus.
TABLE used
1. Size, heat treatments
Test piece
Material
Pin
Nickel (99.45%)
and hardnesses
of test pieces
020100
I
600
300
Slldlng distance
Rotor
Size (mm) da=2 lb=24
Carbon steel W5C)
‘Diameter;
d =40+2 WC=4
Hardness Hv (0.5)
Heat treatment 900 “Cx20 annealed in vacuum as-rolled
min
L
, m
Fig. 2. Relationship between the total amount of wear and the sliding distance under various test conditions.
138 I”
L=BOOm
232
blength; SNidth.
The pin was inserted into the centre of the magnetic coil, the coil constant of which was 5 4840 m-l, and then it was magnetized to various intensities using an a.c. The rubbing surfaces of the pin and rotor were ground to a surface roughness R, of less than 0.05 pm. Then, they were demagnetized sufficiently that they did not have residual magnetization.
0’ c
J
0
0.6 Flux
Fig. 3. Magnetic
2.2. Effect of a magnetic field on wear The repeated dry wear tests were performed at room temperature under several different conditions: load P on the pin, 1 or 3 N; sliding velocity V, 5 or 30 m min-‘; magnetic flux density B, 0, 0.6 or 1.2 mT. An extremely weak magnetic flux density B, such as 1.2 mT or less, was used because stronger magnetization completely prevented wear, which then maintained a low value without any variation [5]. Figure 2 shows the relationship between the sliding distance L and the total amount AW of wear. Figure 3 indicates the effect that the flux density B has on AW at the distance L =900 m. Figure 4 shows the relationships between the flux density B and the normalized reduction C#Iin wear caused by magnetization. The values of C$were defined by the equation
‘=
900
AK-Awn, AW 0
xlOOQ
0
1.2
density
B
, mT
effect on the total amount of wear.
P:N V:mimln 80
J 0
06
1.2
Flux density
Fig. 4. Relationship and flux density.
B
between
.
mT
the normalized
reduction
in wear
(1)
where AW, and AW,,, are the total amounts of wear measured in the absence and in the presence respectively of magnetization. The scale in Fig. 4 was defined so
that the larger C#I values are located at lower positions. As shown in this figure, the decrease in wear with increasing flux density had a marked effect for the higher load P and higher velocity V.
198
K. Kumagai et al. / Magnetization effects on wear
3. Relationship between the size of the wear particles and the amount of wear One of the remarkable phenomena that are caused by the magnetic effect is the refinement of the wear particles. Therefore the variation in the size of the wear particles caused by magnetization was examined and the relationship between the refinement and the amount of wear was discussed as follows. 3.1. Wear particle size distribution The wear particles, which were generated in the wear test for Fig. 2, were sampled and classified corresponding to each sliding interval AL in the ranges O-20 m, 20-100 m, 100-300 m, 300400 m and 600-900 m. It was evident that the size of wear particles became finer on magnetization and for longer sliding distances as shown in Fig. 5. On the assumption that the wear particles are ellipsoids, the long diameter a and the short diameter b of each sampled wear particle were measured with a computer-aided image analyser. The wear particles were assumed to be ellipsoids with a volume V. The nominal diameter of wear particle was defined as the diameter d of the equivalent sphere that had the same volume 2). The values of w were obtained from the following equation:
Tab&
u=
?d3
6
6
Figure 6 shows the particle size distribution for P=3 N and V= 30 m min-I. In this figure, N is the measured total number of wear particles in each AL,, and II is the number of wear particles contained in each range of d. In the case when there is no magnetization, most of the wear particles had a diameter larger than 35 pm for L G 100 m and most of them became smaller than 5 pm for L > 100 m. The inset in Fig. 6 indicates the details of the size distribution in the range da20 pm for wear particles. On the contrary, in the case when the pin was magnetized weakly, e.g. 1.2 mT, most of the wear particles were refined to a smaller size less than 5 pm. Figure 7 shows that the magnetization influenced in particular the size distribution of wear particles generated in the range AL =20-100 m. It is clear that more than 90% of the particles became smaller than 5 pm as a result of magnetization for any test conditions,
21.
20 100 100-300 -RN-tiilfl
zo-
‘,
;y
x
5
10
15
20
Particle size
d
m
u-
8~‘,
0
5
10
15
20
25
30
;
lb
1;
;o
2;
3b
;5
Particle size
. @"I
d
3?
. irm
Fig. 6. Magnetic effect on the particle size distribution for each sliding interval.
(a)
B=O
100 9 0
A L -20-100 D
2 80 \ ‘
A 1. 20--100m 0 v:i v=30 A P:3 v= 5
0 r=3 vz30 n P=3 vm 5 a P-1 v-30
2 60 2 i k p
n P-I VPIV
v-30 5
40
: az
0
(b)B=1.2 mT Fig. 5. Variation in the wear particle size due to magnetization and the sliding distance (P-3 N; V-30 m min-I): (a) B-O T, (b) B = 1.2 mT.
0
5
10
15
20
25
30
4
lb
1;
2b
2;
3b
3;
SLZ~
d
Particle
3?
. ~!m
0
5
10
15
20
25
30
5
lb
lb
;o
2;
3;
3;
Particle size
d
3,5
. iim
Fig. 7. Magnetic effect on the particle size distribution for various test conditions.
199
K IGunagai et al. / Magnetization effects on wear
3.2. Variation in the mean particle diameter Figure 8 shows the relationship between the mean particle diameter ci and the sliding interval AL in the case when V=30 m min-’ for example. The values of d are the arithmetical means within each AL for several conditions. From this figure, it can be seen that the values of d became smaller for longer sliding distances, irrespective of whether magnetization was present or not. The values of d decreased as a result of a magnetic flux density of 1.2 mT at an extremely early stage of sliding. The refining effect of magnetization on wear particles was more marked under a larger load. The trends in d when V=5 m min-’ were similar but the effect of magnetization was smaller than when V=30 m min-‘. 3.3. Relationship between the size of the wear particle and the amount of wear The experimental result that the amount of wear decreased for fine wear particles, which may be understood immediately from a commensense viewpoint, has been discussed quantitatively. The particle size parameter gi, which is defined by eqn. (3), can be calculated from the relationship between n/N and d corresponding to each sliding interval, as shown in Fig. 6:
where dj is the central value of each diameter range of wear particles and Rj is the particle number ratio, i.e. Rj= (n/N),, corresponding to each range of d, and 17.5 pm ( = 35/2 pm) is the median value of the particle size. The particle size index g is defined by the following equation, which represents the size properties of all the wear particles after a sliding distance of 900 m:
B:mT
"0
20
2b Id0
300 1po
where AWi is the total amount of wear at each sliding distance. On the contrary, the total wear volume was calculated from AW,= !%’ PNi
+ % Ps45c
by substituting the amount, AW,, of wear on the pin and the amount AW, of wear on the rotor after sliding a distance L =900 m; pNi and psdsc are the densities of nickel and S45C respectively. Then the specific amount w, of wear was calculated from
ws=
Aw, pL
The relationship between g and w, defined above is shown in Fig. 9 plotted in logarithmic coordinates. A linear relationship exists between g and w,, i.e. clearly the specific amount w, of wear decreases as the wear particle size index g decreases. It is shown, moreover, that the reduction in wear due to magnetization is more marked for more severe wear conditions such as with the larger load and the higher sliding velocity.
4. Oxidation-promoting
300
600
660
960
Slldlng interval
Fig. 9. Relationship between the specific amount of wear and the particle size index.
AL , m
Fig. 8. Relationship between the mean particle diameter and the sliding interval.
effect of magnetization
It seems that the variation in g caused by magnetization directly depends on the oxidation-promoting effect of the magnetization. Two of the present authors and a coworker previously reported [5] that magnetization by an a.c. or d.c. decreased the activation energy of oxidation at high temperatures from 225 to 275 “C.
K. Kumagai
200
et al. / Magnetization effects on wear
In the present case of wear testing at room temperature as mentioned above, the oxidation-promoting effect also could be detected as follows. 4.1. Test piece The test pieces, whose size was 10 mmX 10 mm square, were machined from the SS41 steel, whose thickness was 5 mm, and their surfaces were finished using abrasive paper. SS41 is a rolled steel of general structure according to the specifications of the JIS (Japanese Industrial Standard); it was used for the tests because it seems to be oxidized more easily than nickel or S45C is. Three test pieces were magnetized up to the magnetic flux density range from 49.0 to 58.9 mT using an intensive magnet. Paper was inserted in the contact face between the test piece and the magnet to prevent direct contact between them and to avoid the generation of a corrosive current, as shown in Fig. 10. These test pieces were exposed in air at room temperature with extra two test pieces that had not been magnetized. 4.2. Variation in the oxidizing reaction due to magnetization Figure 11 shows the oxide on the test pieces; it can be seen that the oxide on the magnetized specimen is more marked than on the specimen without magnetization. The results of oxygen analysis using an electron probe microanalyser verified that deposit on the oxidized part of the specimen was an oxide.
i Paperfar lsolatlan
Fig. 10. Shapes and dimensions
(a> B=O
of the specimen
(b)B=51.1
Fig. 11. State of oxide (a) without magnetization (b) with magnetiz$tion (B-51.1 mT).
and magnet.
mT (B=O T) and
Fig. 12. Magnetic effect on the relationship ratio and the lapse of time.
between the oxidized
The oxidized ratio r which is the ratio of the total area of oxidized parts to the measured area of test piece, was computed using the image analyser. From these results, the relationship between the oxidized ratio and the lapse of time shown in Fig. 12 was obtained. From this figure, it can be seen that magnetization promotes oxidation at room temperature, too.
5. Variation in oxygen content in air due to magnetization
It has been reported that oxygen has an unusually high magnetic permeability for a gas [6]. There is a possibility that the oxidation promotion due to magnetization is a result of an increase in the oxygen content in air. This assumption was examined using an experimental procedure to determine the oxygen content C of air. The air was sampled at the edge of an iron core magnetized by the d.c. magnetic coil and was chemically analysed with respect to the magnetic flux density B. Figure 13 shows the relationship between C and B. The oxygen content, as analysed by gas chromatography, tends to rise with increasing flux density, although the standard values of C without magnetization vary according to the day that the measurements were taken. It is clear that the magnetization increased the oxygen content C although the effect was relatively weak. Therefore the increase in oxygen content caused by magnetization is considered to be one of the factors that promotes oxidation.
K timagai
Dateof
ap 21.1
.
et al. / Magnetization effects on wear
measurement
Aug. 29.'91
E . 21.0c) t: m c, 20.9. ," 2 % z 20.8c :: Carrier gas : He (20 milmln)
20.71
I
8
I
0
100 Flux
density
200 B
300
, mT
Fig. 13. Relationship between the oxygen content in air and the flux density.
6. Discussion On the basis of the above experimental results, the mechanism of the drastic reduction in wear caused by the magnetization is considered as follows. First, the transfer elements [3] that were produced in the wear process were strongly oxidized by the oxidation-promoting effect of magnetization. Then, it is considered that oxidation prevents the transfer elements from growing into transfer particles which are caused by mutual transfer. As a result, it is assumed that the magnetic effect generated the fine wear particles. The increase in the oxygen content C caused by the magnetization seems to have an effect on the oxidation of the particles. Next, the refined and oxidized wear particles are attracted and retained at the friction interface by the weak magnetic force. These particles prevent the pin and rotor from direct contact with each other. The particles promote the transition from severe to mild wear at an extremely early stage of wear process, with the result that the wear decreases.
7. Conclusions The effects of magnetization on wear have been examined by operating a repeated-type wear test on
201
a nickel pin-steel rotor combination at room temperature in dry air. The results are summarized as follows. (1) The wear markedly decreased as a result of the effect of the magnetization even when it is extremely weak. The effect appears more pronounced for more severe conditions. (2) Because of the oxidation-promoting effect of magnetization, the wear particles are refined at an extremely early stage of wear process. The particles are attracted and retained at the friction interface, which promotes the transition from severe to mild wear. (3) It is confirmed that the refinement of wear particles causes the wear reduction, which is supported by a linear relationship between the size of the wear particles and the amount of wear. (4) It is considered that the refinement in wear particle is caused by the oxidation-promoting effect of magnetization and that the increase in oxygen content also enhances oxidation.
Acknowledgments The authors express their thanks to Mr. Ashihara, Akita University, for his assistance in carrying out the experiments. The authors offer their deepest thanks to Professor Ohnuma and his assistant Mr. Kitabayashi for their analysis of the oxygen contents of the samples.
References Y. Yamamoto and S. Gondo, Effect of magnetic field on boundary lubrication, Tribal. Znt., 20 (1987) 342-346. M. K. Muju and A. Radhakrishna, Wear of non-magnetic materials in the presence of a magnetic field, Wear, 58 (1980) 49-58. T. Sasada, Fundamental analysis of the “adhesive wear” of metals - Severe and mild wear, Ptvc. JSLE Znt. Conf, Tokyo, 1985, Japanese Society of Lubrication Engineers, Japan, Tokyo, 1985, pp. 623-628. K. Hiratsuka, T. Sasada and S. Norose, The magnetic effect on the wear of materials, Wear, 110 (1986) 251-261. K. Kumagai, M. Takahashi and 0. Kamiya, Research for wear behavior in the magnetic field, Proc. Japan Znt. Tri6ology Conf Nagoya, 1990, Japanese Society of Tribologists, Nagoya, Japan, 1990, pp. 881-886. H. Eyring, J. Walter and G. E. Kimball, Quantum Chemistry, Wiley, New York, 1944, pp. 347-350.
Wear, 162-164 (1993)
Study on reduction
196-201
in wear due to magnetization
Kazuo Kumagai Akita University, 1-I Tegata Gakuen-machi, Akita 010 (Japan)
Koshi Suzuki Tohoku Electric Power Co., Inc, 3-7-l Ichiban-cho, Aoba-lac, Sendai 980 (Japan)
Osamu Kamiya* Akita University, 1-I Tegata Gakuen-machi
Akita 010 (Japan)
Abstract Operating the pin-rotor type wear test using the ferromagnetic materials, an Ni pin and steel rotor combination, even a weak magnetization of the pin decreased the wear. In order to probe that phenomenon, the grain size distributions of wear particles have been analysed by means of the computer-aided image analyser. The experimental results indicate the relationship that wear rate reduces with the fining down of the wear particles. It is concluded that magnetization accelerated the oxidation at rubbing surfaces and wear particles. Oxidation prevents the rubbing surfaces from mutual material transfer and the consequent pile up to form larger particles, therefore the wear particles remain fine. The cause of wear reduction was the fining down of wear particles by accelerated oxidation in magnetic effect, and those oxidized fine-wear-particles existed between rubbing surfaces and were attracted by a magnetic force, which acted as a lubricant and reduced the wear. The observation that the magnetization accelerated oxidation was verified by the fact that the oxidation reaction of a magnetized steel piece was more severe than one without magnetization. Moreover, the oxygen density at the surface of ferromagnetic materials in air was a little increased by magnetization, which also contributed to the acceleration of oxidation at a rubbing surface.
1. Introduction The moving parts of electromagnetic apparatus are somewhat exposed and are operated in a magnetic field. Therefore it is very important to understand the wear behaviour of the rubbing interface under the magnetic field. Several studies on the effect of a magnetic field on wear behaviour have been reported as follows. For example, Yamamoto and Gondo [l] reached the conclusion that the magnetization increased the surface activation energy. Muju and Radhakrishna [2] stated that the magnetic field decreased the wear activation energy. Subsequently, Sasada [3] and Hiratsuka et al. [4] reported that the magnetization decreased the wear of the transition metals. On the contrary two of the present authors and a coworker have made repeated-type wear tests on combinations of several kinds of ferromagnetic material. As a result, the wear drastically decreased under extremely weak magnetization such as 20 mT [5]. We also indicated that the activation energy of oxidation was decreased by magnetization and explained the
0043-1648/93/$6.00
decrease in wear due to magnetization as follows. The magnetization promoted oxidation and refinement of the wear particles that were attracted and held at the rubbing interface, and these particles prevent the rubbing materials from wearing. Sasada and coworkers [3, 41 also recognized the refinement of wear particles by magnetization. The relationship, however, between the refinement of wear particles and the wear rate has not as yet been examined. Therefore in this study the relationship between the refinement of wear particles and the wear rate has been realized quantitatively, and some evidence concerning the oxidation-promoting effect due to magnetization is discussed.
2. Reduction in wear due to magnetization 2.1. Test specimens and experimental apparatus Figure 1 shows schematically the pin-rotor type of repeated-wear-test apparatus. Table 1 shows the materials and sizes of the pin and rotor.
0 1993 - Elsevier
Sequoia.
All rights reserved
197
K. Kwnagai et al, / Magnetization effects on wear
Fig. 1. Sketch of wear-testing
apparatus.
TABLE used
1. Size, heat treatments
Test piece
Material
Pin
Nickel (99.45%)
and hardnesses
of test pieces
020100
I
600
300
Slldlng distance
Rotor
Size (mm) da=2 lb=24
Carbon steel W5C)
‘Diameter;
d =40+2 WC=4
Hardness Hv (0.5)
Heat treatment 900 “Cx20 annealed in vacuum as-rolled
min
L
, m
Fig. 2. Relationship between the total amount of wear and the sliding distance under various test conditions.
138 I”
L=BOOm
232
blength; SNidth.
The pin was inserted into the centre of the magnetic coil, the coil constant of which was 5 4840 m-l, and then it was magnetized to various intensities using an a.c. The rubbing surfaces of the pin and rotor were ground to a surface roughness R, of less than 0.05 pm. Then, they were demagnetized sufficiently that they did not have residual magnetization.
0’ c
J
0
0.6 Flux
Fig. 3. Magnetic
2.2. Effect of a magnetic field on wear The repeated dry wear tests were performed at room temperature under several different conditions: load P on the pin, 1 or 3 N; sliding velocity V, 5 or 30 m min-‘; magnetic flux density B, 0, 0.6 or 1.2 mT. An extremely weak magnetic flux density B, such as 1.2 mT or less, was used because stronger magnetization completely prevented wear, which then maintained a low value without any variation [5]. Figure 2 shows the relationship between the sliding distance L and the total amount AW of wear. Figure 3 indicates the effect that the flux density B has on AW at the distance L =900 m. Figure 4 shows the relationships between the flux density B and the normalized reduction C#Iin wear caused by magnetization. The values of C$were defined by the equation
‘=
900
AK-Awn, AW 0
xlOOQ
0
1.2
density
B
, mT
effect on the total amount of wear.
P:N V:mimln 80
J 0
06
1.2
Flux density
Fig. 4. Relationship and flux density.
B
between
.
mT
the normalized
reduction
in wear
(1)
where AW, and AW,,, are the total amounts of wear measured in the absence and in the presence respectively of magnetization. The scale in Fig. 4 was defined so
that the larger C#I values are located at lower positions. As shown in this figure, the decrease in wear with increasing flux density had a marked effect for the higher load P and higher velocity V.
198
K. Kumagai et al. / Magnetization effects on wear
3. Relationship between the size of the wear particles and the amount of wear One of the remarkable phenomena that are caused by the magnetic effect is the refinement of the wear particles. Therefore the variation in the size of the wear particles caused by magnetization was examined and the relationship between the refinement and the amount of wear was discussed as follows. 3.1. Wear particle size distribution The wear particles, which were generated in the wear test for Fig. 2, were sampled and classified corresponding to each sliding interval AL in the ranges O-20 m, 20-100 m, 100-300 m, 300400 m and 600-900 m. It was evident that the size of wear particles became finer on magnetization and for longer sliding distances as shown in Fig. 5. On the assumption that the wear particles are ellipsoids, the long diameter a and the short diameter b of each sampled wear particle were measured with a computer-aided image analyser. The wear particles were assumed to be ellipsoids with a volume V. The nominal diameter of wear particle was defined as the diameter d of the equivalent sphere that had the same volume 2). The values of w were obtained from the following equation:
Tab&
u=
?d3
6
6
Figure 6 shows the particle size distribution for P=3 N and V= 30 m min-I. In this figure, N is the measured total number of wear particles in each AL,, and II is the number of wear particles contained in each range of d. In the case when there is no magnetization, most of the wear particles had a diameter larger than 35 pm for L G 100 m and most of them became smaller than 5 pm for L > 100 m. The inset in Fig. 6 indicates the details of the size distribution in the range da20 pm for wear particles. On the contrary, in the case when the pin was magnetized weakly, e.g. 1.2 mT, most of the wear particles were refined to a smaller size less than 5 pm. Figure 7 shows that the magnetization influenced in particular the size distribution of wear particles generated in the range AL =20-100 m. It is clear that more than 90% of the particles became smaller than 5 pm as a result of magnetization for any test conditions,
21.
20 100 100-300 -RN-tiilfl
zo-
‘,
;y
x
5
10
15
20
Particle size
d
m
u-
8~‘,
0
5
10
15
20
25
30
;
lb
1;
;o
2;
3b
;5
Particle size
. @"I
d
3?
. irm
Fig. 6. Magnetic effect on the particle size distribution for each sliding interval.
(a)
B=O
100 9 0
A L -20-100 D
2 80 \ ‘
A 1. 20--100m 0 v:i v=30 A P:3 v= 5
0 r=3 vz30 n P=3 vm 5 a P-1 v-30
2 60 2 i k p
n P-I VPIV
v-30 5
40
: az
0
(b)B=1.2 mT Fig. 5. Variation in the wear particle size due to magnetization and the sliding distance (P-3 N; V-30 m min-I): (a) B-O T, (b) B = 1.2 mT.
0
5
10
15
20
25
30
4
lb
1;
2b
2;
3b
3;
SLZ~
d
Particle
3?
. ~!m
0
5
10
15
20
25
30
5
lb
lb
;o
2;
3;
3;
Particle size
d
3,5
. iim
Fig. 7. Magnetic effect on the particle size distribution for various test conditions.
199
K IGunagai et al. / Magnetization effects on wear
3.2. Variation in the mean particle diameter Figure 8 shows the relationship between the mean particle diameter ci and the sliding interval AL in the case when V=30 m min-’ for example. The values of d are the arithmetical means within each AL for several conditions. From this figure, it can be seen that the values of d became smaller for longer sliding distances, irrespective of whether magnetization was present or not. The values of d decreased as a result of a magnetic flux density of 1.2 mT at an extremely early stage of sliding. The refining effect of magnetization on wear particles was more marked under a larger load. The trends in d when V=5 m min-’ were similar but the effect of magnetization was smaller than when V=30 m min-‘. 3.3. Relationship between the size of the wear particle and the amount of wear The experimental result that the amount of wear decreased for fine wear particles, which may be understood immediately from a commensense viewpoint, has been discussed quantitatively. The particle size parameter gi, which is defined by eqn. (3), can be calculated from the relationship between n/N and d corresponding to each sliding interval, as shown in Fig. 6:
where dj is the central value of each diameter range of wear particles and Rj is the particle number ratio, i.e. Rj= (n/N),, corresponding to each range of d, and 17.5 pm ( = 35/2 pm) is the median value of the particle size. The particle size index g is defined by the following equation, which represents the size properties of all the wear particles after a sliding distance of 900 m:
B:mT
"0
20
2b Id0
300 1po
where AWi is the total amount of wear at each sliding distance. On the contrary, the total wear volume was calculated from AW,= !%’ PNi
+ % Ps45c
by substituting the amount, AW,, of wear on the pin and the amount AW, of wear on the rotor after sliding a distance L =900 m; pNi and psdsc are the densities of nickel and S45C respectively. Then the specific amount w, of wear was calculated from
ws=
Aw, pL
The relationship between g and w, defined above is shown in Fig. 9 plotted in logarithmic coordinates. A linear relationship exists between g and w,, i.e. clearly the specific amount w, of wear decreases as the wear particle size index g decreases. It is shown, moreover, that the reduction in wear due to magnetization is more marked for more severe wear conditions such as with the larger load and the higher sliding velocity.
4. Oxidation-promoting
300
600
660
960
Slldlng interval
Fig. 9. Relationship between the specific amount of wear and the particle size index.
AL , m
Fig. 8. Relationship between the mean particle diameter and the sliding interval.
effect of magnetization
It seems that the variation in g caused by magnetization directly depends on the oxidation-promoting effect of the magnetization. Two of the present authors and a coworker previously reported [5] that magnetization by an a.c. or d.c. decreased the activation energy of oxidation at high temperatures from 225 to 275 “C.
K. Kumagai
200
et al. / Magnetization effects on wear
In the present case of wear testing at room temperature as mentioned above, the oxidation-promoting effect also could be detected as follows. 4.1. Test piece The test pieces, whose size was 10 mmX 10 mm square, were machined from the SS41 steel, whose thickness was 5 mm, and their surfaces were finished using abrasive paper. SS41 is a rolled steel of general structure according to the specifications of the JIS (Japanese Industrial Standard); it was used for the tests because it seems to be oxidized more easily than nickel or S45C is. Three test pieces were magnetized up to the magnetic flux density range from 49.0 to 58.9 mT using an intensive magnet. Paper was inserted in the contact face between the test piece and the magnet to prevent direct contact between them and to avoid the generation of a corrosive current, as shown in Fig. 10. These test pieces were exposed in air at room temperature with extra two test pieces that had not been magnetized. 4.2. Variation in the oxidizing reaction due to magnetization Figure 11 shows the oxide on the test pieces; it can be seen that the oxide on the magnetized specimen is more marked than on the specimen without magnetization. The results of oxygen analysis using an electron probe microanalyser verified that deposit on the oxidized part of the specimen was an oxide.
i Paperfar lsolatlan
Fig. 10. Shapes and dimensions
(a> B=O
of the specimen
(b)B=51.1
Fig. 11. State of oxide (a) without magnetization (b) with magnetiz$tion (B-51.1 mT).
and magnet.
mT (B=O T) and
Fig. 12. Magnetic effect on the relationship ratio and the lapse of time.
between the oxidized
The oxidized ratio r which is the ratio of the total area of oxidized parts to the measured area of test piece, was computed using the image analyser. From these results, the relationship between the oxidized ratio and the lapse of time shown in Fig. 12 was obtained. From this figure, it can be seen that magnetization promotes oxidation at room temperature, too.
5. Variation in oxygen content in air due to magnetization
It has been reported that oxygen has an unusually high magnetic permeability for a gas [6]. There is a possibility that the oxidation promotion due to magnetization is a result of an increase in the oxygen content in air. This assumption was examined using an experimental procedure to determine the oxygen content C of air. The air was sampled at the edge of an iron core magnetized by the d.c. magnetic coil and was chemically analysed with respect to the magnetic flux density B. Figure 13 shows the relationship between C and B. The oxygen content, as analysed by gas chromatography, tends to rise with increasing flux density, although the standard values of C without magnetization vary according to the day that the measurements were taken. It is clear that the magnetization increased the oxygen content C although the effect was relatively weak. Therefore the increase in oxygen content caused by magnetization is considered to be one of the factors that promotes oxidation.
K timagai
Dateof
ap 21.1
.
et al. / Magnetization effects on wear
measurement
Aug. 29.'91
E . 21.0c) t: m c, 20.9. ," 2 % z 20.8c :: Carrier gas : He (20 milmln)
20.71
I
8
I
0
100 Flux
density
200 B
300
, mT
Fig. 13. Relationship between the oxygen content in air and the flux density.
6. Discussion On the basis of the above experimental results, the mechanism of the drastic reduction in wear caused by the magnetization is considered as follows. First, the transfer elements [3] that were produced in the wear process were strongly oxidized by the oxidation-promoting effect of magnetization. Then, it is considered that oxidation prevents the transfer elements from growing into transfer particles which are caused by mutual transfer. As a result, it is assumed that the magnetic effect generated the fine wear particles. The increase in the oxygen content C caused by the magnetization seems to have an effect on the oxidation of the particles. Next, the refined and oxidized wear particles are attracted and retained at the friction interface by the weak magnetic force. These particles prevent the pin and rotor from direct contact with each other. The particles promote the transition from severe to mild wear at an extremely early stage of wear process, with the result that the wear decreases.
7. Conclusions The effects of magnetization on wear have been examined by operating a repeated-type wear test on
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a nickel pin-steel rotor combination at room temperature in dry air. The results are summarized as follows. (1) The wear markedly decreased as a result of the effect of the magnetization even when it is extremely weak. The effect appears more pronounced for more severe conditions. (2) Because of the oxidation-promoting effect of magnetization, the wear particles are refined at an extremely early stage of wear process. The particles are attracted and retained at the friction interface, which promotes the transition from severe to mild wear. (3) It is confirmed that the refinement of wear particles causes the wear reduction, which is supported by a linear relationship between the size of the wear particles and the amount of wear. (4) It is considered that the refinement in wear particle is caused by the oxidation-promoting effect of magnetization and that the increase in oxygen content also enhances oxidation.
Acknowledgments The authors express their thanks to Mr. Ashihara, Akita University, for his assistance in carrying out the experiments. The authors offer their deepest thanks to Professor Ohnuma and his assistant Mr. Kitabayashi for their analysis of the oxygen contents of the samples.
References Y. Yamamoto and S. Gondo, Effect of magnetic field on boundary lubrication, Tribal. Znt., 20 (1987) 342-346. M. K. Muju and A. Radhakrishna, Wear of non-magnetic materials in the presence of a magnetic field, Wear, 58 (1980) 49-58. T. Sasada, Fundamental analysis of the “adhesive wear” of metals - Severe and mild wear, Ptvc. JSLE Znt. Conf, Tokyo, 1985, Japanese Society of Lubrication Engineers, Japan, Tokyo, 1985, pp. 623-628. K. Hiratsuka, T. Sasada and S. Norose, The magnetic effect on the wear of materials, Wear, 110 (1986) 251-261. K. Kumagai, M. Takahashi and 0. Kamiya, Research for wear behavior in the magnetic field, Proc. Japan Znt. Tri6ology Conf Nagoya, 1990, Japanese Society of Tribologists, Nagoya, Japan, 1990, pp. 881-886. H. Eyring, J. Walter and G. E. Kimball, Quantum Chemistry, Wiley, New York, 1944, pp. 347-350.