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The influence of grain size on the wear of nickel-zinc ferrite by flexible media
Wear, 31 (1975) lo!-117 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
109
THE INFLUENCE OF GRAIN SIZE ON THE WEAR OF NICKEL-ZINC FERRITE BY FLEXIBLE MEDIA
W. D. KEHR, C. B. MELDRUM and R. F. M. THORNLEY General Products Division, International Business‘ Machines Corporation, Boulder, Colo. 80302 (U.S.A.)
(Received July 3 1, 1974)
SUMMARY
Wear resistance and certain other properties of a nickel-zinc ferrite were determined as a function of grain size in tests with two y-Fe,O, tapes and a CrO, tape. Wear resistance was found to decrease-that is, the depth of wear increasedwith decreasing grain size. The CrO, tape caused about three times as much wear as the y-Fe,O, tapes. Microhardness and moduli of rupture improved with decreasing grain size, but the structure-sensitive magnetic properties did not. Annealing did not improve the wear resistance, but did improve the moduli of rupture and the magnetic properties.
INTRODUCTION
The early work in developing soft magnetic ferrites was concerned with improving their magnetic properties. Coarse-grained materials were desirable, since structure-sensitive magnetic properties-permeability, coercivity, and loss factorusually reach their best values when the number of grain boundaries and residual stress are kept to the minimum. The recent trend toward narrow-track recording, however, necessitates maximum strength in thin sections. Fine-grained materials that have been hot-pressed, or precision cold-pressed and sintered give greater strength than coarse-grained materials. Some decrease in magnetic properties is tolerable, but a significant decrease in wear resistance is not. Unfortunately, wear resistance has been reported to decrease with decreasing grain size’. 2; the smallest grain sizes investigated, however, were only 10 to 15 pm. The effect of grain size on the wear of ferrite by commercially available oxide recording tapes, particularly for grain sizes of less than 10 pm was studied. Certain other mechanical and magnetic properties were also monitored to observe their variations with grain size. PROCEDURE
Preparation of specimens
Grain size was to be the only variable in the material, but processing introduced a small variation in density, whose effect is discussed later. The material
110
W. D. KEHR,
C. B. MELDRUM,
R. F. M. THORNLEY
was that investigated by Secrist and Turk3, Ni0,35Zn0,6sFe, ,,90 4. Bricks of ferrite with a target average grain size of 2 pm were prepared by hot pressing. Brick s with target average grain sizes of 5, 10, 25 and 50 pm were prepared by cold pr,essing and sintering for increasing times at 1230°C. Cylindrical test specin lens were cu t from the bricks with a diamond saw and ground so that all significa nt surfaces had a c.1.a. smoothness of 0.025 to 0.050 pm. Two full sets of specimens were made; those in the second set were annealed for 1 h at 900°C in air after finalI grinding.
Fig. 1. Wear test specimen
mounted
Fig. 2. Wear test rod in holder.
on tape drive. (Magnification
The arrow
indicates
about
0.3 x )
the wear scar. (Magnification,
about
2x)
WEAR OF NICKEL-ZINC
FERRITE
111
Wear resistance
Rods 5 mm diam., cylindrical within 2.5 pm, were wear-tested on an IBM 729 Model VI tape drive (see Figs. 1 and 2). They were tested against reels of tape 12.5 mm wide by 800 m long, two of y-Fe,O, and one of CrO,. Each rod was subjected to the wear test with each tape. A test consisted of 400 passes of the tape at 2.8 m/s and a pressure of about 0.05 MN/m2. All passes were at 19” to 23°C and
Fig. 3. Part of a wear scar on a test specimen with a grain size of 4.1 pm. Note that there are fewer pullouts on the wear scar than on the unworn part of the rod. (Magnification, about 63 x )
20 to 30% relative humidity. The rod was rotated slightly between tests. In all cases, the depth of wear was less than 0.1% of the radius of the rod, and therefore resulted in a negligible change in the average contact pressure. The depth of wear was measured from profile traces taken axially along the wear scars (see Fig. 3) with a stylus having a tip radius of 2.5 pm. Other physical and mechanical properties
For determinations of grain size, lapped sections were thermally etched for l/2 h at 1050°C and 10F6 torr. The number of grains in a known area was then counted by use of an automated image analysis system. Density was measured by use of Archimedes’ principle. Semiquantitative chemical analyses were made by X-ray fluorescence. Modulus of rupture was measured on specimens 1 mm thick according to ASTM B 406-70. Knoop microhardness was measured on these same specimens, with a 500 g load.
112
W. D. KEHR,
Magnetic
C. B. MELDRUM,
R. F. M. THORNLEY
properties
Real and imaginary permeabilities, as a function of frequency, were determined on toroidal specimens. A vector impedance meter with a four-turn winding was used for frequencies up to 0.5 MHz; a coaxial adaptor, which was in effect a single-turn winding, was used for frequencies from 0.5 to 100 MHz. The magnetic field used in these measurements was about 1 mOe. The effective permeabilities were obtained by combining the real and imaginary parts in quadrature. The tangent of the loss angle was obtained from the ratio of the real and imaginary permeabilities. The coercivity and the saturation induction were measured with the toroids in a transformer configuration at 100 Hz, and with four-turn drive and sense windings. RESULTS
AND DISCUSSION
The results of the wear tests are discussed first, since ferrite wear was the primary subject of this investigation. The results of the other physical and mechanical tests are in the latter part of this section. Wear resistance
Figure 4 shows the depths of wear caused by the CrO, tape and the two y-Fe,O, tapes. (For the latter an average is plotted, since there was little difference in the wear they caused.) The trend toward decreasing wear resistance with decreasing grain size was found to hold even for fine-grained ferrite. There does not seem to be a fully satisfactory explanation for the influence of grain size (grain boundary density) per se on wear, but two somewhat related factors may account for it. First,
l
0.51.120)
-O-.--o
0
As ground
.
Anwaled,
9OO’C
Y-Fe203
-O-----_-Q
rapes
.-.@----o,
0 L(0)
I
3.4
of wear scar as a function
25.2
8.1 Average
Fig. 4. Depth
i
I
I
4.1
gran
vze
(urn)
of grain size, for y-Fe,O,
and CrO,
tapes.
I
38.9
WEAR OF NICKEL-ZINC
FERRITE
Fig. 5. Photomicrographs of thermally etched sections. (680 x) (a) Material with a grain si.ze of 3.4 pm. The dark areas are pullouts. (b) Material with a grain size of 38.9 pm.
114
W. D. KEHR,
C. 3. MELDRUM,
R. F. M. THORNLEY
although the test specimens were fabricated carefully, the number of pullouts increased as the grain size decreased. A pullout is a place on the surface where all or part of a grain was dislodged during grinding (see Fig. 5); a high density of pullouts leads to greater in-contact tape pressure, and therefore to deeper wear. The wear test actually reduced the number of pullouts, as can be seen in Fig. 3. Second, bulk density increased slightly with increasing grain size, again permitting greater in-contact pressure on the finer-grained materials. Still, it is questionable if these two factors alone adequately explain the observed trend. CrO, tape, regardless of the grain size of the specimen, caused about three = E
,z
$
0Es ‘3
r,
O-O-
100
.-8
-0
Cl
99.5 0
99.0 t
OAO z
As ground +
241 -05)
Annealed, 900°C
0103L(15)
, 3.4
, 4.1
I 25.2
, 8.1
38.9
Average grain size @ml
310-145)
l
/O’ 103L(15)
0.1
0.2 (Average
Fig. 6. Density,
hardness,
I 0.4
0.3
and moduli
grain size)-lR.
of rupture
0.5
0.6
(~mj-“2
as a function
of grain
size.
WEAR OF NICKEL-ZINC
115
FERRITE
times as much wear as the y-Fe,O, tapes, because it is more abrasive. The comparatively poor wear resistance of ferrite with an average grain size of 4.1 w against CrO, tape was attributed to the relatively low density of this material. It has been claimed4 that grinding ferrite can cause surface damage about 0.1 pm deep, and that this damage can be restored by annealing. The results of the present investigation suggest that a “damaged layer” is beneficial for resisting wear by y-Fe,O, tapes, since annealing led to increased wear. Annealing had no consistent effect, however, when the specimens were worn by CrO, tape. Finally, it is interesting to note that whereas resistance to wear increased with increasing grain size, strength and hardness clearly decreased. Other physical
and mechanical
properties
The measured actual grain sizes appear as abscissae in Figs. 4, 6 and 7. Only a slight increase in density, due to more complete sintering, was detected with increasing grain size (Fig. 6). The exception is the material with a grain size of 4.1 pm; although this material had a high density for a polycrystalline ferrite, it was, less completely sintered than the others. The relatively low density of this material accounted for its lower strength, hardness, and resistance to wear by CrOz tapes. X-ray diffraction revealed that the compositions of all the specimens were identical, and were not changed by the annealing. As was expected, the moduli of rupture decreased with increasing grain size (Fig. 6). The data points represent the average of five tests for samples as ground and after annealing. The moduli of rupture were replotted to determine whether they followed the Orowan-Petch relationship. strength oc C + (grain size)-i
1
10 Average grain size
Fig. 7. Magnetic properties as a function of grain size.
100 (pm)
W. D. KEHR,
116
C. B. MELDRUM,
R. F. M. THORNLEY
where C= 0 in the Orowan analysi?, and Cf 0 in the Petch analysis6. (This expression neglects a small correction factor for porosity.) Although the data suggest a linear relation between the moduli and (grain size)-*, it was not possible to determine whether the behavior was elastic (Orowan) or plastic (Petch). The moduli of rupture were about 15% greater for the annealed specimens than for the as-ground ones, presumably because some surface damage or residual stress was removed. Investigators of a manganese-zinc ferrite’ found that annealing at 600” to 900°C actually caused a reduction in strength, which they attributed to damage by thermal etching. Nickel-zinc ferrite is apparently less susceptible to this form of damage; no etching was found after an hour at 900°C. The microhardness data in Fig. 6 represent the average of live measurements. Only a small linear decrease in hardness with increasing grain size was found; the hardness was unchanged after annealing. Magnetic
properties
As Fig. 7. shows, the structure-sensitive magnetic properties were significantly altered by changes in grain size. Since a reduction in the density of grain boundaries makes domain walls more mobile, the permeability and the coercivity were expected to improve with increasing grain size, as happens in soft magnetic metals’. The reductions in permeability of the two larger-grained specimens at 1 MHz occurred because this test frequency was above the frequency at which the imaginary permeability peaked. The saturation induction remained nearly constant at 3200 G with changing grain size, even after annealing. Even the complete removal of surface damage does not satisfactorily explain the improvements in permeability and coercivity after annealing. Considerable residual stress must have been present in the ferrite bricks before the specimens were fabricated. CONCLUSIONS
For in-contact recording applications, line-grained nickel-zinc ferrite can be expected to wear more rapidly than coarse-grained. Also, the CrO, tapes examined caused more wear than y-Fe,O, tapes; about three times as much was found in this investigation. Annealing did not improve the wear resistance, but did improve the moduli’of rupture and the magnetic properties. Material having a density greater than 99.5% of theoretical and a grain size of 8 to 10 pm should offer the best combination of properties for most applications with flexible media. ACKNOWLEDGEMENTS
The authors are indebted to P. Manley for supplying the ferrite materials, and to H. McCabe for the X-ray analysis and the density determinations.
WEAR OF NICKEL-ZINC
FERRITE
117
REFERENCES 1 H. Watanabe and I. Yamaga, Low noise manganese-zinc single crystal ferrite heads, IEEE Trans. Msg., MAG-8 (1972) 497-502. 2 F. R. Monforte, R. Chen and P. D. Baba, Pressure sintering of MnZn and NiZn ferrites, IEEE Trans. Mug., MAG-7 (1971) 345-350. 3 D. R. Secrist and H. L. Turk, Electrical properties of high-density iron-deficient nickel-zinc ferrites, J. Amer. Ceramic Sot., 53 (1970) 683-686. 4 J. A. L. Potgiesser and J. Koorneef, Wear of magnetic heads, Proc. Conf Video and Data Recording, IEEE hoc., 26 (1973) 203-212. 5 E. Orowan, Fracture and strength of solids, Rep. Prog. Phys., 12 (1948) 186-232. 6 N. J. Petch, Cleavage strength of polycrystals, 1. Iron Steel Inst. (London), 174 (1953) 25-28. 7 R. F. M. Thornley and W. D. Kehr, The permeability-frequency response of4-79 molybdenum permalloy foils, IEEE Trans. Mug., MAG-7 (1971) 672474.
109
THE INFLUENCE OF GRAIN SIZE ON THE WEAR OF NICKEL-ZINC FERRITE BY FLEXIBLE MEDIA
W. D. KEHR, C. B. MELDRUM and R. F. M. THORNLEY General Products Division, International Business‘ Machines Corporation, Boulder, Colo. 80302 (U.S.A.)
(Received July 3 1, 1974)
SUMMARY
Wear resistance and certain other properties of a nickel-zinc ferrite were determined as a function of grain size in tests with two y-Fe,O, tapes and a CrO, tape. Wear resistance was found to decrease-that is, the depth of wear increasedwith decreasing grain size. The CrO, tape caused about three times as much wear as the y-Fe,O, tapes. Microhardness and moduli of rupture improved with decreasing grain size, but the structure-sensitive magnetic properties did not. Annealing did not improve the wear resistance, but did improve the moduli of rupture and the magnetic properties.
INTRODUCTION
The early work in developing soft magnetic ferrites was concerned with improving their magnetic properties. Coarse-grained materials were desirable, since structure-sensitive magnetic properties-permeability, coercivity, and loss factorusually reach their best values when the number of grain boundaries and residual stress are kept to the minimum. The recent trend toward narrow-track recording, however, necessitates maximum strength in thin sections. Fine-grained materials that have been hot-pressed, or precision cold-pressed and sintered give greater strength than coarse-grained materials. Some decrease in magnetic properties is tolerable, but a significant decrease in wear resistance is not. Unfortunately, wear resistance has been reported to decrease with decreasing grain size’. 2; the smallest grain sizes investigated, however, were only 10 to 15 pm. The effect of grain size on the wear of ferrite by commercially available oxide recording tapes, particularly for grain sizes of less than 10 pm was studied. Certain other mechanical and magnetic properties were also monitored to observe their variations with grain size. PROCEDURE
Preparation of specimens
Grain size was to be the only variable in the material, but processing introduced a small variation in density, whose effect is discussed later. The material
110
W. D. KEHR,
C. B. MELDRUM,
R. F. M. THORNLEY
was that investigated by Secrist and Turk3, Ni0,35Zn0,6sFe, ,,90 4. Bricks of ferrite with a target average grain size of 2 pm were prepared by hot pressing. Brick s with target average grain sizes of 5, 10, 25 and 50 pm were prepared by cold pr,essing and sintering for increasing times at 1230°C. Cylindrical test specin lens were cu t from the bricks with a diamond saw and ground so that all significa nt surfaces had a c.1.a. smoothness of 0.025 to 0.050 pm. Two full sets of specimens were made; those in the second set were annealed for 1 h at 900°C in air after finalI grinding.
Fig. 1. Wear test specimen
mounted
Fig. 2. Wear test rod in holder.
on tape drive. (Magnification
The arrow
indicates
about
0.3 x )
the wear scar. (Magnification,
about
2x)
WEAR OF NICKEL-ZINC
FERRITE
111
Wear resistance
Rods 5 mm diam., cylindrical within 2.5 pm, were wear-tested on an IBM 729 Model VI tape drive (see Figs. 1 and 2). They were tested against reels of tape 12.5 mm wide by 800 m long, two of y-Fe,O, and one of CrO,. Each rod was subjected to the wear test with each tape. A test consisted of 400 passes of the tape at 2.8 m/s and a pressure of about 0.05 MN/m2. All passes were at 19” to 23°C and
Fig. 3. Part of a wear scar on a test specimen with a grain size of 4.1 pm. Note that there are fewer pullouts on the wear scar than on the unworn part of the rod. (Magnification, about 63 x )
20 to 30% relative humidity. The rod was rotated slightly between tests. In all cases, the depth of wear was less than 0.1% of the radius of the rod, and therefore resulted in a negligible change in the average contact pressure. The depth of wear was measured from profile traces taken axially along the wear scars (see Fig. 3) with a stylus having a tip radius of 2.5 pm. Other physical and mechanical properties
For determinations of grain size, lapped sections were thermally etched for l/2 h at 1050°C and 10F6 torr. The number of grains in a known area was then counted by use of an automated image analysis system. Density was measured by use of Archimedes’ principle. Semiquantitative chemical analyses were made by X-ray fluorescence. Modulus of rupture was measured on specimens 1 mm thick according to ASTM B 406-70. Knoop microhardness was measured on these same specimens, with a 500 g load.
112
W. D. KEHR,
Magnetic
C. B. MELDRUM,
R. F. M. THORNLEY
properties
Real and imaginary permeabilities, as a function of frequency, were determined on toroidal specimens. A vector impedance meter with a four-turn winding was used for frequencies up to 0.5 MHz; a coaxial adaptor, which was in effect a single-turn winding, was used for frequencies from 0.5 to 100 MHz. The magnetic field used in these measurements was about 1 mOe. The effective permeabilities were obtained by combining the real and imaginary parts in quadrature. The tangent of the loss angle was obtained from the ratio of the real and imaginary permeabilities. The coercivity and the saturation induction were measured with the toroids in a transformer configuration at 100 Hz, and with four-turn drive and sense windings. RESULTS
AND DISCUSSION
The results of the wear tests are discussed first, since ferrite wear was the primary subject of this investigation. The results of the other physical and mechanical tests are in the latter part of this section. Wear resistance
Figure 4 shows the depths of wear caused by the CrO, tape and the two y-Fe,O, tapes. (For the latter an average is plotted, since there was little difference in the wear they caused.) The trend toward decreasing wear resistance with decreasing grain size was found to hold even for fine-grained ferrite. There does not seem to be a fully satisfactory explanation for the influence of grain size (grain boundary density) per se on wear, but two somewhat related factors may account for it. First,
l
0.51.120)
-O-.--o
0
As ground
.
Anwaled,
9OO’C
Y-Fe203
-O-----_-Q
rapes
.-.@----o,
0 L(0)
I
3.4
of wear scar as a function
25.2
8.1 Average
Fig. 4. Depth
i
I
I
4.1
gran
vze
(urn)
of grain size, for y-Fe,O,
and CrO,
tapes.
I
38.9
WEAR OF NICKEL-ZINC
FERRITE
Fig. 5. Photomicrographs of thermally etched sections. (680 x) (a) Material with a grain si.ze of 3.4 pm. The dark areas are pullouts. (b) Material with a grain size of 38.9 pm.
114
W. D. KEHR,
C. 3. MELDRUM,
R. F. M. THORNLEY
although the test specimens were fabricated carefully, the number of pullouts increased as the grain size decreased. A pullout is a place on the surface where all or part of a grain was dislodged during grinding (see Fig. 5); a high density of pullouts leads to greater in-contact tape pressure, and therefore to deeper wear. The wear test actually reduced the number of pullouts, as can be seen in Fig. 3. Second, bulk density increased slightly with increasing grain size, again permitting greater in-contact pressure on the finer-grained materials. Still, it is questionable if these two factors alone adequately explain the observed trend. CrO, tape, regardless of the grain size of the specimen, caused about three = E
,z
$
0Es ‘3
r,
O-O-
100
.-8
-0
Cl
99.5 0
99.0 t
OAO z
As ground +
241 -05)
Annealed, 900°C
0103L(15)
, 3.4
, 4.1
I 25.2
, 8.1
38.9
Average grain size @ml
310-145)
l
/O’ 103L(15)
0.1
0.2 (Average
Fig. 6. Density,
hardness,
I 0.4
0.3
and moduli
grain size)-lR.
of rupture
0.5
0.6
(~mj-“2
as a function
of grain
size.
WEAR OF NICKEL-ZINC
115
FERRITE
times as much wear as the y-Fe,O, tapes, because it is more abrasive. The comparatively poor wear resistance of ferrite with an average grain size of 4.1 w against CrO, tape was attributed to the relatively low density of this material. It has been claimed4 that grinding ferrite can cause surface damage about 0.1 pm deep, and that this damage can be restored by annealing. The results of the present investigation suggest that a “damaged layer” is beneficial for resisting wear by y-Fe,O, tapes, since annealing led to increased wear. Annealing had no consistent effect, however, when the specimens were worn by CrO, tape. Finally, it is interesting to note that whereas resistance to wear increased with increasing grain size, strength and hardness clearly decreased. Other physical
and mechanical
properties
The measured actual grain sizes appear as abscissae in Figs. 4, 6 and 7. Only a slight increase in density, due to more complete sintering, was detected with increasing grain size (Fig. 6). The exception is the material with a grain size of 4.1 pm; although this material had a high density for a polycrystalline ferrite, it was, less completely sintered than the others. The relatively low density of this material accounted for its lower strength, hardness, and resistance to wear by CrOz tapes. X-ray diffraction revealed that the compositions of all the specimens were identical, and were not changed by the annealing. As was expected, the moduli of rupture decreased with increasing grain size (Fig. 6). The data points represent the average of five tests for samples as ground and after annealing. The moduli of rupture were replotted to determine whether they followed the Orowan-Petch relationship. strength oc C + (grain size)-i
1
10 Average grain size
Fig. 7. Magnetic properties as a function of grain size.
100 (pm)
W. D. KEHR,
116
C. B. MELDRUM,
R. F. M. THORNLEY
where C= 0 in the Orowan analysi?, and Cf 0 in the Petch analysis6. (This expression neglects a small correction factor for porosity.) Although the data suggest a linear relation between the moduli and (grain size)-*, it was not possible to determine whether the behavior was elastic (Orowan) or plastic (Petch). The moduli of rupture were about 15% greater for the annealed specimens than for the as-ground ones, presumably because some surface damage or residual stress was removed. Investigators of a manganese-zinc ferrite’ found that annealing at 600” to 900°C actually caused a reduction in strength, which they attributed to damage by thermal etching. Nickel-zinc ferrite is apparently less susceptible to this form of damage; no etching was found after an hour at 900°C. The microhardness data in Fig. 6 represent the average of live measurements. Only a small linear decrease in hardness with increasing grain size was found; the hardness was unchanged after annealing. Magnetic
properties
As Fig. 7. shows, the structure-sensitive magnetic properties were significantly altered by changes in grain size. Since a reduction in the density of grain boundaries makes domain walls more mobile, the permeability and the coercivity were expected to improve with increasing grain size, as happens in soft magnetic metals’. The reductions in permeability of the two larger-grained specimens at 1 MHz occurred because this test frequency was above the frequency at which the imaginary permeability peaked. The saturation induction remained nearly constant at 3200 G with changing grain size, even after annealing. Even the complete removal of surface damage does not satisfactorily explain the improvements in permeability and coercivity after annealing. Considerable residual stress must have been present in the ferrite bricks before the specimens were fabricated. CONCLUSIONS
For in-contact recording applications, line-grained nickel-zinc ferrite can be expected to wear more rapidly than coarse-grained. Also, the CrO, tapes examined caused more wear than y-Fe,O, tapes; about three times as much was found in this investigation. Annealing did not improve the wear resistance, but did improve the moduli’of rupture and the magnetic properties. Material having a density greater than 99.5% of theoretical and a grain size of 8 to 10 pm should offer the best combination of properties for most applications with flexible media. ACKNOWLEDGEMENTS
The authors are indebted to P. Manley for supplying the ferrite materials, and to H. McCabe for the X-ray analysis and the density determinations.
WEAR OF NICKEL-ZINC
FERRITE
117
REFERENCES 1 H. Watanabe and I. Yamaga, Low noise manganese-zinc single crystal ferrite heads, IEEE Trans. Msg., MAG-8 (1972) 497-502. 2 F. R. Monforte, R. Chen and P. D. Baba, Pressure sintering of MnZn and NiZn ferrites, IEEE Trans. Mug., MAG-7 (1971) 345-350. 3 D. R. Secrist and H. L. Turk, Electrical properties of high-density iron-deficient nickel-zinc ferrites, J. Amer. Ceramic Sot., 53 (1970) 683-686. 4 J. A. L. Potgiesser and J. Koorneef, Wear of magnetic heads, Proc. Conf Video and Data Recording, IEEE hoc., 26 (1973) 203-212. 5 E. Orowan, Fracture and strength of solids, Rep. Prog. Phys., 12 (1948) 186-232. 6 N. J. Petch, Cleavage strength of polycrystals, 1. Iron Steel Inst. (London), 174 (1953) 25-28. 7 R. F. M. Thornley and W. D. Kehr, The permeability-frequency response of4-79 molybdenum permalloy foils, IEEE Trans. Mug., MAG-7 (1971) 672474.