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Measurement and prediction of tape edge wear using accelerated wear testing
Wear 259 (2005) 1362–1366
Case study
Measurement and prediction of tape edge wear using accelerated wear testing J.H. Wang ∗ , F.E. Talke Center for Magnetic Recording Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0401, USA Received 2 September 2004; received in revised form 3 December 2004; accepted 31 January 2005 Available online 10 May 2005
Abstract A rotating drum tester was developed to investigate accelerated tape edge wear. The results from accelerated wear measurements were found to be in good agreement with tape edge wear measurements from an actual tape drive. The wear coefficients for the magnetic layer, the back coat, and the base film of typical tapes were determined on a pin-on-disk tester and used to predict the tape edge wear coefficient of a tri-layer composite tape. Predicted tape edge wear results were found to be in good agreement with actual tape edge wear results in a tape drive. © 2005 Elsevier B.V. All rights reserved. Keywords: Tape edge wear; Accelerated wear; Magnetic tapes; Wear coefficient
1. Introduction Sliding motion between the tape guides and the edge of a magnetic tape in a tape drive causes tape edge wear. Wang and Talke [1] have found that tape edge wear causes an increase of lateral tape motion, thereby limiting the maximum track density that can be achieved on tape. Tape edge wear has been studied as a function of contact force, tape speed, tape tension, tape guide surface roughness (Ra ), and tape substrate using atomic force microscopy (AFM) [2,3]. The edge quality of new and worn tapes has been characterized by Bhushan and co-workers [4,5]. Recently, tapes with improved substrates and better mechanical properties have become available [6–8]. Edge wear of these generally thinner tapes needs to be studied since tape edge wear is an important concern for the use of these new tapes in future tape drives. The measurement of tape edge wear in a tape drive is a time-consuming task since the maximum tape speed in a tape drive is only on the order of 4 m/s and the tape has to be moved forward and backward during a wear test. Thus, an apparatus to accelerate ∗
Corresponding author. Tel.: +1 8585347578; fax: +1 8585342720. E-mail address: [email protected] (J.H. Wang).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.048
tape edge wear is highly desirable. In addition, prediction of tape edge wear prior to manufacturing of a new type of tape is important since it can save time and development costs. In this paper, a rotating drum tester is used to study acceleration of tape edge wear as a function of tape substrate and speed for present day tape substrates (polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyaramide (PA)). In addition, tape edge wear of a tri-layer tape is predicted from wear measurements of the base film, the magnetic layer, and the back coat using a pin-on-disk tester. The predicted tape edge wear results are compared to tape edge wear results obtained using a commercial tape drive.
2. Accelerated wear testing 2.1. Experiment and procedure A rotating drum tester shown in Fig. 1 was designed and built to accelerate tape edge wear. The tester consists of a precision drum around which a single layer of tape can be mounted. A cantilever beam with a wear pad attached at the tip of the beam is forced against the protruding edge of the
J.H. Wang, F.E. Talke / Wear 259 (2005) 1362–1366
1363
Fig. 1. (a) Schematic and (b) photograph of the rotating drum tester.
tape on the drum resulting in sliding contact between the tape edge and the wear pad. The wear pad is made of Al2 O3 –TiC (70–30%) with an average surface roughness Ra of 14 nm and a Knoop hardness of 23.5 GPa. The following procedure was used to study tape edge wear. The circumference of the 38 mm diameter drum was first wrapped with a single layer of new tape under 1 N tension. A second layer of 40 m thick polyester tape was then attached under the same tension around the first layer. To ensure that tape edge wear occurred only on the first layer, care was taken so that the top edge of the second layer was approximately 1 mm lower than the top edge of the first layer. The drum was then attached to a spindle rotating at 2000 rpm resulting in a speed of 4 m/s at the tape edge. A force of 20 mN was used to load the slider against the tape edge (Fig. 1(a)). The sliding motion between the stationary slider and the rotating tape causes wear of the tape edge. Tape edge wear was measured using the method described by Wang and Talke [3]. In this method, reference indentations are made in the surface of the tape edge prior to testing. The depth change of these indentations is then measured as a function of the number of wear passes using an AFM. For a rotating drum tester, the number of wear passes equals the
Fig. 2. Tape edge wear as a function of substrate using accelerated wear measurement (drum tester) (20 mN and 4 m/s).
number of revolutions of the drum during a wear test. For a tape drive, each pass of the tape under the wear pad is counted as one wear pass. The depth change of the indentations equals the amount of tape edge wear. The tape edge wear coefficient k is determined from dtH (1) Pn where d is the tape edge wear (determined from the depth change of the indentations), t denotes the tape thickness, H is the hardness of the tape, P is the contact force, and n is the number of wear passes. Three different base films (PET, PEN, and PA) (Table 1) coated with the same magnetic layer and back coat were tested. k=
2.2. Results of accelerated wear testing Accelerated tape edge wear results as a function of tape substrate are shown in Fig. 2, obtained with the drum tester. We observe that PEN tape exhibits the least amount of tape edge wear while PA tape shows the largest amount of tape
Table 1 The specifications of tape samples
Tape sample Base film
A PET
B PEN
C PA
Base Young’s modulus
Machine direction Transverse direction
MPa MPa
6700 5000
8000 6500
11000 15000
Coating layer Young’s modulus
Magnetic layer Back coat
MPa MPa
15200 9300
15200 9300
15200 9300
Thickness
Total thickness Base film Magnetic layer Back coat
m m m m
7.8 6.1 1.32 0.4
8.0 6.4 1.11 0.5
6.1 4.5 1.24 0.4
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Fig. 5. Photographs of tape edge of PEN tape: (a) new tape, (b) worn tape after 12k wear passes at 8 m/s, and (c) worn tape after 4k wear passes at 10 m/s. Fig. 3. Tape edge wear as a function of substrate using a commercially available tape drive (20 mN and 4 m/s).
tape speed. Clearly, tape edge wear increases with increasing tape speed. In particular, at 10 m/s, tape edge wear is seen to reach 2.35 m after only 2000 wear passes. At speeds above 10 m/s, thermal degradation of the tape edge was observed, resulting in large scale damage of the tape edge. This effect can be seen by looking at the photographs in Fig. 5(c), which show the degradation of the tape edge after 4000 wear passes at 10 m/s. 3. Prediction of tape edge wear 3.1. Experiment and procedure
Fig. 4. Accelerated tape edge wear of PEN tape as a function of tape speed.
edge wear. In Fig. 3, tape edge wear is shown using a commercial tape drive with the same Al2 O3 –TiC wear pad as used in the drum tester [3]. Similar to Fig. 2, we observe that PA tape has the largest amount of tape edge wear while PEN tape has the least amount of tape edge wear. The tape edge wear coefficients for PET, PEN, and PA tape can be calculated using Eq. (1). Table 2 shows the average wear coefficients ka obtained from the accelerated drum tester and the average wear coefficients kt obtained using a commercial tape drive. We observe that the tape edge wear coefficient of PA tape is larger than that of PET and PEN tape. In addition, we observe that the ratio of wear coefficients ka /kt lies between 0.96 and 1.32, i.e., the wear coefficients measured with the drum tester are on the same order as the wear coefficients obtained in an actual tape drive. Accelerated wear test results from the drum tester are shown in Fig. 4 for tape edge wear of PEN tape as a function of
In the previous experiments, tape edge wear and tape edge wear coefficients were determined using edge wear measurements of actual magnetic tape. An alternative way to obtain a quantitative value of tape edge wear is to calculate the wear coefficient of tape based on the wear coefficient measurements of each individual layer and use this overall wear coefficient for the tape to calculate and predict tape edge wear. In this approach, it is necessary to first determine the wear coefficients of the base film, the magnetic layer, and the back coat, and then predict tape edge wear for the composite tape (Fig. 6) using
Fig. 6. The schematic of a tri-layer tape. Table 2 Tape edge wear coefficients using drum tester Tape sample Base film
A PET
B PEN
C PA
Accelerated tape edge wear coefficient, ka Tape edge wear coefficient, kt , measured in a tape drive Ratio of wear coefficients, ka /kt
2.94E−04 2.70E−04 1.09
2.58E−04 2.68E−04 0.96
4.09E−04 3.10E−04 1.32
J.H. Wang, F.E. Talke / Wear 259 (2005) 1362–1366
d =
kp Pn tH
1365
(2)
where the wear coefficient for the composite tape kp can be obtained as [3] kp =
E (t1 /t)(E1 /k1 ) + (t2 /t)(E2 /k2 ) + (t3 /t)(E3 /k3 )
(3)
In deriving equation (3), the assumption is made that the wear depth of each layer is the same (d = d1 = d2 = d3 ) and that the contact force is shared by all three layers, i.e., P = P1 + P2 + P3 . In Eq. (3), ti , ki , and Ei are the thickness, wear coefficient, and Young’s modulus of layer i, respectively, t is the total thickness of the tape, and E is the effective Young’s modulus of the tri-layer tape. With the assumption that each material of a tri-layer composite material subjected to an axial force will endure the same strain (ε1 = ε2 = ε3 ), the effective Young’s modulus can be defined as E=
3 ti i=1
t
Ei
(4)
To determine the wear coefficients for the base film, the magnetic layer, and the back coat, we have used a pin-ondisk tester (Fig. 7) and applied the following experimental procedure. First, a circular disk was made for each of the base materials shown in Table 1. The disks, of 90 mm diameter, were then mounted on the pin-on-disk tester. A ceramic wear pad (Al2 O3 ) was attached to a cantilever beam which was loaded against the disk surface. The radius of the wear pad was 12.7 mm. The average surface roughness Ra of the wear pad was 12 nm, and the Knoop hardness was 20.4 GPa. A load of 1 mN was applied as contact force on the disk surface. A speed of 1000 rpm was used, resulting in a linear speed of 4 m/s at the contact point. The sliding motion between the ceramic pad and the disk created a wear track on the base
Fig. 7. The pin-on-disk experiment used to determine wear coefficients: (a) photograph, (b) schematic, (c) 3D AFM image of a wear track, and (d) cross sectional area of a wear track.
Fig. 8. Wear coefficients of magnetic layer, back coat, and various base films.
film, as shown schematically in Fig. 7(b). The cross sectional area of the wear track was measured as a function of the number of wear passes (Fig. 7(c) and (d)) using an AFM. The wear coefficient of the base film was then determined using k=
AH Pn
(5)
where A is the cross sectional area of the wear track, H denotes the hardness of the film, P is the contact force, and n is the number of wear passes. For a pin-on-disk tester, the number of wear passes equals the total number of revolutions of the disk during a wear test. Using the same setup and Eq. (5), we have subsequently determined the wear coefficients of the magnetic layer and the back coat. 3.2. Results of tape edge wear prediction The wear coefficients for the magnetic layer, the back coat, and the various base films investigated are shown in Fig. 8. We observe that all wear coefficients are on the order of 10−4 . The magnetic layer has the largest wear coefficient while the PET film has the smallest wear coefficient. Using equation (3), the predicted wear coefficients kp for magnetic tapes with PET, PEN, and PA substrates were calculated (Table 3). We observe that all values of kp are on the order of 10−4 which is typical for the wear coefficients of base films [9]. We note that the predicted tape edge wear coefficient is highest for PA tape while it is lowest for PET tape. The ratio of the predicted composite wear coefficients from pin-on-disk testing to the measured wear coefficients using tape in an actual tape drive (Table 3) is in the range from 1.26 to 1.99, indicating that the predicted results and the actual measurements agree to within a factor of 2. Thus, it is apparent that tape edge wear can be predicted well using a composite wear coefficient kp obtained from pin-on-disk measurements.
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Table 3 Predicted tape edge wear coefficients and their comparisons Tape sample Base film
A PET
B PEN
C PA
Predicted tape edge wear coefficient, kp , from Eq. (3) Tape edge wear coefficient, kt , measured in a tape drive Ratio of wear coefficients, kp /kt
3.41E−04 2.70E−04 1.26
4.21E−04 2.68E−04 1.57
6.17E−04 3.10E−04 1.99
4. Discussion and conclusion The excellent agreement between tape edge wear measurements using an accelerated wear tester and an actual tape drive indicates that the mechanism of wear in both test systems is similar. Since an accelerated wear test using the drum tester can be completed in 6 min (12,000 wear passes), while a wear test using a commercial tape drive takes at least 5 days, it is apparent that the accelerated drum test set-up is an efficient and desirable tool to study and predict tape edge wear. To improve the data rates in advanced tape drives, work is in progress in the recording industry to increase the speed at which tape is moved in a tape drive. Currently, maximum speeds are approximating 4 m/s, while speeds of 10 m/s up to 20 m/s are under consideration. To determine tape edge wear for those situations, it is necessary to either use an accelerated wear tester, such as the drum tester described in this research, or predict wear using the composite wear coefficient method. Our initial tests have shown that the typical amount of tape edge wear is larger than 2 m at 10 m/s. An amount of tape edge wear of this magnitude can result in a large increase in lateral tape motion [1]. This, in turn, causes increased track misregistration errors [10]. Thus, tape edge wear at high speeds will be an important design issue in future tape drives. In fact, severe tape edge wear of PEN tape is observed at speeds above 10 m/s, due to thermal degradation of the substrate, using contact forces as encountered in typical present day tape drives. Since tape edge wear increases with increasing contact force [2], a smaller contact force is desirable to reduce tape edge wear at high speeds. However, since the thickness of future tapes will decrease, the contact force must be reduced substantially to keep the contact pressure on the edge of a tape in the range encountered presently. The predicted tape edge wear coefficient of a coated magnetic tape was found to be on the order of 10−4 . This value is similar to the wear coefficient of the substrate alone and indicates that tape edge wear of a typical magnetic tri-layer tape is determined predominately by the wear coefficient of its base film. This can be understood with reference to Eq. (3). Since the thickness of the magnetic layer t1 and the back coat t3 is much smaller than the total tape thickness t, the contribution of the second term in the denominator overshadows those of the other terms as long as the ratios of Ei /ki for all three terms in the denominator are comparable. This situation will change in the future. For example, if a thinner substrate is developed, the ratio of substrate thickness to tape thickness
will be reduced. In this situation, prediction of tape edge wear is still possible, but accurate data need to be obtained for all individual layers. Thus, the methodology developed in this paper will become of even more importance. The ratio of the predicted tape edge wear coefficient kp to the tape edge wear coefficient kt in an actual tape drive, kp /kt , was seen to be within the range from 1 to 2 (Table 3). This indicates that tape edge wear of composite tape in an actual drive can be predicted using Eq. (2) with wear coefficient measurements from a pin-on-disk tester. One potential limitation of determining a composite tape edge wear coefficient from pin-on-disk measurements should be pointed out, however. A pin-on-disk tester creates wear on the surface of the tape while tape guides create wear on the edge of the tape. Therefore, a pin-on-disk tester is suitable to predict tape edge wear only for those materials for which the wear coefficient in the plane is similar to the wear coefficient along the edge. This assumption is true for isotropic materials but needs to be modified for anisotropic materials.
Acknowledgement We would like to thank Tsugihiro Doi of Hitachi Maxell for preparing the tape samples used in this study.
References [1] J.H. Wang, F.E. Talke, Tape edge wear and its relationship to lateral tape motion, Microsyst. Technol., in press. [2] J.H. Wang, R.J. Taylor, F.E. Talke, Lateral motion and edge wear of magnetic tapes, Tribol. Int. 36 (2003) 423–431. [3] J.H. Wang, F.E. Talke, Investigation of tape edge wear of magnetic recording tape, Tribol. Int., in press. [4] A.V. Goldade, B. Bhushan, Measurement and origin of tape edge damage in a linear tape drive, Tribol. Lett. 14 (2003) 167– 180. [5] J.J. Topoleski, B. Bhushan, Qualitative and quantitative evaluation of the quality of factory-slit magnetic tape edges, J. Info. Storage Proc. Syst. 2 (2000) 109–116. [6] B.L. Weick, B. Bhushan, Characterization of magnetic tapes and substrates, IEEE Trans. Magn. 32 (4) (1996) 3319–3322. [7] B.L. Weick, B. Bhushan, Shrinkage and viscoelastic behavior of alternative substrates for magnetic tapes, IEEE Trans. Magn. 31 (6) (1995) 2937–2939. [8] B.L. Weick, B. Bhushan, The tribological and dynamic behavior of alternative magnetic tape substrates, Wear 190 (1995) 28–43. [9] A.D. Sarkar, Friction and Wear, Academic Press, New York, 1980. [10] D.B. Richards, M.P. Sharrock, Key issues in the design of magnetic tapes for linear systems of high track density, IEEE Trans. Magn. 34 (4) (1998) 1878–1882.
Case study
Measurement and prediction of tape edge wear using accelerated wear testing J.H. Wang ∗ , F.E. Talke Center for Magnetic Recording Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0401, USA Received 2 September 2004; received in revised form 3 December 2004; accepted 31 January 2005 Available online 10 May 2005
Abstract A rotating drum tester was developed to investigate accelerated tape edge wear. The results from accelerated wear measurements were found to be in good agreement with tape edge wear measurements from an actual tape drive. The wear coefficients for the magnetic layer, the back coat, and the base film of typical tapes were determined on a pin-on-disk tester and used to predict the tape edge wear coefficient of a tri-layer composite tape. Predicted tape edge wear results were found to be in good agreement with actual tape edge wear results in a tape drive. © 2005 Elsevier B.V. All rights reserved. Keywords: Tape edge wear; Accelerated wear; Magnetic tapes; Wear coefficient
1. Introduction Sliding motion between the tape guides and the edge of a magnetic tape in a tape drive causes tape edge wear. Wang and Talke [1] have found that tape edge wear causes an increase of lateral tape motion, thereby limiting the maximum track density that can be achieved on tape. Tape edge wear has been studied as a function of contact force, tape speed, tape tension, tape guide surface roughness (Ra ), and tape substrate using atomic force microscopy (AFM) [2,3]. The edge quality of new and worn tapes has been characterized by Bhushan and co-workers [4,5]. Recently, tapes with improved substrates and better mechanical properties have become available [6–8]. Edge wear of these generally thinner tapes needs to be studied since tape edge wear is an important concern for the use of these new tapes in future tape drives. The measurement of tape edge wear in a tape drive is a time-consuming task since the maximum tape speed in a tape drive is only on the order of 4 m/s and the tape has to be moved forward and backward during a wear test. Thus, an apparatus to accelerate ∗
Corresponding author. Tel.: +1 8585347578; fax: +1 8585342720. E-mail address: [email protected] (J.H. Wang).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.048
tape edge wear is highly desirable. In addition, prediction of tape edge wear prior to manufacturing of a new type of tape is important since it can save time and development costs. In this paper, a rotating drum tester is used to study acceleration of tape edge wear as a function of tape substrate and speed for present day tape substrates (polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyaramide (PA)). In addition, tape edge wear of a tri-layer tape is predicted from wear measurements of the base film, the magnetic layer, and the back coat using a pin-on-disk tester. The predicted tape edge wear results are compared to tape edge wear results obtained using a commercial tape drive.
2. Accelerated wear testing 2.1. Experiment and procedure A rotating drum tester shown in Fig. 1 was designed and built to accelerate tape edge wear. The tester consists of a precision drum around which a single layer of tape can be mounted. A cantilever beam with a wear pad attached at the tip of the beam is forced against the protruding edge of the
J.H. Wang, F.E. Talke / Wear 259 (2005) 1362–1366
1363
Fig. 1. (a) Schematic and (b) photograph of the rotating drum tester.
tape on the drum resulting in sliding contact between the tape edge and the wear pad. The wear pad is made of Al2 O3 –TiC (70–30%) with an average surface roughness Ra of 14 nm and a Knoop hardness of 23.5 GPa. The following procedure was used to study tape edge wear. The circumference of the 38 mm diameter drum was first wrapped with a single layer of new tape under 1 N tension. A second layer of 40 m thick polyester tape was then attached under the same tension around the first layer. To ensure that tape edge wear occurred only on the first layer, care was taken so that the top edge of the second layer was approximately 1 mm lower than the top edge of the first layer. The drum was then attached to a spindle rotating at 2000 rpm resulting in a speed of 4 m/s at the tape edge. A force of 20 mN was used to load the slider against the tape edge (Fig. 1(a)). The sliding motion between the stationary slider and the rotating tape causes wear of the tape edge. Tape edge wear was measured using the method described by Wang and Talke [3]. In this method, reference indentations are made in the surface of the tape edge prior to testing. The depth change of these indentations is then measured as a function of the number of wear passes using an AFM. For a rotating drum tester, the number of wear passes equals the
Fig. 2. Tape edge wear as a function of substrate using accelerated wear measurement (drum tester) (20 mN and 4 m/s).
number of revolutions of the drum during a wear test. For a tape drive, each pass of the tape under the wear pad is counted as one wear pass. The depth change of the indentations equals the amount of tape edge wear. The tape edge wear coefficient k is determined from dtH (1) Pn where d is the tape edge wear (determined from the depth change of the indentations), t denotes the tape thickness, H is the hardness of the tape, P is the contact force, and n is the number of wear passes. Three different base films (PET, PEN, and PA) (Table 1) coated with the same magnetic layer and back coat were tested. k=
2.2. Results of accelerated wear testing Accelerated tape edge wear results as a function of tape substrate are shown in Fig. 2, obtained with the drum tester. We observe that PEN tape exhibits the least amount of tape edge wear while PA tape shows the largest amount of tape
Table 1 The specifications of tape samples
Tape sample Base film
A PET
B PEN
C PA
Base Young’s modulus
Machine direction Transverse direction
MPa MPa
6700 5000
8000 6500
11000 15000
Coating layer Young’s modulus
Magnetic layer Back coat
MPa MPa
15200 9300
15200 9300
15200 9300
Thickness
Total thickness Base film Magnetic layer Back coat
m m m m
7.8 6.1 1.32 0.4
8.0 6.4 1.11 0.5
6.1 4.5 1.24 0.4
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J.H. Wang, F.E. Talke / Wear 259 (2005) 1362–1366
Fig. 5. Photographs of tape edge of PEN tape: (a) new tape, (b) worn tape after 12k wear passes at 8 m/s, and (c) worn tape after 4k wear passes at 10 m/s. Fig. 3. Tape edge wear as a function of substrate using a commercially available tape drive (20 mN and 4 m/s).
tape speed. Clearly, tape edge wear increases with increasing tape speed. In particular, at 10 m/s, tape edge wear is seen to reach 2.35 m after only 2000 wear passes. At speeds above 10 m/s, thermal degradation of the tape edge was observed, resulting in large scale damage of the tape edge. This effect can be seen by looking at the photographs in Fig. 5(c), which show the degradation of the tape edge after 4000 wear passes at 10 m/s. 3. Prediction of tape edge wear 3.1. Experiment and procedure
Fig. 4. Accelerated tape edge wear of PEN tape as a function of tape speed.
edge wear. In Fig. 3, tape edge wear is shown using a commercial tape drive with the same Al2 O3 –TiC wear pad as used in the drum tester [3]. Similar to Fig. 2, we observe that PA tape has the largest amount of tape edge wear while PEN tape has the least amount of tape edge wear. The tape edge wear coefficients for PET, PEN, and PA tape can be calculated using Eq. (1). Table 2 shows the average wear coefficients ka obtained from the accelerated drum tester and the average wear coefficients kt obtained using a commercial tape drive. We observe that the tape edge wear coefficient of PA tape is larger than that of PET and PEN tape. In addition, we observe that the ratio of wear coefficients ka /kt lies between 0.96 and 1.32, i.e., the wear coefficients measured with the drum tester are on the same order as the wear coefficients obtained in an actual tape drive. Accelerated wear test results from the drum tester are shown in Fig. 4 for tape edge wear of PEN tape as a function of
In the previous experiments, tape edge wear and tape edge wear coefficients were determined using edge wear measurements of actual magnetic tape. An alternative way to obtain a quantitative value of tape edge wear is to calculate the wear coefficient of tape based on the wear coefficient measurements of each individual layer and use this overall wear coefficient for the tape to calculate and predict tape edge wear. In this approach, it is necessary to first determine the wear coefficients of the base film, the magnetic layer, and the back coat, and then predict tape edge wear for the composite tape (Fig. 6) using
Fig. 6. The schematic of a tri-layer tape. Table 2 Tape edge wear coefficients using drum tester Tape sample Base film
A PET
B PEN
C PA
Accelerated tape edge wear coefficient, ka Tape edge wear coefficient, kt , measured in a tape drive Ratio of wear coefficients, ka /kt
2.94E−04 2.70E−04 1.09
2.58E−04 2.68E−04 0.96
4.09E−04 3.10E−04 1.32
J.H. Wang, F.E. Talke / Wear 259 (2005) 1362–1366
d =
kp Pn tH
1365
(2)
where the wear coefficient for the composite tape kp can be obtained as [3] kp =
E (t1 /t)(E1 /k1 ) + (t2 /t)(E2 /k2 ) + (t3 /t)(E3 /k3 )
(3)
In deriving equation (3), the assumption is made that the wear depth of each layer is the same (d = d1 = d2 = d3 ) and that the contact force is shared by all three layers, i.e., P = P1 + P2 + P3 . In Eq. (3), ti , ki , and Ei are the thickness, wear coefficient, and Young’s modulus of layer i, respectively, t is the total thickness of the tape, and E is the effective Young’s modulus of the tri-layer tape. With the assumption that each material of a tri-layer composite material subjected to an axial force will endure the same strain (ε1 = ε2 = ε3 ), the effective Young’s modulus can be defined as E=
3 ti i=1
t
Ei
(4)
To determine the wear coefficients for the base film, the magnetic layer, and the back coat, we have used a pin-ondisk tester (Fig. 7) and applied the following experimental procedure. First, a circular disk was made for each of the base materials shown in Table 1. The disks, of 90 mm diameter, were then mounted on the pin-on-disk tester. A ceramic wear pad (Al2 O3 ) was attached to a cantilever beam which was loaded against the disk surface. The radius of the wear pad was 12.7 mm. The average surface roughness Ra of the wear pad was 12 nm, and the Knoop hardness was 20.4 GPa. A load of 1 mN was applied as contact force on the disk surface. A speed of 1000 rpm was used, resulting in a linear speed of 4 m/s at the contact point. The sliding motion between the ceramic pad and the disk created a wear track on the base
Fig. 7. The pin-on-disk experiment used to determine wear coefficients: (a) photograph, (b) schematic, (c) 3D AFM image of a wear track, and (d) cross sectional area of a wear track.
Fig. 8. Wear coefficients of magnetic layer, back coat, and various base films.
film, as shown schematically in Fig. 7(b). The cross sectional area of the wear track was measured as a function of the number of wear passes (Fig. 7(c) and (d)) using an AFM. The wear coefficient of the base film was then determined using k=
AH Pn
(5)
where A is the cross sectional area of the wear track, H denotes the hardness of the film, P is the contact force, and n is the number of wear passes. For a pin-on-disk tester, the number of wear passes equals the total number of revolutions of the disk during a wear test. Using the same setup and Eq. (5), we have subsequently determined the wear coefficients of the magnetic layer and the back coat. 3.2. Results of tape edge wear prediction The wear coefficients for the magnetic layer, the back coat, and the various base films investigated are shown in Fig. 8. We observe that all wear coefficients are on the order of 10−4 . The magnetic layer has the largest wear coefficient while the PET film has the smallest wear coefficient. Using equation (3), the predicted wear coefficients kp for magnetic tapes with PET, PEN, and PA substrates were calculated (Table 3). We observe that all values of kp are on the order of 10−4 which is typical for the wear coefficients of base films [9]. We note that the predicted tape edge wear coefficient is highest for PA tape while it is lowest for PET tape. The ratio of the predicted composite wear coefficients from pin-on-disk testing to the measured wear coefficients using tape in an actual tape drive (Table 3) is in the range from 1.26 to 1.99, indicating that the predicted results and the actual measurements agree to within a factor of 2. Thus, it is apparent that tape edge wear can be predicted well using a composite wear coefficient kp obtained from pin-on-disk measurements.
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Table 3 Predicted tape edge wear coefficients and their comparisons Tape sample Base film
A PET
B PEN
C PA
Predicted tape edge wear coefficient, kp , from Eq. (3) Tape edge wear coefficient, kt , measured in a tape drive Ratio of wear coefficients, kp /kt
3.41E−04 2.70E−04 1.26
4.21E−04 2.68E−04 1.57
6.17E−04 3.10E−04 1.99
4. Discussion and conclusion The excellent agreement between tape edge wear measurements using an accelerated wear tester and an actual tape drive indicates that the mechanism of wear in both test systems is similar. Since an accelerated wear test using the drum tester can be completed in 6 min (12,000 wear passes), while a wear test using a commercial tape drive takes at least 5 days, it is apparent that the accelerated drum test set-up is an efficient and desirable tool to study and predict tape edge wear. To improve the data rates in advanced tape drives, work is in progress in the recording industry to increase the speed at which tape is moved in a tape drive. Currently, maximum speeds are approximating 4 m/s, while speeds of 10 m/s up to 20 m/s are under consideration. To determine tape edge wear for those situations, it is necessary to either use an accelerated wear tester, such as the drum tester described in this research, or predict wear using the composite wear coefficient method. Our initial tests have shown that the typical amount of tape edge wear is larger than 2 m at 10 m/s. An amount of tape edge wear of this magnitude can result in a large increase in lateral tape motion [1]. This, in turn, causes increased track misregistration errors [10]. Thus, tape edge wear at high speeds will be an important design issue in future tape drives. In fact, severe tape edge wear of PEN tape is observed at speeds above 10 m/s, due to thermal degradation of the substrate, using contact forces as encountered in typical present day tape drives. Since tape edge wear increases with increasing contact force [2], a smaller contact force is desirable to reduce tape edge wear at high speeds. However, since the thickness of future tapes will decrease, the contact force must be reduced substantially to keep the contact pressure on the edge of a tape in the range encountered presently. The predicted tape edge wear coefficient of a coated magnetic tape was found to be on the order of 10−4 . This value is similar to the wear coefficient of the substrate alone and indicates that tape edge wear of a typical magnetic tri-layer tape is determined predominately by the wear coefficient of its base film. This can be understood with reference to Eq. (3). Since the thickness of the magnetic layer t1 and the back coat t3 is much smaller than the total tape thickness t, the contribution of the second term in the denominator overshadows those of the other terms as long as the ratios of Ei /ki for all three terms in the denominator are comparable. This situation will change in the future. For example, if a thinner substrate is developed, the ratio of substrate thickness to tape thickness
will be reduced. In this situation, prediction of tape edge wear is still possible, but accurate data need to be obtained for all individual layers. Thus, the methodology developed in this paper will become of even more importance. The ratio of the predicted tape edge wear coefficient kp to the tape edge wear coefficient kt in an actual tape drive, kp /kt , was seen to be within the range from 1 to 2 (Table 3). This indicates that tape edge wear of composite tape in an actual drive can be predicted using Eq. (2) with wear coefficient measurements from a pin-on-disk tester. One potential limitation of determining a composite tape edge wear coefficient from pin-on-disk measurements should be pointed out, however. A pin-on-disk tester creates wear on the surface of the tape while tape guides create wear on the edge of the tape. Therefore, a pin-on-disk tester is suitable to predict tape edge wear only for those materials for which the wear coefficient in the plane is similar to the wear coefficient along the edge. This assumption is true for isotropic materials but needs to be modified for anisotropic materials.
Acknowledgement We would like to thank Tsugihiro Doi of Hitachi Maxell for preparing the tape samples used in this study.
References [1] J.H. Wang, F.E. Talke, Tape edge wear and its relationship to lateral tape motion, Microsyst. Technol., in press. [2] J.H. Wang, R.J. Taylor, F.E. Talke, Lateral motion and edge wear of magnetic tapes, Tribol. Int. 36 (2003) 423–431. [3] J.H. Wang, F.E. Talke, Investigation of tape edge wear of magnetic recording tape, Tribol. Int., in press. [4] A.V. Goldade, B. Bhushan, Measurement and origin of tape edge damage in a linear tape drive, Tribol. Lett. 14 (2003) 167– 180. [5] J.J. Topoleski, B. Bhushan, Qualitative and quantitative evaluation of the quality of factory-slit magnetic tape edges, J. Info. Storage Proc. Syst. 2 (2000) 109–116. [6] B.L. Weick, B. Bhushan, Characterization of magnetic tapes and substrates, IEEE Trans. Magn. 32 (4) (1996) 3319–3322. [7] B.L. Weick, B. Bhushan, Shrinkage and viscoelastic behavior of alternative substrates for magnetic tapes, IEEE Trans. Magn. 31 (6) (1995) 2937–2939. [8] B.L. Weick, B. Bhushan, The tribological and dynamic behavior of alternative magnetic tape substrates, Wear 190 (1995) 28–43. [9] A.D. Sarkar, Friction and Wear, Academic Press, New York, 1980. [10] D.B. Richards, M.P. Sharrock, Key issues in the design of magnetic tapes for linear systems of high track density, IEEE Trans. Magn. 34 (4) (1998) 1878–1882.