Characterization of Nano-wear Journal Mechanisms of ELECTRONIC of Hard MATERIALS, Disk Coatings Vol. 30, No. 5, 2001

Regular Issue Paper 503

Characterization of Nano-wear Mechanisms of Hard Disk Coatings WOO SEOK KIM,1 JANG-KYO KIM,2,3 and PYUNG HWANG1 1.—Yeungnam University, School of Mechanical Engineering, Gyongsan, 712-749, Korea. 2.—Hong Kong University of Science and Technology, Department of Mechanical Engineering, Clear Water Bay, Hong Kong. 3.—e-mail: [email protected]

The wear mechanisms of carbon coated computer hard disks with laser-textured (LT) and mechanically-textured (MT) surfaces were characterized after contact start/stop (CSS) cyclic tests. Various analytical and mechanical testing techniques were employed to study the changes in topography, roughness, chemical elements, mechanical properties, and friction characteristics of the coating and lubricant. These techniques include: the atomic force microscopy (AFM), continuous nano-indentation test, nano-scratch test, time-of-flight secondary ion mass spectroscopy (TOF-SIMS), and Auger electron spectroscopy (AES). The CSS test at 15 k cycles resulted in tangible reductions of surface roughness of approximately 3.0 nm and 5.8 nm, respectively, for the LT bump and MT zone. The elastic modulus and hardness values increased after the CSS test, indicating strain hardening of the top coating layer. A critical load was identified for adhesion failure between the magnetic layer and the Ni-P layer. The TOF-SIMS analysis also revealed reductions in the intensities of all lubricant elements, indicating wear of the lubricant applied on the disk surface. All foregoing results confirm the usefulness of the characterization techniques employed to detect the subtle changes in disk surface characteristics. Key words: Hard disk coating; laser-texture, mechanical-texture, nano-wear mechanisms; interface failure

INTRODUCTION As the recording density of the computer hard disk continues to increase, the flying height of the recording head over the data zone needs to be decreased. The reduction in flying height inevitably introduces many unwanted side effects: direct contact between the head and disk often leads to severe wear of the surface coating and an increase in stiction and friction with increasing the contact start/stop (CSS) cycles, degrading the function of the head/disk interface. The introduction of laser texture (LT) technique makes it possible to control the topography of the landing zone by creating discrete topographical features with round, crater-like protrusions, usually known as bumps, on the contact surface.1–4 The disks containing LT zones can offer a low cost, precise control of the surface topography and zone position. The presence of LT bumps was shown to improve significantly the tribo(Received June 15, 2000; accepted January 25, 2001)

logical performance in contact start/stop testing and glide avalanche prediction.5 It was found that stiction remained almost constant and the statistical wear rate was much lower for the hard disk with LT bumps than for those with mechanically textured (MT) surfaces for the same range of CSS cycles, due to the inherently rough MT zone surface.6 The stiction of the rough MT surface was found to increase with increasing the CSS cycles, whereas the stiction of LT disks remained constant. This was despite the fact that the acoustic emission signal of LT disks was generally higher than that of MT disks. The shape, radius, height, and location of these bumps can be accurately controlled by controlling laser parameters, such as laser pulse energy, pulse width, and laser wave length.7 A smaller focus spot size and shorter pulse width produced a smaller radius of rim curvature, which, in turn is beneficial in reducing stiction.4 Significant progress has been made in the nanotribology of magnetic coating systems.8 Sliding wear mechanisms that are potentially operative in contact 503

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Fig. 1. Microphotograph of cross-section of hard disk coatings.

recording are sensitive to the mechanical properties of the surface coatings, such as shear strength, hardness, elastic modulus, and fracture toughness, as well as surface roughness, topography, and friction. These properties of various coatings have been characterized using state-of-the-art techniques, such as nanoindentation, nano-scratch techniques, atomic force microscopy (AFM);9–11 and stylus profilers or noncontact optical profilers.12,13 In addition to the above techniques, Raman spectroscopy was also successfully used to study the structures of ultra-thin carbon films of thickness down to 1 nm, as well as to identify the non-uniformity in wear rate along the air bearing surface.14 Surface analytical techniques, such as the time-of-flight secondary ion man spectroscopy (TOF SIMS), x-ray photoelectron spectroscopy (XPS), Auger depth profiling,15–17 as well as point contact microscopy,18 have been widely used to evaluate the wear mechanisms and durability of various ultrathin films and textures. The present work is a continuation of our previous study19 on anisotropic tribological and mechanical properties of mechanically textured disk surfaces. The nano-wear, friction, coating failure characteristics, and other tribological properties of both the laser and mechanically textured disks are characterized after 15 k cycles of CSS test. Special emphasis is placed on evaluation of the subtle changes in surface chemistry and adhesion failure mechanisms between the coatings. EXPERIMENTAL PROCEDURE Materials and Contact Start/Stop Wear Test Two types of textured disk surfaces were employed in this study: LT disks containing arrays of small bumps introduced on the start/stop zone; and MT disks. Both disks were made with Al-Mg alloy 5086 substrates (95.4% Al, 4% Mg, 0.4% Mn, and 0.15% Cr) that were electroless-plated with a 10–20 µm thick Ni-P layer to improve the surface hardness.20,21 A magnetic Co-Cr-Ta layer of 70–100 nm in thickness was deposited, on top of which an amorphous carbon

b Fig. 2. Schematics of (a) contact start/stop wear tester and (b) acceleration profile of test.

film of 20–30 nm thickness was coated using the radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) method. On the carbon film, a Zdol-2500 type lubricant with a thickness of approximately 18 ± 4 Å was applied to protect the magnetic layer, according to the hard disk supplier. Figure 1 shows a transmission electronic microphotograph of the cross-section of the hard disk coatings. For the LT disks, crater-shaped bumps were created in the annular contact zone of 3.5 mm in width, with the bump spacing being 35 ± 2 µm and 30 ± 2 µm, respectively, in the radial and circumferential directions of disk. The nominal rim-to-rim diameter of bump was 6.2 ± 0.8 µm, while the nominal rim-to-base height was 20 nm. The wear tests were conducted with the standard CSS testing machine. Figure 2 presents the schematics of a CSS tester, and the acceleration profile used in the experiment. The slider was of the tri-pad type with negative pressure (Voyager3, Samsung Electromechanics Co.) and 34.4 mN of preload. Each CSS cycle was 11.2 sec at a maximum spindle speed of 5400 rpm, which corresponded to a linear velocity of approximately 7.1 m/s. Disks were subjected to 15 K cycles in a clean room at ambient temperature, which was followed by examination using the characterization techniques as described below.

Characterization of Nano-wear Mechanisms of Hard Disk Coatings

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Fig. 3. AFM images of laser textured bump (a) before and (b) after the CSS wear test.

Fig. 4. AFM images of mechanically textured zone (a) before and (b) after the CSS test.

Atomic Force Microscopy and Surface Roughness Analysis

the LT bump and MT zone were treated statistically based on the Weibull cumulative distribution function, F(x), and the probability density function, f(x):19

The surface roughness was measured and the topographic images of disks were taken on an atomic force microscope (AFM, TMX2000, Discovery Probe microscope, TopoMetrix). A piezoelectric (PZT) tube scanner was used to scan the sample in three-dimensions with nanometer resolution at a rate of 40 µm/s. Ten 20 µm × 20 µm square areas inside the MT zone were scanned and over 30000 data points were used for the surface height analysis. For the LT zone, eight bumps were selected for scanning from eight different locations covering the whole annular contact zone. The measurement of the surface roughness of the LT bumps were much more difficult than the flat surfaces of the MT zone, because the rim heights were not uniform between different bumps and the bump height did not relate to the texture direction around the bump. Further, the LT bumps are highly asymmetrical in cross-sectional shape, indicating that only certain parts of the rim would be in contact with the tester head. Therefore, a new method was devised to overcome these problems: the bump top was scanned following lines through the center of bump at 10° intervals over a distance of 20 µm, and a total of 576 bump asperities were used for the analyses to measure the ‘rim-to-base height’ as the surface roughness of the LT zone. The surface roughness data thereby obtained for

  

(1)

  x b b b−1 x exp −   θb   θ  

(2)

  x F(x) = 1 − exp−    θ  f ( x)

b

where b is the shape parameter (or Weibull modulus), and θ is the scale parameter. Nano-Indentation and Nano-Scratch Tests The nano-indentation and nano-scratch tests were performed to measure the hardness and elastic modulus as well as the adhesion characteristics using a nano-indenter (Nanoindenter II by Nano Instrument Inc.).22 The indenter is equipped with a three-sided pyramidal Berkovich diamond indenter tip with a tip radius of less than 100 nm. During indentation, the instrument monitors and records the dynamic load and displacement of the indenter tip. The ‘continuous stiffness mode’ was used where an incremental load was applied continuously until the displacement reached a desired value of 500 nm.23 The output response provided the stiffness and contact area data without discrete unloading cycles. Indentation was

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Fig. 5. Typical surface profiles of (a), (c) laser textured bump, and (b), (d) mechanically textured zone before and after the CSS wear test.

performed before and after the CSS test at two different locations for a given LT bump, namely, one on the top of the bump and the other outside the bump. The nano-scratch tests were conducted to study the friction characteristics and coating-substrate adhesion of the disk surface before and after the CSS test. The ‘ramp load mode’ was employed with an increasing load from 0 to 10 mN at a rate of 100 µN/s. The Berkovich diamond tip was scratched in the faceforward direction, i.e. with the face of the tip facing the scratch direction. After the scratch tests, the topographic images of scratch deformation and depth were generated using the AFM. Surface Analysis The time-of-flight secondary ion mass spectroscopy

(TOF-SIMS, PHI 7200 from Physical Electronics, Inc.) and Auger electron spectroscopy (AES, PhI 5600 Physical Electronics Multi-Technique System) were employed to identify the changes in surface chemistry and atomic compositions after the CSS wear test. The sampling depth of TOF-SIMS was in the range of 10 Å to 20 Å since only the species in the outermost region of a sample had sufficient energy to overcome the surface binding energy and leave the sample. The AES instrumentation involved an ultra-high vacuum (UHV) system, electron gun for target excitation and an electron spectrometer for energy analysis of emitted electrons. The Auger analysis was conducted on the disk surface for 12 min. in a depth-profiling mode. The primary electron energy used was 5.0 keV with a beam current of 0.5 µA, and the beam was focused to

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Fig. 6. (a), (b) Weibull cumulative and (c), (d) probability density distributions of surface roughness.

a spot diameter of about 2 mm. If any wear or damage has occurred at the contact zone, the changes can be detected based on the surface chemistry. RESULTS AND DISCUSSION Surface Roughness Typical AFM 3D topographic images of the LT bump and MT zone before and after the CSS test are presented in Figs. 3 and 4, respectively. The corresponding surface height profiles obtained from the AFM are presented in Fig. 5. There were some clear indications of wear after the CSS test depending on the type of textures. A number of small debris—most likely of carbon coating—were detected inside the bump (Fig. 3b) and the bump rims were moderately

flattened (see Fig. 5a and c), indicating that nanoscale wear took place in the LT bumped sliding zone. Comparisons of the surface height profiles in Fig. 5b and d also suggests that the roughness of MT zone was reduced to a certain extent after the CSS test. The cumulative probability distributions according to the Weibull and probability density distributions presented in Fig. 6 further support the above findings. The Weibull parameters, b and θ, and the mean values of surface roughness are summarized in Table I. It is obvious that the surface roughness decreased while the data scattering remained almost unchanged after the CSS test. The changes in surface roughness parameters were more remarkable for the MT zone than for the LT bump, indicating more serious wear in the former surface. The mean wear rates obtained

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Kim, Kim, and Hwang Table I. Weibull Parameters and Mean ± Standard Deviation (SD) for LT Bump Rim-to-Base Height and MT Zone Surface Roughness LT Bump Rin-to-Base Height

Weibull Parameters Shape parameter, b Scale Parameter, θ (nm) Mean ± SD (nm)

MT Zone Surface Roughness

Before CSS Test

After CSS Test

Before CSS Test

After CSS Test

5.3 27.4 23.8 ± 6.5

4.9 24.5 20.8 ± 5.6

4.9 77.1 64.4 ± 13.8

5.3 69.5 58.6 ± 13.6

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b

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Fig. 7. Elastic modulus of (a), (b) laser textured bump, and (c), (d) mechanically textured zone as a function of indentation depth.

from the changes in surface height and rim-to-base height were approximately 5.8 nm and 3.0 nm, respectively, for the MT zone and LT bump. This obser-

vation is consistent with the previous finding6 that attributed the result to a higher stiction force acting on the MT zone than on the LT zone. The above wear

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Fig. 8. Hardness of (a), (b) laser textured bump, and (c), (d) mechanically textured zone as a function of indentation depth.

rate for the MT zone is considered to be quite moderate compared to the experimental result of 11 nm to 14 nm wear after 15 k cycles of CSS test for a similar type of hard disk coating under similar testing conditions.24 Elastic Modulus and Hardness The continuously varying elastic modulus and hardness values with increasing indentation depth obtained from the nano-indentation tests are presented in Figs. 7 and 8, respectively. Experimental data obtained from at least three runs of the test are shown in these figures, where the results from regression analyses are also superimposed. It is interesting to note that for the LT bump, these mechanical properties were slightly higher at the bump top than the outside area surrounding the bump (Figs. 7a and 8a).

The difference in the mechanical properties between the bump top and base results mainly from the heat treatment of the base Al-Mg alloy during the laser texturing process. The differences in these properties between the bump top and the outside became wider after the CSS test, especially at shallow indentation depths, say less than about 20–30 nm (Figs. 7b and 8b). The elastic modulus and hardness values corresponding to indentation depths lower than about 100 nm were much higher after CSS test (compare Figs. 7c and 8c with Figs. 7d and 8d). All these observations indicate that the CSS cycles caused the carbon overcoat to become denser and harder, giving rise to high elastic modulus and hardness values. It is also likely that the magnetic coating and the Al-Mg alloy substrate underwent strain hardening due to the

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Fig. 9. Coefficient of friction as a function of normal load (a) for laser textured bump before and after CSS test; and (b) for mechanically textured zone before and after the CSS test.

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Fig. 10. Scratch depth as a function of normal load (a) for laser textured bump before and after CSS test; and (b) for mechanically textured zone before and after the CSS test.

contact stresses and the generation of internal stress fields.25 The finite tip radius of the indenter used in the indentation test may also have an influence on the resistance to penetration of the indenter, causing the hardness obtained for very shallow depths to be dependent on surface roughness. The changes of hardness and elastic modulus due to the CSS cycles were even more significant for the MT zone than the LT bump. This observation is quite congruent with the more significant reduction in the surface roughness of the MT zone, as discussed above.

It appears that the effect of plastic deformation was restricted only to the top layer—less than approximately 150∼200 nm from the surface—as indicated by the similar properties at larger indentation depths measured before and after CSS test. Coefficient of Friction and Scratch Depth The coefficients of friction measured from the nanoscratch test are plotted as a function of normal load, as shown in Fig. 9. They increased rapidly at the initial ramp loading, which was followed by a slow

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Table II. Changes in Intensities of Elemental Mass after CSS Test, Measured from TOF-SIMS Intensities for LT Bump Element (m/z) C (12) C2H7 (31) CFO (47) CF2 (50) CF3 (69) C2F3 (81) C2F4 (100) C2F5 (119)

Intensities for MT Zone

Before CSS Test

After CSS Test

% Reduction

Before CSS Test

After CSS Test

% Reduction

5.1 6.0 1.9 2.3 2.3 0.8 4.2 6.5

4.6 5.1 1.5 1.9 1.8 0.5 3.1 3.9

10 15 21 17 18 37 26 40

4.8 6.3 2.5 2.9 3.1 0.8 6.0 9.3

4.1 5.9 2.0 2.5 2.4 0.7 4.3 6.5

15 6 20 14 23 12 28 30

quite consistent with the higher initial elastic modulus and hardness values obtained after the CSS test (Figs. 7 and 8). Surface Elemental Analysis

Fig. 11. Typical TOF-SIMS mass spectrum of laser textured bump before the CSS test.

increase with further increase in the applied load. Of note is that there was a sudden surge or drop of coefficient of friction, when the applied load reached approximately 9.5 mN, for all conditions studied. Basically the identical results were obtained from the scratch depth versus normal load plots, as shown in Fig. 10. The sudden increase or drop of coefficient friction is thought to be a direct reflection of the changes in vertical displacement of the indenter tip while scratching in the ramp load mode. The load corresponded to a scratch depth of about 140 nm, which is roughly equal to the distance from the surface to the interface between Co-Cr-Ta magnetic layer and Ni-P layer (Fig. 1). Judging from this information, the abrupt change in indenter tip displacement was most likely attributed to the failure of the interface bond between these layers. It is also worth noting that for both the LT bump and MT zone, the magnitudes of these changes were higher after the CSS test, probably associated with the strain hardening of these layers, as explained earlier. This result seems

A typical mass spectrum obtained from a TOFSIMS measurement is illustrated in Fig. 11, and the changes in the intensities of various species after the CSS tests are summarized in Table II. It is noted that for both the LT bump and MT zone, the intensities of C(12), C2H7(31), CFO(47), CF2(50), CF3(69), C2F3(81), C2F4(100), C2F5(119) all decreased substantially after the CSS tests, especially so for the latter three species. These species are identified as the components of Z-Dol, the lubricant applied on the surface of disk overcoat. This means that the reduction was mainly attributed to the local depletion of the lubricant. It should be mentioned here that C(12) is not from the carbon overcoat, but belongs to the lubricant. A similar conclusion was also drawn previously26 for the reduction of TOF-SIMS signals from C2F 5(119) due to the reduction of the lubricant level on the bump after 25 k cycles of CSS test. Meanwhile, the AES spectra did not reveal significant changes between measurements taken before and after the CSS tests for both the LT and MT zones. This does not mean that there was no wear taking place, but rather indicates that the AES was not particularly sensitive to detect the type of wear that occurred. This seems because the wear was limited mainly to the lubricant and some reduction of surface roughness due to the internal plastic deformation occurring in the underlying coating materials. CONCLUDING REMARKS To assess the nano-wear mechanisms of the carbon coating and lubricant applied on magnetic recording media after contact start/stop cycles, various mechanical and analytical measurements have been performed, including atomic force microscopy, nanoindentation and nano-scratch tests, time-of-flight secondary ion mass spectroscopy, and Auger electron spectroscopy. The following can be summarized: • The AFM images revealed some local nano-wear of the carbon coating after 15 k cycles of CSS test. The surface roughness was reduced on the

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nano-meter scale. • The elastic modulus and hardness of the LT bump top were slightly higher than the outside region, and the differences in these properties increased after the CSS test, especially at the initial stage of indentation. More significant increases in these properties were observed after the CSS test for the MT zone at indentation depths below about 100 nm. This is attributed mainly to strain hardening of the top coating layer arising from contact stresses and the generation of internal stress fields during the CSS test. There may also be an effect of the finite tip radius of the indenter on the hardness of the rough surface, especially for shallow depths. • The scratch depth and the corresponding coefficient of friction displayed a sudden surge or drop at a normal load of about 9.5 mN (or equivalent scratch depth of about 140 nm) during the nanoscratch test. The characteristic sudden change in scratch behavior was explained as the interface failure between the magnetic layer and the underlying Ni-P layer. • The TOF-SIMS spectra displayed significant reductions in the intensities of lubricant fragments, such as C(12), C2H7(31), CFO(47), CF2(50), CF3(69), C2F3(81), C2F4(100), C2F5(119), after the CSS test. However, the AES was not particularly sensitive to detect the type of wear observed above. ACKNOWLEDGEMENTS The authors wish to thank Samsung Electronics for the supply of hard disks and carrying out the CSS wear experiments. WSK was a visiting scholar from Yeungnam University, Korea to Hong Kong University of Science & Technology (HKUST), Hong Kong when this work was performed. Most experiments were conducted with the technical support of the Material Characterization & Preparation Facilities (MCPF) and the Advanced Engineering Materials Facilities (AEMF) at HKUST. The financial support from the HK Research Grant Council (RGC) is also acknowledged.

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