IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
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Cobalt Oxalate Formation on Thin-Film Magnetic Recording Media T. E. Karis1 , X.-C. Guo1 , B. Marchon1 , V. Raman2 , and Y.-L. Hsiao3 Hitachi Global Storage Technologies, San Jose Research Center, K74/C1, San Jose, CA 95120 USA Hitachi Global Storage Technologies, CITA/050-3, San Jose, CA 95193 USA Hitachi Global Storage Technologies, L3HA/050-1, San Jose, CA 95193 USA Needle-like crystalline nanostructures were formed on the surface of a lubricated experimental thin-film media during exposure to elevated temperature and humidity. Clusters of nodules were formed on unlubricated media during the same exposure. Both the needles and nodules were identified as cobalt oxalate by Micro-Raman spectroscopy. A chemical mechanism is proposed to explain how cobalt hydroxide corrosion product and atmospheric oxalic acid combine to form cobalt oxalate in the presence of moisture. Index Terms—Cobalt alloys, corrosion, disk drives, disks, magnetic disk.
I. INTRODUCTION
A
CARBON overcoat limits the corrosion of metals in the recording layer to pores in the overcoat [1]. Overcoat porosity varies with surface roughness, overcoat thickness, and the type of overcoat deposition process (e.g., ion assisted versus sputtered) [2]. Overcoat pores are also made by asperity removal or abrasion during the burnish process. Pores in electrically conductive overcoats become anodic sites of a galvanic cell, with the overcoat surface acting as a cathode for the oxygen reduction reaction [3]–[5]. In this case, the electrolyte is atmospheric moisture, which adsorbs or condenses on the carbon overcoat surface. Even though the magnetic layers comprise numerous metals, we focus on cobalt because it is the most active metal in the recording layers [6], [7]. Cobalt oxidizes Co 2 . Electrons enter the according to Co electrolyte through the carbon overcoat, and help reduce H O 2 2OH . Combining oxygen as: 1/2 O these electrochemical reactions yields cobalt hydroxide 2OH Co OH . Co Here, we characterize a new type of crystalline needle-like corrosion product that was observed on an experimental thin-film media. II. EXPERIMENT An experimental cobalt alloy metallic layer, especially tailored to yield heavy corrosion products, was deposited on glass substrates and overcoated with carbon. Disks were subsequently lubricated with 1 nm of Ztetraol 2000. The media was exposed to elevated temperature and humidity at 65 C/100% RH (noncondensing) for 48 h (RH/T exposure). III. RESULTS Following RH/T exposure, darkfield optical microscopy revealed that the light scattering sites on unlubricated media were
Digital Object Identifier 10.1109/TMAG.2006.878629
Fig. 1. Micrographs of scattering sites formed during RH/T exposure (a) unlubricated media, (b) lubricated media, (c) media with the overcoat etched away, and (d) synthetic Co oxalate needles.
clusters of nodules [Fig. 1(a)] while needle-like structures were observed on both burnished and unburnished lubricated media [Fig. 1(b)]. The needle-like structures resembled those reported near corrosion sites by others [7], [8]. Atomic force microscopy (AFM) was performed to determine the size and shape of the needles on lubricated media, Fig. 2. They were rectangular shaped 6 to 8 m long, 300 to 600 nm wide, and 30 to 40 nm high. In addition to the prominent needles, many smaller nodules were also observed around the needles, which resemble the hillocks reported in [9]. Sweeping the slider removed up to 99% of the scattering sites from the lubricated burnished media. Excessive head disk interaction was observed during the head sweep, but the slider did not crash. A small amount of fine debris was observed on the slider after sweeping. Chemical analysis was performed to determine the composition of the corrosion sites and slider debris. Before RH/T exposure, angle resolved XPS detected cobalt hydroxide within the overcoat as in [4], and cobalt extraction measurements detected 20 to 45 ng/disk which is comparable to the levels reported in [2]
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
Fig. 2. AFM image of typical needles formed on lubricated media during RH/T exposure.
Fig. 4. FTIR absorption spectra (a) media overcoat etched away and exposed, (b) synthetic Co oxalate needles, and (c) oxalic acid and (d) Co hydroxide reagents.
Fig. 3. Micro-Raman spectra of (a) debris on a swept slider, (b) scattering (corrosion) sites on lubricated and, (c) unlubricated RH/T exposed media, and (d) Co oxalate and (e) Co hydroxide reagents.
and [7]. Time of flight secondary ion mass spectroscopy (TOFSIMS) imaging detected excess cobalt. Micro-Raman spectra of sites on unlubricated and lubricated media and the slider debris are compared with the Raman spectra of cobalt hydroxide and cobalt oxalate reagents (Sigma-Aldrich) in Fig. 3. The Raman spectra of the sites on the unlubricated and lubricated media and the spectrum of the slider debris match the spectrum of the cobalt oxalate reagent. IV. DISCUSSION It is well known that cobalt oxalate forms needle-like crystals [10], [11]. Hydrocarbon ether which is similar to the perfluoropolyether lubricant acts as a “soft template” for crystallization of cobalt oxalate [12]. Thus, cobalt oxalate tends to crystallize on lubricated media while it remains molecularly dispersed or in the form of amorphous nodules on unlubricated media. Carbon dioxide from the air reacts with NiP plating on magnetic recording disk substrates in the presence of moisture to form nickel carbonate [13]. Cobalt or cobalt hydroxide reacts with carbon dioxide in the presence of moisture to form cobalt carbonate. Since carbonate forms so readily, it is surprising that
only cobalt oxalate and no cobalt carbonate was observed on the RH/T exposed media. Conversion of cobalt carbonate to cobalt oxalate is thermodynamically unfavorable. However, either cobalt carbonate or cobalt hydroxide could react with oxalic acid to form cobalt oxalate in the presence of moisture. Cobalt oxalate precipitated out of solution in the form of crystalline needles, Fig. 1(d), when a dilute solution of oxalic acid was combined with either cobalt carbonate or cobalt hydroxide in water. The transmission Fourier transform infrared (FTIR) spectra are shown in Fig. 4(b). The synthetic needles resemble those formed on the lubricated media during RH/T exposure. Of course, cobalt oxalate could also form directly from cobalt metal and oxalic acid, in the presence of oxygen, according to Co H C O 1/2 O CoC O H O. A carbon overcoated media was etched down to the cobalt alloy with the Ar ion beam in the XPS tool. This etched media was then sealed in an impermeable foil bag while exposed to a small amount of oxalic acid dissolved in water for seven days at 65 C. Fig. 4(a) is the FTIR reflection absorption spectrum after exposure relative to that measured before exposure. A cobalt oxalate layer formed on the alloy surface during the exposure. The oxalate was present as nodules, Fig. 1(c), and resembled those found on the unlubricated RH/T exposed media, Fig. 1(a). A disk with the carbon overcoat etched down to the cobalt alloy was placed in a hermetically sealed steel chamber with reservoirs attached to supply moisture and carbon dioxide. The cylinder was evacuated and then allowed to backfill with water vapor and carbon dioxide and stored for seven days at ambient temperature. At the end of this time, reflection FTIR detected no cobalt oxalate. Only adsorbed water and carbonate were present on the alloy surface. The most likely chemical reaction mechanism for the formation of cobalt oxalate on the experimental media during RH/T exposure is shown schematically in Fig. 5. Electrochemical corrosion product cobalt hydroxide on the left can react either with
KARIS et al.: COBALT OXALATE FORMATION ON THIN-FILM MAGNETIC RECORDING MEDIA
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H. Merkins, C. Shebib, L. Brinkman, J. Kraus, M. Powell, J. Burns, Q. Dai, and H-B. Tu.
REFERENCES Fig. 5. Chemical reaction mechanism for the formation of cobalt oxalate from cobalt hydroxide in the presence of moist air and oxalic acid.
carbon dioxide to form cobalt carbonate, or with oxalic acid to form cobalt oxalate. Any cobalt carbonate that has formed subsequently reacts with oxalic acid through dissociation to form cobalt oxalate (the Gibbs free energies of formation were derived from [14]). Therefore, the chemical reactions will proceed in the directions shown by the arrows in Fig. 5. The net free energy change to form cobalt oxalate from cobalt hydroxide is the same for both pathways. Nickel carbonate instead of nickel oxalate is formed on NiP because nickel carbonate has a lower free energy than nickel oxalate. Oxalic acid on carbon overcoats was associated with corrosion mounds that can lead to damaging head disk interaction in [15]. Oxalic acid is ubiquitous in the form of atmospheric particles, and oxalic acid sublimes with a fairly high vapor pressure (4.7 mPa at 25 C) [16], [17]. Even though oxalic acid aerosol is sequestered in the clean room HEPA air filters, oxalic acid sublimes into the air stream. Ion chromatography measurements found that 31 ng/disk of oxalic acid was consumed during RH/T exposure. The amount of oxalic acid consumed by reaction with cobalt hydroxide on the media during RH/T exposure estimated from the site areal density, the average needle volume, and the crystal density from X-ray crystallography [10] is between 10 and 40 ng/disk. V. CONCLUSION In addition to the well-known cobalt hydroxide, a new corrosion product (cobalt oxalate) was identified directly by Raman spectroscopy. Cobalt oxalate that formed on unlubricated media remained in clusters of nodules which were detected as a low density of small light scattering sites. On lubricated media, the lubricant backbone chain acts as a template for crystallization of the cobalt oxalate into microscopic needles, which are detected as a high areal density of light scattering sites. ACKNOWLEDGMENT The authors would like to thank B. Yen, R-H. Wang, V. Yra, R. Wendt, C. Brown, D. Pocker, A. Spool, K. Kuboi,
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Manuscript received March 10, 2006 (e-mail:
[email protected]).
