Corrosion of Thin Film Magnetic Disk: Galvanic Effects of the Carbon Overcoat

Thin film magnetic disks are usually coated with a layer of carbon or silica to improve wear properties, enhance flyability, and reduce environmental degradation of the underlying metal layers. In this paper we investigated the effect of dc and RF sputter deposited carbon overcoats on the corrosion behavior of the magnetic layers using electrochemical techniques and a corrosive gas chamber. Corrosion test samples were characterized by optical microscopy, ellipsometry, scanning electron microscopy (SEM), and scanning Auger spectroscopy (SAM) in order to establish the mechanism of corrosion. Corrosion of the magnetic layer was found to be galvanically coupled to the carbon overcoat and influenced by several disk processing parameters. A functional thin film disk has to have good magnetic performance, wear resistance, and long term stability. As the demand for storage capacity increases, the selection of suitable materials relies more on their magnetic properties and less on their chemical stability. Co alloys and TbFe metal films, which are coming into use for high density magnetic and magneto-optic storage, respectively, have significantly lower chemical stability than the previously used magnetic iron oxide particles. Thin magnetic films of Co alloys are typically coated by a layer of carbon or Si02 up to 50 nm thick to provide the needed wear resistance. If this layer were porefree, it would also provide chemical stability. In reality, films are not perfect and the environment to which they are exposed is not inert. Not only can corrosion occur through imperfections in an overlayer, but it will be enhanced by galvanic interaction if the overlayer is electrochemically active and more noble in potential. One might expect that this is the case with carbon-coated Co alloys. Recently, Novotny et al. (1) have emphasized the galvanic activity of the carbon overcoat. Other investigators, however, have found that carbon decreases the corrosion rate of similar materials by slowing down the lateral growth of corrosion products (2). The purpose of this work is threefold: 1. To define the principal corrosion mechanism in thin film disks, in particular the role of carbon in the corrosion of CoP. 2. To evaluate ways to minimize disk corrosion. 3. To develop and utilize experimental techniques which are most suitable for the evaluation of thin film behavior. Working with structures of 100 nm or less presents special experimental challenges. The choice of surface preparation techniques, passivation treatments, and test methods are all limited by the thickness of the specimens. Furthermore, the layer beneath the magnetic layer may play a role by influencing the structure of the magnetic layer or altering the electrochemistry if this sublayer is also exposed through defects. There are, however, advantages in working with thin films and thin film structures. For example, vacuum deposition techniques provide unique ways of tailoring thin film properties through a judicious choice of both materials and deposition parameters. Experimental Samples.—The thin film disk structure AlMg/NiP/CoP/C was studied. The Ni81P19 and Co92P8 layers were electrolessly plated, and the C layer was dc or RF magnetron sputter deposited from a pyrolytic graphite target in argon under carefully controlled conditions. Also studied were E-beam deposited Co, CoCr, CoTi, and CoNi films or NiP or Si/SiO2 substrates and 99.997% pure Co foil from Johnson Matthey Chemicals Limited. Some of the Co and CoP films were covered by a thin dielectric layer of a dc sputtered glassy oxide and evaluated as such or further over-coated with a thin carbon layer. Carbon films were also deposited in the range of 2560 nm on a nonconducting substrate in order to measure electrical resistivity and evaluate the electrochemical behavior of carbon alone. Procedure.—Electrochemical tests.—Samples with no surface pretreatment were exposed to an electrolyte, and their behavior was monitored by electrochemical techniques using an EG&G Model 350A corrosion console. Two types of experimental cells were used. One is an airtight cell made of Kel-F and equipped with quartz windows to allow for simultaneous ellipsometric measurements. Most of the experiments, however, were conducted in a much simpler miniature cell designed to mimic the conditions of atmospheric corrosion while preserving the advantages of a three-electrode cell. The schematic of the cell is given in Fig. 1. It consists of the sample (working electrode) masked with plating tape to expose only a 0.32 cm 2 area, Pt mesh (counterelectrode), and a mercurous sulfate electrode (reference electrode), with a filter paper disk separating each electrode. A 20 μl droplet of electrolyte was used. Due to small distances between the electrodes, the ohmic resistance in the cell is relatively small even with electrolytes such as DI and triple distilled water. As the ohmic resistance of the latter is only about 800Ω in the first seconds of measurement, insignificant errors in the evaluation of corrosion rates are introduced. Since mild electrolytes such as water may be used and the thinness of the electrolyte brings a lot of oxygen to the surface, this setup is very close to the conditions of atmospheric corrosion. The procedure was to monitor the corrosion potential on exposure for about 15 min and periodically measure the polarization resistance by scanning at a rate of 1 mV/s over ±15 mV from the corrosion potential. The corrosion rate was routinely calculated with the program Fig. 1. Schematic of the "droplet" cell for corrosion measurements provided by the EG&G system assuming anodic and cathodic Tafel slopes of 0.10 and 0.12 V/dec, respectively. The use of the "reasonable estimates" instead of "true values" causes an error in the calculation of the corrosion rate. The maximum possible error, however, is limited to a factor of 2.2 (3) and in this work stayed below 1.5. The potentiodynamic polarization curve was then measured at a rate of 1 mV/s from 0.25V cathodic of the corrosion potential. Polarization resistance measurements in a mild electrolyte, such as water, are very suitable for the study of thin films. In contrast, much or all of a film can be dissolved during potentiodynamic sweeps, but such tests are quite informative as they provide an assessment of the ability of a metal to passivate and of possible galvanic exposure. Some of the results were obtained with the Model 378 electrochemical impedance system by EG&G. All of the potentials are expressed in volts against the mercurous sulfate electrode. Accelerated corrosion chambers.—Samples were also exposed to clean or contaminated air with varying temperature and humidity (T/H) in glass chambers produced by Interfact Associates, Limited. Most of these tests were conducted in an atmosphere of air with 70% RH and 10 ppb of Cl2 at 25°C. An exposure of 24h to this environment was primarily used as a technique for pore decoration in the carbon-covered disk structure, similar to the technique reported by Abbott (4). Quantitative pore counts were made from SEM photographs taken at 1200×. Samples were also characterized by optical microscopy, ellipsometry, and scanning Auger spectroscopy. Results and Discussion Co and CoP.—Electrochemical tests show that CoP readily dissolves in water and slightly acidic electrolytes with an average corrosion rate of 3 × 10 -6 A/cm 2 or 0.06 nm/min. This rate decreases slightly over the first few minutes and then is quite constant with time (Fig. 2). "Steady-state" values are listed in Table I. The anodic Tafel slope is low, 35-60 mV/decade, increasing only at pH's above 9. In contrast, the cathodic Tafel slope is high, from about 120 mV/dec up to several hundred mV/dec, throughout the evaluated pH range from 3 to 11. Typical potentiodynamic polarization curves, obtained in DI water (pH 7.2) and in NaOH/Na2B407 (pH 11.0), are given in Fig. 3 as examples. The cathodic reaction, in most cases, proceeds with mixed kinetics, with both hydrogen and oxygen reduction playing a role. In DI water oxygen reduction is a predominant cathodic reaction proceeding with a diffusion-limited rate. The value of the diffusion-limited current is about 10 -5 A/cm 2 (Fig. 3), i.e., of the order expected for the oxygen diffusion in an unstirred electrolyte. The corresponding anodic process has well-defined Tafel region, with an initial slope of 43 mV/dec changing at higher potentials to even lower value (Fig. 3). The observed reversal of the current-potential slope at the current value of about 10 -3 A/cm 2 is not a sign of passivation, but a consequence of the catastrophic dissolution of the thin film. At Fig. 2. Variation of corrosion rate with time measured in DI water on CoP, NiP/Co, and Si/SiO2/Co samples. higher pH's the anodic currents are considerably lower, and the Tafel region is replaced by a current-potential relationship normally observed on passive electrodes (Fig. 3). Experiments have shown that the basic reactions in the "droplet cell" are similar to those measured in a large three-electrode cell with an unstirred bulk electrolyte saturated with air, as shown in Fig. 4 for Co-foil in NaOH/Na2B4O7, pH 11. Also, it is interesting to note that the Tafel slopes obtained in the "droplet cell" both on Co and CoP are similar to the values reported for Fe (5), Ni (6), and Co (7, 8) in neutral and acidic solutions and for Fe in slightly alkaline solutions (9), all measured in bulk electrolytes. This suggests that the processes determining the anodic portion of V log I curve for cobalt are similar to those evaluated in detail for iron (9), i.e. In the absence of oxide formation Eq. [2] is the rate-determining step resulting in the low Tafel slope observed. When film formation is kinetically possible, Eq. [5] is the rate-determining step. The value of the corrosion potential for CoP in neutral electrolytes (pH 6 and 7) is about −0.84 which is just in the potential range where the oxide formation is possible (10). It is also of interest to know if the air-formed oxide dissolves on immersion in the electrolyte or Table 1. Variation of the steady-state corrosion potential and rate on CoP and Co films as a function of substrate, dielectric, and carbon coating Fig. 3. Potentiodynamic polarization curve on CoP measured in a droplet of Dl water (solid line) and of NaOH/Na2B407, pH 11 (dashed line). The dash-dot line gives an indication of the Tafel region. if it protects Co against further corrosion. In situ ellipsometric measurements made in 0.1N Na2SO4 using a thin film Co sample deposited on a Si/SiO2 wafer have determined that the airformed oxide disappears from the surface, but the process is slow. In the first 4 min at the opencircuit potential ellipsometric parameter ∆ increases gradually by about 1°. This can be interpreted as a removal of 2-3Å of the film. The majority of the oxide is removed by a forceful reduction at -1.2V. At a pH above 9, at the open-circuit potential, ellipsometry detects a growth of a very thin new oxide, which is apparently responsible for a decrease of the corrosion rate and an appearance of "passive" range in the V log I curve (Fig. 3). This oxide is stable only while the cobalt is in an alkaline solution. Passivation in alkaline solution, followed by a test in a neutral or acidic environment (pH 3-6) is short-lived. As this is the pH range of interest for atmospheric corrosion, we conclude that the corrosion of Co is not inhibited by self-passivation. Although CoP, Co films, and Co foil samples show similar corrosion behavior [low anodic Tafel slopes, lack of self-passivation (Fig. 3 and 5)] the corrosion rate on CoP initially decreases with time and levels off to a value typically lower than on Co alone (Fig. 2). This effect is most likely not due to an oxide. Our earlier data on NiB and NiP (11) show that phosphorus and boron tend to decrease the passivating ability of a metal and speed up the removal of the air-formed surface oxide on immersion. And yet in all cases the corrosion rate of Fig. 4. Potentiodynamic polarization curves on Co foil in NaOH; Na2B2O7 in a droplet cell (dashed line) and in a conventional cell (a) with electrolyte open to air and (b) electrolyte stirred by O2 bubbling. Fig. 5. Potentiodynamic polarization curve on Co, Co-18%Cr, Co-18% Ti, and Co-50%Ni alloys in DI