The corrosion performance of very thin carbon overcoat layers in magnetic media

We have comparatively studied the corrosion performance of three different carbon types with 28-Å thicknesses: magnetron sputtered high density (HDC), plasma enhanced chemical vapor deposition (PECVD), and dual-layer overcoat (DLO), where a thin HDC layer is deposited on top of a thicker PECVD layer. Two different techniques are utilized to quantify the corrosion susceptibility for each overcoat type: surface reflectivity analysis (SRA), which is sensitive to localized light scattering centers, and transition metal ion chromatography (TMIC), which is based on surface extraction of corrosion products. SRA results indicate that both DLO and HDC carbons provide lower corrosion counts than PECVD whereas TMIC data show that both DLO and PECVD give lower cobalt counts than HDC. Without tape burnish, all three carbons have low SRA corrosion counts. Auger electron spectroscopy (AES) analysis shows that the corrosion products for PECVD and DLO contain much more Ni compared to HDC, suggesting that the corrosion scattering centers dominant in PECVD are Ni-rich. AES and SRA results strongly indicate that PECVD is more vulnerable to fracture damage giving rise to scattering centers. The TMIC data indicates that the thin HDC layer has many pinholes that provide effective conduits for Co migration leading to high Co surface concentrations. PECVD carbon is expected to be pinhole free. The best performing carbon system is DLO which is resistant to general surface corrosion detectable in TMIC and also to fracture damage giving rise to Ni-rich scattering centers in SRA. We postulate that the presence of sputtered carbon layer on the top of a base PECVD layer in DLO is responsible for the fracture resistance.