Ehsan Rismani

Effect of pre-treatment of the substrate surface by energetic C+ ion bombardment on structure and nano-tribological characteristics of ultra-thin tetrahedral amorphous carbon (ta-C) protective coatings

Depositing an ultra-thin tetrahedral amorphous carbon (ta-C) protective coating on the surface of the recording heads in magnetic tape drives can improve the tribological problems at the head/tape interface. In this work the effect of pre-treatment of the surface of AlTiC substrate (main bearing surface of head in contact with tape) by C+ ions of moderate energy (smaller than 400 eV) on the structural and tribo-mechanical behaviours of the coated surfaces is studied. Sample preparation consisted of two separate stages of surface pre-treatment and deposition of the protective film, and was done by means of filtered cathodic vacuum arc. Structure of the ta-C film and its interface with the substrate were studied by transmission electron microscopy and time-of-flight secondary ion mass spectrometry depth profiling. The results revealed the formation of a broader, dense atomically mixed layer at the ta-C film–substrate interface of the pre-treated samples comparing with that of the samples without pre-treatment. Chemical characterization of thin diamond-like carbon coatings was conducted by means of x-ray photoelectron spectroscopy and the surface pre-treatment was found to have a remarkable effect on increasing the sp3 hybridization fraction in the ta-C overcoat. Nano-tribological properties of the treated surfaces were examined using ball-on-flat wear test at very low load (20 mN). There was a good correlation between the surface and structure characteristics of the film, and the tribological results and the pre-treated surfaces presented a very low coefficient of friction and higher wear life. The experimental results demonstrate the effectiveness of bombardment of the surface with C+ ions of moderate ion energy to improve the structural and tribo-mechanical properties of the protective ta-C films on the magnetic head substrate material.

Enhanced Tribological, Corrosion, and Microstructural Properties of an Ultrathin (<2 nm) Silicon Nitride/Carbon Bilayer Overcoat for High Density Magnetic Storage

An ultrathin bilayer overcoat of silicon nitride and carbon (SiNx/C) providing low friction, high wear resistance, and high corrosion resistance is proposed for future generation hard disk media. The 16 Å thick SiNx/C overcoat consists of an atomically thin SiNx underlayer (4 Å) and a carbon layer (12 Å), fabricated by reactive magnetron sputtering and filtered cathodic vacuum arc (FCVA), respectively. When compared with monolithic overcoats of FCVA-deposited carbon (16 Å) and sputtered SiNx (16 Å), the SiNx/C bilayer overcoat demonstrated the best tribological performance with a coefficient of friction < 0.2. Despite showing marginally less electrochemical corrosion protection than monolithic SiNx, its ability to protect the magnetic media from corrosion/oxidation was better than that of an ∼27 Å thick commercial hard disk overcoat and 16 Å thick monolithic FCVA-deposited carbon. From X-ray photoelectron spectroscopy and Raman spectroscopy analyses, it was found that the introduction of the 4 Å SiNx underlayer facilitated higher sp(3) hybridization within the carbon layer by acting as a barrier and promoted the formation of strong bonds at the SiNx/C and the SiNx/media interfaces by acting as an adhesion layer. The higher sp(3) carbon content is expected to improve the thermal stability of the overcoat, which is extremely important for future hard disk drives employing heat assisted magnetic recording (HAMR).

Ultrathin Si/C graded layer to improve tribological properties of Co magnetic films

An ultrathin Si layer (≤1 nm) deposited on Co magnetic films was bombarded with energetic C+ ions to form a Co/Si/C mixed layer. This layer improved the tribological properties of the Co film as compared with those of a commercial hard disk media with 3.0 nm carbon overcoat and 1.4 nm of lubricant. Formation of a network of C-C and Si-C bonds at the outermost layer and the bulk of the Si/C film as well as formation of chemical bonds between the Co surface and the mixed layer was found as the main factors to improve the tribological properties.

Developing an (Al,Ti)N (x) C (y) Interlayer to Improve the Durability of the ta-C Coating on Magnetic Recording Heads

In this study, a new surface pre-treatment technique has been developed to improve the durability of the ultra-thin tetrahedral amorphous carbon (ta-C) coating for magnetic tape drive read/write heads. In this technique, prior to the deposition of an 8-nm ta-C overcoat, a 2-nm thin TiN interlayer was deposited on the heads surface and bombarded with energetic Ar+ and C+ ions. X-ray photo-electron spectroscopy results revealed that this surface pre-treatment technique would lead to the formation of an atomically mixed (AlTi)NxCy interlayer which can chemically bond the interlayer to the overcoat and substrate. The effect of this atomically mixed interlayer on the wear resistance of the ta-C coating was investigated using ball-on-flat tests as well as a functional tape drive tester. According to the ball-on-flat test results, the application of the (AlTi)NxCy interlayer was able to improve the wear life of the ta-C overcoat by up to 3.3 times as compared to that of the conventional ta-C coating. The results of the wear tests in a real head/tape interface were in agreement with the ball-on-flat results, and showed that while the conventional ta-C film was completely removed from the head surface, the ta-C film with (AlTi)NxCy was able to protect the head surface for wear tests of about 1.6 million meters.

Comparison of Corrosion and Tribological Properties of Ultrathin (<2 nm) Carbon Films on Hard-Disk Media by DC Sputtering and FCVA Processes

Ultrathin carbon overcoats (COCs) are of great interest for future hard-disk media. In view of future requirements, the key research focus lies in investigating/maintaining tribological and oxidation resistance properties of different types of COCs with thicknesses <;2nm. We report on the tribological and oxidation resistance properties of sputtering and filtered cathodic vacuum arc (FCVA) processed ultrathin COCs of thicknesses in the range of ~1.2-1.6nm. FCVA-deposited 1.6nm COC exhibits excellent properties in terms of lower coefficient of friction (COF, 0.2-0.25), higher wear resistance and greater oxidation resistance. In contrast, sputtered 1.2 and 1.5nm COCs show higher COF (~0.4-0.5), poor wear and oxidation resistance properties. The tribological and oxidation resistance performance of 1.2 and 1.6nm thick FCVA-processed COCs are found to be comparable with, or even better than, significantly thicker ~2.7nm commercial COC, demonstrating the effectiveness of the FCVA processed ultrathin protective magnetic media overcoats.