Ultra-thin Diamond-like Carbon Coatings Research Articles

Effects of sp3 bond and modulation ratios on ultra-thin diamond-like carbon coatings using molecular dynamics nanoindentation

Effects of sp3 bond and modulation ratios on ultra-thin diamond-like carbon coatings using molecular dynamics nanoindentation

Dependence of Optimum Thickness of Ultrathin Diamond-like Carbon Coatings over Carbon Nanotubes on Geometric Field Enhancement Factor

The generation of a field emission (FE) cathode electron source has attracted wide attention for its miniaturization, high-frequency operability, and low energy consumption. However, the FE performance of cold cathodes is limited by poor current stabilities of carbon nanotube (CNT) emitters. Coating CNTs with sp3-bonded carbon coatings is considered as a successful approach to stabilize the FE currents. High-quality ultrathin diamond-like carbon (DLC) films, which serve as sp3 carbon coatings, are deposited uniformly on CNTs by filtered cathodic vacuum arc evaporation in this research. The thicknesses of DLC coatings and the field enhancement factors of pristine CNTs affect to a large extent the FE properties. An optimum coating thickness of DLC layers corresponding to the lowest threshold field exists due to space-charge-induced band bending, at which the depletion region of the DLC layer in equilibrium is maximized. The optimum coating thickness increases with the geometric field enhancement factor of p...

Deformation of Ultra-Thin Diamond-Like Carbon Coatings Under Combined Loading on a Magnetic Recording Head

Ultra-thin diamond-like carbon (DLC) coatings are used in precision engineering applications, such as magnetic storage devices, to protect intricate structures from wear and corrosion. A DLC coating typically consists of hard amorphous carbon in combination with an interlayer such as silicon (Si), to improve adhesion to the substrate material. Deformation and delamination of these coatings, even in part, could expose the substrate material and compromise its integrity and functionality. We have implemented a molecular dynamics model to quantify the strength of the interface between an ultra-thin tetrahedral amorphous carbon coating, a Si layer, and a permalloy (NiFe) substrate, under combined normal and tangential loading that mimics accidental contact between the recording head and the disk of a hard drive. We have evaluated the effect of the thickness of the different coating layers on deformation and interfacial strength of the coating during combined loading. The results indicate that deformation occurs primarily in the Si layer, and at the interface between the Ni–Si and the Si–C layers. Permanent separation of the Si and ta-C layers is observed, which gradually increases with multiple combined loading cycles. We find that increasing the Si and carbon layer thickness strengthens the DLC coating. However, increasing the carbon layer thickness has a larger effect on coating strength than increasing the Si layer thickness.

Ultra-thin nano-patterned wear-protective diamond-like carbon coatings deposited on glass using a C60 ion beam

The wear resistance and optical properties of ultra-thin diamond-like carbon (DLC) coatings deposited on glass substrates at room temperature were investigated. The coatings were deposited using a C60 ion beam. A sequence of surface treatments including ion-beam etching, molecular-beam deposition and ion-beam deposition were used for production of ultra-thin DLC coatings with a nano-patterned surface. The goal of the surface nano-patterning was to improve the wear resistance of the DLC coatings while maintaining a low thickness to obtain a high optical transparency. The experimental results demonstrated the excellent ability of the ultra-thin DLC coatings to improve the wear resistance of the glass substrates. A comparison between the wear rate for the DLC coating with nano-patterns and that of a more smooth coating revealed that the nano-patterned surface shows a 39% higher wear resistance. Furthermore, the coatings demonstrated a high transparency (94–97%) in the visible-light wavelength range.

Surface and structural properties of ultrathin diamond-like carbon coatings

Nanoscale wear resistance, friction, and electrical conduction tests using atomic force microscope (AFM) have been conducted on ultrathin diamond-like carbon (DLC) coatings, including tetrahedral amorphous carbon (ta-C) deposited using pulsed cathodic arc (PCA) and filtered-PCA, and hydrogenated amorphous carbon (a-C:H) deposited using electron cyclotron resonance—chemical vapor deposition (ECR-CVD). The low-resistant layers at the surfaces of these thin DLC coatings were revealed by AFM-based nanowear tests. Their thickness is mainly determined by the deposition methods and does not show an obvious variation with the coating thickness decreasing from tens of nm to a few nm. The ∼3 nm ta-C coatings from PCA and filtered-PCA deposition were found to have the stable bulk structure beneath the thin (0.3–0.95 nm) surface layers. The ∼3 nm a-C:H coating from ECR-CVD had the extremely low load-carrying capacity and exhibited the evidence of coating delamination, which can be related to the thicker (1.5±0.1 nm) soft surface layers of a-C:H coatings. The results from conducting-AFM measurements indicate that a-C:H coatings have H and sp 3 C enrichment surface layers while the soft surface layers of ta-C coatings have graphite-like structure. The nanoscale friction coefficients of these thin ta-C and a-C:H coatings were compared by AFM-based lateral force microscope. The lower friction coefficient of ta-C coatings can be attributed to the existence of graphite-like surface structure.

Measurements of elastic properties of ultra-thin diamond-like carbon coatings using atomic force acoustic microscopy

We present a comparative study of the elastic stiffness of ultra-thin (5, 20 and 100 nm thick) diamond-like carbon coatings with a sampling depth less than or comparable to the thickness of the coating. The experiments were conducted using atomic force acoustic microscopy, which is a dynamic operation mode of the atomic force microscope that permits the measurement of elastic properties with high spatial resolution. The method is based on the evaluation of the shift of the cantilever resonance frequencies caused by the contact stiffness between the sensor tip and the sample surface. Two sets of measurements are reported: one in which the silicon sensor tips were employed; and another set in which diamond-coated tips were used. The silicon tips showed considerable wear during data acquisition. Using the diamond-coated tips, however, the wear could be avoided and qualitative measurements of high stability could be performed, thereby permitting the comparison of coatings obtained by different deposition techniques. It is also shown how the obtained results can lead to future quantitative measurements on stiff coatings.

Ultrathin diamond-like carbon coatings used for reduction of pole tip recession in magnetic tape heads

Diamond-like carbon (DLC) coatings were deposited using a commercial direct ion beam deposition technique on thin-film Al2O3–TiC inductive write heads. The coating thicknesses used were 5, 10, and 20 nm. Accelerated wear tests were conducted with metal particle tapes in a linear tape drive. Atomic force microscopy was used to image the thin-film regions to measure pole tip recession (PTR), relative wear of the pole tip with respect to the air bearing surface. It is found that the coating wears off of the head substrate to a significant extent in the first 1000 km of sliding distance. The coating is worn off the substrate long before it wears off of the thin-film region. The existence of the coating on the thin-film region provides close enough wear characteristics between the substrate and thin film that the two wear at similar rates. This results in little growth in pole tip recession. Early in the wear test, the coated substrate wears at a slightly higher rate than the DLC coated thin-film region due to the difference in tape contact pressure between the two materials; decreasing PTR is the result. As the coating on the substrate wears significantly, PTR begins to increase with sliding distance. Failure does not actually occur until the coating has worn off of the thin-film region. Near failure, the coating delaminates locally. Results indicate that coatings of 20 nm thickness may provide protection against PTR in future tape drives.

A study of microstructural and electrochemical properties of ultra-thin DLC coatings on altic substrates deposited using the ion beam technique

Microstructural and electrochemical characterization of diamond like carbon (DLC) ion beam-deposited on AlTiC (70 wt% Al2O3 + 30 wt% TiC) substrate has been carried out. Tapping mode atomic force microscopy (AFM) imaging showed that the island-like topography of DLC-coated substrates is similar to the un-coated one, indicating the uniform coverage of DLC without visible pinholes. Confocal micro-Raman analysis demonstrated that the total Raman intensity, as well as the I-D/I-G ratio, increases with the coating thickness. Electrochemical impedance spectra showed that with the increasing DLC coating thickness, a transition from one-time constant response to two-time constant response occurred when the coating thickness equals 5 nm (IS2), indicating the existence of micro-defects in the coatings which are invisible for AFM. More detailed analysis using the equivalent circuit model revealed that the charge transfer resistance (R-ct) at electrolyte/substrate interface and the resistance (R-p) related to DLC coatings increase significantly with the coating thickness, while the double-layer capacitance (C-dl) and the capacitance (C-co) of DLC coatings decrease dramatically. All these phenomena can be interpreted in terms of the evolution of the subsurface diamondlike phase (sp(3)-bond) and the reduction of micro-defects in the DLC coatings with the growing film. As a result, an increase in the corrosion potential (E-c) with the DLC coating thickness was also detected using the Tafel technique. In consequence, the DLC coatings can improve significantly the anti-corrosion properties of AlTiC substrates when the coating thickness is more than a few tens of nanometres. (C) 1999 Elsevier Science S.A. All rights reserved.

Microscale mechanical and tribological characterization of hard amorphous carbon coatings as thin as 5 nm for magnetic disks

For “near” contact recording applications, the thickness of the protective hard amorphous carbon overcoat and its surface roughness need to be reduced further to increase the magnetic recording density. Mechanical and tribological characterization of ultrathin diamond-like carbon coatings of 5, 10 and 25 nm thickness, deposited onto supersmooth (rms 0.50 nm) and smooth (rms 0.76 nm) magnetic disks was conducted. Durability tests show that the durability of the head-disk interface increases with an increase in the carbon thickness. Static and kinetic coefficients of friction and the durability of supersmooth disks are inferior to those of the smooth disks. On a microscale (AFM measurements), scratching and wear resistance of the magnetic coating without protective coating are higher and the coefficient of friction is lower than that of the diamond-like carbon (DLC) coated supersmooth disks. Thus, on a microscale, polycrystalline magnetic coatings are superior in tribological performance, however, they are inferior on a macroscale, as compared with DLC coatings. Auger electron spectroscopic studies show the magnetic layer is oxidized (possibly with the formation of cobalt oxide), which is responsible for the good tribological performance on a microscale. Based on tests of the head-disk interface, to achieve acceptable tribological performance, supersmooth disks would require the use of a textured landing zone or the use of a load-unload mechanism in the disk drive.

Microscale mechanical and tribological characterization of hard amorphous carbon coatings as thin as 5 nm for magnetic disks

For “near” contact recording applications, the thickness of the protective hard amorphous carbon overcoat and its surface roughness need to be reduced further to increase the magnetic recording density. Mechanical and tribological characterization of ultrathin diamond-like carbon coatings of 5, 10 and 25 nm thickness, deposited onto supersmooth (rms 0.50 nm) and smooth (rms 0.76 nm) magnetic disks was conducted. Durability tests show that the durability of the head-disk interface increases with an increase in the carbon thickness. Static and kinetic coefficients of friction and the durability of supersmooth disks are inferior to those of the smooth disks. On a microscale (AFM measurements), scratching and wear resistance of the magnetic coating without protective coating are higher and the coefficient of friction is lower than that of the diamond-like carbon (DLC) coated supersmooth disks. Thus, on a microscale, polycrystalline magnetic coatings are superior in tribological performance, however, they are inferior on a macroscale, as compared with DLC coatings. Auger electron spectroscopic studies show the magnetic layer is oxidized (possibly with the formation of cobalt oxide), which is responsible for the good tribological performance on a microscale. Based on tests of the head-disk interface, to achieve acceptable tribological performance, supersmooth disks would require the use of a textured landing zone or the use of a load-unload mechanism in the disk drive.