Nanomechanical and Friction Properties of Ultrathin Amorphous Carbon Films Studied by Molecular Dynamics Analysis

An ultrathin (<4 nm) film of amorphous carbon (a-C) is used in contemporary disk drives to protect the magnetic medium of the hard disk from corrosion and mechanical wear due to intermittent impact of the low-flying magnetic head. Because of increasing demands for much higher magnetic storage densities (i.e., >10 Tbits/in2), the a-C film thickness must be decreased to <2 nm. However, the tribological and mechanical properties of such thin a-C films are not well understood and, moreover, are extremely difficult to determine experimentally. The objective of this study was to obtain insight into the tribological behavior of ultrathin a-C films by performing molecular dynamics (MD) simulations. MD results of the hardness and friction properties of nanometer-thick a-C films are interpreted in terms of the ratio of tetrahedral-to-trigonal carbon atom hybridization. A critical thickness for the effective protection of the magnetic medium by the a-C film is estimated from MD results. The results of this study elucidate the nanomechanical and nanotribological properties of ultrathin a-C films used as protective overcoats in extremely-high-density magnetic recording.

Ultra-Thin Film Ultrasonic Characterization

Ultrasonically induced nanoscale fatigue and friction reduction phenomena are researched for nanometer order thick protective overcoats. The newly developed method described here is a synergy of the nanometer scale quantitative nanoindentation/scratch technique and ultrasonic excitation. A standing ultrasonic wave is generated at the sample holder during the quasi-static nanoindentation/scratch test at an excitation frequency of several hundreds of kilohertz. Due to the fact that high strain rates are being generated at the surface of the sample, nanofatigue phenomenon followed by delamination and overcoat chipping can be observed. Statistical investigation of fatigue inducing parameters, such as critical quasistatic load and amplitude of oscillations, leads to a means of comparative sample characterization.

Room-temperature oxidation of ultrathin TiB2 films

Titanium diboride has been claimed as a very promising candidate material for protective applications in the magnetic recording. Its oxidation resistance at room temperature is a critical criterion in assessing this application potential. In this paper, the oxidation characteristics of ultrathin TiB2 thin films, such as overcoat erosion and oxide thickness, are investigated via a combination of x-ray reflectivity, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy. It was found that a <2-h exposure to air at room temperature led to the formation of approximately 15-Å-thick, well-defined oxides at the expense of an approximately 9-Å erosion of the TiB2 overcoats, coupled with the existence of a sharp oxide/TiB2 interface. XPS studies confirmed the existence of the oxides. Considering the decreasing allowable thickness for such protective overcoats, oxidation and the resultant thickness gain negate such a potential of ultrathin TiB2 films. The results in our current report provide a new perspective on its potential as protective overcoats in magnetic recording.

Ultrathin TiB2 protective films

TiB2 thin films demonstrate considerable potential for use as protective overcoats in the magnetic recording industry due to their excellent mechanical and tribological properties and good chemical and thermal stability. In the many studies performed on TiB2 films, the relative effectiveness of ultrathin TiB2 films has not been systematically investigated for very thin TiB2 films. In the present investigation, film stress and microstructure in as-sputtered and annealed ultrathin TiB2 films were investigated as a function of thickness. Ultrathin TiB2 films, as thin as 5 nm, were observed to adequately protect an underlying magnetic layer from oxidation up to 400 °C.

Strongly adhering and thick highly tetrahedral amorphous carbon (ta–C) thin films via surface modification by implantation

Highly tetrahedral amorphous carbon thin films have exceptional mechanical properties that make them ideal for many challenging wear applications such as protective overcoats for orthopaedic prostheses and aerospace components. However, the use of ta–C in many wear applications is limited due to the poor adhesion and the inability to grow thick films because of the large compressive stress. Here we report on a simple modification of the substrate growth surface by 1-keV ion bombardment using a cathodic vacuum arc (CVA) plasma prior to deposition of ta–C films at 100 eV. The 1-keV C+ ion bombardment created a thin intermixed layer consisting of substrate and carbon atoms. The generation of the intermixed carbide layer improved the adhesion and allowed the growth of thick (several μm) ta–C layers on metallic substrates.