Experimental Investigation of Hydrocarbon Contamination at the Head–Disk Interface

Abstract

Hydrocarbon oil contamination of the head–disk interface is investigated. Optical surface analysis, atomic force microscopy, and contact angle measurements are used to study the adsorption characteristics of hydrocarbon contaminants on the disk surface. Optical microscopy, scanning electron microcopy, energy-dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry are used to investigate hydrocarbon contamination at the head–disk interface. Temperature and time were found to significantly influence hydrocarbon contamination. The results agree well with molecular dynamics simulation studies.

Appendix: Hydrocarbon Crystallization Molecular Dynamics Simulation

In the molecular dynamics simulation presented in this work, the so-called united atom model (UAM) was used to simulate the crystallization of linear hydrocarbon chains shown in Sect. 2.2.

As shown in Fig. 18, the UAM approach models CH₂ and CH₃ as beads in order to reduce computational cost while accurately reproducing thermodynamic properties of linear hydrocarbon chains [28]. Using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [34] and appropriate potential functions and corresponding parameters [28–33], we were able to simulate the crystallization of linear hydrocarbon chains.

Fig. 18

Schematic of “united atom model” of a linear hydrocarbon chain

For non-bonded interactions, we used the Lennard–Jones potential function given by

$$U_{\text{LJ}} = 4\varepsilon \left[ {\left( {\frac{\sigma }{r}} \right)^{12} - \left( {\frac{\sigma }{r}} \right)^{6} } \right],\quad r < r_{\text{c}}$$

(2)

where ε is 0.112 kcal/mol, σ = 4.5 Å, and r _c = 12 Å. The bond interaction was described as linear harmonic potential function given by

$$U_{\text{Bond}} = K_{\text{L}} \left( {r - r_{o} } \right)^{2}$$

(3)

where K _L = 350 kcal/mol Å² and r _o  = 1.53 Å. The angular bond interaction for linear hydrocarbon chains was simulated by the following potential function:

$$U_{\text{Angle}} = K_{\theta } \left( {\theta - \theta_{o} } \right)^{2}$$

(4)

where K _θ  = 60 kcal/mol rad² and θ _o  = 109.5°, or 1.91 rad. The dihedral bond interaction for the molecules was given by

$$U_{\text{Dihedral}} = \frac{1}{2}k_{1} (1 - \cos \varphi ) + \frac{1}{2}k_{2} (1 - \cos 2\varphi ) + \frac{1}{2}k_{3} (1 - \cos 3\varphi )$$

(5)

where k ₁ = 1.6 kcal/mol, k ₂ = 0.867 kcal/mol, and k ₃ = 3.24 kcal/mol. Lastly, for the wall–atom interaction was modeled using the following potential function:

$$U_{{{\text{Wall}} {-} {Atom}}} = 2\pi \varepsilon_{w} \left[ {\frac{2}{5}\left( {\frac{{\sigma_{w} }}{z}} \right)^{10} - \left( {\frac{\sigma }{z}} \right)^{4} - \frac{{\sqrt 2 \sigma^{3} }}{{3\left( {z + \left( {0.61/\sqrt 2 } \right)\sigma } \right)^{3} }}} \right],\quad z < z_{c}$$

(6)

where ε _w = 1.0 kcal/mol, σ _w = 3.8 Å, and z _c = 9.5 Å.

The linear hydrocarbon chains were first positioned as shown in Fig. 8a. Then, we imported the position data and applied the above potential functions in LAMMPS [28–34]. The MD simulation was first carried out in the canonical (NVT) ensemble with a time step size Δt = 2 fs. An NVT ensemble is a thermo-statistical system in which the number of atoms (N), the volume of the simulation box (V), and the temperature (T) are kept constant. With the NVT ensemble, we randomly distributed the linear hydrocarbon chains for 100,000 time steps (Fig. 8b). The system was then equilibrated at 450 K for a total of 600,000 time steps. Thereafter, we used the microcanonical ensemble (NVE) and the Langevin thermostat to quench the equilibrated system of linear hydrocarbon chains to 290 K for 1,000,000 time steps, at a cooling rate of $1.5 \times 10^{11} \,{\text{K}}/{\text{s}}$. The system was then equilibrated at 300 K for 50,000,000 time steps in order to grow crystals of linear hydrocarbon chains as shown in Fig. 8c.