High-Sensitivity Low-Energy Ion Spectroscopy with Sub-Nanometer Depth Resolution Reveals Oxidation Resistance of MoS2 Increases with Film Density and Shear-Induced Nanostructural Modifications of the Surface

Abstract

For decades, density has been attributed as a critical aspect of the structure of sputter-deposited nanocrystalline molybdenum disulfide (MoS₂) coatings impacting oxidation resistance and wear resistance. Despite its importance, there are few examples in the literature that explicitly investigate the relationship between the density and oxidation behaviors of MoS₂ coatings. Aging and oxidation are primary considerations for the use of MoS₂ coatings in aerospace applications as they inevitably experience prolonged storage in water and oxygen-rich environments prior to use. Oxidation that is either limited to the first few nanometers or through the bulk of the coating can result in seizure due to high initial coefficients of friction or component failure from excessive wear. High-sensitivity low-energy ion spectroscopy (HS-LEIS) and Rutherford backscattering spectrometry (RBS) are both used to understand the extent of oxidation throughout the first ∼10 nanometers of the surface of pure sputtered nanocrystalline MoS₂ coatings after high-temperature aging and how it is impacted by the density of coatings as measured by RBS. Results show that low-density coatings (ρ = 3.55 g/cm³) exhibit a more columnar microstructure and voiding, which act as pathways for oxidative species to penetrate and interact with edge sites, causing severe surface and subsurface oxidation. Furthermore, HS-LEIS of surfaces sheared prior to oxidation reveals that the oxidation resistance of low-density MoS₂ coatings can be significantly improved by shear-induced reorientation of the surface microstructure to a basal orientation and elimination of pathways for oxygen into the bulk through compaction of surface and subsurface voids.

Scheme 1. Oxidation of MoS₂ under Accelerated Aging Conditions

Figure 1

Transmission electron microscopy (TEM) of the low-density (LD) and high-density (HD) coatings showing voids in LD, resulting in a low measured film density of 3.55 g/cm³ compared to 4.5 g/cm³ for HD (a). X-ray diffraction (XRD) of LD and HD (b) indicates that LD is preferentially (101̅0) oriented (i.e., columnar), whereas HD has peaks relating to (0002), (101̅0), and (112̅0) planes (i.e., randomly oriented).

Figure 2

HS-LEIS spectra showing peaks for O (1150 eV), S (1820 eV), and Mo (2500 eV) as a function of sputter depth for each sample and aging condition studied. (a) Low-density (LD) coating after 12 h of 10 mbar 250 °C O₂. (b) High-density (HD) coating after 12 h of 10 mbar 250 °C O₂. (c) Shear-modified region of the low-density coating. (d) Shear-modified region of the high-density coating after 12 h 10 mbar 250 °C O₂ Note: there is a shift in the energies between (a/c) and (b/d) due to a change in the calibration of the beam energy of the HS-LEIS. (a and c) have a beam energy of 3 kV, whereas (b and d) have a beam energy of 3.1 kV.

Figure 3

Ratio of oxygen to molybdenum (O/Mo) as a function of sputter depth measured using depth profiling HS-LEIS for the as-deposited (a) low-density (LD) and (b) high-density (HD) coatings for unaged as well as aged samples at 250 °C of 1 mbar O₂ for 0.5 h and at 250 °C of 10 mbar O₂ for 12 h. (c, d) Mechanistic hypothesis describing the role of density on the oxidation behavior of pure MoS₂ coatings is presented for (c) low-density and (d) high-density coatings. (c) Low-density coatings allow oxygen to penetrate a coating and cause severe oxidation of the coating. (d) Oxidation is limited to the surface of high-density coatings by minimizing penetration pathways.

Figure 4

(a) Ratio of oxygen to molybdenum (O/Mo) as a function of sputter depth measured using depth profiling HS-LEIS comparing shear-modified and nonshear-modified low-density (LD) and high-density (HD) coatings after aging experiments at 250 °C of 10 mbar O₂ for 12 h. (b) High-resolution transmission electron micrograph showing the basally oriented surface of the shear-modified layer in low-density coatings before aging. (c, d) Mechanistic hypothesis describing the role of density on the oxidation behavior of shear-modified MoS₂ coatings is presented for (c) low-density and (d) high-density coatings. (c) For low-density coatings, sliding imparts additional improvements by compacting voids near the surface and densifying the subsurface of the coating. (d) Oxidation is limited to the surface of high-density coatings by minimizing penetration pathways. In both cases, sliding causes reorientation of MoS₂ crystallites near the surface to become basally oriented, which further limits oxidation due to less-reactive sulfur planes as opposed to more reactive edge sites, preventing sites for oxygen to bond and blocking pathways into the film.