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.

The structure and optical characteristics of thin (∼30 nm) wurtzite AlInN films grown pseudomorphic on free-standing, c-plane GaN substrates are presented. The Al1−xInxN layers are grown by metalorganic chemical vapor deposition, resulting in films with varying In content from x = 0.142 to 0.225. They are measured using atomic force microscopy, x-ray diffraction, reciprocal space mapping, and spectroscopic ellipsometry (SE). The pseudomorphic AlInN layers provide a set where optical properties can be determined without additional variability caused by lattice relaxation, a crucial need for designing devices. They have smooth surfaces (rms < 0.29 nm) with minimum pit areas when the In content is near lattice-matched to GaN. As expected, SE shows that the refractive index increases and the bandgap energy decreases with increased In-content. Plots of bandgap energy vs In content are fitted with a single bowing parameter of 3.19 eV when using bandgap energies for AlN and InN pseudomorphic to GaN, which is lower than previous measurements and closer to theoretical predictions.

As part of improving the tribological properties of TiN-based coatings, researchers have introduced additional elements to the binary TiN system. Addition of a self-lubricating and oxide-forming substitutional element such as Mo to the rock salt TiN system deposited by sputtering has been widely studied. But, the TiN-MoN solid solution system grown by atomic layer deposition (ALD) is yet to be reported. Our current work is motivated by the need to understand and probe the structure of TixMo1−xN solid solutions with respect to the ALD growth process. In this work, thin films of TixMo1−xN (0 ≤ x ≤ 1) were deposited by plasma enhanced atomic layer deposition (PEALD) at 250 °C. Tetrakis(dimethylamido) titanium, bis(tert-butylimido)bis(dimethylamido) molybdenum, and N2 plasma were used as sources for Ti, Mo, and N, respectively. X-ray diffraction revealed nanocrystalline films with a rock salt crystal structure for all compositions of TixMo1−xN except for MoNx, which consisted of multiple phases with cubic MoN being the dominant phase. The elemental composition determined by x-ray photoelectron spectroscopy deviated from the pulse ratio of TiN:MoN. This study revealed that nearly the whole solid solution of the TiN-MoN system can be accessed by PEALD.

In this work, TiMoN thin films were deposited by plasma-enhanced atomic layer deposition with an equal number of Ti and Mo precursor exposures at a substrate temperature of 250 °C. Tetrakis(dimethylamido) titanium and bis(tert-butylimido)bis(dimethylamido) molybdenum were used as sources for Ti and Mo, respectively. N2 and N2/H2 plasma were used, respectively, for TiN and MoN cycles as a source for N. Negative RF substrate bias voltage of magnitude, |Vbias|, of 0, 31, 62, 125, and 188 V were applied during the plasma half cycle. Nanocrystalline rock salt crystal structures were found by x-ray diffraction for films deposited on single-crystal Si and Si-thermal oxide substrates. Applying |Vbias| generated voids by the bombardment of high-energy ions, lowering the density. Further increase of |Vbias| caused the annihilation of voids and a slight increase in density. Four-point probe measurement showed increased electrical resistivity due to a reduction in grain size caused by continuous renucleation during growth. High-energy ions at high |Vbias| sputtered away the films resulting in low growth rates. Stripe test revealed inferior wear rates and coefficients of friction at higher |Vbias| due to low-density porous films. Epitaxial films deposited on c-plane sapphire had (111) orientation and considerable mosaicity with twinned domains rotated at 60° to each other.