Electrodeposited MoxSyOz/Ni Tribological Coatings.

Deposition of molybdenum disulfide (MoS2) coatings using physical vapor deposition (PVD) and mechanical burnishing has been widely assessed for solid lubricants in space applications but still suffers from line-of-sight constraints on complex geometries. Here, we highlight one of the first demonstrations of electrodeposited MoxSyOz and MoxSyOz/Ni thin-film coatings from aqueous solutions of ammonium tetrathiomolybdate for solid lubricant applications and their remarkable ability to provide low coefficients of friction and high wear resistance. Characterization of the coating morphology shows amorphous microstructures with a high oxygen content and cracking upon drying. Even so, electrodeposited MoxSyOz can achieve low steady-state coefficients of friction (µ ∼ 0.05-0.06) and wear rates (2.6 × 10-7 mm3/(N m)) approaching those of physical vapor deposited coatings (2.3 × 10-7 mm3/(N m)). Additionally, we show that adding dopants such as nickel increased the wear rate (7.5 × 10-7 mm3/(N m)) and initial coefficient of friction (µi = 0.23) due to compositional modifications such as dramatic sub-stoichiometry (S/Mo ∼ 1) and expression of a NiOx surface layer, although doping did reduce the degree of cracking upon drying.

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology.

Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.

Solvent-cast 3D printing with molecular weight polymer blends to decouple effects of scaffold architecture and mechanical properties on mesenchymal stromal cell fate.

The biochemical and physical properties of a scaffold can be tailored to elicit specific cellular responses. However, it is challenging to decouple their individual effects on cell-material interactions. Here, we solvent-cast 3D printed different ratios of high and low molecular weight (MW) poly(caprolactone) (PCL) to fabricate scaffolds with significantly different stiffnesses without affecting other properties. Ink viscosity was used to match processing conditions between inks and generate scaffolds with the same surface chemistry, crystallinity, filament diameter, and architecture. Increasing the ratio of low MW PCL resulted in a significant decrease in modulus. Scaffold modulus did not affect human mesenchymal stromal cell (hMSC) differentiation under osteogenic conditions. However, hMSC response was significantly affected by scaffold stiffness in chondrogenic media. Low stiffness promoted more stable chondrogenesis whereas high stiffness drove hMSC progression toward hypertrophy. These data illustrate how this versatile platform can be used to independently modify biochemical and physical cues in a single scaffold to synergistically enhance desired cellular response.

Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS2 upon Aging.

Molybdenum disulfide (MoS2) coatings have attracted widespread industrial interest owing to their excellent lubricating properties under vacuum and inert conditions. Unfortunately, the increase in MoS2 interfacial shear strength following prolonged exposure to ambient conditions (a process referred to as "aging") has resulted in reliability issues when MoS2 is employed as solid lubricant. While aging of MoS2 is generally attributed to physical and chemical changes caused by adsorbed water and/or oxygen, a mechanistic understanding of the relative role of these two gaseous species in the evolution of the surface chemistry of MoS2 is still elusive. Additionally, remarkably little is known about the effect of thermally- and tribologically-induced microstructural variations in MoS2 on the aging processes occurring in the near-surface region of the coating. Here, we employed three analytical techniques, namely, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and grazing-incidence X-ray diffraction (GIXRD), to gain insights into the aging phenomena occurring in sputtered MoS2 coatings before and after tribological testing, while also evaluating the impact of thermally-induced variations in the coating structure on aging. The outcomes of XPS analyses provide evidence that a substantial surface oxidation of MoS2 only takes place under humid conditions. Furthermore, the correlation of XPS, ToF-SIMS, and GIXRD results allowed for the development of a qualitative model for the impact of shear-induced microstructural variations in MoS2 on the transport of water in the near-surface region of this material and on the extent of surface oxidation. These results add significantly to our understanding of the aging mechanisms of MoS2 coatings used in tribological applications and their dependence on environmental conditions.

Characterizing properties of scaffolds 3D printed with peptide-polymer conjugates.

Three-dimensional (3D) printing is a popular biomaterials fabrication technique because it enables scaffold composition and architecture to be tuned for different applications. Modifying these properties can also alter mechanical properties, making it challenging to decouple biochemical and physical properties. In this study, inks containing peptide-poly(caprolactone) (PCL) conjugates were solvent-cast 3D printed to create peptide-functionalized scaffolds. We characterized how different concentrations of hyaluronic acid-binding (HAbind-PCL) or mineralizing (E3-PCL) conjugates influenced properties of the resulting 3D-printed constructs. The peptide sequences CGGGRYPISRPRKR (HAbind-PCL; positively charged) and CGGGAAAEEE (E3-PCL; negatively charged) enabled us to evaluate how conjugate chemistry, charge, and concentration affected 3D-printed architecture, conjugate location, and mechanical properties. For both HAbind-PCL and E3-PCL, conjugate addition did not affect ink viscosity, filament diameter, scaffold architecture, or scaffold compressive modulus. Increasing conjugate concentration in the ink prior to printing correlated with an increase in peptide concentration on the scaffold surface. Interestingly, conjugate type affected final conjugate location within the 3D-printed filament cross-section. HAbind-PCL conjugates remained within the filament bulk while E3-PCL conjugates were located closer to the filament surface. E3-PCL at all concentrations did not affect mechanical properties, but an intermediate HAbind-PCL concentration resulted in a moderate decrease in filament tensile modulus. These data suggest final conjugate location within the filament bulk may influence mechanical properties. However, no significant differences were observed between PCL filaments printed without conjugates and filaments printed with higher HAbind-PCL concentrations. These results demonstrate that this 3D printing platform can be used to functionalize the surface without significant changes to the physical properties of the scaffold. The downstream potential of this strategy will enable decoupling of biochemical and physical properties to fine-tune cellular responses and support functional tissue regeneration.

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.

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.

Nanomechanical Filler Functionality Enables Ultralow Wear Polytetrafluoroethylene Composites.

Surprisingly, certain α-phase alumina filler particles at one to five weight percent can reduce the wear rate of polytetrafluoroethylene (PTFE) by 10,000 times, while other, seemingly comparable α-phase alumina particles provide only modest─by PTFE composite standards─100 times improvements. Detailed studies reveal that size, porosity, and composition of the particles play important roles, but a quantitative metric to support this mechanism is yet to be developed. We discovered the mechanistic importance of friability of the particles, for example, the ability of the particles to fragment at the sliding interface. This work establishes the importance of functionally friable metal-oxide filler particles in creating ultralow wear PTFE-metal-oxide composite systems. We used in situ nanoindentation/electron microscopy experiments to characterize the fracturability of candidate filler particles. A mechanistic framework relating apparent particle fracture toughness and wear is established, where porous low-apparent fracture toughness particles were observed to promote ultralow wear by breaking up during sliding and forming a thin, robust tribofilm, while dense, high-apparent fracture toughness particles abrade the countersurface, limiting the formation of ultralow wear promoting tribofilms. This framework enables use of a new metric to select filler particles for multifunctional, ultralow wear PTFE composites without relying solely on empirical tribological tests of polymer composite materials.

Role of Environment on the Shear-Induced Structural Evolution of MoS2 and Impact on Oxidation and Tribological Properties for Space Applications.

This work investigates the role of water and oxygen on the shear-induced structural modifications of molybdenum disulfide (MoS2) coatings for space applications and the impact on friction due to oxidation from aging. We observed from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that sliding in both an inert environment (i.e., dry N2) or humid lab air forms basally oriented (002) running films of varying thickness and structure. Tribological testing of the basally oriented surfaces created in dry N2 and air showed lower initial friction than a coating with an amorphous or nanocrystalline microstructure. Aging of coatings with basally oriented surfaces was performed by heating samples at 250 °C for 24 h. Post aging tribological testing of the as-deposited coating showed increased initial friction and a longer transition from higher friction to lower friction (i.e., run-in) due to oxidation of the surface. Tribological testing of raster patches formed in dry N2 and air both showed an improved resistance to oxidation and reduced initial friction after aging. The results from this study have implications for the use of MoS2-coated mechanisms in aerospace and space applications and highlight the importance of preflight testing. Preflight cycling of components in inert or air environments provides an oriented surface microstructure with fewer interaction sites for oxidation and a lower shear strength, reducing the initial friction coefficient and oxidation due to aging or exposure to reactive species (i.e., atomic oxygen).

Achieving Ultralow Wear with Stable Nanocrystalline Metals.

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt-Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10-9 mm3 N-1 m-1 , comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt-Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.