MoVN-Cu Coatings for In Situ Tribocatalytic Formation of Carbon-Rich Tribofilms in Low-Viscosity Fuels

Inhibiting the tribological failure of mechanical assemblies which rely on fuels for lubrication is an obstacle to maintaining the lifetime of these systems with low-viscosity and low-lubricity fuels. In the present study, a MoVN-Cu nanocomposite coating was tribologically evaluated for durability in high- and low-viscosity fuels as a function of temperature, load, and sliding velocity conditions. The results indicate that the MoVN-Cu coating is effective in decreasing wear and friction relative to an uncoated steel surface. Raman spectroscopy, transmission electron microscopy, and electron-dispersive spectroscopy analysis of the MoVN-Cu worn surfaces confirmed the presence of an amorphous carbon-rich tribofilm which provides easy shearing and low friction during sliding. Further, the characterization of the formed tribofilm revealed the presence of nanoscale copper clusters overlapping with the carbon peak intensities supporting the tribocatalytic origin of the surface protection. The tribological assessment of the MoVN-Cu coating reveals that the coefficient of friction decreased with increasing material wear and initial contact pressure. These findings suggest that MoVN-Cu is a promising protective coating for fuel-lubricated assemblies due to its adaptive ability to replenish lubricious tribofilms from hydrocarbon environments.

Accessibility of the pores in highly porous alumina films synthesized via sequential infiltration synthesis

Inorganic nanoporous materials with highly accessible pores are of great interest for the design of efficient catalytic, purification and detection systems. Limited access to the pores is a common problem associated with traditional approaches for the synthesis of porous materials, affecting the functionality of the low-density structure. Recently, infiltration of a nanoporous polymer template with inorganic precursors followed by oxidative annealing was proposed as a new and efficient approach to creating porous inorganic structures with controlled thickness, composition and pore sizes. Here, we report an ultra-high accessibility of the pores in porous films prepared via polymer-swelling-assisted sequential infiltration synthesis (SIS). Using a quartz crystal microbalance technique, we show the increased solvent adsorbing capabilities of highly porous alumina films as a result of high interconnectivity of the pores in such structures. The directionality and highly interconnected nature of the pores are demonstrated in experiments with the partial blocking of pore access by the deposition of a single-layer graphene that is not transparent to solvent. 60% of the pores remain accessible when only 20% of the surface is exposed to solvent. Using humidity detection as an example, we also show that highly porous alumina produced by polymer-swelling-assisted SIS is a promising candidate for sensing applications.

Synthesis of Au-MoS(2) nanocomposites: thermal and friction-induced changes to the structure

The synthesis of Au-MoS2 nanocomposite thin films and the evolution of their structures during film growth, in situ transmission electron microscopy (TEM) heating, and sliding contact were investigated. TEM revealed that the films deposited at ambient (room) temperature (RT) consisted of 2-4 nm size Au particles in a matrix of MoS2. With increasing growth temperatures, the nanocomposite film exhibited structural changes: the Au nanoparticles coarsened by diffusion-driven Ostwald ripening to 5-10 nm size and the MoS2 basal planes encapsulated the Au nanoparticles thereby forming a solid Au-core MoS2 structure. However, when the RT deposited film was heated inside the TEM, the highly ordered MoS2 basal planes did not encapsulate the Au, suggesting that MoS2 surface diffusivity during film growth is different than MoS2 bulk diffusion. Increases in MoS2 crystallinity and coarsening of Au nanoparticles (up to 10 nm at 600 °C) were observed during in situ TEM heating of the RT deposited film. Sliding contact during friction and wear tests resulted in a pressure-induced reorientation of MoS2 basal planes parallel to the sliding direction. The subsurface coarsened Au nanoparticles also provide an underlying load support allowing shear of surface MoS2 basal planes.