Atomic layer deposition of solid lubricating coatings on particles

Atomic layer deposition (ALD) has been used to deposit ceramic films on the surfaces of particles to improve the slurry and dry powder rheology of bulk powders. An overview of several studies is presented here, which demonstrates the extent and limitations for this ceramic coating platform technology to improve the tribological and/or flow properties of microfine and ultrafine particles. Direct evidence of the effect of the nanoscale ceramic coatings is shown via a significant improvement in flow properties of dry particles, including dynamic, bulk and shear properties obtained using a FT4 Powder Rheometer, and a marked reduction in slurry viscosity at high solids loadings. Microfine zinc powders, similar to those used in alkaline batteries, have been coated using boron nitride ALD films of sub-nanometer thickness, or about 0.1 wt.%. The low surface energy coatings reduced the cohesion of 1–5 μm particles by 52%. A highly-loaded slurry of the same material in concentrated KOH showed a 10–30% reduction in slurry viscosity over a range of shear rates, with a shear-thinning effect at high shear rates. Boron nitride platelets were coated using Al2O3 and SiO2 films to change the surface properties from hydrophobic to hydrophilic. The coated and uncoated powders were dispersed into an epoxy to evaluate the solids loading to viscosity ratio. The ALD films improved the particle–resin adhesion and decreased the viscosity of equivalently-loaded slurry of uncoated powder. Coated microfine nickel, aluminum and iron powders were also dispersed into epoxies, and lower viscosities and yield stresses were observed due to ceramic–epoxy interactions being more favorable than metallic–epoxy interactions. The ALD platform can be used to modify surfaces of primary particles in order to change the interparticle and particle–liquid forces, which provides a lubricating effect without detracting from the bulk properties of the core particles themselves.

Graphical AbstractALD has been used to deposit ceramic films on the surfaces of particles to improve the slurry and dry powder rheology of bulk powders. The ALD coatings modified the surfaces of primary particles, in order to change the interparticle and particle–liquid forces, and provided a lubricating effect without detracting from the bulk properties of the core particles themselves.Figure optionsDownload as PowerPoint slide