Atomic layer deposition of solid lubricating coatings on particles Oliver J. Kilbury a , Kathryn S. Barrett a , Xiaowei Fu b , John Yin b , Dean S. Dinair c , Christopher J. Gump d , Alan W. Weimer a , David M. King a, d, ⁎ a Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, USA b Freeman Technology, Worcestershire, UK c LUM Americas, Boulder, CO, USA d ALD NanoSolutions, Inc, Broomfield, CO, USA abstract article info Available online 31 January 2012 Keywords: Nanocomposites Powder flow Rheology Atomic layer deposition (ALD) 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 tribo- logical and/or flow properties of microfine and ultrafine particles. Direct evidence of the effect of the nano- scale 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 Al 2 O 3 and SiO 2 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Surface coatings that provide enhanced lubricity to particles in the dry or slurry state would be desirable in a wide variety of industries [1,2]. Improving the “flowability” of powders can lead to reduced wear on the internal surfaces of industrial equipment, conveying lines and similar, which can reduce product contamination, repair/ maintenance downtime and energy consumed during the manufac- ture of products that incorporate powders [3]. Each of these negative- ly impacts the operating costs of industrial manufacturing processes. Oftentimes flow additives, including liquid-phase surfactants and emulsifiers or solid-phase inert particles, will be incorporated into slurries or pastes to improve the flowability of the material, though these can be costly and only serve to decrease the content of the use- ful material that needs to be conveyed [4]. The ALD film thicknesses required to achieve a demonstrable lubricating effect are relatively thin, and as such end up being a very small, oftentimes negligible, weight percentage of the final slurry. The cost of applying the ALD lubricating film can be cheaper than the cost of the flow additives that yield similar performance, even for commodity powders found in cements, paints and other products where consistent dispersion within an end-use product is paramount to product and brand integrity. Ensuring that these flow additives are well dispersed within a system can be a more tenuous process than ensuring the primary phase continues to flow from point A to point B. The chaotic process of slurry destabilization, which can lead to irreversible segregation, can be a costly problem to diagnose in terms of why, how, where and when, and each may vary even with minute batch-to-batch variations within equipment, processing conditions and the slurries Powder Technology 221 (2012) 26–35 ⁎ Corresponding author at: ALD NanoSolutions, Inc., 580 Burbank St., Unit 100, Broom- field, CO 80020-7166, USA. Tel.: +1 720 980 5930; fax: +1 303 410 6520. E-mail address: dking@aldnanosolutions.com (D.M. King). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.12.021 Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec