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The friction and wear behavior of materials are not intrinsic properties, but extrinsic properties; in other words, they can drastically vary depending on test and environmental conditions. In ambient air, humidity is one such extrinsic parameter. This paper reviews the effects of humidity on macro- and nano-scale friction and wear of various types of materials. The materials included in this review are graphite and graphene, diamond-like carbon (DLC) films, ultrananocrystalline diamond (UNCD), transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), boric acid, silicon, silicon oxide, silicates, advanced ceramics, and metals. Details of underlying mechanisms governing friction and wear behaviors vary depending on materials and humidity; nonetheless, a comparison of various material cases revealed an overarching trend. Tribochemical reactions between the tribo-materials and the adsorbed water molecules play significant roles; such reactions can occur at defect sites in the case of two-dimensionally layered materials and carbon-based materials, or even on low energy surfaces in the case of metals and oxide materials. It is extremely important to consider the effects of adsorbed water layer thickness and structure for a full understanding of tribological properties of materials in ambient air.
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Switchable Slippery Surfaces Controlled by a Humidity‐Induced Glass Transition of Polyelectrolyte‐Grafted Brushes
Abstract Polymer brushes have found extensive applications as nano‐scale surface coatings with responsive properties, particularly in achieving tunable friction in solvent environments. Here, a special property of hygroscopic polyelectrolyte‐grafted brushes, where the friction forces change by over two orders of magnitude within a narrow range in humidity is reported. Using mechanical measurements of nano‐scale modulus and water absorption coupled with friction and surface‐sensitive spectroscopy, this sharp change in friction is controlled by a humidity‐induced glass transition that abruptly shifts the mode of sliding is demonstrated. Contrary to expectations based on conventional thinking regarding brush lubrication, friction remains large and humidity‐independent below the glass transition even for systems that absorb as much as 30–40% water by volume. This results in an abrupt change in friction past the glass transition humidity. Tuning the chemistry of brushes and their humidity‐induced glass transition offers the tunability to control the on/off friction (or slipperiness) for nanoactuators, ratchets, and catheters, without the need for externally applied lubricating liquids.
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- Award ID(s):
- 2208464
- PAR ID:
- 10532209
- Publisher / Repository:
- Advanced Materials Interfaces
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 11
- Issue:
- 13
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/admi.202400025
