Abstract
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.
Highlights
Friction and wear are ubiquitous in daily life
This paper aims at reviewing the humidity dependence of the friction and wear behaviors of materials commonly used as solid lubricants or tribo-elements and the underlying mechanisms
When the amount of interfacial water molecules is large enough form multilayers show that the friction between Si3N4 ball and silicon substrate begin to decrease when to humidity is higher of physisorbed water, the degree of bridge bond formation is substantially reduced since the silicon than 80% Relative humidity (RH), which is mainly attributed to the formation of an adsorbate film acting as a boundary lubricant screens the two fromseparated one another, thereby hindering bond formation leading atoms at the that sliding interface aresurfaces physically due to thick water layers
Summary
Friction and wear are ubiquitous in daily life. Under most circumstances, friction and wear come with energy consumption and material loss. During the process of rubbing or sliding, water can react with the tribo-material due to normal load and shear stress, and further influence friction and wear behavior. This paper aims at reviewing the humidity dependence of the friction and wear behaviors of materials commonly used as solid lubricants or tribo-elements and the underlying mechanisms. Beside the conventional tribo-tests at the macroscale, nanoscale tribological tests based on atomic force microscopy (AFM) are involved. It was wished for but not possible to cover all published data on humidity dependence in the literature; in this review paper, overarching trends rather than individual cases are discussed. Some older data can be found in another review paper and references therein [1]
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