Tribological Behavior of NiAl-Layered Double Hydroxide Nanoplatelets as Oil-Based Lubricant Additives

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Layered double hydroxides (LDHs) are a class of naturally occurring inorganic minerals that are composed of divalent and trivalent metal cations. In this study, three different sized NiAl-LDH nanoplatelets were synthesized by varying crystallization time during the microemulsification process. The layered structure and three-dimensional size of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As lubricant additives, their tribological properties in base oil were evaluated by use of a ball-on-disk reciprocating tribometer under three different loads: 50, 100, and 150 N (which created peak Hertz pressures of 1.74, 2.16, and 2.47 GPa). Under contact pressures of up 2.16 GPa, not only did the coefficient of friction (COF) decrease by about 10% after nano-LDHs were added but also the wear performance improved substantially. These improvements resulted from a protective tribolayer formation on the contact interface, as revealed by detailed surface and structure analytical studies. In particular, cross-sectional TEM images revealed that the larger size nanoplatelets (NiAl-24h), rather than the smaller ones (NiAl-6h) showed the best and most stable tribological performance. This was mainly because of their higher degree of crystallinity, which in turn resulted in the formation of a tribofilm with far superior mechanical properties during sliding. Owing to the simple synthetic method and superior tribological properties as oil-based additives, nano-LDHs hold great potential for use in demanding industrial applications in the future.