Graphene nanoplatelet (GNP)-reinforced aluminum oxide (Al 2 O 3 ) composites were sintered by spark plasma sintering in three different compositions (0, 0·5, 5 vol.% GNPs). To investigate the effects of graphene addition on the composites' wear resistance, ball-on-disk wear tests were conducted under very high normal load (40 N) by using a 3-mm-dia. ceramic counterface. Aluminum oxide-0·5 vol.% GNP exhibited 65% improvement in the wear resistance, while aluminum oxide-5 vol.% GNP displayed 53% poorer wear resistance as compared with aluminum oxide. The coefficient of friction was 0·45 for aluminum oxide-0·5 vol.% GNP, 0·40 for aluminum oxide-5 vol.% GNP and 0·60 for aluminum oxide. The highest wear resistance of aluminum oxide-0·5 vol.% GNP is attributed to formation of a continuous, protective and ultrathin graphene tribofilm on the wear surface. Tribofilm formation occurs due to the high shear forces induced by countersurface movement and localized heating, which causes GNP's delamination, overlap and welding together. In the case of aluminum oxide-5 vol.% GNP, poor dispersion and agglomeration of GNP results in a thick and discontinuous graphene tribofilm, which does not protect from the brittle fracture of aluminum oxide grains during wear.
MXene has emerged as an exciting two-dimensional nanomaterial because of its interesting multifunctional properties. In this paper, the authors report the multiscale damping behaviors of pure MAX and MXene. Dynamic loading of a multilayer MXene assembly shows an appreciable loss tangent (tan δ), indicating the energy-dissipation ability of the material. The tan δ value of MXene is recorded to be as high as 0·37, which is about a 200% improvement over that of pure MAX. It is hypothesized that there are multiscale energy-loss mechanisms active in the material. While intralayer bond contraction operates in individual MXene sheets, interlayer compression and sliding/shearing mechanisms are active between the stacked layers. The presence of functional groups, van der Waals interactions and a low coefficient of friction between the MXene sheets provide MXene with an extraordinary energy-dissipation ability. Damping behavior is highly stable in MXene for as high as 50 000 cycles, making it extremely promising for advanced applications requiring superior impact resistance, stability against noise and ability to damp mechanical vibrations.
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