Tribochemically Driven Carbon Release Lubrication in Zr-Incorporated (Ti,Nb)2AlC via Selective M─C Bond Cleavage.

Benefiting from the heterodesmic bond character, MAX phase ceramics are promising candidates for integrating mechanical and self-lubricating properties under harsh conditions. However, the robust M─C bonds cause hard metal carbide debris on worn surfaces, resulting in a persistently high coefficient of friction (COF) at room-temperature (RT) to moderate temperatures. Here, focusing on selectively weakening and promoting dissociation of M─C bonds, Zr is selected as a dopant due to its relatively weakest M─C bond among common constituent metals of MAXs, and the (Ti0.5Nb0.5)2AlC solid solution MAX is chosen as the matrix to overcome the limited solid solubility of Zr in Ti2AlC or Nb2AlC, while compensating for the accompanying strength degradation. As confirmed by Raman spectroscopy and XPS, the Zr─C bonds are preferentially cleaved under the sliding friction at even RT, which in-situ releases amorphous carbon species on the worn surface. Remarkably, this tribochemically driven bond cleavage reduces the COF by 27.3% compared with the (Ti0.5Nb0.5)2AlC matrix, while retaining approximately 94% of the hardness and 101% of the elastic modulus. These findings demonstrate a new design strategy for integrated self-lubricating structural materials based on controlled M─C bond cleavage in MAX phases.