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 (Ti_0.5Nb_0.5)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 (Ti_0.5Nb_0.5)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.