Abstract: Due to strongly tribological atmosphere sensitivity of carbon-based coatings, it is of extreme significance to investigate their friction and wear behaviors in different atmospheres. In this letter, duplex nc-TiC/a-C(Al) nanocomposite carbon-based coating coupled with high hardness, low internal stress and high adhesion strength was successfully fabricated using magnetron sputtering process. The friction and wear behaviors of as-fabricated coating were evaluated in dry N₂, humid N₂, air, dry O₂, and humid O₂ atmospheres, respectively. Results show that the asfabricated coating possesses very high friction and wear due to the strong covalent bond interactions at the sliding interface caused by the free -bonds on the coating surface in dry N₂ atmosphere. Whereas the free -bonds can be efficiently terminated and passivated by water and/or oxygen molecules to weaken the strong covalent bond interactions to result in low friction and wear of coating in humid N₂, air, dry O₂, and humid O₂ atmospheres. The compact and homogeneous carbonaceous tribo-layer on the counterpart is mainly responsible for the lowest friction and wear of coating in humid N₂ atmosphere. Whereas the tribo-layer can be restrained to a certain extent by the tribochemical reaction, especially it results in a nearly negligible carbonaceous tribo-layer on the counterpart in dry O₂ atmosphere, which is mainly responsible for largely increased friction and wear of coating.

Conclusion

HRTEM bright-field images and SAED pattern of nc-TiC/a-C(Al) nanocomposite carbon-based coating

The following conclusions can be drawn from the current study:

1. The as-fabricated nc-TiC/a-C(Al) carbon-based coating exhibits typical nanocrystallite/amorphous microstructure and good combined mechanical properties.

2. In dry N₂ atmosphere, nc-TiC/a-C(Al) coating present very high friction and wear due to the strong covalent bond interactions at the sliding interface caused by the free-bonds on the coating surface.

3. The free -bonds of surface carbon atoms are efficiently terminated and passivated by water and/or oxygen molecules to weaken the strong covalent bond interactions, resulting in a reduced friction, and wear of nc-TiC/a-C(Al) coating in the humid N₂, air, dry O₂, and humid O₂ atmospheres.

4. The compact and homogeneous tribo-layer formed on the counterpart is mainly responsible for the lowest friction and wear of nc-TiC/a-C(Al) coating in humid N₂ atmosphere.

5. The tribo-layers can be restrained to a certain extent by tribo-chemical reaction among the coating, steel ball, and oxygen molecules; especially, it results in nearly negligible carbonaceous tribo-layer on the counterpart in dry O₂ atmosphere, which is mainly responsible for largely increased friction and wear of coating similar to that in dry N₂ atmosphere.

Published in Tribol Lett(2012) 47:435–446