Scratch–wear resistance of nanoscale super thin carbon nitride overcoat evaluated by AFM with a diamond tip

Atomic force microscopy (AFM) was used to investigate the scratch–wear resistance of ultrathin superhard carbon nitride overcoats of thickness 1, 2, 4, 6, 8 and 10 nm. When sliding against a diamond tip of radius less than 100 nm in the mode of line scratch, the thin overcoats of thickness 1–4 nm exhibited poor wear resistance, especially at contact pressures larger than 25 GPa, with a wear depth of 4 nm or larger and a specific wear rate up to 0.8×10−4 mm3/nm. Non-contact mode imaging of a scratched surface has shown that a large amount of nanoscale wear debris was formed along the two sides of the scratched grooves, which indicated that the material removal mechanism of such thin overcoats was due to brittle fracture and abrasive wear, both in the nanoscale. In comparison, the overcoats of thickness 6–10 nm exhibited wear resistance with a specific wear rate less than 0.2×10−4 mm3/nm. Instead, the least debris was observed on the scratched surfaces and only shallow grooves were left after scratching. It means that the grooves were formed by both plough and plastic deformation. The micro/nanowear mechanism and thickness effect of coating on scratch resistance were discussed.