Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

The tribological properties of diamond-like carbon (DLC) coatings produced by pulsed laser deposition (PLD) are investigated. Films are grown onto steel substrates to 0.5 mu m using a 248 nm laser to ablate graphite and polycarbonate targets in high vacuum. Chemical bonding is studied with Raman, XPS and EELS techniques; mechanical and tribological properties are evaluated using microindentation and ball-on-disk friction tests. Coatings grown from graphite targets are amorphous DLC (a-C), while those grown from polycarbonate targets are amorphous hydrogenated carbon (a-C:H). The hardness of the a-C coatings is 55-70 GPa and the hardness of the a-C:H coatings is 12-20 GPa depending on the substrate bias. Friction coefficients of the coatings against steel and sapphire balls are determined in several environments: in air as a function of relative humidity, in dry nitrogen, and in 10 Pa vacuum. For a-C coatings, the friction coefficients are typically below 0.1 and are as low as 0.03 in dry nitrogen. In wear tests, a critical contact pressure of 1.4 GPa led to catastrophic adhesive failure of a-C coatings, whereas failure of a-C:H coatings is by wear-through after 5 x 10(3) cycles, Extremely low wear rates of 10(-9) mm(3) h(-1) m(-1) are found for a-C coatings at the contact pressure of 0.8 GPa.