Structural and tribological characterization of protective amorphous diamond-like carbon and amorphous CNx overcoats for next generation hard disks

Abstract

Further insight into processing-structure-property relationships have been carried out for existing and candidate carbon-based protective overcoats used in the magnetic recording industry. Specifically, 5 nm thick amorphous diamond-like carbon $\mathrm{(a:}\mathrm{C})$ and nitrogenated diamond-like carbon $\mathrm{(a:}{\mathrm{CN}}_x)$ overcoats were deposited by low deposition rate sputtering onto a thin film disk consisting of either CoCrPt/CrV/NiP/AlMg or CoCrPt/CrV/glass. The wear durability and frictional behavior of these hard disks were ascertained using a recently developed depth sensing reciprocating nanoscratch test. It was determined that the ${\mathrm{CN}}_{\mathrm{0.14}}\mathrm{/CoCrPt/CrV/glass}$ disk exhibited the most wear resistance, least amount of plastic deformation, and lowest kinetic friction coefficient after the last wear event. Core level x-ray photoelectron spectroscopy (XPS) results of sputter cleaned overcoats indicated that nitrogen up to 14 at. % incorporated into the amorphous network resulted in these improvements near the overcoat/magnetic layer interface, since there was an increase in the number of $\mathrm{N}{\mathrm{-sp}}^3$ C bonded sites in a predominantly $\mathrm{N}{\mathrm{-sp}}^2$ C bonded matrix. However, nonsputter cleaned overcoats exhibited a more graphitic pyridine-like (nondoping configuration) structure near the surface as evidenced by the increase in $\mathrm{C=N}$ versus C–N bonds and the valence band XPS determined appearance of the $\text{2p-π}$ band near the Fermi level ${\mathrm{(E}}_F\mathrm{).}$ Therefore, XPS sputter cleaning revealed a gradient in the chemical nature of the overcoats through the thickness. In addition, micro-Raman spectroscopy established that a further increase of nitrogen (⩾18 at. %) weakened the overcoat structure due to the formation of terminated sites in the amorphous carbon network, since nitrogen failed to connect the ${\mathrm{sp}}^2$ domains within the network. This, in conjunction with an increase in the intensity of the $\text{2p-π}$ band from the valence band XPS spectra and the increase in the G-band position and $I_D{\mathrm{/I}}_G$ ratio from the Raman spectra, confirmed the increase in the size and number of ${\mathrm{sp}}^2$ bonds in the ${\mathrm{CN}}_{\mathrm{0.18}}$ overcoat.