Observations of depth-sensing reciprocating scratch tests of DLC and nitrogenated-DLC overcoats on magnetic disks

In this investigation, the wear durability of existing and candidate protective overcoats and substrates was examined. Specifically, 5 nm thick diamond-like carbon (DLC) and nitrogenated diamond-like carbon (N-DLC) overcoats were deposited by sputtering onto glass, glass-ceramic, and NiP/AlMg substrates. The magnetic medium was a 15 nm thick layer of CoCrPt deposited on a 50 nm thick underlayer of CrV. The wear resistance of the hard disks was determined by a recently developed depth sensing reciprocating scratch test using the Nano Indenter II. During the scratch tests, a constant normal load of 30 μN was maintained at an indenter velocity of 2μ/sec. It was found the N-DLC/CoCrPt/CrV/glass disk exhibited the most wear resistance and least amount of plastic deformation after the last wear event. Conversely, the N-DLC/CoCrPt/CrV/NiP/AlMg disk displayed the least wear resistance even though the magnitude of the elastic recovery was the greatest. This amount of recovery was influenced by the high elastic modulus of the NiP/AlMg substrate. Consequently, the scratch test failed to isolate the intrinsic properties of the overcoat, however it provided a very powerful means of quantitatively assessing the overall response of the whole magnetic disk. This is more relevant since it simulates the response the disks see in performance. In addition, a discrete amount of nitrogen up to 14 atomic % incorporated into the amorphous network resulted in an increase in overcoat durability compared to the DLC overcoat. This was attributed to an increase in the XPS determined number of N-sp³ C bonded sites in a predominantly N-sp² C bonded matrix. However, with increasing nitrogen concentrations ≥18%, the film structure was weakened due to the micro-Raman spectroscopy determined formation of terminated sites in the amorphous carbon network since nitrogen failed to connect the sp² domains within the network.