Growth, structure, and mechanical wear strength of ultra-thin ion beam and sputter deposited hard coatings

Detailed processing-structure-property relationships have been carried out for existing and candidate carbon-based protective overcoats with possible applications in magnetic recording devices and micro-electromechanical systems. Specifically, ≤6 nm thick amorphous diamond-like carbon (a:C and a:CH_x) and nitrogenated diamond-like carbon (a:CN_x and a:CN_xH_y) overcoats were deposited by sputtering and/or ion beam deposition onto thin film magnetic recording disks. Micro-Raman and x-ray photoelectron spectroscopies (XPS) were performed to study the chemical bond structure near the overcoat surface and near the overcoat/magnetic layer interface for nitrogen doping and nondoping configurations. It was determined the micro-Raman graphitization parameters were sensitive to changes in the bond structure with nitrogen incorporation. The core level XPS analysis showed the near-surface CN_x overcoats consisted of mainly free unbound nitrogen and nitrogen bonded to two neighbors, e.g., C≡N (sp ) bonds. However, sputter cleaning revealed an increase in the contribution of substitutional nitrogen bonded to three neighbors (sp 2 sites) near the overcoat/magnetic layer interface. Also, the valence band XPS analysis showed an increase in the C 2s and C 2s+2p peak positions and areas with increasing nitrogen, which further confirmed the overcoats were becoming more graphitic.; The overcoat coverage of the underlying CoCrPt magnetic medium was determined by core level and angle resolved XPS. The improved coverage of the ion beam deposited overcoats with respect to the sputtered overcoats was due to the increased density of nucleation sites formed before deposition (subplantation) and the increased mobility during deposition resulting in an atomic intermixing zone. Mechanical wear strength was investigated by using nanoscratch and micro-wear scan techniques. The improved strength with nitriding the overcoats was due to the XPS determined wear resistant CrN formation at the overcoat/CoCrPt interface. Surprisingly and contrary to most reports and classical wear laws, the mechanical properties, density, and diamond-like bonding of ultra-thin overcoats did not contribute to the improved mechanical wear strength with nitrogen incorporation. Instead, the interfacial and intrinsic stress of the overcoats and the more probable nitriding of the metal underlayer were responsible for the observed improvements.