Tracing C changes in a C/CrC PVD coating using Raman spectroscopy and EELS

A C/CrC coating was deposited by unbalanced magnetron sputtering of graphite and Cr metal targets in argon atmosphere for low friction tribological applications. The coating possessed a nanocomposite structure with amorphous carbon embedded in a metastable fcc CrC matrix. The nanocomposite structure was annealed at 750°C in Ar+5%H2 atmosphere for 30 minutes to evaluate its thermal stability. Microstructures of the as-deposited and annealed coating were characterised using Raman spectroscopy and cross sectional transmission electron microscopy coupled with electron energy loss spectroscopy (TEM/EELS). Raman spectroscopy suggested the presence of graphitic carbon in the coating after annealing. EELS (ELNES of C K) combined with HRTEM investigation of the cross sections revealed graphitic C at the very top (~200nm) of the coating and amorphous C at the remaining part of the coating after annealing at 750°C. The advantages and limitations of Raman spectroscopy and TEM techniques in studying C/CrC coating are also discussed.