Degradation of a C/CrC PVD coating after annealing in Ar + H-2 at 700 degrees C studied by Raman spectroscopy and transmission electron microscopy

The thermal stability of a hydrogen free C/CrC coating was the focus of this study. The coating was deposited by unbalanced magnetron sputtering of graphite and Cr metal targets in a non-reactive argon atmosphere at high ion irradiation conditions. The coating possessed a nanocomposite structure with amorphous carbon embedded in a metastable NaCl (B1) structure CrC matrix. The nanometre amorphous carbon clusters formed layers within the CrC matrix, producing a self-assembled multilayer structure. In this paper, degradation of the multilayer nanocomposite structure and NaCl (B1) structure CrC phase was evaluated by annealing at 700 degrees C in Ar + 5%H-2 atmosphere for 30 minutes. Microstructures of the as-deposited and annealed coating were characterised using Raman spectroscopy and cross-sectional transmission electron microscopy (TEM) coupled with electron energy filtered mapping. Raman spectroscopy suggested the presence of graphitic carbon in the coating after annealing, together with a trace of Cr2O3 associated with coating growth defects. TEM investigation of the cross sections of annealed coating revealed regions of C enrichment at the very top (similar to 40 nm) and bottom (similar to 10 nm) of the coating, which was confirmed by Raman spectroscopy. However, the central region of the coating retained its composite C/CrC multilayer structure. Tentative mechanisms of C enrichment are proposed. The advantages and limitations of Raman spectroscopy and TEM techniques in studying C/CrC coating are also discussed.