Microstructure and thermal stability of arc-evaporated Cr–C–N coatings

Abstract

The role of C incorporation in the microstructure and thermal stability of arc-evaporated Cr–C–N coatings is explored via reactive growth in a mixed C₂H₄–N₂ environment. C is found to react more readily than N at both the Cr cathode and the coating surfaces, so that a C₂H₄-to-N₂ flow ratio of only 1% yields a C-to-N ratio of approximately 10% within the coatings. The as-deposited microstructures consist primarily of the δ-Cr(C, N) phase and possess high compressive residual stresses, which decrease with increasing C content. Post-deposition annealing up to 700°C results in depletion of lattice defects, and concomitant reductions in stress and coating hardness, together with phase transformations which suggest metastable phase formation during growth. Apparent activation energies for this lattice defect are found to be in the range expected for bulk diffusion of N and C (2.4–2.8 eV). The results suggest that inclusion of small amounts of C in this system offers the ability to reduce internal stresses while maintaining defect-related hardness increases, permitting growth of thicker and thus more wear-resistant coatings.

Notes

† The fabrication procedure of the reference sample has been given by Gall et al. (2002) and its composition was determined by Rutherford back-scattering techniques.

† The lattice parameter of the CrC phase is imprecisely known; experimental observations of ion-implanted Cr coatings report a ₀(CrC) = 4.03 Å (Liu and Cheng 1992) (no possible stress effects were accounted for), while first-principles calculations (using the augmented plane-wave method) yield a value of a ₀(CrC) = 4.07 Å.

† Single-crystal moduli of TiN yield E(100)/E(111) = 556/418, where E(100) and E(111) are in gigapascals (Kim et al. 1992), so that substantial elastic anisotropy exists. Similar values have yet to be reported for CrN.