Effect of the interfacial microstructure on hardness of multi-layer diamond coatings

Abstract

Multi-layer diamond coatings have been widely employed in cutting tools due to their superior mechanical performance, which mainly stems from their complex multi-layer structures and versatile interfaces. However, it is still lack of investigation on how these interfaces may be tuned to enhance the mechanical properties. Herein, the multi-layer diamond coatings with different modulation periods (ʌ = 1.70, 0.87, 0.29 μm), containing microcrystalline diamond (MCD), sub-microcrystalline diamond (SMCD) and nanocrystalline diamond (NCD) as sub-layers, were synthetized by hot filament chemical vapor deposition (HFCVD). The interfacial microstructures were characterized by scanning transmission electron microscopy (STEM). The hardness (H) and Young's modulus (E) of the coatings were evaluated by nanoindentation tests with two different modes. The planar defects including stacking faults (SFs) and twinning boundaries (TBs) are observed at both MCD/SMCD and NCD/MCD interfaces but absent at SMCD/NCD interfaces. Owing to more interfaces, SFs and TBs as barriers inhibiting the dislocation motion effectively, the H and E of the coatings are enhanced with the decrease of ʌ. Hence, the coating with ʌ of 0.29 μm, possessing more SFs, TBs and more sharp interfaces, obtains the highest hardness (78.39 ± 1.10 GPa). In summary, the mechanical properties of the multi-layer diamond coatings can be greatly improved by the elaborately designed interfacial microstructure, which is of great significance to promote the far-ranging application of diamond coatings.