Diamond thin film depostion on reaction bonded silicon carbide (RB-SiC) substrates using a hot-filament chemical vapour deposition (HF-CVD) system has been explored in this study. Microcrystalline (MCD), nanocrystalline (NCD) and functionally graded diamond films of ~5 µm thickness were synthesised, and the structural, topological and mechanical/tribological properties evaluated and compared. Nanoindentation studies revealed the hardness values to lie in the range of ~ 25 GPa for the NCD and FGC coatings to ~ 38 GPa for the MCD film.
Diamond films were deposited on RB-SiC in a single-step continuous process by varying the HF-CVD process parameters such that the initially deposited diamond grains were microcrystalline, which became progressively finer with further deposition (labelled C-MC). In contrast, intermittently grown films with one, two and three-layered structures (labelled Ix-MC, x being the number of layers), each layer with decreasing grain size was deposited with the same recipes as that of the C-MC. Fine grains in the range of 0.2–0.7 μm were identified in the C-MC and sharply faceted diamond crystallites having sizes between 0.5- 2 μm were seen in the intermittently deposited coatings. The hardness values were 71 GPa, 56 GPa and 46 GPa for the one, two and three layered intermittent coatings respectively. In contrast, a lower hardness of 29 GPa was obtained in C-MC. The friction coefficients of all the diamond films were measured up to 106 sliding cycles (530 m) using a ball-on-disc universal tribometer. In the initial stages of sliding, the continuously grown C-MC exhibited a friction coefficient ~0.17, while the intermittently coated three-layered film (I3-MC) showed a friction coefficient of 0.28. However, beyond 40,000 sliding cycles, the friction coefficient of I3-MC showed a value of ~0.05, which was slightly lower than C-MC (~0.06).
The temperature-dependent residual stresses and thermal stabilities of the C-ML and Ix-ML (sliced from C-MC and Ix-MC ) coatings after annealing at 100o intervals from 100 ℃ – 800 ℃ were investigated. 2D/3D Raman mapping, cluster analysis, and sp2/sp3 ratio computations were carried out at all stages and the results compared. The residual stresses of the as-deposited coatings, I1-ML, I2-ML, I3-ML, and C-ML, were compressive with values of ~1.09 GPa, ~0.90 GPa, ~0.62 GPa and ~1.35 GPa, respectively, which progressively shifted to the tensile regime on annealing and exhibited values of ~0.50 GPa, ~0.50 GPa, ~0.61 GPa and ~1.75 GPa respectively, after annealing at 700 ℃. The sp2/sp3 ratios of the C-ML and I3-ML coatings dropped from 0.94% to 0.54% and 1.07% to 0.54%, respectively, as the annealing temperature increased from 100 to 700 ℃. All coatings were completely oxidised at 800 ℃ except I3-ML, where patches of the original coating persisted even after annealing at 800 ℃.