Thick stress-free amorphous-tetrahedral carbon films with hardness near that of diamond

Abstract

We have developed a process for making thick, stress-free, amorphous-tetrahedrally bonded carbon $\mathrm{(a-t}\mathrm{C})$ films with hardness and stiffness near that of diamond. Using pulsed-laser deposition, thin $\mathrm{a-t}\mathrm{C}$ films (0.1–0.2 μm) were deposited at room temperature. The intrinsic stress in these films (6–8 GPa) was relieved by a short (2 min) anneal at 600 °C. Raman and electron energy-loss spectra from single-layer annealed specimens show only subtle changes from as-grown films. Subsequent deposition and annealing steps were used to build up thick layers. Films up to 1.2 μm thick have been grown that are adherent to the substrate and have low residual compressive stress $\text{(<0.2\hspace{0.17em}}\mathrm{GPa}\mathrm{).}$ The values of hardness and modulus determined directly from an Oliver–Pharr analysis of nanoindentation experimental data were 80.2 and 552 GPa, respectively. We used finite-element modeling of the experimental nanoindentation curves to separate the “intrinsic” film response from the measured substrate/film response. We found a hardness of 88 GPa and Young’s modulus of 1100 GPa. From these fits, a lower bound on the compressive yield stress of diamond $\text{(∼100\hspace{0.17em}}\mathrm{GPa})$ was determined.