Understanding of the superior stability of Silicon- and oxygen-containing hydrogenated amorphous carbon in harsh environments
Amorphous carbon-based materials have been used over the last three decades in a wide range of technological applications because of their impressive properties, notably their high strength and strain to failure, ability to form smooth, continuous, ultra-thin, conformal coatings, as well as their outstanding tribological performance (i.e., low friction, wear, and adhesion). This has resulted in their use as coatings for high-performance tools, microelectromechanical systems, atomic force microscope probes, and overcoat materials for hard disk drives.
To enhance the reliability of amorphous carbon-based materials under harsh environmental conditions, their growth procedure can be tailored by introducing dopants or alloying elements. Among the several doped/alloyed versions of a-C:H films that have been synthesized over the last decades, silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) is a particularly promising class of multicomponent materials for several applications. The incorporation of silicon and oxygen increases the thermal stability, while reducing the residual stress and not significantly affecting the mechanical properties. Furthermore, a-C:H:Si:O films have been shown to possess good tribological performance across a broader range of conditions and environments compared to a-C:H films.
Tevis D. B. Jacobs and colleagues have shown the reliability of these films by exposing them to the harsh conditions of the low Earth orbit (LEO) environment (hyperthermal atomic oxygen, thermal cycling, ultraviolet radiation) aboard the International Space Station. By X-ray photoelectron spectroscopy measurements, they have shown the degradation of the near-surface region of a-C:H:Si:O through breakage and subsequent oxidation of carbon-carbon bonds as well as formation of a silica layer (shift of the silicon 2p signal to higher binding energies).
They have investigated, in detail, the chemical reactions and structural changes occurring in the near-surface region of a-C:H:Si:O under harsh environmental conditions (namely elevated temperatures, oxidative environments, and low Earth orbit (LEO) environment). The in situ spectroscopic investigation of the thermally-induced structural changes occurring in the near-surface region of a-C:H:Si:O indicated that introducing silicon and oxygen in a-C:H at a level of, respectively, 6 ± 1 at.% and 3 ± 1 at.% slightly enhances the thermal stability under high vacuum conditions. Photoelectron spectroscopy and transmission electron microscopy measurements suggested that exposing a-C:H:Si:O either to elevated temperatures under aerobic conditions or to the harsh LEO environment (aboard the International Space Station) leads to carbon volatilization in the near-surface region with the formation of a silica surface layer, which prevents the underlying carbon phase from further reacting with oxygen and/or water and being eroded. Our findings shed light, for the first time, on the origin of the superior thermal and thermo-oxidative stability of a-C:H:Si:O compared to undoped a-C:H.
In conclusion, this study demonstrates that the introduction of small amounts of silicon (6 ± 1 at.%) and oxygen (3 ± 1 at.%) in a-C:H moderately enhances the thermal stability under high vacuum conditions, but tremendously increases the thermo-oxidative stability and the resistance to the harsh conditions of the LEO environment. Under high vacuum conditions, the presence of silicon and oxygen in a-C:H:Si:O results in a higher activation energy for the conversion of sp3-to sp2-bonded carbon compared to the activation energy for the same structural transformation in a-C:H. Exposing a-C:H:Si:O either to elevated temperatures under aerobic conditions or to the harsh LEO conditions leads to carbon volatilization in the near-surface region with the formation of a silica surface layer, which protects the underlying carbon phase from further erosion. These findings provide guidance to develop strategies for rationally designing novel carbon-based materials that can withstand harsh environmental conditions.