Long-term stability of hydrogenated DLC coatings: Effects of aging on the structural, chemical and mechanical properties

• Hydrogenated DLC films were analyzed before and after a three years aging period.
• The coatings exhibited significant structural stability over time.
• Hardness and surface morphology remained unaffected.
• Stress decreased due to aging but was not associated with a sp3 to sp2 conversion.
• Increased surface oxidation, higher friction coefficient and loss of hydrophobicity were observed after aging.

Long-term stability is an essential condition for the commercial use of protective coatings, yet often remains overlooked in the literature. Here we report the effects of long-term environmental aging on the properties of hydrogenated diamond-like carbon (DLC) films. A range of DLC coatings produced by plasma-enhanced chemical vapor deposition were first thoroughly characterized and then stored for three years before the second set of analysis. Raman spectroscopy showed that the films exhibited excellent structural stability during aging, observing no sign of sp3 to sp2 conversion. Similarly, the hardness and smoothness of the DLC coatings remained unchanged, despite the observed relaxation of their intrinsic stress with time. However, X-ray photoelectron spectroscopy analyses provided evidence of aging-induced surface oxidation, which was confirmed by reduced hydrophobicity (water contact angle dropped to 65°). Overall, these findings suggest that DLC possesses a suitable long-term stability when exposed to environmental conditions.