Long-term stability of hydrogenated DLC coatings: Effects of aging on the structural, chemical and mechanical properties M. Cloutier a,b , C. Harnagea c , P. Hale c , O. Seddiki c , F. Rosei c,d , D. Mantovani a,b, ⁎ a Laboratory for Biomaterials and Bioengineering, CRC-I, Dept Min-Met-Materials Engineering, Laval University, Québec, QC, Canada b Laboratory for Biomaterials and Bioengineering, CRC-I, CHU de Québec Research Center>, Québec, QC, Canada c Centre Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique, Université du Québec, Varennes, QC J3X 1S2, Canada d Center for Self-Assembled Chemical Structures, McGill University, H3A 2K6 Montreal, QC, Canada abstract article info Article history: Received 8 January 2014 Received in revised form 14 June 2014 Accepted 7 July 2014 Available online 14 July 2014 Keywords: Diamond-like carbon a-C:H Plasma enhanced CVD Aging Long-term stability Surface characterization Microstructure Mechanical properties Coatings 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 hy- drogenated 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, observ- ing no sign of sp 3 to sp 2 conversion. Similarly, the hardness and smoothness of the DLC coatings remained un- changed, 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 suit- able long-term stability when exposed to environmental conditions. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The high stability of amorphous diamond-like carbon (DLC) films has been actively promoted in recent years and has encouraged its use in various applications as a durable protective coating, ranging from bio- active biomedical implants [1–4] to magnetic data storage disks [5,6]. This interest stems from the unique set of properties of DLC, which in- cludes high hardness and elastic modulus, excellent wear resistance, low friction coefficient, high chemical inertness and atomic smoothness [6–9]. Like diamond, several of their beneficial properties are conferred by the strong σ bonds of sp 3 hybridized carbon atoms. However, unlike diamond, they are achieved in an isotropic, amorphous thin film con- taining both sp 2 and sp 3 bonded carbon in nanometer-sized clusters and, in some cases, hydrogen [6]. This particular microstructure arises from the bombardment of energetic species during growth via a physi- cal process termed subplantation [10–12]. Penetration of carbon or hydrocarbon ions creates local quenched-in increases in density, which causes the local bonding to convert to sp 3 , but also generates high compressive stresses in the film. The latter phenomenon can result in delamination, especially on non-optimized interfaces, and generally limits the thickness of deposited films, as stresses can act as a driving force for debonding from the substrate [13]. Although the importance of these intrinsic stresses in the formation of sp 3 bonds is still controversial (McKenzie et al. [14,15] vs Robertson et al. [6,16]), they remain a defining characteristic of DLC coatings, which brings forth questions about their long-term stability. The ther- mal stability of DLC coatings has been extensively studied [17–20], yet very little work has been reported on the effects of long-term environ- mental aging (i.e. stability in environments that are not deliberately harsh), either on stress-related degradation or on the stability of the aforementioned properties. However, this is of particular interest for ap- plications such as protective and barrier coatings where DLC films are expected to maintain their protective behavior for several years [21–23]. Furthermore, it has been extensively reported that plasma- deposited coatings and other thin films materials are particularly prone to progressive surface alterations induced by environmental aging [24–27]. The study of the long-term stability and the occurrence Diamond & Related Materials 48 (2014) 65–72 ⁎ Corresponding author at: Laboratory for Biomaterials and Bioengineering, CRC-I, Dept Min-Met-Materials Eng, University Hospital Research Center, Laval University, PLT1745, Québec, QC G1A 0V6, Canada. Tel.: +1 418 656 2131x6270. E-mail addresses: maxime.cloutier.2@ulaval.ca (M. Cloutier), harnagea@emt.inrs.ca (C. Harnagea), rosei@emt.inrs.ca (F. Rosei), diego.mantovani@gmn.ulaval.ca (D. Mantovani). URL's: URL:E-mail addresses:E-mail address: http://www.lbb.ulaval.ca (M. Cloutier), http://www.lbb.ulaval.ca (D. Mantovani). http://dx.doi.org/10.1016/j.diamond.2014.07.002 0925-9635/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond