Silver surface segregation in Ag-DLC nanocomposite coatings

Highlights

• Analysis of the distribution of Ag nanoparticles through the coatings thickness.

• Silver segregation to the AgDLC coatings surface at room temperature conditions.

• Importance of the carbon matrix morphology on the control of Ag segregation.

Abstract

AgDLC coatings were deposited by magnetron sputtering in order to evaluate: i) the Ag nanoparticle size distribution along the coatings thickness and ii) the silver stability in DLC coatings. Four different coatings were deposited, two AgDLC nanocomposite coatings containing 20 at.% of Ag, with thicknesses of 250 nm and 1000 nm, a 250 nm AgDLC nanocomposite coating with carbon barrier layer of 75 nm and a multilayer coating, consisting of a pure Ag layer with a carbon barrier layer of 75 nm. The coatings were characterized with respect to their structure (by means of X-ray diffraction (XRD)), morphology (by scanning electron microscopy (SEM)) and Ag distribution in depth (by glow discharge optical emission spectroscopy (GDOES)). The AgDLC nanocomposite coatings are composed of Ag nanograins (with a size of about 3 nm) dispersed in an amorphous carbon matrix. The SEM results suggest a bimodal size distribution along the coatings thickness, it was found that Ag forms nanoparticles and aggregates with sizes ranging from 14 nm up to 52 nm on the coating's surface, whereas in bulk, no Ag nanoparticles were visible by SEM. The monitoring of the Ag stability at room temperature conditions during a period of 6 months showed that Ag segregates to the coatings surface, in the AgDLC nanocomposite coating with 1000 nm forming Ag nanofibers, while the thinner AgDLC (250 nm) as well as the multilayer (Ag + DLC and AgDLC + DLC) coatings were stable over time.

Introduction

Diamond-like carbon (DLC) coatings are largely used as wear protective coatings, owing to their low friction coefficient and high hardness; moreover, these coatings are chemically inert, presenting outstanding corrosion resistance. Presently, carbon-based coatings find numerous industrial applications in different fields, namely, microelectronics, optics, manufacturing and biomedical devices [1]. The incorporation of different metal atoms (e.g. Ti [2], Zr [3], W [4], Cu [5], Ag [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], among others) enables us to tailor the carbon coating's functional properties, namely, the tribological behavior, residuals stress state and, consequently, the coating's adhesion, corrosion resistance, electrical resistivity and biological response [21]. In the particular case of silver-doped DLC, it has been reported that these coatings are able to i) reduce the residual stress state, thus improving the coating's adhesion to different substrates [9], [13], [18], ii) provide antibacterial properties [7], [17] and iii) improve the tribological behavior [14], [15], [20]. AgDLC coatings have been pointed as an effective coating for biomedical implants due to their good corrosion resistance, wear resistance [7], [20], antibacterial activity [7], [17] and hemocompatibility [10]. Moreover, Endrino et al. [11] reported that amorphous carbon coatings containing about 5.5 at.% of Ag were not toxic against mouse MC3T3 osteoblastic cells.

Numerous studies report that AgDLC coatings are able to form nanocomposite coatings, where metallic Ag nanoparticles are dispersed in amorphous carbon matrix, with the size of these nanoparticles being strongly dependent on the deposition process and parameters and also on the amount of Ag incorporated in the carbon coating [7], [9], [14], [15]. In fact, the functional properties of these nanocomposite coatings are determined by the size of Ag nanoparticles and their distribution along the coating's thickness; thus, in order to achieve the desired functional properties, the structure and morphology of these coatings must be precisely controlled. Previous works of the group suggested that Ag nanoparticles are not stable in DLC coatings, it was found that silver is able to diffuse and segregate in the coating's surface even at room temperature conditions, which had a strong influence on the tribological behavior [18]. The mobility of Ag in different nanocomposite coatings has been reported in numerous works, it was found that Ag forms a silver-rich layer on the coating's surface. Several reports state a nonuniform Ag distribution along the coating's thickness in as-deposited coatings, where a silver-rich layer is found on the coating's surface followed by an Ag depletion zone a few nanometers below the coating's surface [22], [23], [24]. The surface segregation of Ag has been used by several authors in high-temperature tribological applications, since it is claimed that Ag diffuses to the coating's surface during heat treatments at several hundred degrees, forming a lubricant surface layer [25], [26], [27], [28], [29]; however, in order to guarantee an extended service life, the Ag diffusion must be controlled. Muratore et al. [28] and Mulligan et al. [29] have proposed the use of diffusion barrier layers based on TiN and CrN coatings, with a designed morphology to allow an efficient Ag transport to the coating's surface. Moreover, the Ag surface segregation must exert a strong influence on the coating's antibacterial activity. Of particular importance is the metal ionization, since the bactericidal effect is associated with the release of Ag⁺ ions, which requires direct contact between the biological medium and the Ag when present on the coating's surface.

In the present report, AgDLC coatings were deposited by dual dc magnetron sputtering and i) the Ag nanoparticles size distribution along the coating's thickness was determined and ii) the coating's morphology and stability over time in room temperature conditions were evaluated. Four different coatings were deposited i) and ii) AgDLC coatings with thicknesses of 250 nm and 1000 nm, respectively, and 20 at.% Ag; iii) AgDLC coating (thickness of 250 nm and 20 at.% Ag) with a DLC barrier layer of 75 nm on the top, in order to evaluate the effect of carbon layer on the growth of Ag clusters on the coating's surface; iv) for comparison with iii), a pure Ag layer was covered with a top DLC barrier layer. The main idea in the first two cases was to obtain coatings with different thicknesses and, consequently, with different morphologies, to analyze their influence on the Ag segregation rates. The coatings were characterized with respect to their morphology (SEM), structure (XRD) and chemical composition through coatings thickness (GDOES). The Ag surface segregation over time was evaluated by SEM and GDOES depth profiling analysis.