Influence of Ag content on mechanical and tribological behavior of DLC coatings
Introduction
Diamond like carbon (DLC) coatings often combine high wear resistance with low friction coefficient, which has prompted their use in several industrial applications as wear protective coatings [1], [2], [3]. Moreover, these coatings are biocompatible and chemically inert and thus interesting for biomedical applications, namely hip joints [4], knee replacements [5], coronary artery stents [6] or heart valves [7]. DLC coatings consist on hydrogenated and non-hydrogenated amorphous carbons (a-C:H and a-C, respectively) with different structures according to their chemical composition. The various forms of amorphous C–H alloys can be displayed in a ternary phase diagram and classified according to the amount of sp2/sp3 bonds and H content [8]. a-C coatings present higher hardness, better wear resistance and lower friction in humid environments in relation to a-C:H coatings [9].
Despite their excellent tribological properties, high residual stresses and consequent adhesion failure are pointed as one of the major limitations of both a-C and a-C:H coatings [10]. The incorporation of metal atoms such as Ti [11], W [12], Cr [13], Zr [14], Cu [2] or Ag [15], [16], [17], to carbon films could reduce residual stress and thus the risk of premature coating adhesion failure. These metals might form small nanocrystallites of pure metal or metal carbide nanograins dispersed through the carbon structure [10]. The incorporation of carbide forming metals (e.g. Ti, W, Cr, and Zr) enhances the coating's adhesive strength and hardness. Alloying of DLC with noble metals (such as Ag and Cu) results in the formation of soft and ductile phases able to improve the coating's adhesion and toughness [2], [16]. In fact, DLC coatings alloyed with Ag exhibited enhanced coating adhesion to steel substrates [16] and reduced internal stresses [17]. Moreover, these soft metals could act as solid lubricants further decreasing the friction coefficient.
Several works have proposed doping or alloying of hard-wear resistant coatings (e.g. TiCN [18]; CrN; TiN; ZrN [19]; DLC [15]) with Ag in order to improve the tribological behavior of these coatings. X. Yu et al. [15] found that the incorporation of 4.3 to 10.6 at.% Ag into DLC coatings resulted in a decrease of both wear rate and friction coefficient, which was attributed to the presence of Ag nanograins on the top layer of the Ag–DLC coating. Higher Ag contents lead to unsatisfactory tribological properties with high friction and low wear resistance. In addition to the promising tribological properties, Ag is a potential antibacterial agent often studied and used in the biomedical field [16], [17]. The combination of wear resistant coatings (CrN, TiN, ZrN [19]; TiCN [18]; TaN [20]; DLC [21]) with silver has been proposed as an effective solution able to improve the resistance to bacterial colonization and, simultaneously, the wear resistance of the base coatings.
In the present study, the effect of Ag content on the mechanical and tribological properties of Ag–DLC coatings is evaluated. Ag–DLC coatings were deposited by magnetron sputtering and the influence of chemical composition on coatings' structure, mechanical and tribological properties was evaluated. Pin-on-disk tests were performed in dry sliding conditions against a zirconia counterpart at two different contact pressures.
Section snippets
Experimental procedure
Ag–DLC coatings were deposited by DC magnetron sputtering from a graphite target (200 × 100 mm2) onto polished and ultrasonically cleaned tool steel (used for tribological tests), stainless steel (determination of residual stress and XRD analysis), high-speed steel (AISI M2) (for determination of coating adhesion) and silicon (nanoindentation tests and Raman spectroscopy). High purity silver pellets (with an average area of approximately 20 mm2) were incorporated in the erosion zone of the graphite
Chemical composition and microstructure
The chemical composition of Ag–DLC coatings is shown in Table 1, together with the coatings' thicknesses, deposition rates and mechanical properties. The increase on the number of silver pellets placed into the erosion zone of graphite target resulted in an increase of silver content from 0 (for reference DLC coating) to 13.1 at.%. The level of oxygen originating from residual atmosphere and target contamination was relatively low. To facilitate reading, the coatings were denominated based on
Conclusion
Ag–DLC coatings with silver contents ranging from 1.3 at.% to 13.1 at.% were deposited by DC magnetron sputtering. The Raman results revealed that the incorporation of Ag up to 6.1 at.% did not promote any changes in the carbon matrix and no Ag phases were detected by XRD analysis, which allows to conclude that silver atoms are probably dispersed in the carbon matrix forming very small nanoclusters. The increase of Ag to 13.1 at.% leads to the formation of crystalline Ag phase with a grain size in
Acknowledgments
The work was partially funded by the Czech Science Foundation through the project 108/10/1782 and by the national funds through FCT — Fundação para a Ciência e Tecnologia under the grant SFRH/BD/82472/2011 and co-funded by the FSE. This research is sponsored by the FEDER funds through the program COMPETE — Programa Operacional Factores de Competitividade and by the national funds through FCT — Fundação para a Ciência e Tecnologia in the framework of the Strategic Projects PEST-C/EME/UIO0285/2011
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