Influence of Ag content on mechanical and tribological behavior of DLC coatings

Highlights

• Correlation between composition, microstructure and tribomechanical properties

• At lower contact pressure the wear rate of DLC and Ag containing coatings is similar.

• At higher contact pressure the wear rate of silver containing coatings increases.

Abstract

Ag–DLC coatings with Ag contents ranging from 1.3 at.% to 13.1 at.% were deposited by DC magnetron sputtering. The coatings were characterized with respect to their structure (by means of XRD and Raman spectroscopy), mechanical and tribological properties (by scratch test, nanoindentation, residual stress measurements and pin-on-disk test). The incorporation of 13.1 at.% Ag resulted in the formation of Ag grains of 2–3 nm which promoted the increase of graphite like bonds organized in rings. Regarding the mechanical properties, no variations were found for films with Ag contents lower than 13 at.%; a reduction of both hardness and compressive residual stress was then observed for higher values. Pin-on-disk tests were performed at two different contact pressures (690 MPa and 1180 MPa) in dry sliding conditions against a zirconia counterpart. For the lower contact pressure the variations in the wear rate are well correlated with the coating structure and mechanical properties, while for the higher contact pressure the presence of Ag is relevant and Ag–DLC coatings are showing higher wear rate than DLC one. SEM analysis revealed the formation of Ag aggregates on the wear track and adhesion of silver to the counterpart.

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.