Influence of the Silver Content on Mechanical Properties of Ti-Cu-Ag Thin Films

Abstract

In this work, the ternary titanium, copper, and silver (Ti-Cu-Ag) system is investigated as a potential candidate for the production of mechanically robust biomedical thin films. The coatings are produced by physical vapor deposition-magnetron sputtering (MS-PVD). The composite thin films are deposited on a silicon (100) substrate. The ratio between Ti and Cu was approximately kept one, with the variation of the Ag content between 10 and 35 at.%, while the power on the targets is changed during each deposition to get the desired Ag content. Thin film characterization is performed by X-ray diffraction (XRD), nanoindentation (modulus and hardness), to quantitatively evaluate the scratch adhesion, and atomic force microscopy to determine the surface topography. The residual stresses are measured by focused ion beam and digital image correlation method (FIB-DIC). The produced Ti-Cu-Ag thin films appear to be smooth, uniformly thick, and exhibit amorphous structure for the Ag contents lower than 25 at.%, with a transition to partially crystalline structure for higher Ag concentrations. The Ti-Cu control film shows higher values of 124.5 GPa and 7.85 GPa for modulus and hardness, respectively. There is a clear trend of continuous decrease in the modulus and hardness with the increase of Ag content, as lowest value of 105.5 GPa and 6 GPa for 35 at.% Ag containing thin films. In particular, a transition from the compressive (-36.5 MPa) to tensile residual stresses between 229 MPa and 288 MPa are observed with an increasing Ag content. The obtained results suggest that the Ag concentration should not exceed 25 at.%, in order to avoid an excessive reduction of the modulus and hardness with maintaining (at the same time) the potential for an increase of the antibacterial properties. In summary, Ti-Cu-Ag thin films shows characteristic mechanical properties that can be used to improve the properties of biomedical implants such as Ti-alloys and stainless steel.

Keywords: FIB-DIC; Ti-Cu-Ag thin films; magnetron sputtering; mechanical properties; nanoindentation.

Figure 1

(a) Physical vapor deposition (PVD) chamber of three tilted metallic targets with respect to the substrate holder, and (b) targets ignite during sputtering.

Figure 2

Representative images from (a–e) step-by-step milled pillar showing the semi-automatic milling procedure (scale bar only in figure (e)), and cross section (f), highlighting the thin film thickness.

Figure 2

Representative images from (a–e) step-by-step milled pillar showing the semi-automatic milling procedure (scale bar only in figure (e)), and cross section (f), highlighting the thin film thickness.

Figure 3

X-ray diffraction patterns of the Ti-Cu and Ti-Cu-Ag composite thin films on a silicon substrate with an Ag content varying between 10 and 35 at.%.

Figure 4

Atomic force microscope (AFM) images of the Ti-Cu and Ti-Cu-Ag thin films with 10, 25, and 35 at.% of Ag.

Figure 5

The average surface roughness of the Ti-Cu and Ti-Cu-Ag thin films as a function of the Ag content.

Figure 6

Elastic modulus and hardness of the Ti-Cu and Ti-Cu-Ag thin films as a function of the Ag content.

Figure 7

Representation of first failure LC1, second failure LC2, and complete delamination LC3 for the TiCu and TiCuAg thin films (scratch direction from left to right).

Figure 8

Critical load (LC1, LC2, and LC3) as a function of Ag content in the TiCu thin films.

Figure 9

The relaxation strain determined by the focused ion beam and digital image correlation (FIB-DIC) method for Ti-Cu and Ti-Cu-Ag thin films with a various Ag content as a function of the milling depth.

Figure 10

Average residual stress determined by the FIB-DIC method and from the substrate curvature measurements according to the Stoney’s equation as a function of the Ag content in the Ti-Cu-Ag thin films.