Microstructural Evolution and Crystallization Behavior of Amorphous Medium-Entropy Ti-Nb-Zr-Ag Thin Films

Abstract

Improving the performance of metallic implants increasingly relies on the development of multifunctional surface modifications that combine structural stability, bioactivity, and prevention of bacterial colonization. Medium-entropy alloys (MEAs) represent a promising approach for such coatings, as their chemical complexity allows the formation of structurally stable matrices with tunable properties. In this study, Ti-Nb-Zr and Ti-Nb-Zr-Ag thin films were deposited by magnetron sputtering and subjected to annealing at temperatures of up to 1100 $^{\circ}$C to evaluate the influence of Ag, added for its antibacterial potential, on structural evolution. The as-deposited Ag-free film was fully amorphous, whereas the Ag-containing film exhibited a predominantly amorphous matrix with finely dispersed crystalline nanoparticles, indicating that Ag promoted early-stage crystallization. Both films displayed a fine columnar morphology (column diameter $\sim$15 nm) with dome-like protrusions, a hierarchical surface structure favorable for protein adhesion. Upon annealing, the Ag-free film recrystallized into a granular, loosely packed morphology, while the Ag-containing film retained a compact structure, demonstrating the stabilizing role of Ag. These findings underscore the potential of Ag-containing amorphous MEAs for forming multifunctional coatings with enhanced thermal stability, antibacterial functionality, and biointerface-relevant surface features for advanced biomedical applications.

Figures (5)

• Figure 1: SEM micrographs of as-deposited Ti–Nb–Zr (a,b) and Ti–Nb–Zr–Ag (c,d) thin films. (a,c) plan-view; (b,d) cross-section 45$^{\circ}$ tilt.
• Figure 2: Cross-sectional TEM of Ti–Nb–Zr (a,a$^\prime$) and Ti–Nb–Zr–Ag (b–e) thin films. (a) Cross-section overview; (a$^\prime$) SAED with Si [01$\bar{1}$] reflections; (b) Cross-section overview; (c) HRTEM of $\beta$-(Nb,Zr) [001] with $d$-spacing and FFT inset; (d) HRTEM showing AgZr [2$\bar{2}\bar{1}$] and $\beta$-(Ti,Nb,Zr) [$\bar{1}$11], FFT inset; (e) Crystalline seeds with $d$-spacings and Ag nanoparticle, FFT inset.
• Figure 3: XPS spectra of Ti $2p$, Nb $3d$, and Zr $3d$ core levels for Ti--Nb--Zr (a–c) and Ti $2p$, Nb $3d$, Zr $3d$, and Ag $3d$ + Nb $3p$ for Ti–Nb–Zr–Ag (d--g). Spectra are shown for as-deposited, sputtered, annealed, and annealed + sputtered states (bottom to top).
• Figure 4: SEM micrographs of Ti–Nb–Zr (a–c) and Ti–Nb–Zr–Ag (d–f) thin films after annealing. (a,d) plan-view, lower magnification; (b,e) plan-view, higher magnification; (c,f) cross-section, 45$^{\circ}$ tilt.
• Figure 5: Cross-sectional TEM of annealed Ti–Nb–Zr (left) and Ti–Nb–Zr–Ag (right) thin films. (a) Overview with labelled phases Nb$_{10}$O$_{29}$Ti$_2$, ZrO$_2$, and TiO$_2$; (b) Monoclinic ZrO$_2$ [$3 2\bar{1}$] with FFT inset; (c) Inverse FFT from monoclinic ZrO$_2$ [$100$] and simulated HRTEM images: monoclinic ZrO$_2$ (turquoise frame) and orthorhombic variant (pink frame), differences in $d$-spacings highlighted. Inset: FFT with circled satellite reflections; (d) ZrTiO$_4$ [$1 \bar{1} 2$] with FFT inset; (e) Overview with labelled Ag and ZrO$_2$-based phases; (f) Tetragonal ZrO$_2$-based [$1 \bar{2} 0$]; FFT (upper inset) with circled diffuse reflections of early-stage monoclinic/orthorhombic ZrO$_2$; inverse FFT with simulated image in turquoise frame (lower inset); (g) Orthorhombic ZrO$_2$$[001]$; FFT (upper inset), inverse FFT with simulated image in pink frame (lower inset); (h) Ag [$1 \bar{1} 0$], Ti$_4$Nb [$\bar{1}\bar{1} 2$], and monoclinic ZrO$_2$ with FFT inset. Bottom: elemental maps, with map widths of approximately 700 nm for Ti–Nb–Zr and 600 nm for Ti–Nb–Zr–Ag.