Phase-Selective and Localized TiO2 Coating on Additive and Wrought Titanium by a Direct Laser Surface Modification Approach

Abstract

Titanium has been the material of interest in biological implant applications due to its unique mechanical properties and biocompatibility. Their design is now growing rapidly due to the advent of additive manufacturing technology that enables the fabrication of complex and patient-customized parts. Titanium dioxides (TiO₂) coatings with different phases (e.g., anatase, rutile) and morphologies have shown to be effective in enhancing osteointegration and antibacterial behavior. This enhanced antibacterial behavior stems from the photocatalytic activity generated from crystalline TiO₂ coatings. Anatase has commonly been shown to be a more photocatalytic oxide phase compared to rutile despite its larger band gap. However, more recent studies have suggested that a synergistic effect leading to increased photocatalytic activity may be produced with a combination of oxides containing both anatase and rutile phases. Here, we demonstrate the selective and localized formation of TiO₂ nanostructures on additive and wrought titanium parts with anatase, rutile, and mixed phases by a laser-induced transformation approach. Compared to conventional coating processes, this technique produces desired TiO₂ phases simply by controlled laser irradiation of titanium parts in an oxygen environment, where needed. The effects of processing conditions such as laser power, scanning speed, laser pulse duration, frequency, and gas flow on the selective transformation were studied. The morphological and structural evolutions were investigated using various characterization techniques. This method is specifically of significant interest in creating phase-selective TiO₂ surfaces on titanium-based bioimplants, including those fabricated by additive manufacturing technologies.

Figure 1

Schematic illustration showing the patterned, localized, and phase-selective fabrication of anatase and rutile TiO₂ nanostructures on the titanium samples by a controlled laser surface modification approach.

Figure 2

Comprehensive map (a) of the pure titanium (purity 99.99%) response to different laser powers and scan speeds under atmospheric oxygen pressure and 0.1 SLM oxygen flow. Blue, red, and purple zones show the process regions where anatase, rutile, and mixed phases form during the process, respectively. The effect of the pulse width on the transformation process (b). The lines in (b) indicate the anatase to rutile transition boundary, where mixed rutile and anatase phases coexist. Pure anatase and rutile phases exist to the left and right of the lines for each waveform, respectively.

Figure 3

Representative Raman spectra (a) and XRD scans (b) of the synthesized TiO₂ structures on wrought TAV samples showing the successful formation of anatase, mixed-phase, and rutile peaks.

Figure 4

Laser confocal 2D images and 3D surface profiles of anatase (a, b), mixed-phase (c, d), and rutile (e, f) TiO₂ wrought TAV samples.

Figure 5

SEM images showing the morphology of the wrought TAV TiO₂ anatase (a, b), mixed-phase (c, d), and rutile (e, f) nanostructures. Smaller features with micro/nanostructures and a mesoporous morphology were observed for anatase samples. The feature became larger and denser for rutile samples.

Figure 6

Selective and localized formation of anatase and rutile TiO₂, side-by-side. Optical images show Auburn University logo (a), concentric circular patterns (b), and checkerboard (c) consisting of anatase and rutile TiO₂, side-by-side. Representative Raman spectra obtained from the rutile (d) and anatase (e) regions as labeled.