Effect of Laser Pulse Overlap and Scanning Line Overlap on Femtosecond Laser-Structured Ti6Al4V Surfaces

Abstract

Surface structuring is a key factor for the tailoring of proper cell attachment and the improvement of the bone-implant interface anchorage. Femtosecond laser machining is especially suited to the structuring of implants due to the possibility of creating surfaces with a wide variety of nano- and microstructures. To achieve a desired surface topography, different laser structuring parameters can be adjusted. The scanning strategy, or rather the laser pulse overlap and scanning line overlap, affect the surface topography in an essential way, which is demonstrated in this study. Ti6Al4V samples were structured using a 300 fs laser source with a wavelength of 1030 nm. Laser pulse overlap and scanning line overlap were varied between 40% and 90% over a wide range of fluences (F from 0.49 to 12.28 J/cm²), respectively. Four different main types of surface structures were obtained depending on the applied laser parameters: femtosecond laser-induced periodic surface structures (FLIPSS), micrometric ripples (MR), micro-craters, and pillared microstructures. It could also be demonstrated that the exceedance of the strong ablation threshold of Ti6Al4V strongly depends on the scanning strategy. The formation of microstructures can be achieved at lower levels of laser pulse overlap compared to the corresponding value of scanning line overlap due to higher heat accumulation in the irradiated area during laser machining.

Keywords: Ti6Al4V; ablation threshold; femtosecond laser; laser pulse overlap; scanning line overlap; ultrashort laser pulse.

Figure 1

Schematic illustration of the laser surface structuring using a line-wise scanning strategy leading to a defined laser pulse overlap (PO) and scanning line overlap (LO). Deflecting in the x and y directions of the Gaussian laser beam is evoked by the movement of galvanic mirrors within the scanning system.

Figure 2

Formation of nano- and microstructures at a PO of 50% and LO of 80% with increasing laser fluence F. The y-axis indicates the scanning direction for the PO. The x-axis indicates the line feed direction for the LO. (a) Femtosecond laser-induced periodic surface structures, λ₁ = 0.859 ± 0.056 µm, scale bar 10 µm and in detail 1 µm. (b) Micrometric ripples, λ₂ = 5.038 ± 0.829 µm, scale bar 10 µm. (c,d) Formation of micro-craters, scale bar 10 µm. (e) Pillared microstructures, scale bar 10 µm.

Figure 3

Structured Ti6Al4V surfaces with varied fluence F and laser pulse overlap PO in a matrix of SEM images. The y-axis indicates the scanning direction for the PO. The x-axis indicates the line feed direction for the LO. Formation of microstructures can be observed at a fluence of approx. 0.98 J/cm² and higher. In particular at low fluences, a clear formation of trenches can be observed due to the LO of 50% and inhomogeneous energy distributions within the Gaussian laser beam profile. By comparison of the structures irradiated with a constant PO of, e.g., 90%, it can be observed that the ablated craters become larger in terms of diameter and depth with increasing fluence (compare roughness data in Figure 7 and elevation profile in Figure 5). Scale bar: 10 µm.

Figure 4

Structured Ti6Al4V surfaces with varied fluence F and scanning line overlap LO in a matrix of SEM images. The y-axis indicates the scanning direction for the PO. The x-axis indicates the line feed direction for the LO. Formation of microstructures can be observed at a F of approx. 4.91 J/cm² and higher. With an increasing LO, a more homogeneous surface with smoother trench formation is apparent (confirmed by elevation height profiles in Figure 6). Scale bar: 10 µm.

Figure 5

Exemplary height elevation profiles in x-direction for different PO (40%, 70%, and 90%) and different fluences (F = 4.91 J/cm² and F = 0.49 J/cm²), respectively. (a) A clear formation of stochastic microstructures can be observed at a high level of PO at F = 4.91 J/cm². Width and height of the structures rise with increasing PO. (b) At F = 0.49 J/cm², the height of deterministic structures grows and periodicity remains constant with increasing PO. Structures are covered with nanoscale roughness.

Figure 6

Exemplary height elevation profiles in x-direction at different LO (40%, 70% and 90%) and different fluences (F = 4.91 J/cm² and F = 0.49 J/cm²). (a) Low levels of LO lead to considerable formation of trenches (LO = 40%). Firstly, structures become smaller with increasing LO (LO from 40% to 70%). After reaching the high ablation regime, stochastic microstructures gradually grow with increasing LO at high levels of F. (b) The periodicity and height of structures decreases with increasing LO. High LO leads to a clear homogenous nanoscale roughness.

Figure 7

Average area surface roughness (Sa) after laser processing related to the reference (Ref). (a) A clear increase of the roughness can be obtained with increasing PO at all fluences. (b) A higher scanning line overlap leads also to an increase in roughness above a fluence of approx. 4.91 J/cm². Firstly, a higher LO leads to a decreased roughness at low fluences. Generally, the determined average area surface roughness is less compared to that obtained at corresponding levels of PO (and the same energy input). At fluences from 7.37 J/cm² to 12.28 J/cm², a minor increase in roughness can be detected at a low level of LO due to a clear trench formation.