Graphical abstract
Highlights
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Direct Laser Interference Patterning was applied to introduce micrometric periodical topographies on the surface of 3Y-TZP.
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Pattern geometry and roughness can be tuned independently by adjusting laser setup and parameters.
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High fluence and low number of pulses produce patterns with higher quality.
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Microcracking, porosity and redepostion of material appear at certain conditions.
Abstract
Objective
The aim of this work is to generate micrometric linear patterns with different topography on dental grade zirconia by means of UV laser interference and to assess the quality of the produced surface, both in term of the geometry produced and of the surface damage induced in the material.
Methods
The third harmonic of a Q-switched Nd:YAG laser (355 nm, pulse duration of 10 ns and repetition rate of 1 Hz) was employed to pattern the surface of 3Y-TZP with micrometric-spaced lines. The resulting topography was characterized with White Light Interferometry and Scanning electron microscopy: pattern depth ( H ), amplitude roughness parameters ( S a , filtered- S a ), Fourier spatial analysis and collateral damages were related to laser fluence and number of pulses employed.
Results
With our experimental setup, line-patterning of zirconia surfaces can be achieved with periodicities comprised within 5 and 15 μm. Tuning laser parameters allows varying independently pattern depth, overall roughness and surface finish. Increasing both fluence and number of pulses allows producing deeper patterns (maximum achievable depth of 1 μm). However, increasing the number of pulses has a detrimental effect on the quality of the produced lines. Surface damage (intergranular cracking, open porosity and nano-droplets formation) can be generated, depending on laser parameters.
Significance
This work provides a parametric analysis of surface patterning by laser interference on 3Y-TZP. Best conditions in terms of quality of the produced pattern and minimum material damage are obtained for low number of pulses with high laser fluence. With the employed method we can produce zirconia materials with controlled topography that are expected to enhance biological response and mechanical performance of dental components.
1
Introduction
Ceramic materials are commonly used for dental applications including veneering material for metal substructures, all-ceramic posts and cores, frameworks for crowns and bridge-works . At the moment, there is an increasing interest in using ceramics also as materials for oral implants as an alternative to titanium due to their higher resistance to bio-corrosion and superior esthetic properties . Tetragonal Zirconia Polycrystal stabilized with 3% molar of Yttrium oxide (3Y-TZP) is considered an excellent material for all these applications because of its bio-stability, mechanical properties (good strength, hardness and high fracture toughness) and esthetic appearance in combination with porcelain enamel (white ivory color, close to natural teeth) . Furthermore, it has been demonstrated that zirconia ceramic exhibits low plaque accumulation and displays similar bacterial binding properties to titanium .
Despite the aforementioned advantages, the suitability of 3Y-TZP to be used in the dental prosthetic field depends on the feasibility of functionalizing its surface to improve the biological response or the mechanical adhesion to other materials (like dental cements and resins) . This should be carried out by techniques that do not compromise material’s bulk properties and its long-term stability. Common strategies to achieve these goals are the modification of the roughness or the introduction of a controlled topography. For instance, micrometric silica reliefs on zirconia implant surfaces have been employed to study endothelial cells and fibroblast response in Ref. , where it was demonstrated that parallel grooved surfaces were able to control cells spatial distribution and to guide their growth. Also an increment in roughness has proved to be beneficial in the osseointegration of zirconia-based implants as well as improving the adhesion to dental cements .
The most common physical surface modification methods available for ceramics are machine-aided approaches such as sandblasting , grinding , acid etching or laser micromachining . The last rises above the others as suggested by Holthaus et al. in a comparative study of different techniques to pattern the surface of ceramics materials such as alumina, zirconia, silica and hydroxyapatite. They concluded that laser treatment processes are a suitable alternative to classical methods since they have the advantage of being fast and with a high control on the final desired topography in contrast with contact techniques, as mechanical micromachining or stamp transfer molding. Moreover, using conventional methods the fabrication of defined patterns smaller than 100 μm is still challenging due to the high hardness and brittleness of ceramics.
Direct Laser Interference Patterning (DLIP) offers a fast and accurate alternative to introduce controlled topography at the micrometric and sub-micrometric scale . In this technique a periodical intensity distribution is produced by beam interference on the surface of the material to be treated. Depending on the number of interfering laser beams and the optical setup, different geometries can be produced (lines or dots). Commonly, nanosecond, picosecond and femtosecond pulsed lasers are used in order to reach high energy density at the interference maxima position. This high peak power permits to locally melt, vaporize or ablate the substrate to engrave the desired geometry . Further details about the technique and the achievable patterns can be found in Ref. . DLIP allows great precision and flexibility in the produced topography and is fast enough to modify large areas, especially if compared to other laser micromachining techniques that require the scanning of the beam through the surface by opto-mechanical methods . DLIP has been successfully used to pattern surfaces of different materials: from metals and ceramics to polymers . Most common applications are in the field of tribology and biomaterials: topographical, chemical and microstructural modifications induced by DLIP are exploited to modify surface wettability , to introduce texture or to tune the interaction with biological species .
DLIP technique has been successfully applied to produce micrometric line-patterning onto Yttria-stabilized zirconia with neither significant collateral damage induced by the laser treatment nor detrimental effect on mechanical properties . However, there has not been any systematic study about the influence of laser parameters on the topographies produced with DLIP on 3Y-TZP. As demonstrated for other ceramic materials, changing laser parameters employed often results in different surface topographies . Moreover, a detailed roughness analysis at different scales is essential to accurately describe the topography of the produced patterns, especially when dealing with biomedical applications .
The objective of this work is to correlate the morphology and the quality of the generated pattern to the laser parameters employed (fluence and number of pulses). The topography and roughness of the patterns are characterized by means of 2D- and 3D-amplitude parameters and spatial distribution analysis. Surface finish and quality are assessed in terms of collateral damages induced by laser treatment. Finally, the processability ranges are determined as a function of laser parameters.
2
Experimental
2.1
Material processing
Commercially available powder of Tetragonal Polycrystalline Zirconia stabilized with 3% molar Y 2 O 3 (TZ3YSB-E, Tosoh Co.) was employed. The powder was isostatically pressed at 200 MPa in a cylindrical mold and subsequently sintered in an alumina tube furnace at 1450 °C for two hours (heating rate: 3 °C/min), as described in previous work . The resulting rods (10 mm in diameter) were cut into discs of approximately 2 mm thickness. The surface of the discs was grinded and polished with diamond suspensions of decreasing particle size (30–6–3 μm) and with colloidal silica as a final step. The samples had a final density of 6.03 ± 0.02 g/cm 3 (99.67% of theoretical density) and a grain size (intercept distance) of 0.31 ± 0.08 μm. The obtained material has biomedical grade, according to ISO 13356:2013 .
2.2
Laser treatment
A Q-switched Nd:YAG laser (Spectra Physics Quanta-Ray PRO210) with a fundamental wavelength of 1064 nm and an output wavelength of 355 nm obtained by 3rd harmonic generation was employed to pattern the surface of zirconia discs. The repetition rate and pulse duration of this laser were 10 Hz and 10 ns, respectively. A mechanical shutter (nmLaser LSTXY-W8) was used to control the number of laser pulses and to send one pulse per second. The optical setup employed to produce the DLIP is illustrated in Fig. 1 . Further details about the technique and the setup employed can be found in Refs. .
By the use of a beam spliter two coherent sub-beams are generated from a primary one. Mirrors guide the two beams to the sample surface, where they are left interfere. The resulting intensity distribution is a plane sinusoidal and can be mathematically described by:
I ( x , y ) = I o [ cos ( 4 π x λ sin α ) + 1 ]
where I o is the intensity of the laser beam before splitting, λ is the laser wavelength and α is the half angle between the interfering beams. The same relation can be expressed in terms of fluence ( F ( x , y )), which represents the laser energy density distribution on the surface of the sample [J/cm 2 ]. In this case, fluence can be calculated from the intensity just by multiplying for the pulse duration, as it is given in [W/cm 2 ] . The fluence distribution consists of alternating lines of energy maximum ( F max = 2 F o ) and minimum ( F min = 0). From now on in the paper we will refer to F o as “fluence”.
The line-pattern then has a periodicity ( P ) of:
P = λ 2 sin α
Varying the incidence angle of the two beams, different line-periodicities were tested (1–4–7–10–15 μm). A beam attenuator (Altechna UAB) and a mechanical shutter (nmLaser LSTXY-W8) were employed to vary the energy of the beam and the number of pulses reaching the surface of the sample. A square mask of 1 × 1 mm 2 placed before the sample allowed the selection of a homogeneous portion of the incoming beam for patterning the entire surface of the discs with multiple adjacent spots. The beam power was measured with a power detector (Gentec-EO UP19-H) both after the beam attenuator and after the mask, in order to precisely take into account energy losses within the components of the optical setup. Knowing the illuminated area (corresponding to the mask area) and the pulse duration, the measured laser power was transformed to laser fluence. Then, the influence of laser fluence and pulse number was investigated in the range of 0.15–7.15 J/cm 2 and 1–10 pulses, respectively.
2.3
Topography characterization as a function of laser parameters
A preliminary analysis was done to determine the range of achievable periodicities and to choose the optimal one to characterize in detail the influence of laser parameters on pattern morphology. The surface of laser-treated samples was characterized with White Light Interferometry (WLI, Veeco Wyko 9300NT) in order to verify if the striped pattern was formed. WLI images of 250 × 250 μm 2 were obtained by stitching of nine images taken with 50× magnification (total resolution of 1263 × 1263 pixels). To assess the effect of laser parameters on the morphology of the treated surfaces, a detailed topographical characterization was carried out on samples with constant line periodicity of 10 μm (the reason for this is later discussed). In order to separate the effect of fluence and number of pulses, each series of samples treated with n pulses was exposed to increasing fluence.
With the intention of describing the quality of the achieved pattern, significant roughness parameters were selected (see Table 1 for definition and description). To evaluate those, WLI images of 633 × 474 μm 2 (10× magnification and resolution of 640 × 480 pixels) and of 127 × 96 μm 2 (50× magnification and resolution of 640 × 480 pixels) were acquired. Height maps and profiles were treated with commercially available software after surface tilt correction. For pattern depth ( H ) and average roughness ( S a ) calculations no filtering was applied in order to evaluate the real depth of the structure and take into account the contribution of all scales features, as it has already been done in a similar study . In a second stage, a Robust Gaussian filter (high-frequency pass with cut-off length of 2.5 μm) was employed to separate the pattern produced with DLIP from smaller size features ( f-S a ). Finally, power spectral density (PSD), a Fourier spatial parameter, was employed to describe the neatness of the stripes produced. The chosen roughness parameters were calculated for all the conditions and plotted as a function of laser fluence and number of pulses.
