A parametric study of laser interference surface patterning of dental zirconia: Effects of laser parameters on topography and surface quality
Graphical abstract
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 [1]. At the moment, there is an increasing interest in using ceramics also as materials for oral implants [2] as an alternative to titanium due to their higher resistance to bio-corrosion and superior esthetic properties [3], [4]. 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) [4], [5]. Furthermore, it has been demonstrated that zirconia ceramic exhibits low plaque accumulation and displays similar bacterial binding properties to titanium [6].
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) [7]. 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. [8], 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 [9] as well as improving the adhesion to dental cements [10], [11], [12].
The most common physical surface modification methods available for ceramics are machine-aided approaches such as sandblasting [13], grinding [14], acid etching [15] or laser micromachining [16], [17]. The last rises above the others as suggested by Holthaus et al.[18] 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 [19] of ceramics.
Direct Laser Interference Patterning (DLIP) offers a fast and accurate alternative to introduce controlled topography at the micrometric and sub-micrometric scale [19]. 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 [20]. Further details about the technique and the achievable patterns can be found in Ref. [21]. 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 [22]. DLIP has been successfully used to pattern surfaces of different materials: from metals [23], [24], [25], [26] and ceramics [27], [28], [29] to polymers [30], [31]. 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 [23], to introduce texture [25] or to tune the interaction with biological species [30], [31].
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 [28] nor detrimental effect on mechanical properties [32]. 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 [27]. 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 [33].
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.
Section snippets
Material processing
Commercially available powder of Tetragonal Polycrystalline Zirconia stabilized with 3% molar Y2O3 (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 [34]. 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
Results
Fig. 2 presents the surface topographies obtained by DLIP, where the linear pattern can be clearly seen. The resulting period, depth and quality depend on the laser parameters and setup used.
The pattern consists of an alternation of peaks and valleys with regular periodicity and sinusoid-like shape. A valley corresponds to laser fluence maximum while a peak corresponds to a minimum. Varying the incidence angle of the laser setup, striped patterns of 4–10–15 μm periodicities were produced on the
Discussion
DLIP exploits the inhomogeneous fluence distribution created by interference to engrave a pattern on the surface of the treated material. Due to the relatively long pulse duration (in the order of nanoseconds), patterning will be obtained by thermal mechanisms, implying melting and evaporation of the material accompanied by liquid flow [24], [37]. However, some contribution of photo-physical mechanisms may not be discarded [23], [25].
The traces of molten material (Fig. 5) indicate that the
Conclusion
The surface of 3Y-TZP has been treated with DLIP producing periodic line-like pattern in a relatively fast and inexpensive way. Varying the optical setup it was possible to produce patterns with periodicities in the range between 3 and 15 μm.
The role of laser parameters (fluence and number of pulses) on the produced topography has been investigated for patterns with 10 μm periodicity. From the obtained results, we can conclude that:
- 1.
Tuning laser parameters allows varying independently pattern
Acknowledgments
Work supported in part by the Ministry of Economy and Competitiveness (MINECO) of Spain (project ref. MAT2014-60720-R) and the CREATE-Network Project, Horizon 2020 Program of the European Commission (RISE Project No. 644013). E. Roitero acknowledges the Erasmus Mundus Doctoral Programme DocMASE of the European Commission (FPA 2011-0020). F. Lasserre. and F. Mücklich wish to acknowledge the EFRE Funds of the European Commission for support of activities within the AME-Lab project. The authors
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