Large-area fabrication of low- and high-spatial-frequency laser-induced periodic surface structures on carbon fibers

Abstract

The formation and properties of laser-induced periodic surface structures (LIPSS) were investigated on carbon fibers under irradiation of fs-laser pulses characterized by a pulse duration τ = 300 fs and a laser wavelength λ = 1025 nm. The LIPSS were fabricated in an air environment at normal incidence with different values of the laser peak fluence and number of pulses per spot. The morphology of the generated structures was characterized by using scanning electron microscopy, atomic force microscopy and Fast-Fourier transform analyses. Moreover, the material structure and the surface chemistry of the carbon fibers before and after laser irradiation was analyzed by micro Raman spectroscopy and X-ray photoelectron spectroscopy. Large areas in the cm² range of carbon fiber arrangements were successfully processed with homogenously distributed high- and low-spatial frequency LIPSS. Beyond those distinct nanostructures, hybrid structures were realized for the very first time by a superposition of both types of LIPSS in a two-step process. The findings facilitate the fabrication of tailored LIPSS-based surface structures on carbon fibers that could be of particular interest for e.g. fiber reinforced polymers and concretes.

Introduction

Carbon fibers are commercially used to reinforce polymers (carbon fiber reinforced polymers, CFRP) and concrete (engineered cementitious composites, ECC) [[1], [2], [3], [4]]. The outstanding mechanical properties of single carbon fibers are a result of the specific structure consisting of graphitic and turbostratic carbon with a crystal structure very similar to graphite [5,6]. Consequently, the strong covalent bonds between the carbon atoms of the graphene layers result in a high tensile strength in fiber direction. This anisotropic behavior also manifests in other fiber properties like crystallinity or electrical conductivity [1]. Regarding CFRP, the interface between carbon fibers and surrounding polymer matrix is in the focus of research activities aiming to increase fiber-matrix bonding strength and thereby to enhance the mechanical properties of the composite material. For this purpose, carbon fiber surfaces were modified by plasma oxidation, chemical and electrolytic etching as well as chemical vapor deposition [[7], [8], [9], [10]]. It was shown that the bonding strength is improved by increasing the surface roughness and the resulting interface area between fiber and polymer matrix [[11], [12], [13], [14], [15]].

Concerning the engineering of surfaces with tailored functional properties, ultra-short pulsed laser processing gained rapidly increasing attention in the past decades. In this context, the fabrication of laser-induced periodic surfaces structures (LIPSS) in a single-step, direct-writing process emerged as a flexible and versatile technique [16]. LIPSS have been demonstrated as a universal phenomenon on almost all types of materials [17] providing outstanding properties of the laser-structured surface such as wettability, optical performance, bioactivity, and tribology [18,19]. In particular, mimicking structures and functional principles provided by nature is an intensively studied field of current scientific research [20]. As a main advantage, the large variety of influencing parameters including laser wavelength λ, number of pulses N, pulse duration τ, laser peak fluence F, angle of incidence θ, and beam polarization, allows to control the specific properties of LIPSS such as their alignment and spatial period. Regarding the spatial period Λ, LIPSS are classified into low-spatial frequency LIPSS (LSFL) and high-spatial frequency LIPSS (HSFL). LSFL are characterized by a period Λ close to the initial laser wavelength λ for strong absorbing materials (metals, semiconductors) and close to λ/n for dielectrics, where n refers to the refractive index of the dielectric material [19,21]. It is well-accepted, that the formation of LIPSS can be explained by spatial modulated intensity pattern resulting from interference of the incident laser radiation with surface electromagnetic waves that are generated by scattering at the rough surface [22]. This might include the excitation of surface plasmon polaritons (SPP) [[22], [23], [24]]. An alternative approach to explain LIPSS formation is given by a self-organization of the irradiated material via laser-induced thermal instabilities resulting in material redistribution [25]. HSFL with spatial periods much smaller than λ are still controversially discussed in literature, and a deep understanding of the formation mechanism is still missing. Possible explanations include self-organization [26], chemical surface alterations [27] and second harmonic generation [28].

While the LIPSS formation on graphite was studied by several groups in dependence on different influencing parameters (beam polarization, laser peak fluence) on the flat surface of bulk materials [[28], [29], [30], [31], [32]], the formation of LIPSS on the surface of carbon fibers is less investigated [11,33]. By performing single-spot experiments on strongly curved fiber surfaces, Sajzew et al. [33] demonstrated the formation of LSFL and HSFL within the Gaussian intensity distribution of the focal spot (diameter 50 μm) upon the irradiation of N = 50 linearly polarized fs-laser pulses with a peak fluence F = 4 J/cm². The study revealed, however, that the large number of laser pulses in combination with the utilized peak fluence lead to a remarkable ablation in the intense center of the focal spot and therefore to a damage of the fibers accompanied by a deterioration of their mechanical properties.

Based on the experimental findings of Sajzew at al., the objective of the present study is the homogenous manufacturing of HSFL and LSFL, respectively, on large areas of carbon fiber arrangements without damage by unidirectional scanning the focused laser beam over the sample surface. For this purpose, the nanostructure formation process was studied in dependence of the fs-laser peak fluence. The surface morphologies of the prepared samples were subsequently characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The material structure and the surface chemistry of the carbon fibers was studied before and after laser irradiation by micro Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Tensile strength measurements of single carbon fibers were performed in order to evaluate the impact of the fs-laser irradiation on the mechanical properties.