Abstract
This contribution discusses the ablation phenomena observed during laser treatment of carbon fiber-reinforced plastics (CFRPs) with pulsed lasers observed employing laser sources with wavelengths of 355 nm, 1064 nm and 10.6 µm and pulse durations from picoseconds (11 ps) to microseconds (14 µs) are analyzed and discussed. In particular, the threshold fluence of the matrix material epoxy (EP) and the damage threshold of CFRP were calculated. Moreover, two general surface pretreatment strategies are investigated, including selective matrix removal and structure generation through indentation (ablation of both, matrix material and fibers) with a cross-like morphology. The surfaces obtained after the laser treatment are characterized by means of optical and scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy is employed for the analysis of composite and constituent materials epoxy and carbon fibers. As a result, different ablation mechanisms, including evaporation and delamination are observed, depending on the employed laser wavelength and pulse duration. For both 355 nm and 1064 nm wavelength, the laser radiation produces only partial ablation of the carbon fibers due to their higher absorption coefficient compared to the epoxy matrix. Although a selective matrix removal without residues is achieved using the pulsed CO2 laser. Differently, both constituent materials are ablated with the nanosecond pulsed UV laser, producing indentations. The sum of the investigations has shown that existing theories of laser technology, such as the ablation threshold according to Liu et al., can be applied to composite materials only to a limited extent. Furthermore, it has been found that the pronounced heterogeneity of CFRP mostly leads to an inhomogeneous ablation result, both when creating grooves and during selective matrix removal, where the carbon fibers influence the ablation result by their thermal conductivity, depending on fiber direction. Finally, despite the material inhomogeneity, a scanning strategy has been developed to compensate the heterogeneous ablation results regarding structure depth, width and heat affected zone.
Highlights
Processes and products with a very low environmental impact are required in all aspects of human life, as in the automotive sector [1]
We investigate the interaction of various laser sources with the bare material components of carbon fiber-reinforced plastics (CFRPs), carbon fibers and epoxy resin
The ablation phenomena observed during the laser treatment of carbon fiber-reinforced plastics (CFRPs) with pulsed lasers operating at different wavelengths were investigated
Summary
Processes and products with a very low environmental impact are required in all aspects of human life, as in the automotive sector [1]. FRPs show different challenges regarding production processes and applications compared to traditional common metal components These comprehend inhomogeneous material structure (fibers and matrix), lower thermal resistance as well as low wear resistance [2]. Latest investigations show promising results of laser structuring of FRP before applying a metal coating, which is necessary for local functionalization, such as wear-resistance [8] In this framework, short and ultra-short pulsed lasers offer a clean surface pretreatment on fiber-reinforced plastics that can be utilized to reach the required surface roughness for mechanical bonding without thermally or mechanically damaging the composite material, especially the reinforcing fibers [8,18,19]. Pulsed laser processing has been applied on FRP for different purposes, ranging from surface cleaning [20,21], to roughening or selective matrix removal [8,10,11,12,16,18,21], hole drilling [22,23,24] and laser cutting [25,26], among others
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