Nano-squares and regular LIPSS on YSZ coating by picosecond UV laser beam: Thin film mediated and direct texturing

The laser texturing of high bandgap ceramic materials, such as Yttrium Stabilized Zirconia (YSZ), is very chal-lenging due to their structural complexity, their mechanical fragility as well as the risk of a chemical decom-position under laser irradiation. The objective of this paper is to fabricate, in a controlled manner, well-organized nanostructures on microscale undulating YSZ coatings, and thus evaluate the feasibility to form laser-induced periodic surface structures (LIPSS), via two experimental approaches: the first one consists in laser structuring of YSZ through a mediated thin film (gadolinium doped ceria -700 nm or Au -20 nm), while the second one involves the direct structuring of the YSZ surface under static and scanning modes. A picosecond Nd: YAG laser (355 nm, 40 ps, 10 Hz) with a relatively large laser beam spot (-550 mu m) was used. In the GDC and Au mediated texturing of YSZ under static conditions, we have observed nano-squares (-250 nm) and typical LIPSS perpendicular to the laser beam polarization, respectively. In the case of laser irradiation of the bare YSZ surface for fluences higher than 310 +/- 10 mJ/cm2, LSFL and HSFL structures were formed depending on the number of pulses (from 102 to 103). The average spatial period measured for the thus-obtained HSFL is about 136 nm with an amplitude of 40 nm, as determined by Scanning Electron Microscopy (SEM) and Atomic Force Microscope (AFM). We have also demonstrated the ability to reproduce LIPSS on YSZ under scanning mode. A "darkening" phenomenon was observed on the YSZ surface at laser fluences exceeding 600 mJ/cm2, resulting from the for-mation of micro-spikes (with sizes close to 1 mu m). XRD measurements demonstrated a slightly lower crystallinity of the laser irradiated YSZ material compared to the untreated YSZ surface.