# On the Detection and Removal of Oxides from Laser-Treated Surfaces
## 2 Materials and Methods  
Three types of materials were used for testing, all of them as substrates for laser texturing as a pre-treatment for thermal spraying. These were AISI 304 stainless steel and W-based materials (pure W and W-Cu composite with \(\sim\)39 vol% Cu), as substrates for plasma-sprayed W coatings for potential use in plasma-facing components in fusion reactors, and a Ti6 Al4V alloy as a substrate for plasma-sprayed Ti for orthopedical implants.  
On the W-based materials, laser texturing was performed with a nanosecond (ns) laser (YLP-HP, wavelength 1070 nm, max. optical power 200 W; IPG Photonics, Oxford, MA, USA), creating a hexagonal dimple pattern. The dimples had a 30 \(\upmu\)m depth, 30 \(\upmu\)m width, and 50 \(\upmu\)m spacing. Besides having the same dimple pattern, the steel samples were textured by a continuous-wave (cw) fiber laser (YLR-3000-SM, wavelength 1070 nm, max. optical power 3000 W; IPG Photonics, Oxford, MA, USA), forming grooves of 25 \(\upmu\)m depth and 70 \(\upmu\)m distance in parallel or perpendicular orientation. On the Ti alloy, again, a dimple pattern was formed by a nanosecond laser, with a 55 \(\upmu\)m depth, 50 \(\upmu\)m width, and 70 \(\upmu\)m spacing.  
Surface morphology was observed with a scanning electron microscope (SEM) EVO MA15 (Carl Zeiss SMT, Oberkochen, Germany). EDS was carried out with the same microscope, using a Quantax system with an XFlash 5010 detector (Bruker, Berlin, Germany). EDS measurements were performed at 500\(\times\) magnification (to encompass a number of surface texture features), with an 8.5 mm working distance, a 1 nA emission current, and a 10 kV acceleration voltage. XRD was carried out using a D8 Discover diffractometer (Bruker, Karlsruhe, Germany). Phase analysis was performed with full-profile Rietveld refinement using TOPAS 5 software.  
Annealing for oxide removal on the W-based materials was performed in a furnace filled with hydrogen at 800 \({}^{\circ}\)C for 1 h (similar treatment was used previously to clean W powder from oxides accumulated from long-term storage [21]). Acid etching of the W-Cu composite material was performed by soaking the samples in 40% hydrofluoric acid (HF) for 20, 40, and 60 s (similar to the treatment used in [22]). Surface cleaning by 'gentle ablation' with ultra-short laser pulses and low pulse energies was carried out with a femtosecond slab laser (FX 200; wavelength: 1030 nm; average power: 80 W; pulse duration: 650 fs; Edgewave GmbH, Wurselen, Germany) in a variety of conditions. On the W-Cu composite material, the frequency of the laser pulses was varied between 6 and 24 MHz, the traverse velocity was varied between 30 and 480 m/s, and the number of passes was varied between 1 and 3 (specific parameter combinations will be mentioned below together with the results). On the Ti alloy, only the 10 MHz at 100 m/s combination was used.  
Fast-temperature measurements during laser ablation were carried out with the LabIR Ultrashort measurement system [23]. An InGaAs detector with a 5 MHz bandwidth was used for the detection of infrared (IR) radiation through the polygon scanning head [24]. As the pulse repetition frequency was higher than the detector bandwidth (6 to 24 MHz compared to 5 MHz), the effect of separate laser pulses was not resolved, and only the average value of heat accumulation was evaluated. To decrease noise, averaging from many laser scanning lines was performed (from 412 to 7, depending on the scanning speed, varying from 30 to 480 m/s). Calibration to obtain temperature values from the voltage was carried out by static heating using a small heating element and measuring signal through the scan head.