Wednesday, May 23, 2012

Laser Ablation: Results

The figure below shows the ablation window depth as a function of stage scan speed.  The removal rate shows exponential decay, but the obvious fact points towed lower removal rates at lower energies; what's important is that the user can still control how stuff is removed by laser.
 Fig.1 Calibration profiles for three different energy levels at several scanning speeds (dwell times). Note absence of ablation window at 35 mJ energy level regardless of scan speed. 
Fig. 1 shown below, shows signs of a minimum window depth feasible within any pulse energy given the maximum scanning speed available. The most important curve is the seemingly flat 35 µJ pulse energy profile, (1 µJ = 10-6J) which stays flat no matter the scan speed, as seen above.  The other two energies (167 and 73 µJ) show clear decay slopes, which can be interpreted as progressive erosion by sample speed reduction (going backwards on the x-axis in Fig. 1 above).  The calibration profiles on the left conform to this finding.

 Fig.2 SEM micrograph showing surface detail of baseline (unablated) coating sample.    
Figs 2-4 are scanning electron microscope (SEM) micrographs, AKA, closeups, of the coating after different laser energies.  Fig. 2 is unabated; Fig. 3 depicts the coating after the 35 µJ laser; Fig. 4 is after 167 µJ.  The coating itself is a TiO2 pigment (small white particles) embedded within the polyurethane binder (uniform dark background); the large spheres and blocky objects are siliceous and talc fillers, respectively.  
Fig. 3 SEM micrograph showing surface detail of coating sample after 35 mJ ablation, 10K mm/s scan speed. Note selective ablation of TiO2 pigment (evidenced by presence of ovaloid cavities) with no apparent disruption to surrounding organic binder. 
Fig.4 SEM micrograph showing surface detail of coating sample after 167 mJ ablation, 10K mm/s scan speed.    
 The laser darkens the pigment to various degrees, even for the 35 µJ laser, which did not ablate the coating.  The 167 µJ scan destroyed most of the pigment and considerably altered the binder morphology to the point of creating a large crater on the surface.  Fig. 5 is a cross-section of the coating and shows a sharp distinction between the ablated and non-ablated regions.
Fig. 5 SEM micrograph of paint cross-section showing transition between unablated and ablated surfaces. Boxed areas are shown in detail in the lower two images. On the left is detail from the unablated surface showing uniform distribution of pigment (small white particles). The right image shows detail from the ablated surface. A 5–8 µm pigment-depleted layer is indicated by the red line. 
It's clear from Fig. 5 that the ablated region has lost much of the pigment and differs significantly in morphology.  TiO2 is a semiconductor with a relatively small band gap which makes it more susceptible to ablation than the surrounding polyurethane binder.

It's going to get a little technical from here on out, but this is where it gets quantitative.  Fig. 6 shows the transmission spectra of a baseline (pristine) sample plus spectra from all three ablation energies (35, 73, 173 µJ).
Fig. 6. Transmission-mode spectrum of unaged baseline and ATR-mode spectra for 35, 73 and 167 µJ ablation. The ester and urethane peaks experienced little change from the ablation process.
The peaks contained within the box on the left did not change much after ablation; this is to be expected since polyurethane binder is mostly what's left in the ablated regions.  There's a large gain in the peak on the right (indicates a gain in quantity for that functional group), but that's a region of considerable overlap between the C–O–C ether and O–Si–O; the authors guess it to be Si–O, since there's a lot of new SiO2 filler exposed.  

Now I compare samples that differ by the amount of simulated weather that they've been exposed to.  Below is Fig. 7.
Fig. 7 ATR-FTIR spectra for baseline, 6-week and 18-week QUV exposed samples of coating. Near-extinction of amide II peak (1523 cm-1) is noted, evidence of photooxidation of urethane groups in the organic binder of the coating. (b) OH absorption (3364 cm-1) is seen to broaden substantially, whereas hydrocarbon peaks (2937, 2863 cm-1) simultaneously decrease, as a result of QUV exposure. 
Keep in mind that cm-1 (wavenumbers) is a unit of frequency just like Hz (s-1); in fact it's proportional by Eνc/λ for the infrared radiation (IR).  You could see from Fig. 7 that the IR is compared for the baseline, 6-weeks QUV exposed, and 18-weeks QUV exposed.  The authors emphasize the complete extinction of the so-called amide II functional group, which means drastic decay for the polyurethane binder.  In part (b) of Fig. 7, the OH/NH band broadens, perhaps due to carboxyl group formation.  Simultaneously,the QUV process reduces the hydrocarbon peaks in the region close to 3000 cm-1.

Fig. 8 presents depth profiles of the ablated window for the baseline and 18-week QUV samples.
 Fig. 9 (a) Ablation window depths for 140 µJ pulse energy, baseline and 18-week QUV samples. (b) Ablation window depths for 80 µJ pulse energy, baseline and 18-week QUV samples. At all scan speed/ pulse energy combinations material removal rate for UV-aged sample exceeds baseline sample 
The depth windows show a greater rate of material removal for the QUV-aged sample than for the baseline; probably due to the weathered urethane binder becoming more broken down.

Fig. 10 shows FTIR depth-profile measurements.  This is the nitty-gritty of the study, as it shows why this method falls short.  
Fig. 11 a) Amide II/C]O ratio for baseline vs. UV-aged coating samples using cross-section transmission-mode FTIR. UV-zone extending 30 mm into topcoat is readily apparent. (b) Amide II/ C]O ratio for baseline vs. UV-aged coating samples using femtosec- ond ablation-assisted depth profiling. UV-damaged zone not apparent. 
Fig. 11 a) repeats a result from an older study where they used transmission-mode FTIR on 3 µm thick cross-sections of the same two samples.  It's mentioned here for the sake of comparison.  Peak ratios are used here to eliminate fluctuating values due to varying sample thicknesses.  Part (a) shows a clear change in the ratios as the depth is increased, meaning the near-surface region is more weather-damaged than the deeper regions in the bulk.  That is not the case for part (b); if the ablation method did its job, it would show a damage gradient just like in part (a).  The next blog will discuss why this is so.