Laser Ablation: Experimental

The experiment went through two phases.

Phase 1: physical and spectroscopic characterization of the ablation process performed at three different pulse-energy levels and five separate dwell times for determination of the proper process parameters.  

Phase 2: applying these parameters for optimal depth profiling of the coating as seen in the real world.  

The general approach of femtosecond ablation profiling generally meant they ablated a series of craters, AKA"windows", at increasingly greater depths into the sample; they (Keene et al) made the key assumption that the insanely fast ablation process won't appreciably modify the material to the point of changing its spectrum.  The window base makes it relatively convenient to acquire a vibrational spectrum (maybe a mass spectrum too).  Fig. 1 is a schematic comparing the cross-section transmission-mode approach with the ablation method discussed in this entry.  Infrared (IR) data is acquired by micro-ATR at the window base.  The window bases had 3mm × 3mm to allow for the capture of at least 4 IR spectra/window.  Note also that all ablations were done in standard atmosphere.  

 Schematic depicting cross-sectional transmission-mode FTIR approach to depth profiling vs. (b) femtosecond ablation-assisted ATR- Mode FTIR depth profiling. 

They used a chirped-pulse amplified multi pass Ti:Sapphire laser with a repetition rate of 1 kHz, wavelength centered at 780 nm (nanometers) and a pulse duration of about 30 femtoseconds.  An aluminum alloy applied with a composite navy coating system of thickness ~100 µm (100 micrometers, topcoat and primer) was used to optimize the ablation parameters.

Parameter Optimization
Three rows of five 3mm × 3mm windows were generated, the windows in each row being ablated at a different stage scanning speed and each row ablated at a decreasingly lower pulse-energy level.  They used five linear scanning speeds (10K, 8K, 6K, 4K& 2K µm/s); the slower you scan, the longer the laser ablates the surface.  Incident energy levels of 167, 73, & 35 µJ plus a baseline (unweathered) sample were used.

The researchers scanned the ablation windows to characterize the crater topography and depth at each fluence/scan speed.  Fluence refers to the number of particles flowing into an area/unit of time.  Each ablation window was raster-scanned with data collected every 10 µm to generate a dense matrix of data corresponding to the topology of the ablation window.  The values/window were then averaged together to give the average ablation depth at the given scan speed/energy combination.

Remember when I mentioned the assumption regarding surface integrity following ablation?  That was put to the test, first with more IR scans, then Raman spectroscopy, finally with scanning electron microscopy.  In the case of IR spectra, five spectra were collected from the base of each of the three rows via micro-ATR mode.  Those five spectra were then averaged to generate a prototypical amalgamation spectrum to minimize regional variations in chemistry.   The collected spectra from all three windows were compared to collected spectra from unablated regions on the sample in the same way.  

2nd set of data comes from dispersive Raman Spectroscopy.  Similar to IR, they collected raman spectra from the base of the three ablated windows at each energy level (167, 73, & 35 µJ) to see if ablation had induced formation of organic peroxide or derivative species (R–OH^– ) due to energetic interaction with atmospheric oxygen.  

Lastly, SEM was used to further characterize the effects of über-fast ablation on the composite coating surface.  They selected the top and bottom energy levels (167 & 35 µJ) for best comparisons of appearance and elemental composition with the baseline (unspoiled) sample.  Samples were sputtered with gold particles to encourage charge compensation during analysis.  SEM images were collected with 5000×, 10,000×, and 50,000× magnifications.  The samples were immersed in liquid N2 to obtain cross-sections.  The now-separated coatings were embedded in histological wax (the same kind used to embed tissue), then microtomed to a thickness of 30 µm.  Then the samples were place in vacuum-compatable copper tape for SEM analysis.

Spectroscopic depth-profile collection


For proper evaluation of the ablation technique as depicted in Fig. 1b) above, two separate samples were chosen.  Both had been analyzed by transmission-mode FTIR and are of the same military coating system.    One sample was the baseline (unaged) sample while the other underwent an 18-week QUV exposure protocol in an army research lab.   QUV is a brand for accelerated weather testing, and is inpart  summarized by this YouTube video from the company.

For 18 weeks, the sample was subjected to a varying spectral power distribution of noon summer daylight (295-450 nm) in conjunction with a constant temperature of 60°C.

Collection of laser profiles was enabled by discrete ablation depths to allow a spectroscopic sampling at regular intervals into the topcoat (see previous entry for schematic of typical vehicle coating).  Ablation was needed at both the near surface and the window, so they had to ablate two columns of windows at two completely different energy levels: 140 µJ/pulse (column of six windows) and 80 µJ/pulse (five window column).  Both columns used an identical scan speed.  This procedure was done for the aged & unaged samples.  IR spectra were collected at the base of each window via micro-ATR FTIR (link found in an earlier paragraph).

Next entry will cover the results.  Expect lots of Ifrared spectra.  Hopefully I can explain their significance at a level appropriate for this blog.  Cheers.