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.  

Femtosecond laser ablation

My research will be on organic coatings.  This is the first such post.

As commonly as coatings are used, their weathering-induce degradation is complex and thus remains poorly understood, as coatings can fail in a variety of ways (I think that's what they mean by 'failure modes').  Barrier properties of the coating system can be compromised, along with its mechanical properties.  Proper evaluation of durability and longevity of modern composite coating systems requires characterization of weather/aging phenomena as a function of their spatial distributions.  In the case(s) of homogenous organic coatings, or for coatings with low pigment-to-volume concentrations (PVC), this is typically performed by spectroscopic analysis to cross-sectional samples or by a form of confocal spectroscopy.  Those techniques are fine and dandy if the coating cross-sections are transparent.  If opaque, better get creative, which a team of researchers from SUNY Stony Brook did so.

Background

UV-induced aging of polymeric coatings significantly changes the chemical and morphological structure within the organic matrix of the coating.  Many coatings are based on aromatic ring structures, and are prone to UV absorption and yellowing.  Even aliphatic binders are prone to UV-induced weathering, although to a lesser degree.  Either way, our understanding of these physiochemical changes remain murky; we can't predict if a polymer chain within the coating will cross-link or undergo scission.  

Degradation of composite coatings are even harder to understand; these are coatings that contain a disproportionate amount of inorganic components.  The mechanism depends on both the base chemistry of the organic binder, and on the characteristics of the inorganic additives contained within the coating.  Below is a schematic of such a coating system.  

Most of my research may focus on coatings for military vehicles, so this should be a good model to use in this article.

The closer you get to the surface coating, the more degradation–in other words, weathering/aging also depends on depth.  

Thus, it's best to design a method that yields spectral data and data of the spatial distribution throughout the coating system.  We may a few such depth-profiling techniques already: confocal focusing of an energy beam probe (Confocal Raman Spectroscopy), Secondary Ion Mass Spectroscopy (SIMS), and transmission-mode Fourier Transform Infrared Spectroscopy (transmission-mode FTIR), and UV-Visble spectroscopy.  CRS requires transparent samples, thus out of the question; SIMS won't work for dielectric coatings (practically all organic coatings).  

Transmission-mode FTIR and UV-Visible spectroscopy work well, but they require intact cross-sections with dimensions too small for practical preparation.  A simpler preparation involves microtoming at very small angles to the surface of the coating–effectively extending the depth of the sample to a degree that enables usage of attenuated total reflectance FTIR, but that opens up another can of worms: interlayer mixing due to the microtome blade.  Materials that are too hard and incohesive to microtome successfully are equally vexed when a spectroscopic profile is demanded.  To meet this demand, Keenes et al devised a novel use of a femtosecond (10^-15 sec) pulsed laser as an enabler for depth-profiling coatings of this type.
This isn't the first time they used lasers for ablation; previous time scales include nanoseconds (10^-9 sec) and picoseconds (10^-12 sec).  Laser ablation isn't entirely understood, but it's believed to be the result of a coulombic explosion, in which near-instantaneous ionization of atoms via bright laser beam excites electrons to the point of escaping from their host atoms.  The atoms have become so ionized as to be repulsed by the neighboring positive charges on the irradiated material–repulsions with pressures 10 million times that of standard atmospheric pressure.  This results in a direct solid-to-vapor transition characterized by a violent release of ions from the surface.
Now, people have used ultra-fast laser ablation before, but in the context of mass spectroscopy.  Keenes et al extend this technique to infrared spectroscopy, which is the kind of data that explains the depth profile of the coating.  They hope no post-ablation thermal disruptions will alter the coating composition.
Why do they care about what a coating looks like underneath its surface?  Well, when your client is the U.S. military, you have many reason$ to care.  The scientific reasons will be explained in the next paragraph.  The coatings in question are commonly used on fixed wing/rotary aircraft and naval vessels.  Femtosecond laser ablation is supposed to reveal the underlying layers of the coating to be analyzed ATR-mode FTIR to investigate the depth to which photooxidation of the organic binder has occurred.  A binder is a resin used to keep particles together and supply mechanical strength or to ensure uniform consistency, solidification, or adhesion to a surface coating.
The military coating studied here is a chemical agent resistant coating (CARC) comprising of low-gloss automotive-grade aliphatic polyurethane highly loaded with a pigmentation/filler package.  It seems that this coating in question in part stems from a 1970s patent on tri-functional isocyanate cross linking agents (a bit too technical to delve into for a blog entry).  The main pigment in this coating is a nanoscale spherical titanium dioxide that bestows a light gray color to the coating for optical camouflage.  Diatomaceous earth is added to the coating to lower the gloss (high gloss is bad for camo). SEM micrographs of the coating in question is provided below.

 SEM micrograph (top) of coating cross-section. High volumetric proportion of pigmentation/inorganic fillers is evident from the three EDS elemental maps of Ti (pigment), Si (pigment C flattening agent) and Ca (flattening agent). 

In addition to optical/infrared camouflage, the military uses coatings for protection of metal substrate from corrosion/weathering and resistance to perforation by chemical decontamination agents.  Uncle Sam also demands that its colors don't wash or fade out and it can maintain its mechanical integrity according to appropriate specifications.  This coating has seen extensive action in weathering studies by Keene et al, so it serves as a useful model for this ablation technique to investigate.  Experimental Results and discussion will be detailed in the next entry.  

It's nice to be back!