Sunday, March 11, 2012

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.

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!  

1 comment:

  1. This is a very beautiful and interesting research
    The most educating one i have read today!

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