On the use of femtosecond laser ablation to facilitate spectroscopic depth profiling of heterogeneous polymeric coatings

Polymer Degradation and Stability 89 (2005) 393e409 www.elsevier.com/locate/polydegstab

On the use of femtosecond laser ablation to facilitate spectroscopic depth profiling of heterogeneous polymeric coatings Lionel T. Keenea,*, Thomas Fierob, Clive R. Claytona, Gary P. Haladaa, David Cardozab, Tom Weinachtb a

Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, USA b Department of Physics and Astronomy, State University of New York at Stony brook, Stony Brook, NY, USA Received 2 November 2004; received in revised form 5 January 2005; accepted 20 January 2005 Available online 19 March 2005

Abstract Spectroscopic depth profiling of organic coatings is useful for characterizing the nature and spatial distribution of weatherinduced aging on a coating system. Due to their highly heterogeneous compositions and general opacity, high-solids organic coatings can be extremely difficult to depth profile using established techniques. This work evaluates the feasibility of using femtosecond time-scale laser pulses as a method for preparing coating samples for the purpose of depth-resolved spectroscopic analysis. Samples were prepared by ablating square regions of various depths in military coating samples. Ablated regions were characterized morphologically using a custom scanning confocal profilometer and spectroscopically using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and dispersive Raman spectroscopy. Depth profiles collected from ablated samples were compared with profiles of the same material collected using a cross-sectional transmission-mode FTIR technique. A threshold level was found, below which a compound-selective ablation process was observed. Additional ether cross-linking was found to occur at moderate/high energy levels. Material in the residual interaction volume showed indications of reconstituted urethane functionality. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Multiphoton-absorption photopolymerization; FTIR; Polyurethane; Depth-profile; Photodegradation

1. Introduction The tendency of natural weathering to disrupt certain organic coating chemistries and induce degradation of both coating and underlying substrate is well known [1e3]. This phenomenon of weathering-induced failure is complex and can lead to a plethora of failure modes in any given coating formulation. These changes can be profound and have consequences not only to the barrier

* Corresponding author. E-mail address: [email protected] (L.T. Keene). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.01.033

properties of the coating system [4e7], but also its mechanical properties in general [8e13]. To properly evaluate the durability and longevity of modern composite coating systems it is necessary to characterize weathering/aging as a function of their spatial distributions throughout the coating layers. For homogenous coatings consisting exclusively of organics, or coatings with a very low pigment-volume concentration (PVC), this is typically accomplished either by the application of spectroscopic analyses to cross-sectional samples [14] or (in the case of highly transparent clearcoat systems) by some form of confocal spectroscopy. Of course, the successful application of these techniques hinges on the

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transparency of the coating or the ease with which a suitably thin cross-section can be prepared. While it has been previously shown that even coatings with a very high PVC can be successfully cross-sectioned and analyzed via transmission-mode FTIR [15,16], certain composite coating systems may not exhibit the necessary flexibility for the successful preparation of intact crosssectional samples. In cases such as these and other cases where a material’s opacity precludes the application of a confocal spectroscopic approach, alternative methods of depth-resolved spectroscopic analyses are required. It is the purpose of this paper to evaluate the use of ultrafast laser ablation as an enabler of spectroscopic depth profiling in materials that are too opaque, or otherwise not mechanically suitable, for the preparation of crosssectional samples.

2. Background The effect of weathering on organic coatings is an active area of study. In particular, UV-induced aging of polymeric coatings has been shown to result in significant chemical and morphological alterations to the organic matrix. It is well established that polymeric coatings based on aromatic ring structures are prone to UV absorption and yellowing due their extensive system of conjugated bonding [1,17]. It has also been shown that, to a lesser degree, aliphatic-based organic binders are also subject to UV-induced weathering, although in this case the absorption is thought to result from residual chromophoric impurities introduced during the manufacturing process [1,18]. In either case, the physicochemical consequences of such weathering are complex and difficult to predict; both cross-linking as well as chain scission may result [12]. In the case of highly-filled composite coatings (coating systems containing a proportionately large amount of inorganic components), the mechanism by which these modifications occur can depend on both the base chemistry of the organic binder as well as the nature of the inorganic additives contained in the formulation [1,19e21]. Additionally, the degree to which photochemical degradation occurs, i.e. the magnitude and depth of the modifications, will directly impact the barrier properties of the coating system. When evaluating performance anticipation on coatings of this type it is therefore desirable to employ an analysis technique that affords not only spectroscopic data of the breakdown, but also information regarding the spatial distribution of these data throughout the coating. There exist a number of spectroscopic depth-profiling techniques; most of these invoke one (or more) of the following methods: (1) confocal focusing of an energy beam probe (e.g. confocal Raman spectroscopy), (2) Secondary Ion Mass Spectrometry (SIMS), and (3) transmission-mode

Fourier Transform Infrared spectroscopy (transmissionmode FTIR) and UVeVisible spectroscopy. The confocal use of an optical beam requires that the coating system be optically transparent to allow sufficient transmission of the beam through the underlying layers. The use of SIMS, or any charged-particle beam technique for that matter, will usually present practical problems concerning charge compensation if the coating system is a dielectric (almost always the case in organic coatings). Transmission-mode FTIR and UVeVisible spectroscopy are the most common methods of depth profiling an organic coating; transmission-mode FTIR has proven to be a viable approach even for systems with very high PVC [15]. In certain cases, however, application of the transmission-mode approach may prove impractical. Such cases include organic coating systems whose volumes of organic binder are so small (or so brittle) that preparation of an intact cross-section of requisite dimensions is impossible. In cases such as these, an alternative (and usually simpler) method of sample preparation is to microtome at very small angles to the surface of the coating, in effect extending the depth of the sample to such a degree that spectroscopic depth profiling is possible via ATR-mode FTIR. The potential drawbacks to this approach are the possibility of interlayer mixing as a result of the action of the microtome blade, and the necessity in some cases to employ the micro-ATR method of spectral collection if the sample is too absorbent to allow spectral collection via diffuse reflectance mode (which allows for a smaller analysis spot size). Since the currently available microATR accessory (Thermo-NicoletÔ) has a contact area of approximately 50 mm diameter, a very low microtome angle is required to resolve small depth changes. Materials that are too hard and incohesive to microtome effectively are equally problematic when a spectroscopic depth profile is desired. These cases provide the motivation for evaluating alternative approaches to spectroscopic depth profiling. In this paper we evaluate the novel use of a femtosecond-class pulsed laser as a facilitator for depth-profiling coatings of this type. The ultra-fast optical phenomenon of femtosecond laser ablation differs from ablation conducted at other time-scales (nano and picosecond) primarily in the observed absence of a zone of photothermal degradation surrounding the ablation crater [22]. The ablation mechanism, while not entirely understood, is thought to consist essentially of a coulombic explosion at the irradiation site [23e26]. Coulomb explosion is the result of a near-instantaneous ionization of atoms initiated by multi-photon absorptions that allow electrons to achieve the necessary escape energy to dissociate entirely from their host atoms [26]. The residual coulombic pressure in the now highly-ionized material (a computer simulation performed by Cheng et al. [26] predicts the electrostatic pressure that results from the mutual repulsion in the

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ionized region to be on the order of 1000 GPa) results in a direct solid-to-vapor transition characterized by an explosive release of ions from the surface [23,24,27]. Since the duration of the entire photoexcitation event is much smaller than the frequency of typical lattice perturbations, thermal conduction into the target material is thought to be negligible [22,23,28]. While ultra-fast laser ablation has been used in the past for spectroscopic studies, the capacity in which it has been applied was typically in the form of a primary excitation source for mass spectroscopy. This work intends to evaluate the potential of femtosecond laser ablation for facilitating vibrational spectroscopic depth profiling by leveraging its supposed ability to remove material with no thermal input. We attempt to exploit the ultra-fast ablation effect by using it to remove successive layers of a military composite coating system commonly employed on fixed wing/rotary aircraft and naval vessels. In so doing, underlying layers of the coating system are exposed and then analyzed by Attenuated Total Reflectance (ATR)-mode FTIR in an attempt to discern the depth to which photooxidation of the organic binder has occurred. Results will be compared with those previously reported for this material using the cross-section transmission-mode FTIR approach [15] in order to evaluate the suitability of this technique to the depth-profile problem. The military coating system under study is a Chemical Agent Resistant Coating (CARC) consisting essentially of low-gloss automotive-grade aliphatic polyurethane (approximately 60% volume) highly loaded with a pigmentation/filler package. The polyurethane binders in use in these systems involve trifunctional hexamethylene diisocyanate (HDI) and polyester-polyol precursors leading to a highly cross-linked poly(ester-urethane) structure. The primary pigmentation consists of nanoscale (250e400 nm) spherical titanium dioxide (rutile phase) that imparts a light gray color to the coating for the purpose of optical camouflage. Larger siliceous extenders consisting of diatomaceous earth are added to the coating in order to lower gloss levels. A cross-section electron micrograph and EDS elemental maps of the major inorganic constituents are shown in Fig. 1; the high degree of inorganic fillers and pigments is readily visible from this figure. For more detailed information regarding this family of coatings the reader is referred to Refs. [29,30]. The primary functions of military coating systems include optical/infrared camouflage, protection of substrate from corrosion/weathering and resistance to penetration by chemical decontaminating agents. In addition, they are expected to demonstrate a high degree of colorfastness and mechanical integrity as mandated by military specifications. Previous work by this group has detailed extensive photooxidative modification to the polyurethane binder under certain weathering conditions [15], making it the logical choice for an

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evaluative study on the feasibility of the femtosecond ablation approach by allowing for a direct comparison with known results derived via an established method.

3. Experimental Experimental work was carried out in two phases, the first involving physical and spectroscopic characterization of the ablation process conducted at three different pulse-energy levels and five separate dwell times in order to determine the proper process parameters. The second phase involved the application of these parameters so as to duplicate as closely as possible the depth profile derived in the earlier study [15]. The general approach of femtosecond ablationassisted depth profiling is to ablate a series of craters, or ‘‘windows’’, at progressively greater depths into the sample, the key initial assumption being that the ultrafast ablation process will not induce any appreciable modifications to the surrounding material’s spectrum. The base of each window allows for the capture of a vibrational and/or possibly a mass spectrum at that particular depth. A schematic comparing the crosssection transmission-mode approach with the ablationassisted approach is shown in Fig. 2. We employ the micro-ATR method for collecting infrared spectra at the base of the ablation windows and, therefore, the dimensions of each ablation window must be large enough to accommodate the physical geometry of the micro-ATR accessory fitted to the infrared microscope. For this reason, ablation windows of 3 mm ! 3 mm dimensions were chosen, the size allowing for the capture of at least four separate IR spectra (ATR mode) per window. All ablation was conducted in atmosphere. 3.1. Optimization of process parameters The laser used in this study was a chirped-pulse amplified multipass Ti:Sapphire type (KM Labs) with a repetition rate of 1 kHz, wavelength centered at 780 nm and a pulse duration of w30 fs. An aluminum alloy (AA2024-T3) coupon applied with a composite navy coating system (military specification MIL-PRF2337 Type II Class C) of approximately 100 mm combined thickness (topcoat and primer) was used to optimize the ablation parameters. The coupon, with coating system facing the laser, was mounted on a vertically oriented platform consisting of orthogonally coupled Coherent LabMotionÔ series 640 micro-stepping stages to allow for XY manipulation of the sample under the focused beam. Optimum focus could not be maintained throughout the course of the experiment due to the absence of a third stage to automatically advance the sample platform as material is ablated. Therefore,

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Fig. 1. 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).

a focusing lens with a Rayleigh range of 3.8 mm was used to focus the beam and ensure adequate fluence for ablation at all window depths. Three rows of five 3 mm ! 3 mm windows were generated, the windows in each row being ablated at a different stage scanning speed and each row ablated at a progressively lower pulse-energy level (pulse energy was approximately halved for each row). Linear stage scanning speeds used were 10K, 8K, 6K, 4K, and 2K mm/s (the progressively slower scan speeds corresponding to greater laser/ surface dwell times). Pulse-energy levels were altered using a neutral density filter placed between the laser and the sample surface. Incident energy levels of 167, 73 and 35 mJ in conjunction with a baseline (unaged) coating sample were used. The ablation windows were scanned to characterize the crater topography and depth at each fluence/scanspeed combination using a custom confocal laser

scanner. The scanner consisted of a KeyenceÔ LT-8010 confocal scanner head mounted above a pair of Coherent LabMotionÔ Series 640 micro-stepping stages arranged in an XY configuration. The scanner sampling spot size (in-plane spatial resolution) is 2 mm and the vertical height resolution is G100 nm. Custom control software was written to coordinate the motion of the stages with the capture of data from the sensor head. Each ablation window was raster-scanned underneath the scanner head with data collected every 10 mm to generate a dense matrix of data corresponding to the microtopography of the ablation window. The average of these values per window gave the average ablation depth at the given scan-speed/energy combination. In order to qualify the initial assumption regarding the lack of photothermal/photochemical degradation occurring as a direct result of the ultra-fast ablation process (in particular, the absence of additional

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Fig. 2. (a) Schematic depicting cross-sectional transmission-mode FTIR approach to depth profiling vs. (b) femtosecond ablation-assisted ATRMode FTIR depth profiling.

formation of the strongly IR-absorbing carbonyl group at approximately 1700e1600 cmÿ1, a concern given the fact that the ablation was conducted in the presence of atmospheric oxygen and involvement of this functional group has been implicated in at least one common polyurethane photooxidation model [18,31], as well as reduction of the amide II peak at 1523 cmÿ1, also symptomatic of photooxidation of polyurethanes [18,31]), IR spectra were collected from the first window in each of the three rows via micro-ATR mode using a diamond micro-ATR attachment fitted to a ContinuumÔ IR microscope (50 mm ! 50 mm aperture) in

conjunction with a Thermo-NicoletÔ model 560 FTIR spectrometer. A total of five spectra were collected from different locations at the base of the ablation windows. These five spectra were then averaged to generate a representative composite spectrum to minimize the effect of regional chemistry variations. The collected spectra from all three windows were compared with a composite average spectrum collected from an unablated area of the sample in the same fashion. All spectra were collected with 256-scan summations and 4 cmÿ1 resolution using MCT detector and software corrected for ATR-mode collection to allow for direct

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comparison with transmission-mode spectra. A pressure sensor was used in conjunction with the ATR accessory to ensure consistent sample pressure between the diamond ATR crystal and the sample surface. In addition to IR analysis, dispersive micro-Raman spectroscopy was conducted at the base of the first three ablation windows at each energy level to determine whether the ablation process had induced the formation of organic peroxide and/or hydroperoxide species due to energetic interaction with atmospheric oxygen. Five spectra were collected from the sample using 32 scan summations and averaged to generate a final spectrum. Finally, SEM/EDS microscopy was used to further characterize the effects of ultra-fast ablation on the composite coating material. Ablation windows at the two energy level extremes (35 mJ and 167 mJ) were selected for comparisons of appearance and elemental composition with the baseline sample. Samples were sputter coated with gold to facilitate charge compensation during analysis. SEM images were collected at 5000!, 10,000! and 50,000! magnifications. Crosssections of the ablated samples were prepared by immersing the sample in liquid N2, causing intact separation of the coating system and the substrate. The coating sample was then embedded in histological wax (ParaplastÔ X-TRA Tissue Embedding Medium) and microtomed to a thickness of 30 mm and placed on vacuum-compatible copper tape for SEM analysis. Electron micrographs as well as EDS maps were collected of the areas surrounding the ablation window.

inside the test chamber for the duration of the exposure and the irradiance level is kept stable via automatic sensor control. Total exposure duration was 18 weeks. To enable the collection of depth profiles from the two samples, laser energy levels and scan speeds were chosen (based on the results from the process parameter experimental phase) so that a series of windows were ablated at discreet depths to allow a spectroscopic sampling at (approximately) regular intervals into the topcoat. To achieve window ablation at both the near surface as well as deep into the coating, the windows were ablated at two different energy levels: one column of six windows ablated using a beam energy of 140 mJ per pulse and a second column of five windows ablated at 80 mJ per pulse (beam energy attenuated via neutral density filter). Both columns of windows used an identical sequence of scan speeds. The procedure was performed on both samples (baseline and 18-week QUV). Ablation window topographies were scanned using the aforementioned confocal scanner to determine the average window depths. IR spectra were collected at the base of each window using micro-ATR FTIR technique, with four spectra collected and averaged per window. All spectra were collected with 256-scan summations and 4 cmÿ1 resolution using MCT detector. A pressure sensor was used in conjunction with the ATR accessory to ensure consistent sample pressure between spectral collections.

3.2. Spectroscopic depth-profile collection

4.1. Process parameter optimization results

In order to evaluate the application of the ablation technique to the spectroscopic depth profiling of composite coatings, two separate samples were chosen. Both have been successfully depth profiled using the transmission-mode FTIR approach in a previous study [15], and both consist of the MIL-PRF-2337 Type II Class C military coating system. One sample was used as the baseline (unaged) coating while a second sample had been subjected to an 18-week QUV exposure protocol at the Army Research Laboratory in Aberdeen, MD. This protocol is intended to simulate accelerated photooxidative aging and consists of placing samples in Q-PanelÔ (Q-Panel Laboratory Products, Ohio) accelerated weathering chambers conforming to ASTM G-53 requirements (‘‘Standard Practice for Operating Light and Water Exposure Apparatus e Fluorescent Ultraviolet (UV)/Condensation Type e for Exposure of Nonmetallic Materials’’) [32]. UV-340 fluorescent lamps emitting a spectral irradiance of 0.77 W/m2, measured at 340 nm, are used in conjunction with a daylight filter to closely approximate the spectral power distribution of noon summer sunlight in the range of approximately 295e450 nm [33]. A temperature of 60  C is maintained

Fig. 3 shows the ablation window depth as a function of stage scan speed. Material removal rate is clearly nonlinear. As expected, lower energy pulses result in less material removal per scan speed and, therefore, by attenuating the pulse energy the depth of the ablation window can be controlled to a degree. The data in Fig. 4 also indicate the existence of a minimum window depth that can be achieved at any pulse energy given the maximum scanning speed available (10K mm/s). Of particular interest is the 35 mJ pulse energy profile, which remains seemingly flat despite the progressively increasing laser/surface dwell time (progressively slower scanning speed) imposed by the experimental parameters. Both the 167 mJ and the 73 mJ pulses caused a progressive erosion of the surface as the sample scan speed was reduced; this trend was not duplicated with the 35 mJ pulse parameter, indicating the presence of a fluence threshold below which the material remains unablated irregardless of the number of pulses per spot. This is verified by inspection of the three-dimensional surface plots of the ablation windows for the 4K mm/s scan speed in Fig. 3: the window generated via the 167 mJ pulse energy is clearly the deepest, followed by

4. Experimental results

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Fig. 3. 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.

the 73 mJ window (not as deep but still discernable). The 35 mJ window cannot be discerned by its topography, indicating a lack of large-scale material removal. Visually, all windows ablated at all energy/scan-speed combinations were seen to have a darkened appearance, including those ablated at the 35 mJ level. This would indicate an ablation process wherein certain coating components, namely the TiO2 pigmentation which is responsible for imparting color to the coating, have been selectively ablated while the polyurethane binder remains intact, thereby causing a darkening of the chromic characteristics of the coating. This is further verified by examination of the electron micrographs of Figs. 4e6 that show the surface of the baseline (unablated), 35 mJ ablated and 167 mJ ablated samples, respectively. The pigment is visible as small bright particles whereas the large spheres and blocky objects are siliceous and talc fillers, respectively. The uniform dark background is the poly(ester-urethane) binder. From the images it can be seen that, in comparison to the baseline unablated sample (Fig. 4), under the 35 mJ ablation conditions (Fig. 5) there is a removal of TiO2 pigment with no apparent damage to the surrounding binder. At higher levels (167 mJ pulse energy, Fig. 6) there is both a general reduction of TiO2 pigment concentration as well as clear morphology modifications to the organic constituent of the coating system. This would imply that the volume of sample material in which the laser interacts (the ‘‘interaction volume’’) extends below the ablation crater. This is further indicated in Fig. 7, which shows the cross-section view

of the transition between unablated surface and ablated window. The regions of interests are taken from surfaces outside as well as inside the ablation window. It can be clearly seen from the figure that the coating surface inside the ablation window has been depleted of TiO2 pigment particles (again, the TiO2 pigments being the small bright particles whereas the SiO2 flattening agents are the large spherical objects). This is not altogether surprising if one accepts the assumption that the underlying ablation mechanism involves a simultaneous multi-photon absorption process; the TiO2 pigment is a semiconductor which, by reason of its relatively low bandgap energy, is more susceptible to ablation at attenuated energy levels than the surrounding organic binder. Fig. 8 further illustrates the effect by showing cross-sectional overlapping EDS maps of titanium and silicon. The titanium signal is indicative of the presence of pigment, whereas the silicon was chosen as a reliable indicator of the physical extents of the topcoat due to its ubiquity in this formulation. The figure shows crosssections for baseline (upper image), 35 mJ (lower left) and 167 mJ (lower right) ablated samples. There are no readily discernable differences in the baseline vs. 35 mJ EDS maps. Since the resolution of the EDS technique is roughly 1 mm, this implies the TiO2 depletion effect does not extend beyond approximately 1 mm below the surface during 35 mJ ablation conditions. The 167 mJ ablation cross-section, however, shows a clear pigment depletion zone, indicating that under these conditions the interaction volume extends approximately 5e7 mm below the ablation surface.

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Fig. 4. SEM micrograph showing surface detail of baseline (unablated) coating sample.

Fig. 9 shows the transmission spectra of a baseline sample as well as spectra from all three ablation energies (35, 73, 167 mJ). Peak assignments are given in Table 1. Very little alteration to the characteristic urethane and ester carbonyl peaks (1685 and 1725 cmÿ1, respectively), as well as the amide II functionality (1523 cmÿ1) of the urethane groups, is noted, a promising result for depthprofiling urethane functionality. While an increase in the absorption peak at approximately 1065e1068 cmÿ1 can be seen, attribution of this behavior is complicated due to the strong CeOeC ether as well as OeSieO absorptions at this frequency. Due to the lack of new peak formations in the organic range (an indication of the athermal nature of the process) as well as the moderate surface roughening noted in Fig. 6, this peak growth is tentatively attributed to progressive uncovering of previously unexposed SiO2 filler agents as a result of the ablation. Fig. 10a,b compares the spectra from a baseline, 6-week and 18-week UV-exposed samples. Near complete extinction of amide II functionality is noted after 18

weeks QUV exposure (Fig. 10a) indicating a stark reduction of in-chain urethane functionality. In Fig. 10b the OH/NH absorption band is seen to broaden, most likely due to carboxyl group formation and increasing absorption of primary amine functionality as a result of the photooxidation mechanism proposed by Wilhelm and Gardette [18]. Likewise, the reduction of the hydrocarbon peaks as a function of QUV exposure seen in the figure is consistent with oxidation of the CH2 group in alpha position to the secondary amine of the inchain urethane as proposed by the Wilhelm model. Fig. 11 shows the dispersive Raman spectra collected from baseline, 35, 73 and 167 mJ surfaces. A steady reduction of the rutile phase TiO2 peaks [34] at 621, 455 and 258 cmÿ1 corresponds to earlier results indicating preferential ablation of pigment. Notable is the complete lack of any peroxides (potentially arising as a result of energetic interaction between atmospheric oxygen and the sample), known to generate a large peak in the 900e800 cmÿ1 range [35].

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Fig. 5. 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.

4.2. Depth-profile results Fig. 12a,b shows the depth of the ablated windows prepared for both baseline as well as the 18-week QUV sample. Data is shown in two plots with 12a showing the 140 mJ series and 12b showing the 80 mJ series. Plots are constructed as depth vs. scan speed. In all cases, the material removal rate was greater for the aged sample than for the baseline sample. This is attributed to a probable general relative decrease in cross-link density of the UV-exposed sample vs. the baseline sample and hence the ablation rate between the two samples differs, an effect that has been previously observed elsewhere [36]. Fig. 13a,b shows the results of the actual FTIR depth-profile measurements. Fig. 13a depicts the previous result, collected using transmission-mode FTIR in conjunction with a 3 mm thick cross-section of the same two samples. These results have been previously reported [15] but are shown here for the purpose of comparison. In order to eliminate fluctuating values due

to possible non-uniform sample thickness, results are shown as the ratio of amide II (1523 cmÿ1) peak height to urethane carbonyl (1685 cmÿ1) peak height. Any perturbations to this ratio (as compared to baseline values) are indicative of a photo-induced shift of the baseline ratio value. The zone of UV damage to the poly(ester-urethane) binder can be seen to extend to approximately 30 mm into the topcoat, a result attributed to the optical transparency and potentially catalytic properties of the siliceous fillers [15]. In contrast, the profile of Fig. 13b (generated using the same samples but using the ablation window technique) shows a uniform suppression of the amide II/carbonyl ratio with no apparent UV damage gradient.

5. Discussion It is apparent from the data presented in Fig. 13a,b that our attempt to depth-profile UV-aged heterogeneous

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Fig. 6. SEM micrograph showing surface detail of coating sample after 167 mJ ablation, 10K mm/s scan speed.

poly(ester-urethane) coatings using the femtosecond ablation approach described herein has failed. Despite its apparent athermal removal process with very little alteration to key urethane IR signals (see Fig. 9), the ablation approach was unable to resolve the UVdamaged region in the topcoat. While, strictly speaking, the primary goal of this work (a feasibility study of the ablation approach to depth profiling) has been accomplished, it is apropos to posit a theory regarding the fundamental source of the failure to properly replicate the known depth profile. As has been noted earlier, it is apparent from Fig. 9 that there is relatively very little change to the depthprofile organic peaks of the baseline topcoat sample (relative to the changes to peak heights that have been shown to occur from prolonged UV exposure, see Fig. 10a) as a result of femtosecond laser ablation. The major peak alteration at 1068e1065 cmÿ1 was attributed to progressive uncovering of SiO2 fillers as the overlying polymer was ablated away. To further probe

the issue, a comparison was made between transmissionmode and ATR-mode spectra of the UV-aged sample rather than the baseline sample. The transmission-mode spectrum was selected such that the 10 mm high aperture was centered at a depth of 10 mm into the topcoat of the aged sample (due to the vertical dimension of the transmission aperture the collection zone contained material from a depth of 10e20 mm; since the depth profile captured using the transmission technique clearly indicates the presence of a UV-damaged zone extending approximately 30 mm into the topcoat, it is safe to assume that the material at 10 mm will detract from the strength of the profile peaks at 15 mm whereas the material at 20 mm will contribute to the strength of these peaks, and therefore the spectrum will portray the approximate peak heights existing at the average of these two values, i.e. 15 mm). A second spectrum was collected from the base of the 15-mm-deep ablated window of the UV-aged sample. These two spectra are shown together in Fig. 14 with a baseline spectrum from

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Fig. 7. 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 5e8 mm pigment-depleted layer is clearly visible.

the topcoat of an unaged sample. Progressive peak growth at 1065e1068 cmÿ1 is again noted. In this case, however, growth of this band cannot be attributed solely to increasing SiO2 exposure since Fig. 14 contains spectra collected from ATR and transmission modes. Since the transmission-mode spectrum was collected subsurface and is of sufficient dimensions to ensure strong exposure to the silica signal, the earlier supposition (based on the data portrayed in Fig. 9) that the large relative peak height differences in this band were due to progressive silica signal exposure no longer appears reasonable. The new data suggest that the ablation process induces ether (CeOeC) functionality (the other source of strong IR absorption in this band) and, therefore, the ablation process would not appear to be as benign as was earlier thought. A further observation regarding urethane peak behavior during the ablation process can also be made using the data in Fig. 14. It has been shown elsewhere [15] that these coatings respond to long (O300 nm) wavelength UV exposure in a manner consistent with the photooxidative mechanism proposed by Wilhelm

and Gardette [18], i.e. long-wavelength UV induces a series of oxidative reactions initiated at the in-chain urethane group (specifically the methyl group in alpha position to the secondary amine of the urethane group) which ultimately lead to chain scission at this site, the final products being carboxyl and urethane end groups. Knowing full well the physical extent of the UVdamaged volume of the topcoat from previous studies, and knowing that the ablated window depth lies firmly within this zone, we can fully expect attenuated urethane signals (i.e. a reduction in the amide II at 1523 cmÿ1 as well as reductions in the urethane carbonyl band at 1685 cmÿ1 and methylene group (2937 cmÿ1) in a-position to in-chain urethane amine [18]. Inspection of the spectra in Fig. 14, however, shows a relative increase in these bands between the transmission-mode spectrum and the ablation spectrum, with the ablation spectrum signals much closer in peak height to the unaged baseline signals than the aged (transmissionmode) signals collected from the same depth. Clearly, this discrepancy is the reason for the failure to generate the expected UV depth profile using the ablation

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Fig. 8. Overlapped EDS maps of titanium and silicon for paint cross-sections of (a) unablated paint, (b) sub-threshold ablation at 35 mJ, and (c) ablation at 167 mJ. Pigment depletion zone near surface (as evidenced by absence of titanium signal) can be clearly seen in (c).

technique. A more fundamental observation is made by recognizing that growth of the urethane bands appears to occur after ablation of the UV-aged sample but not after ablation of the baseline sample (see spectra of Fig. 9 vs. spectra of Fig. 14). The implication is that the UV-aged sample is in a metastable state relative to its original condition, that is, the poly(ester-urethane) binder has been ‘‘pre-activated’’ by its exposure to (and subsequent disruption by) UV energy. Upon irradiation by femtosecond pulses, the data would suggest a degree of reversal of this UV damage occurring within the interaction volume. The relative stability of the baseline urethane groups, coupled with the lack of ready reactants, makes this effect in the unaged samples unlikely. It should be noted that the likely end products of the photooxidative reaction proposed by Wilhelm and Gardette, namely carboxyl and endechain urethane, are not necessarily expected to react readily with one another to regenerate the in-chain urethane functionality. Indeed, the presence of the

carbonyl group adjacent to the primary amine end product introduces a resonance stabilizing effect that all but eliminates the basicity of the amine group [37]. There are three factors which we theorize may account for the (apparent) reactivity of this group: (1) the end products of the photooxidation are bonded to their host polymeric chains, therefore this steric hindrance should ensure the general proximity of the two reactants everywhere within the ablation interaction volume, (2) the multi-photon absorption phenomenon allows for the excitation of electronic states which would otherwise not be available to take part in chemical reactions, and (3) the shockwave propagation (from the coulombic explosion) which has been predicted by computer models [26], and later shown experimentally [23], may provide the physical disruption necessary to force photooxidative products into even closer proximity such that (in their photoexcited state) they react and revert to their original configurations (albeit not necessarily via the same reaction path). The attempt at depth profiling fails, therefore, because the

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Fig. 9. Transmission-mode spectrum of unaged baseline and ATR-mode spectra for 35, 73 and 167 mJ ablation. Very little height alteration of signature peaks (relative to UV-induced alteration, Fig. 10) is evident as a result of the ablation process.

ablation interaction zone (extending approximately 7e8 mm below the base of the ablation window, Fig. 8) consists of an advancing zone of partially reconstituted polyurethane that consistently presents the spectrometer with a modified amide II/carbonyl ratio. While initially it may seem unlikely (or at least unintuitive) that the interaction between the incident laser energy and the organic binder would result in induced cross-linking of a photodegraded polyurethane while simultaneously avoiding disruptions to the intact urethane matrix, it should be noted that a similar effect has been observed in previous studies involving photo cross-linking at the femtosecond time-scale between proteins and DNA with no apparent disruption to the

bulk protein chemistry [38e40]. The example is particularly meaningful given the chemical similarities between the peptide bonds (eCOeNHe) of protein and the urethane bonds (eOeCOeNHe) present in polyurethane. In addition, the phenomenon of multi-photon absorption photopolymerization is currently being investigated as a method of fabricating microscale structures from isocyanate-bearing prepolymer components [41,42], the success of which requires a net increase in the establishment of structural cross-links, i.e. the simultaneous formation (polymerization) and preservation of the objects’ structural matrix. This hypothesis also accounts for the topography profiles shown in Fig. 12a,b. The initial incident pulse

Table 1 IR peak attributions Frequency (cmÿ1)a

Relative intensityb

Main assignmentsc

References

3364 2937 2863 1725 1685 1523 1464 1430 1376 1234 1068, 1065

m s m vs vs s s w, sh m s, sh vs

n (OH) H-bonded nas (CH2) in HDI nsym (CH3) in HDI n (C]O) ester n (C]O) urethane n (CeN) C d (NeH) (amide II) d (CH2) d (CH3) d (CH3) n (C]O) C n (OeCH2) ester, n (CeN) nas (OeSieO), n (CeOeC)

[35,43] [18,35,43] [18,35,43] [31,35,43] [18,31] [18,31,44] [18,31,35,43] [35] [43] [18,31,35,43] [35,45]

a b c

All spectra collected at 4 cmÿ1 resolution. m Z medium; s Z strong; sh Z shoulder; vs Z very strong. n Z stretching; nas Z asymmetric stretching; nsym Z symmetric stretching; d/das Z bend/asymmetric bend.

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Fig. 11. Dispersive Raman spectra for coating sample (baseline) showing progressive reduction of TiO2 signal as a function of increasing ablation energy. Note absence of peroxide peak (900e 800 cmÿ1).

Fig. 10. (a) ATR-FTIR spectra for baseline, 6-week and 18-week QUV exposed samples of coating. Near-extinction of amide II peak (1523 cmÿ1) is noted, evidence of photooxidation of urethane groups in the organic binder of the coating. (b) OH absorption (3364 cmÿ1) is seen to broaden substantially, whereas hydrocarbon peaks (2937, 2863 cmÿ1) simultaneously decrease, as a result of QUV exposure.

removes the UV-degraded polyurethane at a greater rate than the baseline polyurethane (due to the UV-induced chain scission of the organic matrix in the UV-aged sample and, presumably, lower cross-link density as a result). This initial pulse leaves behind a residual interaction volume with a partially reconstituted polyurethane matrix. This new matrix responds to subsequent ablation pulses in much the same manner as the baseline polyurethane, the result being two depth

profiles that exhibit the same general shape but are separated by a linear offset due to the initial differences in ablation rates. Finally, a comment should be made regarding the activity of the CeOeC absorption at 1065 cmÿ1 in response to irradiation by the laser. The reaction path responsible for the increase in this functional group is unclear. However, by examining the higher frequencies of the IR spectra for the transmission-mode aged, the ablated-aged, and the baseline samples (Fig. 15), a possible reaction route is hinted at. According to the model proposed by Wilhelm and Gardette [18], one photooxidative product of aliphatic poly(ester-urethane) is a carboxyl group. We can therefore expect the IR spectra to indicate a sample rich in OeH functionality. Examination of Fig. 15 shows that both the transmission, as well as the ablated window spectra, exhibit greater OeH absorption than the baseline. To continue with the hypothesis presented in this work, i.e. that the ablation effect partially restores the urethane functionality, then the OeH absorption level of the ablationwindow sample should reside somewhere between the transmission-mode level and the baseline level. From the figure it is clear this is not the case. One possible explanation is that greater OeH functionality arises as a result of a condensation reaction between in-chain methyl groups and oxygen radicals (arising either

407

50 45 40 35 30 25 20 15 10 5 0 4000

140 microJoule ablation

Transmission-mode depth-profile

0.6 0.5 Amide II / C=O

Depth (microns)

L.T. Keene et al. / Polymer Degradation and Stability 89 (2005) 393e409

Baseline 5000

6000

7000

8000

9000

QUV 10000

0.4 0.3 0.2 0.1 0

11000

Scan speed (microns/sec)

Baseline

0

10

80 microJoule ablation

40

50

60

Ablation-assisted depth-profile

0.6

20

0.5 amide II / C=Ol

Depth (microns)

30 Depth (microns)

25

15 10 5 Baseline

0 4000

20

18 wks QUV

5000

6000

7000

8000

9000

QUV

10000

11000

Scan speed (microns/sec)

0.4 0.3 0.2 0.1 0

Baseline

0

10

20

30

40

18 wks QUV

50

60

Depth (microns)

Fig. 12. (a) Ablation window depths for 140 mJ pulse energy, baseline and 18-week QUV samples. (b) Ablation window depths for 80 mJ 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.

Fig. 13. (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 femtosecond ablation-assisted depth profiling. UV-damaged zone not apparent.

through energetic interaction between atmospheric oxygen and the laser, or recombination of ionic oxygen from any of the oxygen-bearing inorganics in the coating that have demonstrated preferential ablation behavior with free electrons present immediately after the coulombic explosion, to produce an oxygen radical that is driven into the underlying interaction zone) to generate eCeOeCe cross-links and H2O and, therefore, accounting for both the increase in ether absorption as well as OeH absorption.

6. Conclusion It is clear from the data that depth profiling by ablating multilevel windows in a coating system in order to facilitate spectroscopic analysis of the underlying regions is not feasible at this time. The apparent predisposition to photo-induced chemistry alteration caused by photooxidative disruptions in the material under study prevents accurate determination of relative peak heights after exposure to ablation. The data

Fig. 14. FTIR spectra of major organic and inorganic bands for baseline sample (1), 15-mm-deep transmission-mode spectrum in UV-aged sample (2) and 15-mm-deep ablation window in UV-aged sample (3).

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Fig. 15. FTIR spectra showing (OeH) and (CH2) bands for baseline sample (1), 15-mm-deep transmission-mode spectrum in UV-aged sample (2) and 15-mm-deep ablation window in UV-aged sample (3).

presented suggest the cause to be an apparent photoetherification/photoamidization of the degraded polyurethane matrix which obscures the amide II/carbonyl ratio used for monitoring the depth of photooxidation. Furthermore, process parameter optimization is difficult given the preferential ablation effects as well as the sensitivity of the material removal rates to the focusing parameters used during the ablation. Sample roughness causes incident fluence instabilities, making it difficult to generate predictable window depth series on rough coatings or coatings applied to warped substrates. The data does show the feasibility of non-modifying ablation of unexposed polyurethane however, as well as the aforementioned apparent ability to partially reconstitute the urethane structure of UV-exposed coatings. Lastly, a critical fluence level was indicated by the selective ablation process observed at 35 mJ wherein the more photoactive components of the coating system (i.e. titania pigment) were ablated with no apparent thermal damage to the surrounding residual polyurethane binder.

Acknowledgements This work was supported through SERDP Program PP-1133.

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