Surface Analytical Techniques for Analysis of Coatings Mary Jane Walzak, Mark Biesinger and Brad Kobe The University of Western Ontario, Surface

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1 Surface Analytical Techniques for Analysis of Coatings Mary Jane Walzak, Mark Biesinger and Brad Kobe The University of Western Ontario, Surface Science Western 999 Collip Circle, Room LL31, London, ON N6G 0J3 CANADA Understanding and solving manufacturing issues associated with coatings often requires the use of multiple analytical techniques. A number of case studies will be discussed to illustrate how different techniques such as scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be applied to defect analysis, to identify surface contaminants, and to elucidate the cause of adhesion failures. For each problem we will discuss the approach used, sample preparation needed, choice of technique(s) and the type of results produced. Introduction Advanced analytical methods that can be utilized to help understand and eliminate manufacturing issues associated with contamination of raw materials, optimizing new products, corrosion, adhesion failures, cosmetic discoloration and staining will be discussed. Most samples are first assessed using a stereo or compound optical microscope to get a feel for the defect and to determine which analytical technique would be most appropriate to diagnose the problem. In most cases, more than one analytical technique is needed to solve a particular problem. Each technique provides a small window about the chemistry and topography of the surface. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX) is a fast method to obtain high resolution images and elemental information about a sample. However, if one needs to know more about the chemistry or functional groups on a surface, FTIR and laser Raman spectroscopies would be more appropriate. Techniques such as ToF-SIMS and XPS are very surface sensitive and are great tools for examining adhesion failures and staining. Understanding how to prepare samples for analyses is also a very important skill. Being able to properly section large parts, such as an automotive car door, in order to properly expose a buried defect less than a millimeter in size is invaluable. The techniques discussed below can be applied across many industry sectors including aerospace, semiconductor, energy, medical, automotive and mineralogy. Ultimately, these techniques can reduce costs associated with downtime, scrap material and product returns. Overview of Common Analytical Techniques The most commonly known surface technique is SEM, which provides high resolution images, revealing details on the order of several nanometers, with great depth of field. The SEM images presented in Figure 1 are from a thin antibacterial coating and illustrate the fine detail that can be observed using high resolution SEM. Combined with an energy dispersive X-ray (EDX) spectrometer, one can also identify the elemental composition of features on the order of several microns. EDX spectroscopy is a semi-quantitative technique that can detect elements from carbon to uranium with a detection limit of approximately 0.5 weight %. In general, SEM/EDX is

2 a relatively fast and inexpensive approach. It is often the first analytical method used before attempting more surface-sensitive and specialized techniques. Figure 1 SEM image of antibacterial coating (left) cross-section and (right) top down. FTIR and Raman spectroscopies are complementary optical techniques that are useful in analyzing and identifying organic and inorganic compounds. During FTIR analysis, the sample is exposed to infrared light, which is absorbed at specific frequencies representing the vibrations of bonds in the molecule. FTIR spectroscopy is sensitive to components that are present in concentrations greater than approximately 3-5%. Samples as small as 20 µm can be analyzed. Laser Raman spectroscopy focuses a laser of a particular wavelength on areas approximately 1-2 microns in size and is sensitive to molecular vibrations. For both techniques, electronic and online databases of reference spectra are commonly accessed to identify chemicals and products. A typical FTIR spectral comparison is presented in Figure 2. A liquid contaminant was identified in a paint defect. FTIR spectroscopy identified a hydrocarbon present in Polysporin that was found to be a match to a liquid material found in the center of the defect crater. Figure 2 FTIR spectra of a defect material in a paint crater (blue) with a reference material Polysporin (red). A hydrocarbon in the Polysporin was found to be a match to the liquid material found in the centre of the defect crater by FTIR spectroscopy.

3 XPS measures the energies of photoelectrons that are emitted from atoms when they are irradiated by soft X-ray photons (1-2 kev). XPS provides elemental (lithium to uranium) and chemical information about the outer few nanometres of a sample surface. XPS can provide both spectral and imaging data. Figure 3 shows a high resolution XPS scan of an electrochemicallygrown film with a 6 nm film thickness. The film consists mostly of nickel hydroxide with a smaller component of nickel oxide. Figure 3 High resolution XPS scan of an electrochemically grown coating. ToF-SIMS is a very surface sensitive analytical technique that uses an ion beam to probe the outermost layer of a surface. A short pulse of primary ions bombards the surface of a sample generating secondary ions. The positive and negative secondary ions produced during the sputtering process are extracted from the sample surface and mass separated in a time-of-flight analyzer. ToF-SIMS can detect ions over a large mass range, from 1-10,000 atomic mass units, with a high sensitivity (ppm and ppb). The technique is capable of generating mass spectra, depth profiles and secondary ion images with a spatial resolution on the order of a micron. A chromium-rich defect on a stainless steel coating illustrates the type of ion maps that can be generated while imaging using ToF-SIMS (Figure 4). Figure 4 Positive ion images showing chromium-rich defect on stainless steel coating.

4 The five techniques introduced above are only a few of the many analytical techniques available to characterize and understand coating morphology and chemistry. In many cases, these techniques can provide valuable answers to manufacturing problems within only a few hours. Below is a summary of some real world problems experienced by manufactures. Case 1: Pinhole defect in chrome coatings A manufacturer who routinely vacuum deposits chromium coatings on polymer substrates suddenly experienced pinhole defects. An optical microscope image of a typical defect is presented in Figure 5. The pinhole defects were visually obvious when the parts were held against a bright light. Several representative pinholes were cut from the parts and examined using SEM/EDX spectroscopy. Initial SEM analysis did not show any obvious debris, residue or contamination associated with the pinhole defects. It was then decided to use ToF-SIMS, which is a much more surface sensitive technique. The ToF-SIMS results showed nitrate species (NO2 - and NO3 - ) at the centre of the pinhole defects on the substrate. No ions associated with other common contaminants such as salts and silicones were detected. Thus, ToF-SIMS indicated that the pinhole defects resulted from a thin nitrate contaminant on the polymer substrate that prevented the chromium deposition. Figure 5 Optical microscope image (left) of pinhole defect in chromium coating. ToF-SIMS ion image (right) of nitrate contaminant. Case 2: Contamination of extruded material A company with experience extruding many different types of materials wanted to identify tiny black defects appearing in their polycarbonate products. Polished metallographic cross-sections were prepared to expose the buried defects (Figure 6). The defects were first examined using SEM/EDX spectroscopy and found to consist of a layered material composed of sodium, magnesium, oxygen and silicon. FTIR analysis of the same defects confirmed the presence of talc. A final check was also made using laser Raman spectroscopy, which found amorphous carbon (degraded polymer) associated with the defect. The defects are most likely degraded polymer and filler material from a previous extrusion, indicating that the extruder was not properly cleaned.

5 Figure 6 Optical microscope image (left) and BSE image (right) showing a buried defect in cross-section. Case 3: Reverse engineering of food packaging material A manufacturer of food packaging materials requested that the layers in a particular product be identified so that they could attempt to reverse engineer the product. The sample is a cup product designed to be used in a single serve coffee system. Initial examination was by optical microscopy and showed the presence of a number of layers in the cross-section (Figure 7). It was apparent that there were numerous thinner layers present at the inner diameter of the sample cross-section. The sample was analysed by SEM/EDX, laser Raman and FTIR spectroscopy in order to identify the layers in the cross-section. The sample cross-section was found to contain nine individual polymer layers with four of these comprising the 50 microns of thickness nearest the inner diameter surface. The four layers on the inner diameter were found to be consistent with polyethylene, a copolymer of polyethylene and polyvinylalcohol, a copolymer of ethylene and vinyl acetate, and a high impact polystyrene. The remaining five bulk layers were found to be consistent with layers of polystyrene with TiO2 and without TiO2. Figure 7 Optical microscope image (left) and composite EDX maps (right) showing multilayer coating.

6 Case 4: Adhesion failure of body side molding An automotive company was experiencing problems with a body side molding that was not sticking properly. At considerable time and expense, the automotive company had to hand wipe each car prior to installing the body side molding. By using XPS, a very surface sensitive analytical technique, the company was able to identify the root cause of the problem as the presence of excess silicone on the clear coated auto body panels and not the foam adhesive on the trim. XPS survey spectra comparing reference surfaces and failed surfaces are presented in Figure 8. The silicone was impeding the proper adhesion of the foam body side molding. After testing, it was found that the excess silicone was due to a mixing problem in one of the clear coat totes. Figure 8 Overlay of XPS survey spectra comparing silicon levels. Case 5: Laser Raman spectroscopy of pigment A company was experiencing dark blue defects in their basecoat and wanted to know if the formulation was correct. The defects were identified using a stereo microscope and thin slices were prepared using a scalpel (Figure 9). The thin sections were then examined by SEM/EDX and laser Raman spectroscopy. EDX spectroscopy detected high levels of carbon, oxygen, nitrogen and iron in the defect. Laser Raman spectroscopy showed that the dark blue defect material matched that of ferric ferrocyanide, a common blue pigment. Ferric ferrocyanide was not the blue pigment specified for use in this basecoat. In this case, the pigment supplier was found to be at fault.

7 Figure 9 Optical microscope image showing dark blue defect. Case 6: Analysis of crater defects Many automotive companies experience paint crater defects from time-to-time. For this example, the client wanted to know what was causing craters on their car panels. The craters were first assessed optically to determine if any obvious particles or residues are visible. If large and obvious contaminants are visible we would go directly to SEM/EDX and FTIR. However, with more subtle defects, ToF-SIMS is often used to search for extremely thin contaminants. In this case, even though the craters appeared to contain very small particles, it was decided to use ToF- SIMS for the analyses (Figure 10). The particle was consistent with Teflon. The crater was also found to contain silicone with a ring of hydrocarbon at the edge of the crater. These two species could be associated with oils and lubricants. Figure 10 ToF-SIMS ion images of fluorinated particle, silicone and hydrocarbon species associated with a crater defect. Case 7: Analysis of Corrosion Products As part of a collaborative research study to improve corrosion testing, zinc galvannealed aluminum test panels were scratched, the panels were strapped on vehicles and driven around various cities in North America for two years. Selected areas were cut from the test panels and initially examined by SEM combined with elemental X-ray mapping. The SEM analysis showed sulfur and other salts associated with the corroded regions (Figure 11). In order to learn more about the type of corrosion species present, XPS imaging and high resolution small spot analysis was conducted on specific areas. High resolution S 2p spectra were collected from 220 micron spots and they identified sulfate. The surface analysis results helped define new corrosion test

8 parameters for the automotive industry that use salt sprays or salt baths (containing sodium and chlorine). Sulfur was not originally part of the corrosion testing. Figure 11 BSE image and elemental X-ray intensity maps of corroded region. Summary Surface analytical techniques such as SEM/EDX, FTIR and Raman spectroscopy, XPS and ToF-SIMS provide fast and accurate information to diagnose coating and material issues, such as surface contamination, staining, corrosion and adhesion failures. In general, it is necessary to cut and cross-section parts in order to expose the defect for analysis. A combination of techniques is often needed to fully understand and solve a manufacturing issue. These techniques can be applied to solve issues from various sectors including aerospace, automotive, energy, packaging and semiconductor.