Alexander Calder and George Stanley Gordon 1973

Subsurface Analysis of Paint Layers

Soyoun Kim

University of Rochester, Department of Chemical Engineering

 

Objective: Examine the composition of paint samples from Alexander Calder's scale model for Braniff International Airways' plane using various EM techniques: SEM (SE2, BSD, EDAX, FIB) and TEM (BF, STEM-HAADF)

1. Introduction

A. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM)

FIB-SEM is one of the nanofabrication methods that uses focused Ga+ ion beam to mill into a surface with high precision. Below are examples of potential applications:

Figure 1: Application of FIB-SEM (www.zeiss.com/fib-sem)

B. Application Value in Cultural Material Heritage Science (Art Conservation)

  • Preparation at Nanoscale
    • Minimum sampling of material is optimal for art conservation purpose
  • Subsurface Analysis
    • This allows the examination of uncontaminated surface
  • Use of Scanning Transmission Electron Microscopy High-Angle Annular Dark-Field (STEM-HAADF) mode
    • Spectral mapping collects spectral data for each pixel
    • Superior technique for elemental analysis of modern painting formulations

C. Techniques

  • Au Sputtering
  • Light Microscopy
  • Scanning Electron Microscopy (Secondary, Backscattered, Characteristic X-ray with Elemental Mapping)
  • Focused Ion Beam (Mill, Platinum Deposition)*
  • Transmission Electron Microscopy (Bright Field, STEM-HAADF with Spectral Mapping)**
  • Colorization (Photoshop)

* Lift-out was performed by Ralph Wiegandt ** TEM was operated by Brian McIntyre

 

2. Sample Preparation

A. SEM Sample Preparation

1) Paint Fragment Sample:

Crumbs of paint samples were gathered and mounted on SEM stubs with carbon tape on. Then, the sample was sputtered with gold. This method really pushes the limit of how little material can be used for analysis purpose.

2) Paint Layer Sample: A cylindrical shape of epoxy was prepared in a mold and halved. On one side of the cut epoxy piece, a small chip of paint samples was mounted vertically with aluminum paste and the rest of the mold was filled with more epoxy so that the paint was embedded in the epoxy. The surface of the epoxy was grated just enough to expose the paint sample. Light microscope image was taken for reference purpose and the sample was mounted on SEM stub and sputtered with gold.

Figure 2: Schematic of SEM Sample Preparation

B. TEM Sample Preparation

Step 1: In order to use FIB, the stage has to be adjusted for eccentricity, tilted to 54 degrees, and set with a working distance of 5.1mm for alignment. The top of the lift-out section was marked by retangular Pt deposition. After that, two trapezoidal pockets were milled out, followed by another pocket on the side to give room for the probe to come in.

Step 2: The stage was returned to 0 degrees tilt in order to cut the bottom of the trimed side. Probe was brought to contact with the sample and was welded with Pt. Then the thinned section was detached by milling away the right side that was still attached to the bulk. The sample welded on the probe was retracted out of the way and TEM grid was loaded in SEM for transfer. Probe was brought back in carefully so that the right side of the sample makes contact with the finger grid then more Pt was deposited for welding at the spot of contact. Then, the tip of the probe was cut using the FIB mill to complete the transfer.

Step 3: More Pt was depositions to reinforce the welding and the section was thinned to electron-transparent thickness for TEM analysis. Some sections were not thinned to provide some stability and structure to the sample.

Figure 3: Schematic of TEM Sample Preparation (Colorized)

3. Sample Analysis

A. Paint Fragment Sample Analysis

Cross-section of the fragment was exposed using FIB milling, which created uncontaminated, flat surface to observe. While the SE2 mode was used for general imaging, BSD mode was useful for differentiating composition of various paint pigment particles and providing general guidance for EDAX analysis. Darker areas contained more K, Si, and C while brighter areas contained more Ba, S, and Zn for red pigment paint.

Figure 4: Characterization of Fragment Sample using BSD and EDAX

B. Paint Layer Sample Analysis

Embedding the paint layer vertially enables ovservation of both the color pigment and the white primer layer at the same time. FIB cross-section clearly shows the particle diffference in the layers.

Orange pigment sample using EDAX spot analysis: The orange pigment area contained coarser particles rich in Zn, S, and Ba while the white pigment area had a lot of fine, round Ti particles.

Figure 5: Characterization of Orange Cross-section using EDAX Spot Analysis

Green pigment sample using EDAX elemental mapping: The orange pigment area contained particles of S and Si with varying shape and sizes while the white pigment matched with the previous result.

Figure 6: Characterization of Green Cross-section using Elemental Mapping

C. Paint Layer Sample Analysis

BF and EDAX: Due to its electron-transparent thickness, TEM samples can clearly illustrate individual particles and their size and shape. Red pigment has particles with varying size and elemental composition (Zn, Ti, S, and Ba) and fine textural details. EDAX spectra on various spots in the red area showed below. Tthe white pigment has rounder and smoother particles of Ti.

Figure 7: TEM Bright Field Images and EDAX

STEM-HAADF: Accompanied with EDAX, STEM-HAADF mode can create spectral mapping images that correlates with a high contrast reference image, which is optimal for particle compositional analysis.

Figure 8: STEM-HAADF Spectral Mapping Images

7. Conclusions

The goal of this project was to examine paint samples using various EM techniques and demonstrate successful lift-out of TEM sample using FIB.

  • Paint samples were examined using various modes (SE2, BSD, EDAX) with SEM
    • BSD can be used as a complimentary technique for x-ray spot analysis
  • TEM thin section was successfully lifted out and mounted onto a TEM grid using FIB milling and Pt welding
  • Bright field images and spectral mapping images were collected with TEM
    • Spectral images via STEM mode had higher resolution than elemental images from SEM

 

8. Acknolwedgement

Thank you to Ralph Wiegandt for providing me to such a cool project and the FIB techqniues!
I would also like to thank Brian McIntyre for the awesome EM learning experience and help with the TEM. Lastly, I would like to thank my TA, Caleb Whittier, for making sure that we are not breaking the instruments during the lab.

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