MICROSCOPY AND X-RAY ELEMENTAL SPECTROSCOPY IN FAILURE ANALYSIS: CASE STUDIES

Kevin P. Battjes

Impact Analytical A Michigan Molecular Institute Center of Excellence

Midland, MI 48640

Abstract

Optical and Scanning electron microscopy (SEM) techniques, coupled with energy dispersive X-ray analysis (EDS) are effective tools in investigating the cause of failures in articles made from plastics. Practical examples and findings are reviewed that have been used to solve customer problems in real world applications.

Introduction

It is common that even in the most stable manufacturing processes, defects and product failures will occur. The cause of the failure and its correction is of great concern to the manufacturer to improve profitability and protect product reputation as well as liability issues. Addressing the failures is not always straightforward as there are many possible analytical methods from which to choose. The first decisions to be made are how does the failure get analyzed and what should be looked for in the analysis.

Optical and electron microscopy imaging techniques along with elemental analysis applied to failure or defect problem solving can be powerful tools to locate, characterize, and identify the cause of failures in fabricated parts. Preparation of specimens for SEM/EDS analysis is relatively straightforward as long as one is careful in handling and choice of preparation methods to not introduce artifacts [1, 2].

When molded plastic parts experience wear and fracture, definitive patterns appear in the fracture or wear surface that give clues to the mechanism and often the ultimate cause of the failure. Imaging the failure surface and analyzing the elemental composition provide information one can interpret to often define the failure cause and mechanism [2, 3, 4]. Detection of foreign particles, non-homogeneous mixing, cold welds during molding, cyclic loading, poor adhesion to reinforcing fibers, and void formation are just a few of the potential failure causes observable by microscopy techniques.

Experimental

Preservation of the failure region or defect is critical during specimen preparation. Handling is kept to a minimum and prevention of contact with finger oils is often a concern. As most parts that fail, generally do so in an in-use situation, environmental elements that are present can be either clues or contaminants. Examination of the part with a low power stereomicroscope often provides clues and suggestions for further analysis. For optical microscopy the samples are generally examined in the state as received with no preparation. This helps preserve the failure in its original state and retains orientation for future reference.

Cleaning the specimens is generally to be avoided or to be done carefully with selected solvents, so as not to damage the failure region or create artifacts. In this study no cleaning was necessary.

Specimens for SEM/EDS evaluation are first reduced in size by carefully cutting the defect area from the bulk part. Specimens were mounted to the support stub using conductive adhesive tabs and/or carbon paint. A conductive carbon coating was applied to the specimen surfaces using a planetary motion stage and carbon (graphite) rods in a Kinney Model KSE-2A-M Vacuum Evaporator. Specimens that would not be examined by EDS were also sputtered with 10 to 20 nanometers of gold/palladium (80/20) in an Anatech Hummer VII sputter coater to improve topographic contrast. The prepared specimens were examined in an Amray Model 1820 SEM equipped with an EDAX Falcon EDS system. Operating voltage ranged from 5 to 20 kV, using the lowest voltage whenever possible.

Discussion

To illustrate the application of these techniques to problem solving, several case studies will be reviewed.

Case 1. Bolt Flange Cracking During Assembly

Mounting flanges on injection molded polypropylene reservoirs failed when the part was fastened in place with a

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screw or bolt. The flange had an appropriate sized hole for the fastener, but split as the fastener was tightened. Optical examination revealed whitened material and a substantial crack in the outer radius of the flange where the fastener made contact during assembly. It was noted the crack appeared in the vicinity of an expected weld line during the molding process. Whitening in polypropylene is generally indicative of crazing or stress micro-cracking and is often tolerated where the fastener makes contact, but should not undergo large cracks. A reference area of this material was prepared by cryo-fracturing a good reservoir to produce a clean, controlled fracture surface. Examination of the reference surface by SEM showed the material to be two phase as seen in Figure 1. The second phase is most likely rubber reinforcement added to the polypropylene matrix.

The corresponding matching halves of the crack failure region were then examined to look for defects, contaminants, large phase irregularities, or other clues. Voids or gas bubbles, 20 to 50 microns in diameter were observed in the fracture surface at the approximate center of the flange. Close to the outer diameter of the flange, a large region showing ductile failure behavior and apparent orientation was observed as shown in Figure 2. Careful examination of both sides of the fracture revealed the lock and key matching features seen in Figure 3. The most striking feature observed in the figure is the clear differences in direction of the orientation in the layers. This is a highly undesirable occurrence in the molding, as it indicates a high degree of shear in the too low temperature of the melt. No foreign particles or inclusions were observed. Ductile deformation and fibrillation was observed accounting for the crazing and whitening in the stress zone. Improper molding conditions resulted in a too-cold weld of the polymer melt in the flange.

Case 2. Molded Bushing Failing During Installation

A manufacturer experienced part failures when self-lubricating Nylon® bushings were being installed during their manufacturing process. The manufacturer was now faced with solving a problem. Was something wrong with the way the bushings were installed, or was there indeed a problem with the parts? The installation process was evaluated and found to be the standard process that had been in place for some time. The investigation pointed to the bushings, not the process.

Examination of new and failed bushings using a low power stereo microscope revealed the appearance of a dark line running through the parts as shown in Figure 4. This occurred adjacent to the failure and 180° to the fill sprue location. This is coincident with the expected general location of a weld line that forms during molding.

In this case, the resin flow was split into two flow fronts in order to fill the mold, rejoining on the opposite side. The presence of the dark line suggests a variation in the material, either from filler orientation, filler migration, or possibly from mold filling characteristics.

Higher magnification imaging by SEM permits viewing details of the fracture surface. The radial lines emitting from the rounded central feature in Figure 5 indicate the failure initiated from this location. Closer inspection of the central feature in Figure 6 shows a smooth skin on the surface, relative to a typical hackle region in the rest of the fracture, and even a gap between this feature and the surrounding material. This is a convincing sign that the flow fronts did not fuse during the molding process, resulting in a cold weld. This cold weld defect is approximately 200 by 400 microns in size. A non-bonded defect of this size will act as a significant stress concentrator and initiation point for failure when the material is under load.

X-ray spectra and mapping experiments did not indicate clumping, deposits, or significant inhomogeniety of the lubricant filler was present to cause a “defect” effect.

The molding conditions at the time of the bushing manufacture set the stage for this failure by not achieving a good weld.

Case 3. Low Performance of Equivalent Grade Glass Reinforced Resin.

During manufacturer qualification of glass reinforced Nylon® resins, a significant performance-to-fail difference was observed between two brands of resin. Samples of fracture surfaces of parts tested to failure from the good and bad resins were examined. The manufacturer was concerned that a metal containing additive might be present in the samples. The surfaces were examined and analyzed by EDS. None of the suspect metal was detected. During the EDS analysis, examination of the glass fibers at the fracture surfaces did reveal differences between the two brands of resin. The poor performing resin was observed to have a much cleaner glass to resin interface on the exposed fibers than the good resin as shown in Figure 7. It is apparent the failure mode of the interface is adhesive failure, indicative of a weak or no bond. This difference in bonding of the glass to the resin significantly contributed to the ultimate strength and performance of the finished product. In separate GC-MS studies, an organic fatty acid was detected in the poor performing resin. This saturated fatty acid acts as a lubricant and release agent. Its presence in the system would be detrimental to the adhesion of the nylon/glass interface. This evidence corroborates the conclusion from the microscopic evidence of poor adhesion. The

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manufacturer repeated the qualification tests using a different lot of the resin brand in question, and found the lot to meet specifications.

Pre-qualifying the resins used in a critical application and the resulting troubleshooting analysis resulted in the manufacturer producing quality parts and no need for a costly recall.

Case 4. Color Defects Observed in Molded Parts.

A brown injection molded polypropylene automotive panel was found to have streaks of discoloration and failed inspections for use. The parts were rejected and the molder needed to fix the problem to reduce waste and lost product. After visual examination, the discolored region was cut out and simply cross-sectioned with a razor blade. Imaging by SEM revealed a contrasting band running through the panel. Backscatter image mode produced significant Z-contrast, indicating the band should contain inorganic elements. X-ray spectra revealed the presence of titanium, iron, and zinc, and suggested the presence of antimony and chromium. Figure 8 shows the band and its EDS spectrum. An element map of the region confirmed the detected elements to be located heavily within the band.

The identified elements are common in brown pigments commonly used in coloring plastics [5]. This evidence suggests the blending of the pigment, by dry powder, flake, or color concentrate, was incomplete, and the pigment did not fully disperse, resulting in pigment rich bands causing a color defect.

Summary

These case studies show the usefulness of microscopy and elemental spectroscopy techniques applied to failure

analysis. Clues left in the material from a failure are analyzed and interpreted to determine a cause of failure of the manufactured article. These simple and cost-effected tests can be done relatively quickly in cases of production or assembly failures. In addition, they can be applied to a wide variety of failure types and materials.

References

1. Sawyer, L. C. and Grubb, D.T., Polymer Microscopy, Chapman and Hall, New York (1987)

2. Goldstein, J., Newbury, D., Joy, D., Lyman, C.,

Echlin, P., Lifshin, E., Sawyer, L., and Michael, J., Scanning Electron Microscopy and X-ray Analysis, Third Edition. Kluwer Academic/Plenum Publishers, New York, 565 (2003)

3. Woodward, A. E., Atlas of Polymer Morphology,

Hanser Publishers, Oxford University Press, New York, (1988)

4. Engel, L., Klingele, H., Eherenstein, H, and Schaper,

H., An Atlas of Polymer Damage, Prentice-Hall, New Jersey, (1981)

5. Webber, T. G., Editor, Coloring of Plastics, John

Wiley and Sons, New York, 41, 139 (1979)

Key Words

Failure analysis, microscopy, SEM, EDS, polymers.

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Figure 1. Two phase nature of the polypropylene blend use to mold the reservoir.

Figure 2. Ductile failure and apparent orientation in the fracture surface.

Figure 3. Lock and key matching features in fracture. Orientation of layers readily shows shear behavior during molding. Layers have not fused. Left half of figure: “lock” feature in side one of fracture; right half: “key” feature in side two of fracture.

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Figure 4. Failed bushing showing fill sprue, dark “interface” line, and failure split.

Figure 5. Fracture surface of failed bushing. Origin is at center circular feature.

Figure 7. Glass fibers in Nylon matrix showing poor adhesion. Note clean fiber and matrix areas.

Figure 6. Smooth skinned region showing no bonding to surrounding material.

Figure 8. X-ray spectrum of banded region in colored polypropylene. Band is from undispersed pigment.

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