Applying SEM-EDS to Practical Tribology Problems

Working in an oil analysis/tribology laboratory can be challenging. The technical challenges generally involve selecting the right tools to perform an effective analysis in a timely and economical manner.

An instrument that combines a high-powered microscope with an emission spectrometer is an old tool that is just now making its way into many oil analysis/tribology laboratories.

This tool, the scanning electron microscope and energy dispersive spectrometer (SEM-EDS), makes it possible to quickly resolve tough analytical problems effectively, timely and economically.

The SEM-EDS is the ultimate tool for:

• Deposits and wear debris analysis,
• Particle sizing and characterization,
• Failure analysis,
• Contaminant analysis, and
• Metallurgical studies.

Background

In 1978, at a tribology conference at the Massachusetts Institute of Technology, Douglas Godfrey, a Herguth research associate, presented a paper on the usefulness of a new surface analysis instrument.¹

In this paper, he concluded that a scanning electron microscope (SEM) paired with an energy dispersive spectrometer (EDS) yielded the most useful instrument for investigating boundary lubrication. Eight years later, in 1986, the Nobel Prize for Physics was awarded to Ernst Ruska for his 1931 contributions to the invention of the electron microscope.

Why is it that the SEM was invented in 1931, but not recognized as a Noble Prize worthy invention until 1986, and it is just now being publicized as a helpful tool in used oil analysis?

The primary reason is that most SEMs are hidden in the research centers of major corporations, universities and government agencies like the U.S. Department of Justice. SEM was expensive (hundreds of thousands of dollars) and historically considered not practical for a commercial laboratory to own and operate.

However, today’s systems are less expensive (not much, but some) and much more user-friendly. The technological advances have made the SEM-EDS an everyday necessity in problem resolution.

SEM Imaging Capabilities

The SEM permits the observation of materials in macro and submicron ranges. The instrument is capable of generating three-dimensional images for analysis of topographic features (Figures 1 and 2). When used in conjunction with EDS, the analyst can perform an elemental analysis on microscopic sections of the material or contaminants that may be present.

Figure 1. Three-Dimensional View of Sewing Needle
Note the damage to the tip of the needle.
Source: Herguth Laboratories Inc.

An SEM generates high-energy electrons and focuses them on a specimen. The electron beam is scanned over the surface of the specimen in a motion similar to a television camera to produce a rasterized digital image.

Electrons are speeded up in a vacuum until their wavelength is extremely short, only one hundred-thousandth that of white light. Beams of these fast-moving electrons are focused on a sample and are absorbed or scattered by the specimen and electronically processed into an image. Most electron microscopes used to study materials can focus on an image down to about 10 angstroms (0.001 microns).

Figure 2. SEM of 52100 Steel Ball Abraded by Silicon
Note the 52100 Steel is very hard (62 Rockwell).
The image on the left is an overview of the area of interests.
The image on the right shows a furrow. At the top end of
this furrow is an embedded silicon particle that caused
the abrasion. Source: Herguth Laboratories Inc.

EDS Analytical Capabilities

Viewing three-dimensional images of microscopic areas solves only half the analysis problem. It is often necessary to identify the different elements associated with the specimen. This is accomplished by using the “built-in” spectrometer called an energy dispersive X-ray spectrometer. EDS is an analytical technique that uses X-rays that are emitted from the specimen when bombarded by the SEM electron beam to identify the elemental composition of the specimen.

To explain further, when the sample is bombarded by the electron beam of the SEM, electrons are ejected from the atoms on the specimen’s surface. A resulting electron vacancy is filled by an electron from a higher shell, and an X-ray is emitted to balance the energy difference between the two electrons.

The EDS X-ray detector measures the number of emitted X-rays versus their energy to produce the EDS spectrum. Just like ICP, used in oil analysis for conventional elemental spectroscopy, the energy of the emitted radiation - in this case X-ray radiation - is characteristic of the element from which the X-ray was emitted. A spectrum of the energy versus relative counts of the detected X-rays is obtained and evaluated for qualitative and quantitative determinations of the elements present.

Figure 3. Mapping of Surface Elements²

Modern SEM-EDS instruments are operated using sophisticated software. These software programs allow unattended feature analysis and “mapping” of the composition of the elements on the specimen’s surface. The top row of Figure 3 shows black and white SEM images of the surface of the test specimen. They are intentionally out of clear focus to optimize the EDS analysis.

The black and white images are computer colored with assigned color schemes for specific elements. These images reveal that the iron (Fe) and aluminum (Al) are clearly separate elemental phases than the primary surface element nickel (Ni).

The conclusion is that metal transfer has occurred, an indication of scuffing due to metal-to-metal contact. There are several practical solutions to this problem, including possible mechanical solutions, as well as lubricant solutions. Some possible mechanical solutions include reducing load, speed and temperature, improving oil cooling, using compatible metals, applying surface coatings such as phosphating, and modifying the surface (for example, ion implantation).

The possible lubricant solutions include using more viscous oil to separate surfaces and/or using extreme pressure (antiscuff) additives such as sulfur-phosphorous or borate compounds.

A particular advantage of EDS is the detection of low atomic number elements such as carbon and oxygen, which are ubiquitous in our environment. Like many other analytical tools, only elements are detected by EDS, so investigators must deduce the compounds of which they are a part.

Particle Sizing and Characterization Tool

Case Study No. 1: Organic versus Inorganic Particle Contamination

Recently the laboratory was tasked with determining the source of an excessive number of particles as measured by laser particle counting in both new and used gearbox lubricants. The decision was made to use SEM-EDS analysis to try to determine the elemental composition of these particles. The oil sample was prepared by filtering through a filter patch, with the solids collected on the patch then subjected to SEM-EDS analysis. Figures 4 and 5 show the results.

Figure 4. SEM of Large Carbon-rich
Polymer (fiber-like) Substance in Fresh Oil
Source: Herguth Laboratories Inc.

Figure 5. SEM of Large Carbon, Sulfur and
Phosphorus-rich Varnish Particle
Note the background shows 0.2 micron holes
in silver filter pad. Source: Herguth Laboratories Inc.

The new unused oil contained large amounts of agglomerated carbon-rich components. While the used samples contained large amounts of carbon, sulfur and phosphorus-rich particles.

The conclusion was that the new lubricant contained organic polymers that were agglomerating to form large particles, while the used lubricants, although absent the polymer type particles, contained varnish-like material that was composed of antiwear (AW) additives. The square area with holes in the center, seen in Figure 5, is the area where the electron beam rastered over the organometallic surface and the energy from the beam decomposed the specimen.

The solution to the analytical problem was to continue to count the particles in the used and new oil samples by the conventional laser light technique, but add preparation methods that would differentiate the hard abrasive particles from the softer, decomposed additive constituents. Both types of particles are problematic for the system engineers; however, the actions of the maintenance team will be clearly different depending on the structure and source of the particles.

Wear Debris Analysis Tool

Case Study No. 2: Carbon versus Wear Metals

A nuclear utility expressed interest in analyzing visible debris in an oil from a critical pump application. The analysis of the oil using emission spectroscopy did not show any of the standard metals of interest - silicon, iron, tin, etc. A bichromatic, optical microscope was used in an effort to characterize the debris. The results from the optical microscopy were astounding!

The sample was laden with highly reflective nonferrous wear particles. Prior to “jumping to a conclusion,” the lab performed an SEM-EDS analysis. It was quickly discovered that the particles were carbon and had the topographical features expected of carbon seal wear (Figure 6). In the end, these analyses saved a great deal of guess work that would have otherwise cost thousands of dollars in manpower and possible delayed operation.

Figure 6. SEM of Carbon-rich Platelet that Appears
Reflective Under Transmitted Light Optical Microscopy
The rectangular yellow line on the particle is the area
analyzed by the EDS. Source: Herguth Laboratories Inc.

Case Study No. 3: Severe Wear versus Mild Wear

A sample of oil was submitted to determine the source of large visible ferrous particles in the oil. Again, the SEM-EDS instrument was employed to solve the problem. The results shown in the images were demonstrative. The large particles were not large at all; rather they were small iron and sulfur-rich platelets that had agglomerated to combine into larger visible particles (Figure 7).

This proved to be quite a different problem than if they had been large wear particles. In fact, this did not constitute a problem at all. These types of 1 to 2 micron iron sulfate particles are ubiquitous in a gearbox oil and are considered to be mild corrosive wear.

Figure 7. Sulfur and Iron-rich, 1 to 2 Micron
Platelets Agglomerated Appearing to Be a Large Particle
Source: Herguth Laboratories Inc.

Figure 8. Connecting Rod Bearing SEM-EDS Analysis Result
(Upper left shows wide view of damaged surface. Upper right
shows closer view of oil balls ~ 10 to 20 microns in size. Lower
graph shows EDS results.) Source: Herguth Laboratories Inc.

Case Study No. 4: Connecting Rod Bearing Failure Analysis

Several bearings were submitted for analysis. The objective of the analysis was to determine the cause of the unusual circles of discolored overlay material. Using optical microscopy, it was determined that these areas were “raised” much higher than the surrounding overlay (Figure 9).

Figure 9. Discolored Raised Portions of Bearing Overlay
Source: Herguth Laboratories Inc.

The decision was made to use the SEM-EDS to determine the cause of this abnormality. Figure 8 shows the results of the SEM-EDS analysis. The raised areas were saturated with embedded “oil balls” and voids that apparently contained similar balls.

The oil balls are believed to be caused by coolant contamination of the lubricant and subsequent degradation of the oil additive into hard particles that can abrade and embed in the softer bearing overlay.^{3, 4} In this case, the EDS identified the oil balls to be rich in calcium, zinc and phosphorus consistent with the oil’s additive package. The recommendation was to replace the bearings and monitor the oil samples for any sign of coolant in the future.

Conclusion
The correct tools are critical to getting the job done right and on time. Often, needless man-hours, downtime, engineering and lubricant studies occur due to a poorly defined problem. SEM-EDS are the super instruments in modern up-to-date testing facilities. When properly applied, an SEM-EDS analysis reveals the surface and particle shape and associated elemental analysis.

References

1. Godfrey, Douglas (1980). Review of Usefulness of New Surface Analysis Instruments in Understanding Boundary Lubrication. Proceedings of the International Conference on the Fundamentals of Tribology, (N.P. Suh and N. Saka, editors). Cambridge Mass.: MIT Press, pp. 945 -967.
2. Echlin, P., Goldstein, J., Joy, D., Lifshin, E., Lyman, C., Michael, J., Newbury, D. and Sawyer, L. (2003). Scanning Electron Microscopy and X-ray Microanalysis. New York: Kluwer Academic /Plenum Publishers.
3. McGeehan, J.A. and Ryason, P.R. (1999). Million Mile Bearings: Lessons from Diesel Engine Bearing Failure Analysis. SAE Technical Paper # 1999-01-3576. 4. Patel, Magan J. (1981) Influence of Oil Balls on Premature Overlay Removal of Diesel Engine Connecting Rod Bearings. SAE Technical Paper # 810501 1981.