Analytical Ferrography, when performed with other analysis tests, is capable of determining the Root Cause of failure, which can lead to failure prevention. Analytical Ferrography utilizes microscopic analysis to identify the composition of the material present. This technology will differentiate the type of material contained within the sample and determine the wearing component from which it was generated. This test method is used to determine characteristics of a machine by evaluating the particle type, size, concentration, distribution, and morphology. This allows a skilled diagnostician to determine the root cause of a specific tribological problem.
Analytical Ferrography can predict potential equipment failures and is an effective tool in determining the root cause of machine component failure. Analytical Ferrography is a qualitative, rather than quantitative analysis that provides digital imagery of the actual particles present. Powerful magnets trap the ferrous particles, which are then placed on slides for microscopic analysis. Particles are analyzed based on being metallic or non-metallic, alloy via heat treatment, shape, size, color, and if possible, source.
Analytical Ferrography is one of the tools of fluids analysis in the group called Wear Debris Analysis (WDA) or Wear Particle Analysis (WPA). Other WDA/WPA tests include Particle Count, Micropatch, Direct Reading Ferrography, and the Particle Quantifier.
The technique of Wear Debris Analysis (Analytical Ferrography) is gaining popularity in the field of Condition Based Maintenance System. WDA is a method of predicting health of equipment in a non-intrusive way, by the study of worn particles. The continuous trending of wear rate monitors the performance of Machine / Machine components and provides early warning and diagnosis. Oil condition monitoring can sense danger earlier than Vibration technique. This technique holds good for both oil and grease samples.
Analytical Ferrography, with supporting physical and chemical tests, can help to determine-
- The start of abnormal wear
- Root cause of wear/failure
- The component(s) that are wearing
- Usability of lubricant beyond its rated life
The particles contained in a lubricating fluid carry detailed and important information about the condition of the machine components. This information can be deducted from-
- Particle shape
- Particle composition
- Particle size distribution
- Particle concentration
When a fluid analysis report indicates a problem, it can be characterized in two dimensions: ambiguity and importance. When the problem is ambiguous and important, root cause analysis can be justified. For many reasons, fluids analysis is a powerful root cause tool, yet few take full advantage of its capabilities. Despite the fact that hundreds of fluids analysis tests are available and useful to the analysis, few venture beyond the 10 to 12 tests most common to used fluids analysis.
Vernon C. Westcott is credited with inventing the ferrograph in the early 1970s. Mr. Westcott passed away in September 2003 at the age of 84. Initially, the ferrograph was used mainly by the military. Today, Ferrography is a fundamental tool of used fluid analysis and reliability maintenance.
Analytical Ferrography is among the most powerful diagnostic tools in fluids analysis today. When implemented correctly it provides a tremendous return on your fluids analysis dollars. Yet, it is frequently excluded from fluids analysis programs because of its comparatively high price and a general misunderstanding of its value.
In his article Wear Analysis, Mark Barnes states, Complete analytical Ferrography is often referred to as the oil analysis equivalent of criminal forensic science. The test method relies on a visual, microscopic evaluation of particles, extracted and deposited on a microscope slide called a ferrogram. Based on an examination of the shape, color, edge detail, the effects of a magnetic field and other diagnostic tests such as heat treatment and the addition of chemical reagents, an assessment of the active wear mechanism can be made. This allows a skilled diagnostician to determine the root cause of a specific tribological problem.
While ferrographic analysis is an excellent tool when attempting to diagnose an active wear problem, it too has its limitations. The test is a qualitative test, which relies on the skill and knowledge of the ferrographic analyst. While this can have definite advantages, the interpretation is somewhat subjective and requires detailed knowledge, not just of analytical chemistry, but also machine and tribological failures. Also, because of the time and skills required to perform the test, it is usually considered too expensive for routine oil analysis. Nevertheless, used as an exception tool when a wear problem is suspected based on other test results, complete ferrographic analysis is one of the most enlightening of all wear analysis methods.
The test procedure is lengthy and requires the skill of a well-trained analyst. As such, there are significant costs in performing analytical ferrography not present in other fluids analysis tests. But, if time is taken to fully understand what analytical ferrography can uncover, most agree that the benefits significantly outweigh the costs and elect to automatically incorporate it when an abnormal wear condition is encountered.
As with all fluids analysis samples, I cannot overstress the importance of a properly taken sample of the fluid. Samples should be taken that are representative of the conditions that are going on inside the equipment. Representative samples are dependent on the way the sample is taken and the location where the sample is taken from. This is especially important when using Analytical Ferrography.
Another critical factor in fluids analysis, and Analytical Ferrography in particular, is the need of the customer to provide as detailed as possible the specific information about the machine/component the sample was taken from. This includes lubricant information, component manufacturer, model and type of component. The more detailed the machine/component information, the better the diagnosis of the test results.
To perform analytical ferrography, the solid debris suspended in a lubricant is separated and systematically deposited onto a glass slide. The slide is examined under a microscope to distinguish particle size, concentration, composition, morphology and surface condition of the ferrous and non-ferrous wear particles.
This detailed examination, in effect, uncovers the mystery behind an abnormal wear condition by pinpointing component wear, how it was generated and often, the root cause.
Analytical ferrography begins with the magnetic separation of machine wear debris from the lubricating fluid in which it is suspended using a ferrogram slide maker. The lubricating fluid sample is diluted for improved particle precipitation and adhesion. The diluted sample flows down a specially designed glass slide called a ferrogram. The ferrogram rests on a magnetic cylinder, which attracts ferrous particles out of the oil (Figure 1).
Due to the magnetic field, the ferrous particles align themselves in chains along the length of the slide with the largest particles being deposited at the entry point. Nonferrous particles and contaminants, unaffected by the magnetic field, travel downstream and are randomly deposited across the length of the slide. The deposited ferrous particles serve as a dyke in the removal of nonferrous particles. The absence of ferrous particles substantially reduces the effectiveness with which nonferrous particles are removed.
After the particles are deposited on the ferrogram, a wash is used to remove any remaining lubricant. The wash quickly evaporates and the particles are permanently attached to the slide. The ferrogram is now ready for optical examination using a bichromatic microscope.
Figure #1. Ferrogram Slide Maker Separates Particles from the Oil
The ferrogram is examined under a polarized bichromatic microscope equipped with a digital camera. The microscope uses both reflected (top) and transmitted (bottom) light to distinguish the size, shape, composition and surface condition of ferrous and nonferrous particles (Figure 4). The particles are classified to determine the type of wear and its source.
Particle composition is first broken down to six categories: white nonferrous, copper, Babbitt, contaminants, fibers and ferrous wear. In order to aid the identification of composition, the analyst will heat-treat the slide for two minutes at 600F.
- White nonferrous particles, often aluminum or chromium, appear as bright white particles both before and after heat treatment of the slide. They are deposited randomly across the slide surface with larger particles getting collected against the chains of ferrous particles. The chains of ferrous particles typically act as a filter, collecting contaminants, copper particles and Babbitt.
- Copper particles usually appear as bright yellow particles both before and after heat treatment but the surface may change to verdigris after heat treatment. These also will be randomly deposited across the slide surface with larger particles resting at the entry point of the slide and gradually getting smaller towards the exit point of the slide.
- Babbitt particles consisting of tin and lead, Babbitt particles appear gray, sometimes with speckling before the heat treatment. After heat treatment of the slide, these particles still appear mostly gray, but with spots of blue and red on the mottled surface of the object. Also, after heat treatment these particles tend to decrease in size. Again, these nonferrous particles appear randomly on the slide, not in chains with ferrous particles.
- Contaminants are usually dirt (silica), and other particulates that do not change in appearance after heat treatment. They can appear as white crystals and are easily identified by the transmitted light source, that is, they are somewhat transparent. Contaminants appear randomly on the slide and are commonly dyked by the chains of ferrous particles.
- Fibers, typically from filters or outside contamination, are long strings that allow the transmitted light to shine through. They can appear in a variety of colors and usually do not change in appearance after heat treatment. Sometimes these particles can act as a filter, collecting other particles. They can appear anywhere on the ferrogram, however they tend to be washed towards the exit end.
Ferrous particles can be broken down to five different categories, high alloy, low alloy, dark metallic oxides, cast iron and red oxides. Large ferrous particles will be deposited on the entry end of the slide and often clump on top of the other. Ferrous particles are identified using the reflected light source on the microscope. Transmitted light will be totally blocked by the particle.
- High Alloy Steel - particles are found in chains on the slide and appear gray-white before and after heat treatment. The distinguishing factor in the identification between high alloy and white nonferrous is position on the slide. If it is white and appears in a chain, it's deemed to be high alloy. Otherwise, it's considered white nonferrous. The frequency of high alloy on ferrograms is rare.
- Low Alloy Steel - particles are also found in chains and appear gray-white before heat treatment but then change color after heat treatment. After heat treatment they usually appear as blue particles but can also be pink or red.
- Dark Metallic Oxides - deposit in chains and appear dark gray to black both before and after heat treatment. The degree of darkness is indicative of the amount of oxidation.
- Cast Iron - particles appear gray before heat treatment and a straw yellow after the heat treatment. They are incorporated in chains amongst the other ferrous particles.
- Red Oxides (Rust) - polarized light readily identifies red oxides. Sometimes they can be found in chains with the other ferrous particles and sometimes they are randomly deposited on the slide surface. A large amount of small red oxides on the exit end of the slide is generally considered to be a sign of corrosive wear. It usually appears to the analyst as a beach of red sand.
Figure 2. The Metal Alloy of the Particles Determines Whether They Line up on or Adjacent to the Magnetic Field
After classifying the composition of particles the analyst then rates the size of the particles using a micrometer scale on the microscope. Particles with a size of 30 microns or greater are given the rating of severe or abnormal. Severe wear is a definite sign of abnormal running conditions with the equipment being studied.
Often, the shape of a particle is another important clue to the origin of the wear particles. Is the particle laminar or rough? Laminar particles are signs of smashing or rolling found in bearings or areas with high pressure or lateral contact. Does the particle have striations on the surface? Striations are a sign of sliding wear, perhaps generated in an area where scraping of metal surfaces occurs. Does the particle have a curved shape, similar to drill shavings? This would be categorized as cutting wear that can be caused by abrasive contaminants found in the machine. Is the particle spherical in shape? To the analyst, these appear as dark balls with a white center. Spheres are generated in bearing fatigue cracks. An increase in quantity is indicative of spalling.
According to Jim Fitch in his article Today's Oil Detectives Have a New Bag of Tricks, The truth is, oil analysis is detective work, plain and simple. Today's detectives are empowered with a growing bag of tricks but frankly, only a few of these tricks involve traditional oil analysis. Let's take a closer look at what's involved in real oil detective work. But before we do, remember that the primary job of the oil analyst is not troubleshooting chronic machine problems but rather the activity of machine health management, that is, maintaining and controlling machine wellness. Proactive maintenance is always where the big payoff is found. Still, even the best proactive maintenance programs can't completely rid machines of random failures and occasional abnormal wear conditions. It is in these cases when the oil detective earns his keep.
- A problem is still a problem whether it is detected early or kept out of sight. Out of sight may be of momentary convenience, but for process-critical machines, problem penalties can grow if not corrected early. Compounding and/or chain-reaction failures can cost millions of dollars or even one's life. You've seen if before - the worse things get, the faster they get worse.
- By the time a problem has been detected and localized, the cause of the problem is often discovered as well, but not always. A suspect cause (misalignment, degraded oil, etc.) may need further confirmation or there may be two or more causes working in concert. Knowing the true root cause is vital to prescribing a remedy that works. Slowing the rate of progress may, in many cases, be the best response, enabling complete correction at the next scheduled outage.
- Defining the wear mode is where the real strength in microscopic Wear Particle Analysis (Analytical Ferrography) lies. Properly sampled lubricants often contain particles of unique shape and size that characterize how they were created. The skillful eye of a well-trained wear particle microscopist can be invaluable.
In the hands of a skilled analyst, Analytical Ferrography is capable of detecting active machine wear and can often provide a root cause based on the morphology of the wear particles. Used in conjunction with treatments of the ferrogram like heating and chemicals, it can pinpoint the root cause of specific wear problems. The advantage of Analytical Ferrography is that the source, cause and scope of equipment wear can easily be determined. The analysis determines both the type and metallurgy of the wear particle, allowing the analyst to 'see' inside operating equipment to identify abnormal wear conditions.
Due to the method of sample preparation, Analytical Ferrography is biased but not necessarily limited to ferrous particles. The test is non-quantitative and its effectiveness is critically dependent on the knowledge and experience of the analyst. Due to the analyst skills required and the time the analysis takes, it can be fairly expensive compared with other test methods.
Used as an exception test based on results from other less expensive tests, Analytical Ferrography can be an effective fluids analysis tool for most machine components.
Wear Particle Atlas, published by Predict/DLI
- A Tribute to Vernon C. Westcott, Inventor of the Ferrograph, Teresa Hansen, Noria Corporation, Practicing Oil Analysis magazine
- Analytical Ferrography - Make It Work For You, Michael Barrett and Matt McMahon, Insight Services, Practicing Oil Analysis magazine
- Converting to Condition-Based Oil Changes - Part I, Raymond Thibault, ExxonMobil Lubricants & Petroleum Specialties Company, Practicing Oil Analysis magazine
- Today's Oil Detectives Have a New Bag of Tricks, Jim Fitch, Practicing Oil Analysis magazine
- Tricks to Classifying Wear Metals and Other Used Oil Suspensions, James C. Fitch, Practicing Oil Analysis magazine
- Wear Analysis, Mark Barnes, Practicing Oil Analysis magazine