September 2009

Analytical Ferrography
AKA Wear Particle Analysis

by Patrick Kilbane, CLS, Sales Manager
& David Hagle, Regional Sales Manager

Wear Particle Analysis is just what it sounds like:  the identification of wear particles in a lubricant and the assessment of those particles to determine the condition of the equipment. As the sizes and quantities of wear particles increase, so does the risk of failure due to wear.

Ferrography allows us to determine if metal particles are ferrous or non-ferrous based on how they align in the magnetic field. We can also heat treat the particles in order to identify the basic metallurgy. Lubricant degradation products can be seen on the slides as thickened deposits of translucent materials. Particles such as fibers, corrosive wear materials, sand, dirt, and other elements can also be identified.

Analytical Ferrography is suggested when wear problems are suspected, especially with steel or ferrous components. One example is when iron (Fe) content or particle count results have increased at a higher rate than expected. An Analytical Ferrography test will provide the information needed to determine the type of iron (wear, rubbing, or corrosion) being produced or the composition of the increased particles from the particle count.

Particles in oil arise from a number of different sources. Contamination can enter the system during maintenance, through breathers, during oil top offs, in the course of oil changes, and through various other means. Wear particles are generated during machine operation. These wear particles can be caused by contamination, improper lubrication, a change in operating conditions, or other factors that cause metal-to-metal contact.

Depending upon the types of particles, the quantities, and their respective sizes, we can determine if an abnormal wear mode exits, identify particulate contamination, and assess the potential for failure.

Typical wear particles found in lubricant samples are:

Cutting Wear Fatigue Wear Lubricant Degradation
Severe Sliding Wear Rubbing Wear Red Oxides
Fatigue Spheres Black Oxides Fibers
Contaminants Sand and Dirt  
Cutting Wear particles are generated due to abrasive contaminants or a misalignment. This form of wear can be highly destructive if not identified early.
Fatigue Wear is generated due to contact fatigue and is typically associated with bearings or gears. Since gear sets have both sliding and rolling contact, these systems will contain wear particles.  The size and quantity will identify an abnormal condition.
Severe sliding wear is created as particles that should be in rolling contact are instead in sliding contact due to improper lubrication or excessive loads/speed.
Rubbing wear is just that: wear due to rubbing, and is typically considered normal. Particles from 1to 15 microns in size are considered rubbing wear. At the onset of abnormal wear, the amount of rubbing wear will quickly increase prior to actual spalling.
Fatigue Spheres are developed from fatigue cracks in rolling element bearings prior to spalling.
Black oxides are generated during metal-to-metal contact where micro-welding occurs and are indicative of boundary lubrication. Contaminants are created by welding, grinding, arcing, and blast furnace operations. These contaminants are ingested or left in the barrel or unit.

Sand and dirt particles are typical contaminants found in most systems.The quantity and sizes of the particles create problems, especially in systems such as turbines and hydraulics, which require a specific level of cleanliness to operate.

Lubricant degradation products can come in many forms and sources. Sludge, varnish, and gelling are all forms of lubricant degradation.

Fibers can also be identified, indicating outside contamination or, more concerning, a possible filter rupture.

Red oxides are an indication of present or past water contamination. Water in the system will greatly reduce the load-caring capacity of the lubricant and should be eliminated from the system.

The recommended frequency to perform an Analytical Ferrography depends upon the criticality of the equipment, typical failure modes, past history, and current oil analysis results. In most cases, Analytical Ferrography is an exception test. We perform this test to supplement the data provided to customers and, consequently, produce an even better assessment of the equipment’s condition and improved overall recommendations.

Although our trained diagnosticians will recommend an Analytical Ferrography test when the data obtained by performing the test will clarify or determine which maintenance action is appropriate, the test can be added to any regular analysis at the customer’s request.

Based on the type, quantities, and sizes of particles, Analytical Ferrography--combined with regular oil analysis--will provide the information needed to make vital maintenance decisions.

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