Microscopic Particle Examination
by Michael Holloway, CLS, OMA I
North America Sales Manager

Microscopic Particle Examination (MPE) aka Analytical Ferrography is one of many useful tools used to understand various wear mechanisms in an oil sample. Using ferrography proactively helps reduce or eliminate impending wear failure by examining the various wear debris particles. Different forms of wear particles can occur when a lubricated component experiences various conditions. The analysis will identify the particles by type and estimate the concentration of each particle type. Abnormal wear particles are formed a number of ways.

 

Abrasive Wear
Abrasive wear particles are typically formed from cutting of a hard, sharp particles or from severe sliding. Abrasive particles can also form as a result of a bearing or gear misalignment. Two body abrasive wear can be due to a misalignment or an asperity of a harder metal component which allows gouging the opposite rotating softer metal. Three body abrasive wear is due to foreign particles in the oil. The harder dirt particle imbeds itself in a softer metal and gouges the metal away from the rotating metal separated by the lubricant film.

Abrasive contamination can produce cutting wear particles which are long and curly with length to width ratios ranging from 5:1 to 50:1. These particles are never considered normal wear particles. A very common abrasive particle would be sand or dirt. Sand is made of silicon dioxide, dirt is typically ferrous or aluminum oxides with organic material. Sand and dirt particles are typically induced through a compromised breather element or through seals, O-rings, gaskets, etc. They can be transparent, translucent, or opaque crystalline or a birefringent material.

Adhesive Wear
Adhesive wear occurs when the speed, temperature, and load allowing metal to metal contact. As the two surfaces meet, the metal welds together. The metal surface appears scuffed or scored with uneven metal chunks attached to the metal surface.

Adhesive wear from severe sliding from excessive speed or load are typically rectangular particles with striations parallel to the direction of elongation.

Spalling or Fatigue Wear
Spalling or fatigue wear is seen when loading or contamination is heavy. Maximum sheer stresses occur below the rolling contact surface. This produces a pit or dent also known as a spall that begins as a crack below the rolling surface and propagates over time to the point that metal particles are generated from the fatigued surface and are found in the oil. These particles are typically round spheres and contribute to abrasive wear as well. The spall site produces a weakened state on the surface and failure can ensue.

Ferrography is a very valuable tool that complements an in-service oil analysis program. While it may not be necessary for all assets, critical equipment will benefit greatly with ferrography analysis. ALS Tribology offers Microscopic Particle Examination (MPE) testing for critical assets. Contact your local laboratory for additional information about this service.