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. The particles are produced by the cutting or deformation of the metal surface by protrusion (two body) or by hard particles in the oil (three body) which can be indicative of misalignment or presence of abrasive contaminants. The surface has deep parallel grooves appear only on contacting surfaces. The particles are referred to as cutting, gouging, ploughing, scratching particles.
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.
Erosive wear particles are generated by the loss of material due to the repeated impact of hard particles at high velocity. Often the extent of erosion varies on velocity, angle of inclination, particle type and concentration. This occurs in pumps, valves, nozzles and even elbows.
Cavitation is the removal of material by repeated implosion of bubbles near or on the metal surface. The void or bubble produces a violent shock wave that fatigues the surface until material is removed. This is common in pumps and journal bearings.
Polishing is the continuous removal of material (on one or both surfaces) by very fine abrasive particles. This produces a shiny mirror like surface. The width of groove (or scratch) generated by abrasive is 5-10% of the grit diameter (5um grit < 0.5um groove).
Sliding, Rolling, and Impact 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.
Adhesive wear particles occur during the removal of material due to the adhesion (welding) of asperities in slight contact in the mixed and boundary lubrication regimes. This occurs when high loads, temperatures or pressures cause the asperities on two contacting metals surfaces, in relative motion, to weld together then immediately tear apart, shearing the metal in small discrete areas. In its mild state, this causes a very low rate of wear and typically not a major concern. This is also known as ‘frosting’. In the severe state, adhesive wear is the removal of material due to the substantial metal to metal contact and is sometimes referred to as sliding wear or galling. Excessive speeds and loads on critical contacts within the equipment can produce these particles. They appear as 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 particles are usually 5-10 microns in size; indicative of bearing fatigue prior to a spalling condition. The spall site produces a weakened state on the surface and failure can ensue. These particles can also appear as black circles with shiny centers under the microscope
Fretting corrosion wear occurs between two surfaces resulting from small-amplitude oscillations. These produce oxide debris, which has the appearance of rust or corrosion, hence the term fretting corrosion. Under microscopic analysis, the particles are characterized by a red oxide color and uniform pattern. Another form is a complex type of wear which occurs in static (not rotating) but oscillating (vibrating) systems. This also occurs in concert with corrosion. Combines abrasion by wear debris particles with oxidative corrosion is sometimes called ‘false brinelling’ due to its similar appearance to small scale plastic deformation. This commonly occurs in idle equipment subject to vibration from transportation or adjacent equipment.
Corrosive wear is due to acidic attack on the internal surfaces of equipment creating a layer of corrosion products that are subsequently removed by the sliding action. This is not a wear mechanism from a mechanical process, but rather a chemical process. The particles appear as small particles <1 µm in size which align themselves on the outside edges of a ferrogram. The surface wear pattern is usually even and uniform.
Wear debris analysis 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 wear debris analysis.
Michael D. Holloway, MLA I, MLA II, OMA1, MLT I, MLT II, CLS, LLA I
Principle Consultant, Certified Reliability Leader