Wear Particle Analysis Methods
David Doyle, OMA, CLS - General Manager, Western Division

A variety of laboratory methods are available to measure and trend wear particles in used lubricants which enable end-users to monitor the health and operating life of equipment. By using a variety of methodology, used oil analysis can better predict the type of wear occurring and the component part experiencing wear.

Spectrochemical Analysis: The most common method employed is spectrochemical analysis, which directly measures the concentration of wear metals such as iron, chrome, tin aluminum, copper, lead, etc. ICP and Emission Rotrode are the two most common instrumentations for used oil trending analysis. Though there is a particle size limit of 8 to 10 micron for absolute concentration measurement for these instruments, the methodology is relevant and applicable since wear limits established for different equipment designs are based on measurements using this type of instrumentation. The fact that wear particles are larger than 10 micron does not mean the spectrometer does not see the particle, the instrument just sees less of the overall surface area. There are special sample preparation methods that can be employed to measure the absolute concentration of wear particles within an oil sample but these methods are not cost effective to most end-users.

Direct Reading Ferrography: Direct Reading Ferrography is a trending tool employed by Staveley Services used to monitor the relative level of ferrous wear material within an oil sample. The method uses an instrument that passes the sample through a magnetic field to capture and quantify the relative amount of ferrous wear particles. The number obtained by the instrument is not quantified by weight, percent or ppm. It quantifies ferrous wear particles in relative concentration levels. DR Ferrography reports numbers on an open ended scale, having no upper reporting limit. The higher the test result number determined by the DR Ferrograph, the higher concentration of ferrous wear material in the oil. Increase in the DR Ferrograph reading over time shows an increase in ferrous wear particle concentration. Sudden increase in the numbers can predict abnormal wear conditions or impending catastrophic failure. DR Ferrography quantifies particles into two sizes ranges, which are density large (DL) and density small (DS). The large density particles are in the size range of 5 micron and greater, while the small density particle are in the range of 1-2 micron.

This analysis tool has advantages over particle count determination in applications such as gearboxes and drive trains. DR Ferrography is an excellent trending tool that will compliment a standard oil analysis package for rolling stock and industrial applications

Wear Particle Concentration (WPC): Not as widely used within Staveley, the methodology is similar to DR Ferrography in that a magnetic field is used to capture and quantify ferrous wear particles within a used oil sample. Wear particles are reported as large and small concentration groups on a scale from 1 to 100. WPC is also an excellent trending tool with applications similar to DR Ferrography.

Wear Particle Analysis – Patch Test: A common method for making a more detailed determination of wear occurrence, especially for non-ferrous materials is to employ a Patch Test examination using a microscope for Wear Particle Analysis. A measure portion of used oil is filter through a filter patch and visually examined microscopically for a qualitative report on the wear material captured. Observation will generally be accompanied by a photo of the filtered wear material on a test report   

LazerNet Fines (LaserView™): Some Staveley laboratories provide Lasernet Fines instrumentation, which was developed by Lockheed Martin with the Naval Research Laboratory for military application. Using direct digital imaging LaserView test results classify particles larger than 20 micron into cutting wear, severe sliding wear, fatigue wear, and nonmetallic material. The analysis economically combines features of particle count determination with quantifying wear particle classification for industrial, gear and drive train components without subjective interpretation. More information is provided than just using DR Ferrography or Wear Particle Concentration Analysis. The test data compliments other wear analysis techniques by using laser imaging and advanced image processing software to identify and measure

• Type of wear mechanism
• Rate and severity of wear processes
• Wear particle size distribution
• Particulate contamination  and oil cleanliness

Data Reported: Wear mode statistics for particles >20 um which include:

• Cutting wear, size range number/ml, mean size (um), maximum size (um)
• Severe sliding wear, size range number/ml, mean size (um), maximum size (um)
• Fatigue wear, size range number/ml l, mean size (um), maximum size (um)
• Nonmetallic particles, size range number/ml, mean size (um), maximum size (um)

Particle counting and industry cleanliness codes include:

• Particle ISO Cleanliness Rating for >4, >6, >14 micron
• Maximum Particle Size 
• Mean Particle Size

Analytical Ferrography: Analytical Ferrography utilizes a skilled analyst examining a prepared ferrogram slide with a computer aided microscopic to identify the composition of the material present in a used lubricating oil sample.
Wear material and other debris suspended in a lubricant is deposited and separated onto a ferrogram slide maker. The sample is diluted to improve particle separation onto the ferrogram slide. Magnetic separation of wear material from the lubricating fluid attracts ferrous particles out of the oil onto the ferrogram slide maker. Though the method is biased to ferrous material, other nonferrous wear particle and contaminants are also captured and identified. The slide is examined under a microscope to distinguish composition, morphology, particle size and relative concentration of the ferrous and non-ferrous wear particles. Treatment of the ferrogram with heating and chemicals will further distinguish identification of the metallurgical composition of the wear material.

The skilled analyst performs the analytical ferrography to provide a root cause for wear mechanisms based on the morphology and composition of the particles. The analyst will report material composition and wear morphology that will include, but is not limited to the following:

• Ferrous wear particles
• High alloy steel 
• Low alloy steel 
• Dark metallic oxides and cast iron        
• Red oxides (rust) 
• White nonferrous metal particles
• Yellow metals wear particles       
• Contaminants, dirt (silica), fibers and other particulates
• Fatigue Wear
• Sliding Wear
• Cutting Wear - Abrasive Wear
• Adhesive Wear
• Corrosive Wear

Other Methodology
Particle Count – ISO Cleanliness Rating: Some laboratories use particle count for determining wear material. This methodology would have some disadvantages if employed strictly for this purpose. Many times equipment such as gear boxes and drive train components inherently contain a large concentration of particles because they may not be filtered and are not regarded as “clean systems”. Therefore, the particle count determination does not necessarily correlate with wear conditions since the particles counted can be wear material along with anything else in the oil system. On the other hand, “clean systems” such as hydraulic and turbine oils should be monitored for particle count determination in order to maintain the required level of oil cleanliness the equipment specifies. There is a direct correlation between oil cleanliness in “clean systems” equipment life.

Particle Quantifier (PQ): PQ is somewhat similar to the Direct Reading Ferrography employed by Staveley. The PQ is a magnetometer that measures the mass of ferrous wear debris in a sample and displays this as a PQ Index. As with Direct Reading Ferrography, a relative number is quantified and can then be trended for useful wear monitoring.