The ICP is however limited to the size of particle it can detect, which is about 5 to 8 microns in size. As the surface size of a wear metal particle increases the percent of the total mass of the measured particle decreases.
To get a perspective on how small 5 microns is, a red blood cell is 6 microns in size. A million one micron particles lined in a row would equal one meter in length. Wear particles this size are considered rubbing wear particles, which are generated from surface to surface contact typically during startup of the equipment when boundary and mixed lubrication occurs.
The source of elements reported in the spectrochemical analysis are dependent on the compartment type sampled. Not all elements will be present in all compartments. Each relevant element has a source. In an engine for example, Iron has several sources whereas Lead may only have a few. In general, the source of an element is typically the same for a particular compartment type. It is advisable however to check with the OEM to see if they provide a guide for oil analysis and possibly a list of sources for wear metals.
Each individual element on the report is a data point that provides useful information. An individual element can alone identify a source of wear, but in evaluating wear metals also look for certain combinations of elements. For instance Iron, Copper, and Lead, that is elevated on a first time sample or shows a notable increase from the previous sample would probably indicate shaft (Iron) and bearing/bushing wear (Copper and Lead). Combinations of wear metals will also identify alloys like Bronze (Copper and Tin), or Brass (Copper and Zinc).
Time on oil is an absolute key factor in evaluating wear metals. 20 ppm Lead may be completely fine for a compartment with 300 hours oil time, but would probably be a cause for alarm at 30 hours oil time. Sump capacity is another factor, especially in industrial and off-road equipment. 400 ppm Iron in a 5 gallon gearbox may be considered a normal condition at the recommended service interval, where 400 ppm Iron in a 50 gallon gearbox would most likely not be a normal condition. Also remember when reviewing wear metals, that “the trend is your friend”. Look for notable increases or even decreases which may indicate an oil change or possible poor sampling technique.
Look for “supporting” data on the report if wear metals are elevated. While an abnormal wear condition going on within the compartment may be the cause, look at other test data that may be contributing to or causing elevated wear.
The most common cause of wear is abrasive (dirt) contamination. If the report shows elevated Silicon and Aluminum, then the cause of elevated wear would be from dirt ingestion. Look at time on the oil to see if the oil was over extended and the elevated wear may be just considered accumulative. Viscosity, Oxidation, Acid Number (AN)/ Base Number (BN) test results may support that the oil is beyond its useful service life and is the cause of wear.
The PQ Index is also part of evaluating wear metals as it should, and also be an included test on every sample. As stated with the limitation of the ICP spectrometer only reading microscopic size particles, the PQ Index is not limited to particle size. The PQ Index analyzes ferrous (iron) metal content in the sample and produces a relative number based on how much ferrous material is present in the sample. To get a perspective of the PQ Index value, a ¼ inch piece of a normal sized paper clip introduced into a sample will produce a result of 10,000. The PQ Index number and level of Iron from the ICP results complement each other. A high PQ result with a corresponding low Iron result indicates probable large severe (> 15 micron size) wear particles are being generated within the compartment. On the flip side, a low PQ result with a high Iron result would indicate the sample contains rubbing wear or corrosive wear particles. An elevated PQ can also be used as a “trigger”, to make a decision on whether the sample may warrant the addition of an MPE (Microscopic Particle Examination), for a more in-depth analyses of the type and amount of wear particles being generated.
For further information or to establish a proactive testing program for your oil, coolant, or fuel in critical operating equipment feel free to contact ALS Tribology laboratory or any of our regional testing labs.
Edward F Eckert, CLS, OMA I
Diagnostics Manager, Tribology