After oxygen, silicon is the most abundant element in the earth's crust. Silicon does not occur naturally in elemental form but rather combined with oxygen in a compound called silica (silicon dioxide). Silica occurs in a free form (quartz, sand, etc.) or combined with a variety of metallic oxides, in which case it is called a silicate (e.g. Felspar). Another class of silicon compounds that should not be confused with silica and silicates is silicones. Silicones are man-made organic compounds that find extensive application in the polish, paint and lubrication industries.
Silica and silicates make up a large proportion of the earth's crust and as such are present at high concentrations in natural soils and dusts. It is for this reason that silicon is used as the main indicator of dust entry into a component. There have been several studies done on the causes of premature wear in components. The figures vary from study to study but one thing is clear: external contamination of lube oil by silicon (dust) is a major cause of accelerated wear.
Figure 1 - The effects of particle size and film thickness on wear rates.
Particles of airborne sand and dust vary in size, shape and abrasive properties. In an engine the ingress of atmospheric dust takes place primarily through the air intake. Efficient air filters remove 99% of the dust that an engine ingests. The remaining 1 % consists of very small dust particles that pass through the air filter. These vary between submicron size particles to particles up to well over 10 microns in size. This dust will pass between piston, rings and cylinder and eventually become suspended in the lubricating oil.
Those particles similar in size to the oil film clearance do the maximum damage. A particle smaller than the clearance will pass straight through doing little harm; a particle larger than the clearance will be unable to enter and do any damage. In an engine the clearance between the piston ring and liner bore is extremely small, therefore, it is very small dust particles that are the biggest threat when a leak occurs in an induction system. Once the dust particle has entered an oil film it forms a direct link between the two surfaces, nullifying the effect of the oil film (see Figure 1).
The first and immediate effect is a "scratching" of the surface as the particle is dragged and rolled across the surfaces. The second and potentially more serious problem is that once the dust particle is introduced in between the two surfaces, it changes the loading of the surface from an even distribution to a load concentrated on the particle with a tremendous increase in pressure at this point. The increase in pressure causes a deflection of the surface, which will eventually result in metal fatigue and the surface breaking up. Among other problems, the increased wear will increase the oil consumption rate (see Figure 2). The solution is to keep the dust out. To do this, design engineers use air cleaners, breathers and seals at any point that dust may enter.
Using Oil Analysis to Detect Dust Early. Before the use of oil analysis, a dust entry problem would go undetected until a routine teardown or a failure occurred. Even then, often the wear would be attributed to lubricant breakdown or normal wear and tear. With the use of oil analysis the picture changes. As soon as a dust entry problem occurs there is an increase in the silicon level of the oil and an acceleration of the wear pattern. As long as the oil samples are being taken at regular intervals in the correct manner, the dust entry will be detected at a very early stage. If effective corrective action is taken, the life of the component will be significantly increased, reducing maintenance costs.
It is beyond the scope of this article to discuss every possibility in all machines and components, so engines will be used as the example application. Engines are at high risk to dust entry as large volumes of air are taken into the system and the close tolerances make it susceptible to even the smallest dust particle.
When an engine has a dust entry problem the type of wear that takes place is related to the manner in which the dust enters. Therefore by examining the type of wear taking place it is possible to discover how the dust is entering the system. When studying an oil analysis report there are four possible wear patterns: (1) normal wear, (2) increased top-end wear, (3) increased bottom-end wear, and (4) all wear rates increased. These are discussed below and illustrated in Figure 4. The table in Figure 3 shows examples of data as it might appear on an oil analysis report.
Figure 3 - Examples of Samples Showing High Silicon
Normal Wear. It is unlikely that there will be a dust entry problem without an increase in wear rates. If normal wear patterns combine with high silicon readings, there are two main possibilities: (1) a silicone sealant, grease or additive is in use, and (2) accidental contamination of the sample.
Action to be taken: Check if an additive, grease or sealant has been used recently on this engine. If they haven’t, make sure the correct sampling technique was used. If an additive, grease or sealant has been used, call your oil analysis lab to find out if the substance could cause a high silicon reading. If there is still doubt as to what caused the high silicon reading, a confirmation sample should be taken.
Was the oil changed when the first sample was taken? If not send a confirmation sample and, as a precaution, change the oil. If the oil was changed, send a confirmation sample only after the engine has run for 50 hours or 1000 km (600 miles).
Increased top-end wear (e.g., iron, chromium, and aluminum). Increased top-end wear is caused by airborne dust that has been drawn into the combustion chamber being forced down between the ring, piston and cylinder. This is caused by a defective air cleaner or a damaged induction system.
Action to be taken: Inspect the air filter element thoroughly, and check its seals and support frame for damage and distortion. Check the pleats for damage. If there is any doubt about a filter element, it should always be changed.
Next, check the induction hosing for damage, cracks etc., and make certain that all hose clamps are secure. The breather and compressor are often connected to the induction system but are frequently overlooked. Check that both are functioning normally and their hoses are sound and secure.
Next, check the inlet manifold for cracks and verify that the gaskets are sound and secure.
Was a leak found? If not, run the engine at idle and block off the air intake. The engine should stall within three seconds. If the engine does not stall, listen carefully at the joint for air being sucked in. Take a confirmation sample after 50 hours or 1000 km (600 miles).
If the leak was found, determine the condition of the engine, check compression and blowby and repair the leak. If all of these parameters are found to be normal, monitor the oil consumption of the engine as a safeguard. If any of these parameters are abnormal, schedule to have the piston rings replaced and the piston and liners examined.
Increased bottom-end wear (e.g., iron, lead, tin, copper, aluminum). This indicates that dirt is getting into the lube oil directly and not past the pistons and rings. The likely sources are: (1) leaking seals, (2) defective breather, (3) damaged seal on oil filler cap or dipstick, or (4) dirty storage containers and/or top-up containers.
Action to be taken: Any dust that is in the oil will be pumped through the oil filter before entering the bearings. Therefore, the first step is to examine the oil filter for: (1) dust contamination and (2) bearing material. If excessive dust is found, thoroughly check all seals and breathers, etc. Check the oil storage containers and top-up containers for the source of contamination.
If excessive dust is not found, check the sampling technique and examine the oil filter at the next service. If bearing material was found drop the sump and inspect all bearings. Replace as necessary. If bearing material was not found, monitor the oil pressure and inspect the oil filter at the next service.
All Wear Rates Increased. This is the worst case. Carry out all checks for top and bottom wear. Several other checks must also be done:
1. Was any repair work done to the engine? It is possible that the increased wear rates are due to a “rebedding in” process and silicon comes from contamination while the engine was open for repairs.
2. Piston torching. This is a rare occurrence but if a piston is torched by a misdirected or badly timed spray from an injector nozzle, silicon is released from the piston itself and all the wear rates will be increased (silicon is alloyed to aluminum to reduce its rate of expansion.)