Monitoring the silicon level in a lubricating oil is a useful way to track damaging dirt ingression into a lubrication system, because dirt [silica] is mostly made from the element silicon. If an increase in silicon level of an oil sample is noted, it is a positive indicator that dirt is entering the system and turning your oil from a lubricant to grinding paste!
However, things are never that simple, silicon can also come from sources other than dirt. There are three common sources of Silicon in lubricating oil and they are:
- Silica [Dirt]
- Silicone [Sealant, Grease, Hose-material]
- Silicon-based anti-foam additive
Fortunately for us, Silica has what Poker players would call a ‘tell’ if you know what to look for. The trick is that the second most common element in silica is aluminium. By monitoring silicon and aluminium together, it becomes easy to tell dirt from ‘silicone sealer’ for example, as the Silicon and Aluminium will move in ‘lock-step’ with each other, with aluminium roughly a quarter to a half of the silicon value.
Another attribute to be aware of about dirt entering a lubricating system, is that even a small amount of dirt in our lubricating oil will trigger a noticeable increase in the wear rate, this is why it is so important to keep oils clean!
In summary, if the silicon value in the oil increases, and the aluminium values increases a little bit, and wear [iron] level increases; you most certainly have dirt contaminating the oil, and action will be required to correct the issue to prevent premature wear from taking place.
Silicone, as in silicone-sealer or silicone-grease, on the other hand does not contain any aluminium and it will seldom change the wear rate when present. Therefore; if silicon increases but aluminium does not increase, and wear levels do not increase, then it is unlikely to be dirt, and is more likely to be contamination from a silicone-based sealant or similar compound and no action is required.
Lastly silicon-based anti-foam additive, this is normally found in most lubricating oils at low concentrations of around 2ppm possibly up to 10ppm. It will be at a similar level for all systems using the same fluid. However, if a system is dosed with anti-foam in an attempt to reduce foam formation, this will lead to a sudden change in the silicon level but similar to Silicone, there will not be as notable change in aluminium, or other elements. When this is suspected, review recent work records to determine if anything has recently been added to the system that could contain silicon.
If you have any questions regarding silicon, or other oil analysis data, please contact ALS Tribology where a technical representative will be happy to provide guidance.
Technical Director, North America