Changes in the Composition of Aircraft Lubricants Undergoing Thermal Breakdown

Turbine oils often encounter high surface temperatures during operation that cause their breakdown and the formation of solid, carbonaceous coke deposits. These deposits may cause serious operational problems, delays, and loss of revenue. Recognizing changes in a lubricant’s composition as it undergoes the high-temperature break down reactions may allow better lubrication maintenance practices to be implemented. Fourier Transform Infrared (FTIR) spectroscopy is often listed as a way to detect changes in oil composition. This study therefore explored any changes in FTIR spectra caused by the formation of coke deposits in a laboratory-controlled, inert environment. In addition, the spectra of new samples of various lubricants (both mineral and synthetic) were compared. It was found that the spectra of synthetic lubricants differ from those of mineral-based lubricants. In addition, synthetic lubricants formed fibrous deposits that clogged oil lines (as opposed to coke deposits like in the case of mineral-based oils) after being exposed to high surface temperatures. Regardless of the mode of failure (coke vs fibrous deposits), the FTIR spectroscopy only showed small changes in the oil spectra. From the present study, it was concluded that FTIR spectroscopy alone may not be sufficient to monitor a lubricant’s degradation when little oxygen is present.