Thermal Degradation

Canonical Definition

The breakdown of lubricant or hydraulic fluid molecules at elevated temperatures through cracking, polymerisation, and volatilisation, independent of oxygen availability. Thermal degradation produces low-molecular-weight volatile compounds (flash point reduction), insoluble carbonaceous deposits (carbon black and coke), and very high-molecular-weight polymers (lacquer and varnish). It occurs at localised hot spots such as pump discharge zones, hydraulic actuator rod seals, and turbocharger bearing housings where temperatures can transiently exceed 200–300°C.

Engineering Context

Thermal degradation is distinct from oxidative degradation because it can proceed without oxygen — it occurs in fully sealed hydraulic systems at high local temperatures even when bulk oil temperature is within specification. Synthetic base oils (PAO, ester) withstand higher temperatures before thermal cracking than mineral oils, making them superior for high-temperature service such as turbocharger oil supply circuits.

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