In-Situ Measurement of ZDDP Films in Concentrated Contacts

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Abstract
The film formation behavior of lubricating oils containing zinc dialkyldithiophosphate (ZDDP) additives has been studied in rolling, concentrated contacts using ultra-thin film interferometry. It has been shown that ZDDP-containing oils form chemical films which are additional to the elastohydrodynamic (EHD) films produced by the base oil. ZDDP film formation occurs at elevated temperatures and begins in the range of 130°C to 170°C, depending upon the base oil type. The thickness of the films increases with temperature and test time. The ZDDP films formed are typically 5 to 25 nm thick and appear to be solid-like surface coatings.