Lead naphthenate additive tribochemistry in hydrocarbon oils

Add time:09/06/2019    Source:sciencedirect.com

There is often ambiguity concerning the antiwear mechanisms of soluble lubricant additives used in bearings. For example, the common spacecraft lubricant additive lead naphthenate is thought to be effective only as long as a measurable concentration is present in the oil lubricant throughout the mechanism lifetime. Recent mechanism tests with several days of operation indicated that a hydrocarbon lubricant was depleted of lead naphthenate, even during successful operation. To elucidate this apparent contradiction we conducted tests on 440C stainless-steel thrust bearings lubricated with Apiezon C hydrocarbon oil that was formulated with 5 wt% lead naphthenate. The remaining oil in the bearings was analyzed using Fourier Transform Infrared Spectroscopy (FTIR) after several tests with varying durations up to 336 h (2 wk). FTIR showed that lead naphthenate chemically changes within 24 h and begins to disappear in 336 h. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the bearing surfaces after testing. XPS spectra indicated that lead naphthenate chemisorbs onto the bearing surface with minimal bearing operation, and further reaction to elemental lead (a solid lubricant) occurs at relatively short interaction times (two weeks in our testing). Ball bearing test data indicate that, despite the early loss of additive from the oil, the lead-containing surface coatings provide continued lubrication for the ball bearing.

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