Synergetic relationships between solid lubricants and extreme pressure additives on the tribological performance of lubricating oils

PRIETO, G.; PILOTTI, BRUNO; CAMILA MÜLLER; DENNEHY, J.; OVANDO, E.; DENNEHY, M.; TUCKART, W. R.

Simposio; VII Simposio Nacional de Recubrimientos, Ingeniería de Superficies y Tribología; 2021

Red de Ingeniería de Superficies y Tribología

The current trend of increasing the power density of machinery and equipment contributes to the reduction in CO2 emissions and an overall increase in their operating efficiency. However, this process implies a challenge for lubricating oils, mainly due to the increase in working temperatures and contact stresses between sliding components. To address this scenario, lubricating oils are additivated with compounds that protect the surfaces from direct metal-to-metal contact in boundary lubrication regimes. For this purpose, additives such as zinc dialkyldithiophosphate (ZDDP) have been traditionally used, thanks to their ability to form protective films on steel surfaces. Similarly, metal sulfides, such as MoS2, can effectively reduce friction and wear by forming chemically bound layers to steel substrates. However, the joint action of metal sulfides and ZDDP has not been yet fully characterized.I will discuss the underlying mechanisms of ZDDP lubrication, focusing on the formation of tribofilms on steel substrates and some basic concepts of the associated tribochemistry processes involved. Then, the fundamentals of solid lubrication will be presented and commented on, paying attention to the variations in the tribological response of the most commonly used solid lubricants when the sliding or environmental conditions are changed. Also, I will present results of the lubricating performance of Bi2S3, a sulfide with enormous potential for being incorporated into multiple technological applications.The presentation will then continue with the discussion of the results of the performance of base PAO oils additivated with different combinations of ZDDP, MoS2, and Bi2S3 in a Reichert test. Our results showed a significant synergetic effect, with wear rate reductions in the range of 30 to 70% depending on the additive formulation. The evaluation of the results led us to infer multiple and competing operating lubrication mechanisms that take place not only between the compounds and the steel substrates but also between the compounds and the tribofilms that are being formed on the surfaces.