Synergistic Effect of Bismuth Naphthenate and Metal Passivator as Additives in Lubricating Grease

In this work, three metal passivators, benzotriazole type metal passivator (T551) and thiadiazole derivative type metal passivators (DMTD-8 and DMTD-12) were selected as additives to complex with bismuth naphthenate. The tests such as copper corrosion, dropping point, and penetration were utilized to investigate the physicochemical properties of lithium complex grease with different additives. And the synergistic effects between additives on extreme pressure and friction reducing and antiwear performances were also researched systematically by tribological tests and surface analyses. The physicochemical property test results indicated that three compound additives had little effects on physicochemical properties (such as thermal stability and colloidal stability) of base grease. The grease with and without compound additives processed high dropping point over 300℃, which indicated the grease have potentials to be used under high temperature. In the copper corrosion experiment, results indicated that bismuth naphthenate showed corrosion on copper. While after blending three metal deactivators with bismuth naphthenate in base grease respectively, the corrosion of the grease with three compound additives on copper was significantly reduced. Three metal passivators in compound additives could effectively inhibit the corrosion of bismuth naphthenate on copper. The extreme pressure performance of grease was studied by standard test methods. Test results indicated that there was no synergistic effect on extreme pressure performance between benzotriazole type metal passivator T551 and bismuth naphthenate. The synergistic effects between thiadiazole derivative type metal passivators and bismuth naphthenate were remarkable because the values of P_B and P_D increased obviously after blending the individual additive in grease. The friction-reducing and antiwear properties of bismuth naphthenate, three kinds of metal activators and their compound additive package for lithium complex grease were systematically investigated by tribological tests on SRV-V. The results showed that bismuth naphthenate could reduce the friction coefficient and wear volume of base grease. However, after blending the benzotriazole type metal passivator T551 and bismuth naphthenate, the friction coefficient curve was similar with that of grease with bismuth naphthenate, and the wear volume was larger than that of grease with individual additive. This indicated that there was no synergistic effect on friction-reducing and antiwear performances in lithium complex grease between bismuth naphthenate and T551. For the two thiadiazole derivative type metal passivators, they showed low friction coefficients and wear volumes when added in grease individually. When the two thiadiazole derivative type metal passivators were blended with bismuth naphthenate, respectively, the friction-reducing and antiwear performances were effectively improved. The results showed that bismuth naphthenate and thiadiazole derivative type metal passivators had synergistic effects on tribological performances. To explore the mechanism of synergistic effect, worn surfaces lubricated by grease with compound additives were investigated systematically by surface analyses. SEM and element mapping results indicated that there were amounts of S and Bi elements in the worn surface lubricated by grease with bismuth naphthenate and thiadiazole derivative type metal passivators. XPS results indicated that the excellent synergistic effects could be ascribed to the tribochemical products such as Fe₂O₃, Li₂O, FeS₂ and Bi₂S₃, which were produced during the friction process.