Effect of Nickel Acetyl Acetonate as Lubricant Additive in Base Oils with Different Molecular Structure on In-Situ Formation and Tribomechanism of Carbon-Based Tribofilms of Steel-Steel Sliding Pair

Abstract

Nickel acetyl acetonate (Ni(acac) 2 ), a metal-organic compound, was directly dispersed in base oils alkylated naphthalene (AN-5), diisooctyl sebacate (DIOS), poly-α-ole n (PAO6), and 150N in the presence of commercial dispersant RF1151 (monoallyl poly(isobutylene succinimide). The tribological properties of the lubricants were tested with a four-ball friction and wear tester. The friction-induced in-situ formation of carbon lms on rubbed steel surfaces under the catalysis of Ni(acac) 2 was investigated, and the as-formed carbon lms were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results show that Ni(acac) 2 added in the base oils can release metallic Ni to form nickel layer on the rubbed metal sub-surfaces and catalyze the degradation of the base oil molecules adsorbed to form carbon-based tribo lms. The carbon lm formed from AN-5 with six-membered ring structure has a high degree of graphitization and the best frictionreducing and antiwear abilities, and those formed from PAO6 and 150N with linear structure have a low degree of graphitization as well as good tribological properties. Under the lubrication of DIOS with Ni(acac) 2 , however, there is no carbon lm formation while the tribological properties of the lubricant are relatively poor, due to the absence of the catalytic metallic Ni and nickel oxide layer on the rubbed metal sub-surface. Thanks to the catalytic effect of metallic Ni released from Ni(acac) 2 for the degradation of various base oils with different molecular structure, the present approach could provide a rational pathway to tune the in-situ formation of carbon-based tribo lm on rubbed steel surfaces so as to effectively reduce the friction and wear of steel-steel sliding pair.