Research on Influence Mechanism of Polyisobutylene Succinimide Dispersant on Low-speed Friction Control of ATF

LI Yuan; DI Zechao; MA Yonghong; HUANG Dongsheng; CUI Haitao; JIANG Yu; ZHAO Zhiyu; XU Jingjing; ZHANG Weiguang

doi:10.16490/j.cnki.issn.1001-3660.2026.11.022

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Surface Technology ›› 2026, Vol. 55 ›› Issue (11) : 260-272. DOI: 10.16490/j.cnki.issn.1001-3660.2026.11.022

Friction, Wear and Lubrication

Research on Influence Mechanism of Polyisobutylene Succinimide Dispersant on Low-speed Friction Control of ATF

LI Yuan, DI Zechao^*, MA Yonghong, HUANG Dongsheng, CUI Haitao, JIANG Yu, ZHAO Zhiyu, XU Jingjing, ZHANG Weiguang

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Abstract

This study systematically investigates the synergistic effects of two dispersants, polyisobutylene succinimide (DIS1) and boron-phosphorus modified polyisobutylene succinimide (DIS2), on the low-speed friction control performance, particularly the anti-shudder durability, of an automatic transmission fluid (ATF) formulation. The base formulation consists of a partial ester friction modifier (FM0) and a calcium-containing detergent (DET0).
Experiments are conducted using a comprehensive multi-analytical approach. Low-speed friction performance is evaluated by measuring the anti-shudder failure time under controlled conditions with a high-precision disk-on-disk friction tester (WAZAU, Germany). To decouple the effects of adsorption from functional performance, the adsorption characteristics of the additive systems onto steel surfaces are quantitatively analyzed in real-time with a Quartz Crystal Microbalance by Dissipation monitoring (QCM-D). This technique, independent of the friction process, provides detailed insights into adsorbed mass, the viscoelastic properties of the adsorbed layer, and kinetic processes. Post-test analysis involves detailed surface characterization: the morphology of the friction surfaces is examined by Scanning Electron Microscopy (SEM), and the chemical composition of the tribofilms is determined by Energy Dispersive X-ray Spectroscopy (EDX).
The key findings reveal a synergistic interaction between DIS1 and DIS2. The individual incorporation of either DIS1 or DIS2 into the FM0+DET0 system significantly degrades its anti-shudder performance: DIS1 causes nearly immediate failure, while DIS2 only extends the failure time to 48 hours, far below the 120-hour benchmark set by the baseline FM0+DET0 system. In contrast, the combined addition of DIS1 and DIS2 at a 1:1 mass ratio (with a total concentration of 6wt.% in Oil D) leads to a dramatic recovery in performance, achieving an anti-shudder failure time of 118.2 hours. This indicates that the negative effects of the individual dispersants are counteracted when they coexist at this specific ratio.
QCM-D analysis provides a critical breakthrough in understanding this phenomenon. The results show that the total adsorbed mass on the steel surface depends primarily on the combined concentration of DIS1 and DIS2. Furthermore, the boron-phosphorus modification in DIS2 does not alter its fundamental adsorption sites compared with DIS1. This leads to the conclusion that the physical adsorption process itself is not the primary determinant of anti-shudder performance; rather, the molecular structure of the additives and the chemical reactions they undergo during the friction process are decisive.
Based on the integrated data, a novel mechanism is proposed. In the FM0+DET0 system, the long polyisobutylene (PIB) chains of the succinimide dispersants are believed to orient and form a surface layer. During prolonged low-speed sliding, this oriented structure undergoes an unfavorable configuration change, leading to the loss of friction control and the onset of shudder. The role of DIS2 is primarily tribochemical. Its boron (B) and phosphorus (P) elements react with sulfur (S) and calcium (Ca) originating from DET0 under frictional heat and pressure, generating a robust tribofilm with a P-B-Ca-S composition in situ. This film provides stable shear properties, thereby enhancing anti-shudder durability.
Additionally, a secondary yet significant mechanism is hypothesized. It is speculated that the tribochemical reactions involving DIS2 also produce a controlled amount of nano-scale borate microspheres within the contact zone. These particles may act as microscopic spacers or supports within the interfacial layer, physically hindering the detrimental reconfiguration of the long PIB chains from DIS1/DIS2, thus helping to maintain a stable friction-modifying structure.
This study presents several contributions. First, it clearly distinguishes between the roles of physical adsorption and tribochemistry in additive functionality, using QCM-D to validate that adsorption is a necessary but insufficient condition for performance. Second, it identifies a specific synergistic mass ratio (1∶1) between conventional and boron-phosphorus modified succinimide dispersants that recovers and maintains optimal low-speed friction control performance. Third, it proposes a dual mechanism for anti-shudder performance: the formation of a robust P-B-Ca-S tribochemical film and the structural stabilization of the adsorbed dispersant layer through in situ generated nano-borates. These insights provide a refined theoretical framework for understanding additive interactions in ATF and offer practical, formulation-level guidance for designing next-generation lubricants with superior low-speed friction control performance and extended durability.

Key words

polyisobutylene succinimide / borated and phosphated polyisobutylene succinimide / automatic transmission fluid / low-speed friction control / additive adsorption

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LI Yuan, DI Zechao, MA Yonghong, HUANG Dongsheng, CUI Haitao, JIANG Yu, ZHAO Zhiyu, XU Jingjing, ZHANG Weiguang. Research on Influence Mechanism of Polyisobutylene Succinimide Dispersant on Low-speed Friction Control of ATF[J]. Surface Technology. 2026, 55(11): 260-272

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