Gaiqing Zhao

Tribological Behavior of Amorphous and Crystalline Overbased Calcium Sulfonate as Additives in Lithium Complex Grease

The aim of this study was to examine the tribological behavior of amorphous overbased calcium sulfonate (AOBCS) and crystalline overbased calcium sulfonate (COBCS, transformed from the AOBCS) as additives in lithium complex grease. The transformation product of the calcium carbonate polymorph from AOBCS was calcite, as determined by Fourier transform infrared spectroscopy. Tribological properties were evaluated by an oscillating reciprocating friction and wear tester and a four-ball tester. The results showed that the addition of COBCS can dramatically improve both the antiwear performance and the friction-reducing and load-carrying properties of the base grease. However, improvement of the tribological properties of the base grease by AOBCS was highly dependent on the concentrations added and the loads applied. The tribological properties of the base grease were improved more by the addition of COBCS than by the addition of AOBCS. X-ray photoelectron energy spectrometry and thermogravimetric analysis revealed that both AOBCS and COBCS underwent complicated tribochemical reactions in the base grease and that chemically reactive films consisting of CaCO3, CaO, iron oxide and organic compounds were formed on the worn surfaces. Taken together with the results of the tribo-tests, we suggest that transformation of the calcium carbonate polymorphs was the main factor in improving the tribological properties of lithium complex grease. The transformation of calcium carbonate polymorphs can broaden the application of AOBCS as an extreme pressure/antiwear additive in greases under boundary lubrication conditions.

Tribological properties of castor oil tris(diphenyl phosphate) as a high-performance antiwear additive in lubricating greases for steel/steel contacts at elevated temperature

Castor oil tris(diphenyl phosphate) (CODP) was synthesized using an environmentally friendly and renewable resource – castor oil, and its tribological properties were evaluated in lithium 12-hydroxystearate greases (LHG) and lithium complex greases (LCG) at 150 °C. The tribological behaviors of the additive for LHG and LCG application in steel/steel contacts were evaluated on an Optimol SRV-IV oscillating reciprocating friction and wear tester as well as on a MS-10J four-ball tester. The worn steel surface was analyzed by a JSM-5600LV scanning electron microscope and a PHI-5702 multifunctional X-ray photoelectron spectrometer. The results indicated that CODP as the additive could effectively reduce the friction and wear of sliding pairs in the two base greases. The tribological performances were also better than the traditional used zinc dialkyldithiophosphate (ZDDP) based additive package in LHG and also in LCG. Boundary lubrication films composed of Fe(OH)O, Fe3O4, FePO4 and compounds containing P–O bonds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance.

Investigating the tribological performance of nanosized MoS2 on graphene dispersion in perfluoropolyether under high vacuum

We report for the first time that nanosized MoS2 on graphene (MoS2/Gr) was used as an additive in perfluoropolyether (PFPE) base oil. The nanocomposite can not only form a stable dispersion in PFPE for two weeks but also dramatically improve the friction-reduction and antiwear properties of the base oil for steel/steel contact under high vacuum.

Treelike polymeric phosphate esters grafted onto graphene oxide and its tribological properties in polyalkylene glycol for steel/steel contact at elevated temperature

We report the first synthesis of a high-performance lubricant additive by grafting treelike polymeric phosphate esters (PPEs) onto graphene oxide (GO) nanosheets. Characterization of the PPEs modified GO (GO–PPEs) was performed by spectral (FT-IR, Raman, SEM, TEM, XPS), and thermogravimetric analysis (TGA). Performance evaluations of GO–PPEs as a high-temperature friction-reduction and anti-wear additive in polyalkylene glycols (PAG) were conducted by an Optimol SRV-IV oscillating reciprocating friction and wear tester. The results indicated that GO–PPEs possessed excellent tribological properties in PAG compared with pure GO and PPEs under the same conditions, which may be attributed to the good dispersion of GO–PPEs in PAG, and the excellent tribological properties of phosphate at elevated temperature. In addition, the wear mechanism was tentatively discussed according to the surface composition.

Reinforcing effect on the tribological behaviour of nanoparticles due to a bimodal grain size distribution

In this study, two kinds of calcite calcium carbonate nanoparticle additives (CN), with a grain size of ca. 30 nm and 80 nm, respectively, were synthesized via the carbonation method. Then, a series of additives were prepared through tuning the grain size distribution, and the corresponding tribological performances were also carefully investigated. Compared to the uniform size additives, the performance of the additives with a bimodal grain size distribution was obviously improved and the application range was further extended. Moreover, it can be observed that CN with a large grain size could improve tribological behavior under high frequency conditions, whereas the CN with a small grain size determined the load bearing capacity. The corresponding lubrication mechanisms were also investigated according to the characterization results of the wear scar surface, such as the morphology, composition and microstructure. The results indicated that a continuous protective film was formed on the contact surface and the corresponding mechanical properties determined the final lubricity. The enhancement of the tribological performance can be attributed to the improved toughness of the protective film, due to the reinforcing effect of the bimodal grain size distribution.

Mechanical synthesis of chemically bonded phosphorus–graphene hybrid as high-temperature lubricating oil additive

Red phosphorus (P) was covalently attached to graphene nanosheets (Gr) using high-energy ball-milling under a nitrogen atmosphere. Benefiting from the formation of phosphate and P–O–C bonds on graphene surfaces, the resulting phosphorus–graphene (P–Gr) hybrids exhibited excellent dispersion stability in polyalkylene glycol (PAG) base oil compared with graphene. Moreover, tribological measurement indicated that addition of 1.0 wt% P–Gr into PAG resulted in significant reduction in friction coefficient (up to about 12%) and wear volume (up to about 98%) for steel/steel contact at 100 °C, which was likely due to the formation of a boundary lubrication film on the sliding surfaces during the friction and wear processes. XPS analysis demonstrated that the tribofilm is composed of FeO, Fe3O4, FeOOH, FePO4, and the compounds containing C–O–C and P–O bonds.

New insight to the tribology-structure interrelationship of lubricating grease by a rheological method

By controlling the heat-treatment process, three types of lithium greases with different thickener fiber morphologies were synthesized via the saponification reaction. With increasing the cooling rate, the dimension of the thickener fiber decreased. The relationship between microstructure and tribological performance of the lubricating greases was investigated via a rheological method. The results indicated that the fiber dimension determined the level of physical entanglement and the evolution of fiber network in the friction process, further influencing the final lubricity. The grease with the large fiber dimension displayed good tribological performance under low frequency and high load conditions due to its large-scale strongly crosslinked structure. In addition, the grease with the small fiber dimension yielded a low level of fiber physical entanglements and displayed low structure strength and fast response in the rheological test. It could obviously improve the tribological performance under high frequency conditions. Through tuning the microstructure of the thickener fiber according to the lubricating conditions, the tribological performance could be obviously improved.

Tribological Performance of Lubricating Greases Based on Calcium Carbonate Polymorphs Under the Boundary Lubrication Condition

In this article, we synthesized the calcium sulphonate grease (CSG) based on the calcite using the bright stock (150BS) as the base oil. In order to investigate the tribological performance of lubricating grease containing different calcium carbonate polymorphs under boundary lubrication condition, a calcium sulphonate complex grease (CSCG) based on the vaterite was used as a reference. An oscillating reciprocating friction and wear tester set at a series of applied loads and frequencies was adopted to evaluate the tribological performance under boundary condition. Results showed that the lubricating grease that was composed of crystalline calcite as the partial thickener had excellent friction-reducing and antiwear (AW) properties, regardless of the applied loads and frequencies. The vaterite in CSCG easily experienced a polymorph transformation into calcite or aragonite characterized by Raman spectroscopy. This polymorph transformation was attributed to the highly local friction temperature and activated hydrogen from water or acids oxidated in the rubbing process at high load or frequency. The physical polymorph transformation corresponded to the fluctuations of the friction coefficients, then contributed to the severe wear. XPS analysis indicated that two calcium sulphonate lubricating greases occurred a tribochemical reaction and boundary tribofilms consisted of CaCO3, CaO, iron oxide and FeSO4 were formed on the rubbing surfaces. The tribofilm formed by the introduction of the CSG that mainly depended upon the thickeners of calcite structure contributed to an excellent AW protection. The possible boundary friction mechanism for greases based on various calcium carbonate polymorphs was also proposed. Effect of calcium carbonate polymorphs on the tribological performance was discussed.

Preparation of WS2 nanocomposites via mussel-inspired chemistry and their enhanced dispersion stability and tribological performance in polyalkylene glycol

Abstract A novel biomimetic surface modification method utilizing mussel-inspired chemistry was used to prepare tungsten disulfide (WS2) nanocomposites, which enhanced the dispersion stability and tribological performance of WS2 in polyalkylene glycol (PAG). Herein, WS2-polydopamine-methoxypolyethylene glycol amine (WS2-PDA-MPGA) was first synthesized via mussel-inspired chemistry and used as a lubricant additive in PAG. After modification, the dispersion stability of WS2 nanosheets in PAG was obviously improved. Moreover, the tribological performance of WS2-PDA-MPGA in PAG at high temperature was evaluated by the oscillating reciprocating tribometer. Compared to pure PAG, the lubricant composition containing WS2-PDA-MPGA exhibited excellent performance in friction reduction and anti-wear properties at high temperature. The optimal tribological performance could be obtained when the percentage of additives was 0.9 wt%. The tribological results indicate that WS2-PDA-MPGA, with its good dispersion stability, has better friction reduction and anti-wear properties than does WS2 in PAG base oil. The chemical composition analysis of the wear surface indicated that a stable protective film had been formed by physical adsorption and tribo-chemical reactions. Therefore, the surface modification strategy is an effective way to improve the dispersion stability of WS2 in PAG, which can be expanded application of WS2 in the tribological field. Graphical Abstract