Preparation And Tribological Behavior Mechanism Of Nano ZnO/Grease Composite System

Nanoparticles have been the focus of development in the field of lubrication since the beginning of last century due to their good tribological properties,and have been studied for nearly 30 years now.Among them,ZnO nanoparticles have shown great potential as lubricant additives in friction reduction and anti-wear.However,there is a lack of research on the addition of ZnO particles in greases,so it is necessary to systematically study the effects of ZnO particles in different greases and their mechanism of action,which can provide experimental and theoretical support for the application of ZnO particles in greases.Based on this,ZnO particles with different morphologies were synthesized by hydrothermal method and used as additives in lithium greases,lithium complex greases and polyurea greases,and the physicochemical properties and tribological behavior of ZnO/grease complex system were systematically investigated by comparing with commercially available ZnO particles of different particle sizes in greases,and further suggesting the possible lubrication mechanism.are as follows:（1）Preparation of nano-ZnO/lithium grease composite system and tribological behavior mechanism studyFirstly,three kinds of ZnO particles with different morphologies were synthesized by a simple hydrothermal method using Zn（AC）₂ and NaOH as raw materials.The three ZnO particles with different morphologies and five commercially available ZnO particles with different particle sizes were added into the lithium grease at different concentrations to investigate the effects of different morphologies and particle sizes on the physicochemical properties and tribological behavior of the lithium grease,and the connection between the microstructure and tribological behavior of the grease was investigated by rheometry.The experimental results showed that the addition of ZnO particles had little effect on the physicochemical properties of the basic lithium grease,and the ZnO particles could improve the load-bearing capacity of the lithium grease;among the ZnO particles of different particle sizes,the 50 nm ZnO particles had better friction reduction and anti-wear effect,and the average friction coefficient was reduced by 32.7%compared with that of the basic lithium grease.This is due to the different particle sizes of ZnO particles and lithium grease soap fiber suitability degree,resulting in superiority differences.Among the prepared ZnO/lithium grease complex systems with different morphologies,the flake ZnO/lithium grease has excellent friction and anti-wear properties,with the lowest average friction coefficient（0.097）at 0.6 wt.%addition,which is about 40%lower than the base grease.The flake ZnO particles added to the lithium grease reduce the wear volume by 97%compared to the base lithium grease.This is due to the fact that flake ZnO particles are more likely to adsorb on the friction surface and continuously form new planes to reduce wear.（2）Preparation and tribological behavior of ZnO/compound lithium grease systemThe ZnO particles with different particle sizes and ZnO particles with different morphologies were added into the composite lithium grease at different concentrations to investigate the effects of different morphologies and ZnO particles on the physicochemical properties and tribological behaviors of the composite lithium grease.The results showed that the addition of ZnO particles had little effect on the cone penetration,dropping point and onset oxidation temperature of the composite lithium grease.The basic lithium complex grease can be loaded with more than 1800 N.The ZnO particles are not suitable for the fiber network of the lithium complex grease and do not have a good synergistic friction reduction effect.（3）Preparation and tribological behavior of ZnO/polyurea grease composite systemThree different morphological ZnO particles and commercially available ZnO particles with different particle sizes were added into polyurea grease at different concentrations to investigate the effects of different morphological and particle sizes on the behavior of polyurea grease.The microstructure,physicochemical properties and tribological behavior of the grease composite system were characterized.The experimental results showed that the addition of ZnO particles had little effect on the cone entry,dropping point and onset oxidation temperature of the polyurea grease,but the addition of ZnO particles destroyed the soap fiber structure of the originally stable granular agglomerates of the polyurea grease.It is not suitable with the fiber network of polyurea grease,and does not play a better effect of friction reduction.In summary,it is proved that ZnO particles exhibit certain friction reduction and anti-wear properties in different types of greases and have potential application prospects.The research in this paper lays down the data support and theoretical guidance for the future application of ZnO particles in greases.