Tribological Behavior Of Cellulose Nanocrystal As A Lubricating Additive In Lithium-Based Greases

Nanoparticles are thought to be eco-friendly lubricant additives that will eventually replace traditional sulfur and phosphorus compounds.However,the high cost of nanoparticle preparation limits their practical application in industry.On the other hand,while some nanoparticles containing heavy metal elements pose no direct toxic hazards,their accumulation in living organisms remains a problem that needs to be addressed.As a result,it is critical to develop new greener and less expensive nanolubricant additives.Cellulose Nanocrystal(CNC)is a type of nanomaterial derived from natural cellulose resources that is non-toxic,biodegradable,low in density,and high in strength.It has been produced at an industrial level and widely used in green coatings,environmental protection packaging,and other fields,owing to its abundant source and mature preparation process.In the field of lubrication,CNC has been demonstrated as an excellent additive in both water and lubricants for its friction reduction and antiwear properties.To investigate the mechanism of CNC in grease,CNC particles were extracted from cotton by acid hydrolysis method and added to four lithium greases(411,412,421,and 422)with different consistency(NLGI grade 1 and 2)and different base oil viscosity(100 c St and 200 c St at 40 °C)from 1wt.% to 15 wt.%,and the following findings were obtained to investigate the effect law and lubrication mechanism of CNC on grease consistency,rheological characteristics,and tribological properties:(1)Infrared spectroscopy,X-ray electron spectroscopy,transmission electron microscopy,X-ray diffraction,and thermal analysis were used to examine the chemical structure,element type,microscopic morphology,crystallinity,and thermal stability of the prepared CNC,which confirmed the rod-like nanoparticles and high crystallinity of CNC.(2)The cone penetration,viscoelasticity,apparent viscosity,and viscositytemperature performance of the compound grease containing CNC were tested,and it was discovered that the CNC doping concentration should be 10%(for 411 grease with low consistency and apparent viscosity,the doping amount should be 5%),in order to avoid the change of CNC on the base properties of the grease.(3)The tribological performance of CNC compound grease was tested using a four-ball friction machine.Due to the low consistency and low base oil viscosity of 411 grease itself low carrying capacity,the lubricating film was weak and the lubrication improvement by adding CNC was limited.When CNC was incorporated into the greases with high consistency such as 412 and 422 grease,on one hand,the dense soap fibers themselves could be partially deposited on the surface to facilitate the lubrication;on the other hand,the dense fibrous network hindered the transportation of CNC to the contact area.In the case of 421 grease,CNC could be effectively deposited on the substrate and protect the underlying surface.Together with the strong oil film,CNC additive resulted in the biggest performance enhancement of friction reduction(16%)and anti-wear(29%).