Study On Film-forming Mechanisms And Tribological Properties Of Grease In The Harsh Conditions

In modern industry, grease is widely employed in many mechanical motion systems, especially under harsh conditions, such as low speed, heavy load and non-steady state. It is of great significance to study the grease lubrication in depth under these conditions to save energy, reduce wear and extend the service life of machinery.In the present thesis, a tribological multi-functional apparatus has been developed by using the technique of relative optical interference intensity to study the lubrication behavior under harsh conditions. The apparatus can simulate the cases at constant speed and variable speed, etc. In addition an ultra-low-speed of 0.04mm/s and a high contact pressure of 2 GPa can be achieved. The dynamic film thickness of the contact area can be measured. The lubrication behavior can be observed and the friction force can be measured online.A sapphire disc and a steel ball (GCr15) are selected as the tribo-pair for studying the film-forming mechanism and the lubrication properties of the grease under the conditions of low speed and heavy load. When the speed is larger than 4mm/s, the variation of the grease film thickness is very close to the theoretical prediction. However, the soap fiber plays an important role in the lubrication process when the speed is lower than 4mm/s. The film forming capacity of the grease at low speeds is far higher than the theoretical prediction, and is not affected by the variation of the entrainment speed. The variation of the contact pressure has no obvious effect on the film forming capacity of grease.During oscillation, the central film is thicker than its peripheries at the moment of zero entrainment speed, and the minimum film thickness is larger than of the static central film thickness. The central film thickness at this moment increases with increasing the rate of the change of entrainment speed. In one oscillation cycle, the minimum central film thickness appears at the initial stage of acceleration. Under micro oscillation sliding condition, there are two thin layers near the interface of the entrapped grease and the two surfaces of the tribo-pairs. The shear flow has a certain effect on the film formation. The rest entrapped grease between the two shear layers shows a solid like state. No shear flow occurs in this layer and the grease in this layer has the same velocity.Micro/Nano MoS₂ and TiO₂ particles are used as additives for the lithium grease. The rheological properties and tribological properties of the modified lithium grease are changed correspondingly. For example, the yield shear stress decreases but the apparent viscosity of the lithium grease increases with the increasing content of the particles. The friction-reduction and anti-wear properties of the lithium grease are improved by adding the particles, and the underlying mechanisms are related to the formation of protective layers on the wear scar and the structure and physicochemical characteristics of the particles.