PhD Defence Yuxin Zhou

On the mechanical aging of lubricating greases 

Yuxin Zhou is a PhD student in the Surface Technology and Tribology group (OPMTR) his supervisor is P.M. Lugt from the faculty Engineering Technology. 

This thesis focuses on the mechanical degradation of lubricating greases, including the change of thickener micro-structure and rheological properties as well as the influence of the mechanical degradation of grease on the bearing lubricant film thickness.

Grease is a multi-phase material, where the base oil is trapped within the thickener network by a combination of Van der Waals and capillary forces. Inside a bearing, grease will act as a reservoir releasing lubricant into the contact area. During bearing operation, mechanical degradation of the grease is observed, reflected by the change of grease consistency, grease bleed, apparent viscosity, etc. This will lead to a loss of lubricant, severe starvation and ultimately the failure of the bearing.

The thesis starts with the ageing mechanism for lubricating greases with a fibre-like thickener structure (including lithium-based, lithium-complex-based and polyurea-based greases). Under shear, grease softening is found. The change of the rheological properties of the grease shows a two-phase ageing behaviour. The degradation is initially fast but subsequently slows down. This degradation is closely related to the change of the thickener micro-structure. In addition to shear, high temperatures will accelerate the ageing process following an Arrhenius behaviour. Based on these observations, an Ageing Master Curve is constructed using an energy concept. This model is later validated using a grease worker and applied to the grease ageing process inside a rolling bearing.

As a comparison to the fibre-like thickener structure greases, the ageing of calcium sulphonate complex grease is investigated. Different from the greases mentioned above, calcium sulphonate complex grease has a particle-like thickener structure that is difficult to break under shear, hence no shear softening is observed. As a result, the Ageing Master Curve is not applicable for this type of grease.

The ultimate goal of grease lubrication is to provide the bearing contacts with a separating film. This is why the influence of grease mechanical ageing on the film thickness is studied as well. The grease film thickness deviates from the calculated elastohydrodynamic film thickness (assuming lubrication by the base oil under fully flooded conditions). The grease film thickness  appears to be influenced by churning, channelling, change of grease bleed and rheological properties caused by shear and/or temperature. In addition, the thickener fragments generated by mechanical degradation also contribute to the film thickness.

The results from the work described in this thesis give an important contribution to the development of rolling bearing grease life models. These models can then be used for the calculation of maintenance intervals.

This thesis is divided into two parts. The first part (Part I) is a short description of the work. The second part (Part II) consists of the papers in which the details are described.