Key result
Shear degradation of lithium grease occurs in two phases—a progressive early phase and a slower subsequent deterioration—driven by the fragmentation of the original fibrous network under shear.
The study formulates an aging equation to describe the degradation behavior of lithium-thickened grease under shear, linking rheological changes to the fragmentation of its fibrous microstructure.
May guide maintenance intervals for sheared lubricants; leaves open validation of the aging equation in service conditions.
In this article, the shear degradation of lithium 12-hydroxy stearate grease will be measured using an in-house-developed Couette aging machine. In this device the shear rate is well defined. The aging is related to the generated entropy density as described in Rezasoltani and Khonsari's work (Tribology Letters, Vol. 56, No. 2, pp. 197–204, 2014). The rheological properties of the aged samples were evaluated using a parallel-plate rheometer. The results showed that there are two aging phases with different degradation rates: a progressive degradation phase at the early stage, followed by a rather slow deterioration afterwards. Based on this observation, an aging equation was formulated to describe the aging behavior of lithium-thickened grease. Atomic force microscopy results of the fresh and aged greases showed that the variation in thickener microstructure provides a good explanation for the lithium grease degradation mechanism: under shear, the original fibrous network is progressively destroyed and becomes fragmented, leading to the loss of consistency and a change in the rheological properties.
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Zhou et al. (2016) studied Shear degradation of lithium 12-hydroxy stearate grease. Shear degradation using Couette aging machine was evaluated on Rheological properties and thickener microstructure variation. Shear degradation of lithium grease occurs in two phases—a progressive early phase and a slower subsequent deterioration—driven by the fragmentation of the original fibrous network under shear.
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