Effect of Base Oil and Thickener on Texture and Flow of Lubricating Greases: Insights from Bulk Rheometry, Optical Microrheology and Electron Microscopy

Abstract

The structure and flow behavior of lubricating greases depend on the base oil and the type and concentration of the dissolved thickener. In this study, the linear viscoelastic properties of greases were characterized by combining oscillatory shear and squeeze flow covering a broad frequency range (0.1–105 rad s−1). Multiple-particle tracking (MPT) microrheology and scanning electron microscopy (SEM) provided further insight into local viscoelastic properties and sample structure on a submicron-length scale. The type and viscosity of the base oil did not affect the absolute value of the complex viscosity and the filament shape formed by a given thickener. High-frequency shear modulus data, however, indicated that the thickener lithium 12-hydroxystearate formed stiffer networks/filaments in poly-α-olefins than in mineral oils. As expected, the viscosity increased with increased thickener concentrations, but microscopy and high-frequency rheometry revealed that the thickness, length, and stiffness of the individual filaments did not change. In mineral oil, the 12-hydroxystearate thickeners yielded higher viscosity than the corresponding stearates with the same metal ion. The filamentous lithium thickeners created stronger networks than the roundish aggregates formed by magnesium and zinc stearate. Network mesh sizes varying between approximately 100 nm and 300 nm were consistently determined from SEM image analysis and MPT experiments. The MPT experiments further disclosed the existence of gel-like precursors of approximately 130 µm at thickener concentrations far below the critical value at which a sample-spanning network resulting in a characteristic grease texture is formed.

Flake Graphene as an Innovative Additive to Grease with Improved Tribological Properties

“…The grease is non-Newtonian liquid, classified as thixotropic liquid. Under the influence of shear forces, its internal structure is destroyed and viscosity is reduced [ 33 ]. Oil is separated as a result of breaking the bonds of the cross-linker, which enables better lubrication.…”

Effect of Grease Viscosity on Channeling Properties of Ball Bearings

“…In the range of yield stress, grease viscosity decreases rapidly with increasing shear rate stress, and in the high shear rate range, viscosity asymptotically approaches a value like that of oil. These changes are influenced by changes in the grease thickener network [26], so the flow differs from grease to grease even at the same shear rate. In addition, Abdulrazaq et al [27] have analyzed viscoelastic fluids, such as grease, during elongational deformation and found that the yield stress threshold, surface tension coefficient, and viscosity ratio in the material during elongation affect the elongation characteristics.…”