Purpose -The aim of this paper is to characterize some selected formulations based on castor oil and a variety of biogenic thickeners from a tribological point of view and compare them with some traditional lithium greases. Design/methodology/approach -The evolution of the friction coefficient in several tribological tests performed using several ball-on-disc configurations and coupling materials was monitored for the different oleogels proposed as biodegradable lubricating greases. Both a rotational ballon-disc classical tribometer designed at MuT laboratory and a nanotribometer were used in rotational and oscillatory modes. Findings -Generally, the use of castor oil-based formulations potentially applicable as biodegradable lubricating greases provides similar or lower values of the friction coefficient than traditional lithium greases, depending on the nature of thickener agent employed and tribological contact. In all cases, biodegradable formulations provide significantly lower values of the friction coefficient in tribological tests performed in the oscillatory mode. Weak oleogels like those thickened with glyceryl and sorbitan monostearates or acylated chitosan, provide the lowest values of the friction coefficient in every type of configuration or frictional test analysed. Biogrease formulations containing cellulose or chitin derivatives as thickener agents generally yield higher values of the friction coefficient, which may be comparable to those obtained with the reference lithium greases depending on the thickener and tribological configuration. In frictional tests performed in the rotational mode, the inclusion of ethyl cellulose in the formulation yields high values of the friction coefficient, which was attributed to the castor oil viscosity modification exerted by this additive. Wear results depend on the balance between the frictional behaviour, especially in the initial transient regime, and oleogel mechanical stability. Practical implications -This investigation proposes different new alternatives to replace the traditional thickener agent in lubricating greases with others based on renewable resources in order to obtain a completely biodegradable formulation for different industrial applications. Originality/value -This paper provides a resource of new practical friction coefficient data as well as a comparative analysis of the tribological response of these new formulations based on biogenic thickeners and other traditional lithium greases.
PurposeThe aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.Design/methodology/approachLubricating greases having the same composition but differing in processing protocols have been manufactured and characterized in order to isolate the rheological behaviour from the formulation.FindingsSome successful empirical correlations between rheological (viscous and viscoelastic) and technological standardized parameters, with the friction factor obtained from a ball‐disc tribometer, have been established in order to elucidate the role of the rheological behaviour of lubricating greases on the friction process. In addition to this, an energetic evaluation of the structural degradation of greases during the friction process has been carried out by performing stress‐growth experiments. Thus, the storage energy density, which is related to the grease capacity to accumulate energy in the elastic deformation, and the limiting energy density, which represents the dissipation of energy in the flow process, have been satisfactorily correlated with the friction factor.Research limitations/implicationsThe complex rheological behaviour of lubricating greases, the extreme deformations and the high‐shear stresses resulting in a tribological contact imply that it is difficult to develop a model to describe their behaviour in an elastohydrodynamic lubricating contact.Originality/valueThis paper provides a resource of practical data to be applied in tribological systems.
Lubricating greases are colloid disperse systems consisting of a base oil and a thickener (additional additives). The lubricant is modeled as a tribological system, and the reaction of a fluid friction stress is investigated. The energetic situation of the volume element is analyzed and the system entropy described. The description of the structural degradation and the used entropy was realized with the help of rheometer tests.
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