Abstract
AbstractA first-ever friction–wear model for four-ball extreme pressure (EP) lubrication is developed in this work based on 12 oil samples comprising minerals, esters, and other formulated lubricants. The model considers the rate of entropy generation and dissipation within the lubricated tribosystem to describe the interaction between the friction and the wear behaviors at extreme pressure conditions. The model can be used to calculate the probability to pass or fail at a specific load to estimate the weld point of a lubricant. The calculated probability exhibited a similar trend as a load-wear index from the ASTM D2783 EP test method. Besides, the model is able to estimate the EP performance of an unknown lubricant based on the model parameter, namely the dissipative coefficient. This parameter describes the proportionality between the friction and the wear phenomena from the perspective of thermodynamic analysis. This work provides useful tools to better understand the fundamentals of EP lubrication and to characterize the lubricants without overly relying on tribotest machines.
Highlights
Tribological modelling has significantly contributed to the development of various applications: lubrication and friction, wear minimization, coating, additivation, functional lubricants, and surface material
In order to keep up with the above developments, tribological modelling has advanced from a simple contact mechanic approach to a complex multiphysical approach, involving different time scale, length scale and conditional state [1]
One of the simplest approach of NonEquilibrium Thermodynamics (NET) is based on the assumption of local equilibrium [36], which states that in any small region of the occurring tribological system, its thermodynamic properties are related to the state variables in the same manner as in equilibrium
Summary
Tribological modelling has significantly contributed to the development of various applications: lubrication and friction, wear minimization, coating, additivation, functional lubricants, and surface material. One of the simplest approach of NET is based on the assumption of local equilibrium [36], which states that in any small region of the occurring tribological system, its thermodynamic properties are related to the state variables in the same manner as in equilibrium Another NET approach is known as the second law analysis [37, 43], which states that the lost available energy is directly proportional to the entropy production due to irreversibility in the process. According to the second law of thermodynamics, the entropy production in the tribosystem is always positive In this case, due to the generation of frictional energy during the sliding activity, the value of entropy in the tribosystem increases with time, in order to drive the tribosystem to the equilibrium state [40, 44]. Eq (6) characterizes the deterioration of tribosystem based on the heat transfer coefficient U, similar to those reported in [36, 40, 46], which represents the efficiency of the tribosystem (domain) to conduct the frictional heat away from the system
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