Evaluation of Temperature-Dependent Boundary Lubrication Behavior of Stearic Acid Using a Force-Controlled Tribometer

A quick evaluation of the effectiveness of additives is important in lubricant formulation. In this study, we employed a friction-force measurement approach using a newly designed lateral force-controlled tribometer. This tribometer evaluates the lubricant properties under boundary lubrication. In this lateral force-controlled tribometer, the absence of a stiffness-altering sensor enables the modeling of the actual contact conditions without altering the contact stiffness. Indirect friction force measurement ensures precise measurements of friction properties while avoiding common measurement errors encountered in conventional tribometers, such as sensor misalignment and changes in the stiffness of the machine due to the sensor. The tribometer designed and built consists of a pendulum that measures the rate of dissipation of the oscillation amplitude as a function of time. The unique characteristics of the machine are the possibility of changing the energy input into the tribosystem without altering the tribo-contact conditions and the capability to do experiments at higher temperatures. To evaluate the capabilities of this tribometer, the impact of temperature on the frictional properties of a base oil and a blend of base oil and stearic acid (SA) (a prominent Organic Friction Modifier) is investigated. The test result shows that frictional energy dissipation decreases when stearic acid is present in the lubricant. And, as the temperature of the oil increases, the energy dissipation increases for pure base oil but reduces for the blend. The observed frictional trends are attributed to the decrease in the viscosity of the base oil with an increase in the temperature. The decrease in friction for the SA blend is attributed to tribofilm formation. Fourier transform infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analyses confirm the presence of the SA tribofilm on the surface. XPS indicates an increase in tribofilm quantity with rising temperatures. The kinetics of film formation and thickness increase with temperature, consequently reducing the friction in the SA blend.