Evaluation of Boundary Lubrication Properties of Fatty Acids Using a Lateral Force-Controlled Tribometer

In tribological contacts, fatty acids are widely utilised as organic friction modifiers. However, their impact on the initial regions of boundary lubrication needs to be better understood. This study suggests a new friction test method using a novel Lateral Force-Controlled Parallel Pendulum (LaFCoPP) Tribometer. The tribometer identifies the energy dissipation contribution from tribo-contact and machine components. Using a load cell for friction measurement alters the nature of actual contact by introducing sensor stiffness. In the LaFCoPP tribometer, the friction measurement is performed without using stiffness-altering sensors. Instead, the friction force measurement employs the energy decay curve of the single degree of freedom parallel pendulum. A sphere-on-flat steel surfaces in contact with lubricant containing additives of saturated fatty acids with different numbers of carbons in the alkyl chain is studied. The Stribeck curve and a new qualitative form represent the velocity-dependent friction force. The study also investigated the sliding to fretting transition at the tribo-contact. In all aspects, higher carbon chain fatty acids showed better lubricity and a lower fretting transition amplitude. Tribo-contact operating conditions and the number of carbons in the alkyl chain influenced the lubrication mechanism. The highly underdamped parallel pendulum with non-contacting sensors ensured an in-depth analysis of the boundary lubrication regime.