Dynamic Behavior of Surface-Adsorbed Molecules Under Boundary Lubrication

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Abstract
Friction under boundary lubrication was measured using a pendulum-type friction machine. Long straight-chain fatty acids with even carbon numbers, amine, and alcohol were used as additives, and n-hexadecane was used as the base oil. Effect of the temperature, chain length of additives and additive concentration were investigated. The phenomena obtained under 373 K were as follows: 1) “Transition concentration” wherein a drastic change of friction coefficient appeared were observed. 2) “Transition concentration” of amine or alcohol solutions appeared at higher concentration than that of acid solution. 3) In “high concentration region,” the friction coefficient gradually increased with the decrease of the concentration. 4) Temperature raise caused a high friction coefficient. 5) Friction coefficient gradually decreased with an increase in chain length. 6) “Transition concentration” shifted toward low concentration with an increase in chain length. 7) No chain-matching effect was observed. These results are explained introducing a new concept of adsorption mechanism, where both mechanical disturbance, i.e. friction, and adsorption-desorption process of molecules are taken into consideration.

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