Properties of polyol esters–-lubrication of an aluminum silicon alloy

The chemical and physical properties of lubricants significantly affect their performance in industrial applications where aluminum is used as a sliding material. The inertness of aluminum contributes to its poor wear resistance. The wear characteristics of a series of polyol esters were evaluated in terms of their chemical structure for the lubrication of an aluminum-silicon (Al-Si) alloy using a pin-on-disc-type wear tester. The molecular structure and the number of acid groups of the polyol esters strongly affected their ability to lubricate the Al-Si alloy. The wear and scuffing performances of the polyol esters with the longer linear-chain acids were better than those of the esters with shorter branched acid chains. For esters of the same number of carbon atoms, neopentylglycol-type esters had higher scuffing loads than trimethylolpropane esters which in turn show better antiscuffing properties than the pentaerythritol esters. The effect of the polarity of the molecules on the friction and wear properties of the polyol esters is discussed using a relative polarity index. Less polar polyol esters were found to have better wear and scuffing protection behavior in lubricating the Al-Si alloy. Oxidation tests in the Penn State Microreactor with Al-Si catalyst specimens showed that the polyol esters were depleted mainly by evaporation losses under an air atmosphere. A consistent trend was observed between the volatility of the polyol esters and their lubrication properties for the Al-Si alloy.