A study of the antiwear action mechanism of alkoxy aluminium in lubricating oil

Abstract

Aluminium is situated beneath boron in the same group of the periodic table. As borate is a good antiwear agent for lubricating oils, it is possible that aluminium compounds maybe possess antiwear properties similar to borate. After testing seven alkoxy aluminium compounds, the results support this assumption. The most efficient compound is triethoxy aluminium. When 2% mass of triethoxy aluminium is added to ISO VG 68 oil, the load wear index of a four-ball machine increases from 219 N to 519 N and the weld point from 1568 N to 4900 N. This compound also exhibits good corrosion protection property. In the present work, after steel balls of a four-ball machine were run in aluminium-containing oil, the wear scars were analysed by scanning electron microscope and X-ray energy dispersion analysis. The main compositions discovered on the scar surface were 5.7–48.3 atom % of iron, 4.4–11.2 atom % of aluminium, 37.1–59.1 atom % of oxygen, 8.7–23.5 atom % of carbon, and 0.3–1.1 atom % of chromium, under 3087 N–39 200 N of axial loads. When the loads are lower than 2450 N, the contents of oxygen are almost equal to zero and the contents of iron increase to 68.6–93.8 atom %. Based on the results of surface analysis and using thermodynamic principles, an antiwear action mechanism for triethoxy aluminium is suggested: under low loads, the weaker bond C-C (350 KJ /mol) is broken, and the broken fragments may form polymer film during friction processes. As the loads increase, the stronger bond Al-O (484.5 KJ /mol) is broken to create active aluminium and oxygen. The oxygen reacts with aluminium and iron to form a complicated solid solution containing AlxOY, FExOy on the surface. This complicated surface layer raises the weld point of the steel balls.