Tribochemical mechanism of alumina with water

Abstract

Water has been found to exhibit significant effects on the tribological behavior of alumina. A film-like substance was found on the surfaces of water lubricated alumina wear surfaces, suggesting the possibility of tribochemical reaction between water and alumina in the contact junction. This paper describes an investigation of the alumina/water trib- osystem to determine the chemical interaction between these two materials under rubbing conditions. A combination of x-ray powder diffraction and thermogravimetric analysis (TGA) has been used to investigate the kinetics of aluminalwater reactions. These experiments have determined that transition (gamma) alumina reacts with water to form hydroxides of aluminum. At high temperature (:ss200°C) aluminum oxide hydroxide (boehmite—AIO(OH)) is formed, while at lower temperature (-100°C) the formation of aluminum trihydroxide (bayerite—Al(OH)s) is favored. Aluminum trihydroxide (bayerite) was found in the wear debris from water lubricated wear tests. A mechanism for lubrication of alumina with water is proposed whereby stresses and temperatures in the contact junction cause phase transformation from alpha alumina to a transition alumina. The transition alumina subsequently reacts with water to form a lubricous hydroxide layer and reduce friction and wear. © 1989 Taylor & Francis Group, LLC.