Investigation of sliding wear surfaces in alumina using transmission electron microscopy

In this study, the subsurface microstructure of alumina wear surfaces and the microstructure of agglomerated debris generated from unlubricated sliding wear at room temperature have been investigated through transmission electron microscopy (TEM). Specimens were thinned through the use of a focused ion beam miller (FIB). TEM studies, including analysis of electron diffraction patterns from the agglomerated region of the specimen, revealed the presence of an aggregate of nano crystalline particles embedded in an amorphous phase, together with some larger alumina particles. These larger alumina particles appear at the base of pits in the alumina surface, whereas the finer material appears at the contact surface. The agglomerated debris was readily distinguished from the alumina substrate, which contained localised dislocation damage and microcracking. It is proposed that the wear process involves the removal of 'large' alumina particles from the surface by a combination of trans- and intergranular microcracking. These particles are then ground into very fine, nanometer-sized particles that react on the surface with moisture in the air to form an amorphous hydroxide film. These are then compacted to form a nanocrystalline structure within an amorphous matrix that may also be viewed as a grain boundary phase.