Investigation of sub-surface damage during sliding wear of alumina using focused ion-beam milling

In this work contact pressure and sliding distance-induced changes in wear rate and mechanism, and friction behaviour of alumina are investigated by consideration of sub-surface damage. Sub-surface cracking and the formation of tribochemical reaction products are observed through the use of a novel sectioning technique; focused ion-beam milling. Initially wear rate experiments with three different grain sizes and different contact loads were undertaken. These were followed by extensive sub-surface microscopic investigations. Wear experiments, undertaken on a pin-on-disc tribometer over a range of contact pressures, revealed that wear mechanisms changed as contact pressure decreased. Both wear rate and friction were monitored. Debris and surface layers which formed were characterised using X-ray photoelectron spectroscopy (XPS). This identified an oxide/hydroxide layer which formed during wear and which was found to significantly affect friction. The nature of this layer varied significantly with contact pressure and sliding distance. It was found that mild wear of alumina in air involves the continual formation and removal of a surface layer at differing rates.