Chemomechanical effects in ion-implanted MgO

Abstract

Experiments have been undertaken to explore the effects of ion implantation on the low-load hardness (Knoop), friction (with a steel ball counterface) and absorbed water content (as measured by infrared spectroscopy) of single-crystal MgO surfaces. While the hardness behaviour shows the usual hardening and softening associated with implantation-induced structural changes leading to radiation hardening and eventual surface amorphisation, a large increase in friction at low implantation doses of Ti⁺ has been found to be associated with the removal of both chemisorbed and physisorbed water. Further, water readsorption has been found not to recur even several years after implantation leading to the postulation that implantation-induced neutral charge defect clusters are responsible for the change. This effect seems most marked for Ti⁺ ions which are known to both substitute for magnesium ions in MgO and to segregate to the surface of MgO samples. The experimental observations are reported and the implications for both the tribology of ion-implanted ceramics and the authors' understanding of water absorption on MgO are discussed.