Modifications to the mechanical properties of ceramic surfaces by ion implantation have been investigated as a means of producing increased wear resistance for tribological applications. It has been established that ion implantation can result in a number of tribologically relevant parameters (e.g. hardness and indentation fracture toughness) being modified in various ways. Examples of these changes will be given for ion implanted sapphire. Further, these modifications have been related to critical structural changes occurring in or near the surface. Radiation damage, amorphisation and surface stress generation are the most important of these and affect both hardness and indentation fracture behaviour. This paper will describe how the observed mechanical properties are related to the structural changes occurring in the surface and presents a number of criteria designed to enable prediction of the mechanical property modifications that will ensure after implantation of a given substrate with a given ion. These include the application of a bond-type criterion to determine the amorphisation dose (amorphisation critically affecting mechanical properties) and an elasticity criterion to determine the maximum likely implantation induced surface stress.