Thermal effects on the microstructure and mechanical properties of ion implanted ceramics

The effects of varying the substrate temperature on the implantation-induced structures and surface mechanical properties of single crystal sapphire and MgO have been investigated for a range of 300 keV implanted ions. As the implantation temperature is lowered, the dose at which amorphization occurs is reduced and thus, for the same doses, more amorphous material is produced at lower temperatures. Quantitative modelling shows that the activation energy for annealing of the amorphous material during implantation is very much lower than might be expected for post-implantation thermal annealing of the same material. Also, as the implantation temperature increases there is a small amount of damage annealing in the damaged-but-crystalline material.Both the microhardness and implantation-induced stresses depend critically on the presence of amorphous material since this is relatively soft and can support only small stresses. However, while the hardness behaviour in the damaged-but-crystalline material is dominated by radiation hardening, the substitutionality, ionic misfit and charge state of the implanted ions have also been found to contribute to the further solid solution component of the hardening produced by ion implantation. These effects are also observed to be temperature dependent. Crazing of the implanted layers has also been reappraised. It has been established that the formation and configuration of crazes is a sensitive function of implantation temperature, and it is now proposed that crazes form in response to the stresses generated as a result of the thermal expansion mismatch between the amorphous layer and the substrate.