The self-repairing of ion irradiation damaged Mo-S-Ti (MoST) lubricant films by thermal annealing

Mo-S-Ti composite films deposited by magnetron sputtering were irradiated by 2 MeV Au²⁺ with a high fluence of 3.3 × 10¹⁵ ion cm^-2. Upon intense bombardment by 2 MeV Au²⁺, the Mo-S-Ti thin film exhibits sensitivity to radiation-induced amorphization at room temperature. Afterwards, thermal annealing experiments on the irradiated thin films were conducted over the temperature range of 200 °C-900 °C. Subsequent annealing experiments found that the effect of heavy ion irradiation on Mo-S-Ti films was a reversible reaction, with the irradiation-damaged MoS₂ lattice being completely self-repaired by thermal annealing. In the annealing temperature range of 200 °C-600 °C, no evidence of an amorphous to recrystallized transition was observed. When the temperature was increased to as high as 700 °C, the damaged MoS₂ molecules were well recrystallized. HRTEM images of irradiated samples annealed at 700 °C strikingly found that the self-repaired MoS₂ crystallites preserving long range ordering were still rearranged in an intrinsic (0 0 2) crystal orientation. MoS₂ nanocrystals that survived the ion bombardment step may provide some nucleus for subsequent MoS₂ grains growing dominantly in the (0 0 2) direction in thermal annealing. Compared with the difficult recrystallization in thermal annealing, the irradiation-amorphized MoS₂ phases were easily self-repaired under friction. The low transition potential barrier from irradiation-induced amorphization to crystallization, achieved by mechanical working, may be an important reason for the quick recrystallization.