Materials with large fractions of interfaces (e.g. nanocrystalline materials and multilayered nanofilms) have attracted attention because of their significantly enhanced ability to tolerate severe irradiation environments. In this paper, the microstructure and nanohardness evolutions of Ti-Si-N nano-amorphous composite coatings with grain sizes from 5 nm to 12 nm were investigated using 50 kev He+ ion bombardment at room temperature. All samples contained amorphous layers that caused decreases in hardness. The amorphous layer thickness and hardness decrement first decreased with the grain size and then increased. Results suggested that the nano/amorphous interfaces in Ti-Si-N coatings can effectively promote defect annihilation during irradiation. However, radiation tolerance was optimized in materials with 9 nm grains due to the competition between the increase in free energy and the energy stored in defects (E-st(o)). In addition, this paper proposed a mechanism that explained the associated decrease in hardness. These results provided the theoretical underpinnings for an understanding of nano-composite material radiation behavior under irradiation.