The structural transitions and resultant changes in the tribological properties of HT/HP-sintered polycrystalline diamond compacts (PDCs) irradiated by 3.25 MeV Xe15+ ions with ion fluence of 4.3 × 1014−4.3 × 1015 ions/cm2 have been examined using various characterization techniques. A significant destruction of sp3 bonds was observed when the damage increased to 0.5 dpa. Thereafter, with a further increase in the irradiation damage, the samples stepped into a quasi-saturation amorphous state at a threshold value of damage, critically depending on the size of diamond grains. Thereafter, no other difference could be found between the samples composed of diamond grains in different sizes, especially in terms of the chemical structure and lubrication performance in an ambient air atmosphere. Thanks to the irradiation-induced transitions of the morphology and chemical structures, the anti-wear property of the PDCs degraded significantly in a damage range of 0–3.5 dpa, and then kept stable in a quasi-saturation state. But overall, owing to the formation of lubricious tribofilms with a rich presence of multilayer graphene on sliding interfaces, the wear rates of all the quasi-saturation samples were below the value of 3.0 × 10−7 mm3/N‧m, which suggests that the PDCs can be applied as one of the potential materials for solid lubrication in nuclear applications.