Quasi-saturation dependent wear resistance of nanocrystalline diamond films under heavy-ion irradiation with large displacement damage

Abstract

Here two groups of experimental studies were performed to investigate the irradiation hardness and tribological performance of nanocrystalline diamond (NCD) films synthesized using hot-filament chemical vapor deposition. Extensive characterizations including the Raman, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy were applied, to explore the structural and property evolution of heavy-ion radiated films with a gradual increase of the displacement damage. Results indicated that films with different original structures possessed the same starting point of a quasi-saturation state at 2.0 dpa, above which films underwent a significant transformation from the sp3 to sp2 bonds within the depth of the projected range, forming an amorphous layer with an identical bonding structure among the different samples. The radiation-induced amorphization resulted in a change in the wear mechanism in dry sliding movements, i.e., from the abrasive wear in the pristine films to the adhesive wear in the radiated films. Furthermore, a formation of graphene nanocrystals was observed in the wear scars of radiated films, which favored the stability of friction coefficient curves. As the displacement damage further increased to 5.0 and 10.0 dpa, the bonding structure of the amorphous layer was well maintained, and in the meantime, the low-friction characteristics of the radiated films were also well maintained, even as the tribotests were continued for 5 × 105 laps and the wear depth had already exceeded the projected range of incident ions. Furthermore, the larger the irradiation damage, the greater the graphitization transformation occurring in the tribofilms, and the lower the film wear rates in an ambient air atmosphere. These results provide insights into what changes in the film nanostructures and sliding interfaces are responsible for the ultrahigh wear resistance of NCD films under nuclear irradiation conditions with large displacement damage.