Mechanism of room-temperature nitriding in pure titanium by the scanning cyclic press

Nitriding enhances the wear resistance of titanium by forming nitrogen compounds such as TiN and Ti 2 N on its surface layer. However, conventional nitriding requires high temperatures to facilitate the diffusion of nitrogen into the substrate. This study aims to form titanium nitride at room temperature using a novel surface modification technique called scanning cyclic press (SCP). Using SCP, titanium surfaces were scanned under a nitrogen atmosphere using a vibrating indenter that applied cyclic compressive loads. After SCP, we conducted surface observations, nanoindentation testing, electron probe microanalyzer elemental analyses, and transmission electron microscopy (TEM) crystal structure analyses to characterize the modified layer. Surface observations revealed the characteristic golden color of TiN in the modified areas. TEM analyses showed that a nitride layer consisting of aggregated TiN particles was successfully formed on the Ti specimen surfaces, at room temperature. The TiN particles were composed of TiN crystallites with grain sizes of a few nanometers, and a portion of them exhibited partial amorphization. Based on the morphological features and nitrogen diffusion analysis, we determined that this layer is a type of mechanically mixed layer formed by mechanical driving forces rather than thermal diffusion. The formation of the nitride layer is believed to proceed in the following sequence: the exposure of fresh titanium surfaces by vibrational compressive loading from the indenter, creation of TiN clusters via nitrogen chemisorption on the exposed surfaces, ultrafine grain refinement caused by excessive dislocation accumulation from the pinning effect of the TiN clusters, and repeated deposition of TiN particles.