A key to tune the grain size gradient of the TiC coating on titanium by interstitial carburization: The timing for pressing

Herein, we fabricated TiC coatings on titanium via the interstitial carburization technique. By hot-pressing the titanium substrate with cast iron at 1100 °C, the interstitial carbon atoms in the cast iron diffused into titanium, forming a TiC coating. The timing of starting pressing affected the spatial distribution of TiC grain size, namely starting pressing at 1100 °C generated coatings with a clear gradient microstructure, while starting pressing at room temperature generated coatings with a relatively homogeneous microstructure. The TiC grain size in the gradient coating increased from 0.3 μm to 1.1 μm with increasing distance from the surface, giving rise to a nanohardness gradient varying from 25 GPa to 32 GPa. The relatively homogeneous microstructure resulted in a constant nanohardness of 32 GPa in the coating. Both coatings exhibited an extremely high micro-hardness of 2100–2300 HV and strong adhesion with the substrate. These findings provide a route for controlling the spatial distribution of the carbide grain size by designing the time evolution of temperature and pressure. These results should be applicable to more substrate materials and may also be valuable for other hot-pressing based technologies.