Performance of various tool coatings in titanium cryogenic machining

Cryogenic machining is a machining approach that uses liquid nitrogen to minimize the rise in cutting temperature caused by plastic deformation of the workpiece. It is widely employed when machining difficult-to-cut alloys, particularly those with low thermal conductivity, which can lead to significant temperature rises. It represents a sustainable alternative to conventional coolants, being non-toxic, environmentally friendly, and capable of enhancing product quality. Despite evidence of improved tool life and surface integrity, no clear guidelines are yet available regarding the most suitable coating type for tools used in this application. This study aims to evaluate, for the first time, the performance of different tool coatings on tool life under cryogenic conditions. Various coatings, based on nitrides, chromides, and their combinations, were deposited on conventional tungsten carbide tools through physical vapour deposition. These coatings were characterized in terms of crystalline phases and mechanical properties, the latter assessed by scratch tests at different temperatures. Finally, cutting trials were performed on a grade 2 titanium workpiece using liquid nitrogen as the coolant to identify the coating that provides the best tool life. Results show that single-phase coatings perform better than multi-phase ones.