ON THE OPTIMIZATION OF THE MICROSTRUCTURE AND THE ADHERENCE OF TiC/TiN COATINGS DEPOSITED AT MODERATE TEMPERATURE FOR MILLING APPLICATIONS

Titanium carbide and nitride coatings can be deposited at a moderate temperature of about 850°C, respectively from butane and nitrogen and a mixture of titanium chlorides containing high concentrations of the subchlorides which are generated in-situ by the prereduction of TiCl4 by Ti metal. Under optimized deposition conditions, a very hard titanium carbide with a polynucleated fine grained structure is obtained which is maintained inside the entire layer. However all conditions which lead to high microhardness do not allow the preservation of a good adherence with the cemented carbide substrate. The process was therefore optimized from a morphological, structural, and solid composition study and also from the point of view of microhardness and scratch test measurements. Titanium nitride deposition was also optimized from a composition, morphological and microhardness study but also in terms of surface roughness and friction coefficient. The value of this process which allows one both to obtain an improved bilayer coating and to maintain a high toughness of the substrate is demonstrated when compared to the performance of a classical TiC/TiN coating deposited at about 1000°C using the same severe conditions of a milling test