It was previously shown that SiC-based films deposited at 870 K on a 4135 steel substrate (SS) had promising properties : H approximate to 30 GPa, E = 270 GPa, evaluated adhesion : 30-40 J m(-2). Nevertheless, the remaining challenge is to increase the layer/substrate mechanical stability. With this view, a TiN layer was introduced between the substrate and the SiC-based films. TiN was prepared from a TICl4-H-2-N-2 mixture by radio frequency (rf) PACVD in the cold wall reactor used for the deposition of the SiC-based films. The interfaces were characterized by various spectroscopies and mechanical behavior of the films was determined by nanoindentation and scratch test measurements. With the conditions used to deposit TiN onto the SS, an interfacial zone was detected by SIMS and Auger. In the case of SiC/TiN/SS stacks, the debonding which is activated by the scratch test depends strongly on the layer thickness : with thin TiN, it occurs at the TiN/SS interface, whereas with thicker layers, failure arises at the SiC/TiN interface. The results are discussed in relation to calculated stresses (shear and tensile stresses at the interface) and chemical interactions. A combination of SIC (4 mu m)/TiN (1 mu m)/SS and/or a nitrogen treatment just before the SiC deposition promotes the best mechanical stability whereas double stage deposits or a too large thickness has a bad effect on the mechanical behavior of the bilayers.