Effect of Substrate Treatment for Fracture Behavior of TiN Film during Three-Point Bending Test

Three-point bending tests have been carried out in order to observe the fractures behavior of coated TiN films and to evaluate adhesion strengths at interfaces between the films and substrates. The substrates were hot work tool steel, SKD61, with and without nitriding treatment, which were coated with TiN films by physical vapor deposition (PVD). Two types of specimens were, therefore prepared, one of which consisted of TiN surface layer and the substrate of SKD61 (TiN/SKD61), while in another one nitride layer was formed between them (TiN/Nitride layer/SKD61). Scratch tests were also carried out for comparison. Specimens with 20L mm×6W mm×0.5t mm were cut from the coated specimens including the coated surface as L-W planes for using in bending tests. The specimen was set on the small bending device which had been developed by the authors, and then bent with referring to strain obtained from a strain gauge glued on the specimen. The specimen was set into SEM for observation. The L-W and L-t surfaces of the specimen were observed. Bending deformation and observation on the same areas were intermittently carried out until the substrate was fractured. On the specimen without the diffusion layer, cracks initiated at the surface of the TiN film and penetrated into the film in the perpendicular direction to the interface. The cracks arrested at the interface. Delamination of the TiN film from the substrate did not occur at the interface, but cracks initiated in the interior of the TiN film and propagated parallel to the interface, which caused fragmentation of the TiN film. Adhesion strength between the TiN film and the substrate was higher than the fracture stress of the TiN film.