Abstract
Abstract The development of hard, multi-layer coatings is an effective strategy to enhance the wear resistance of cutting tools and so extend their service life. In the present study, a sandwich structured TiN/g-TiSiN/TiSiN film (where a graded (g-) TiSiN layer with an increasing Si content from 0 to 10 at% was inserted as a transitional layer between the TiN layer and the TiSiN layer with a fixed silicon content of 10 at%) was prepared on to a M42 tool steel substrate. Its mechanical properties were compared to both a dual-layered TiN/g-TiSiN film and a monolithic TiN film. Nanoindentation testing, assisted by focused-ion-beam (FIB) microscopy, was employed to evaluate contact-induced deformation and the mode of fracture of these films. Indented regions created on samples by a 5 μm radius indenter were examined by transmission electron microscopy (TEM). Finite element analysis was used to model the stress distributions within these films and predict the regions where crack initiation and growth may occur. The deformation of the monolithic TiN film was found to be predominantly accommodated by shear sliding along columnar grain boundaries, leading to a lower resistance to deformation. For the bilayer TiN/g-TiSiN film, the g-TiSiN layer hindered the propagation of columnar cracks, however, this bilayer film exhibited a stress concentration together with radial cracks at the bottom of the film. Compared with the former two films, the sandwich-structured film that contained the graded TiSiN interlayer exhibited the highest resistance to contact damage. This is because the graded TiSiN interlayer altered the stress distribution in the film and lowered the overall stress concentration level.
Highlights
Owing to its high hardness, good adhesion, chemical stability and low friction coefficient, titanium nitride (TiN) is an attractive candidate material to prolong the service lifetime and enhance the performance of various cutting tools [1,2,3,4,5]
High-speed cutting is necessary for improved processing efficiencies and machining cost reduction, but the resultant high temperature generated at the contact area between cutting tool and workpiece and the intense friction that results can readily lead to failure of the TiN film, due to its poor oxidation resistance and insufficient hardness [6,7,8,9]
The selected area electron diffraction (SAED) patterns inset in each micrograph were obtained from the TiN layer in the TiN film (a), the TiSiN gradient layer in the TiN/g-TiSiN film (b) and the TiSiN out layer in the TiN/g-TiSiN/ TiSiN film (c), respectively
Summary
Owing to its high hardness, good adhesion, chemical stability and low friction coefficient, titanium nitride (TiN) is an attractive candidate material to prolong the service lifetime and enhance the performance of various cutting tools [1,2,3,4,5]. High-speed cutting is necessary for improved processing efficiencies and machining cost reduction, but the resultant high temperature generated at the contact area between cutting tool and workpiece and the intense friction that results can readily lead to failure of the TiN film, due to its poor oxidation resistance and insufficient hardness [6,7,8,9]. The incorporation of Si, forming a nanocomposite (nc) structure consisting of nc-TiN embedded in amorphous a-Si3N4 matrix [12,13,14], leads to significant increases in both hardness and oxidation resistance. The addition of Si is beneficial to the oxidation resistance of TiN, because the SiO2 formed on the surfaces of the material has a lower oxygen diffusion rate than TiO2 [18,19]
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