Abstract
TiN coatings were deposited onto mirror polished stainless steel substrates by reactive DC magnetron sputtering using a pure Ti target and Ar+N2 atmosphere. The deposited TiN coatings were thermally treated in ambient air at temperatures ranging from 500 to 700°C for times between 1 and 16 h. The as-deposited and thermally treated coatings were characterized using glow discharge optical emission spectroscopy, x-ray diffraction and scanning electron microscopy. Titanium oxide layers were identified at the surface of thermally treated TiN coatings, which grow according to oxygen diffusion controlled parabolic time law. Phase composition of the oxide layers is found to depend strongly on temperature and exposure time. At low temperatures and shorter exposure times the oxide layers were found to be a mixture of anatase and rutile polymorphs of TiO2, while at high temperatures and longer exposure times the oxide layers consisted only of the rutile polymorph of TiO2. The results show that the microstructure of the oxide layers is porous and non-uniform across the oxide layer thickness. The porous microstructure is explained by the accumulation of nitrogen by short-range diffusion and transition into a gaseous state. Key words: TiN, coating, magnetron sputtering, rutile, thermal treatment, X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES).
Highlights
Transition metal nitrides have become universally accepted coatings owing to their exceptional physical and chemical properties, like high hardness, excellent wear resistance, high melting points and chemical stability (Chen et al, 2012; Fateh et al, 2007; Gong et al, 2012; Huang et al, 2006; Mo and Zhu, 2009; Sundgren, 1985; Vaz et al, 2003)
The depth at which titanium and iron glow discharge optical emission spectroscopy (GD-OES) signals intersect each other was used as a measure of the thickness of as-deposited coatings
Based on the results of GD-OES, X-ray diffraction (XRD), and SEM the following conclusions can be made: i) Thermal treatment in air leads to the formation of a TiO2 oxide layer on top of the underlying titanium nitride (TiN) layer
Summary
Transition metal nitrides have become universally accepted coatings owing to their exceptional physical and chemical properties, like high hardness, excellent wear resistance, high melting points and chemical stability (Chen et al, 2012; Fateh et al, 2007; Gong et al, 2012; Huang et al, 2006; Mo and Zhu, 2009; Sundgren, 1985; Vaz et al, 2003). Among the transition metal mononitrides, titanium nitride (TiN) is the best-known and -studied coating solution. TiN has been widely used as a wear-resistant hard protective coating on cutting and punching tools, drills, dies and injection molds, and so on (Cunha et al, 2002; Omrani et al, 2012; Zhang and Zhu, 1993). As a hard coating material on cutting tools and machinery parts, TiN coatings are frequently subjected to extreme conditions such as high temperatures, corrosive and oxidizing environments, stresses or a combination of these. For TiN coatings subjected to such hostile environments, the understanding of the behavior at elevated temperatures in various oxidizing environments is of increasing importance in applications
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