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||Materials Science and Engineering|
|Type of Document
||Interfacial Mechanical Properties, Oxidation Properties and Microstructure|
of Titanium-Based Nitride Films Deposited on Steels
|Date of Defense
|| Titanium-based nitride films are deposited on steels by magnetron reactive sputtering for cutting, forming and decorative applications. The nitride films about 2 m thick with interlayer for better adhesion are grown at a working pressure of 0.8 Pa, discharge power of 220 watt, Ar/N2 gas ratio of 0.93, substrate temperature of 300 ℃ and deposition time of 40 minutes. Four different targets (Ti, Ti-6Al, hot-pressed Ti-6Al-4V, commercial Ti-6Al-4V, where Al and V are in wt.%) and two different steels ( stainless steel (SS) and high speed steel (HSS)) as substrate materials are tested. |
The indentation technique is used to evaluate the adhesion properties of the nitride films to the substrate and elasticity analysis of indentation to analyze the distribution of the stresses at the interface. The differences in indentation properties
between nitride/ SS and nitride/ HSS are examined and correlated with the elastic/plastic analysis. It appears the cracks and spallings observed inside the indentation cavity in nitride/SS system are due to the higher poisson’s ratio of stainless steel but those around the indentation cavity observed in nitride/HSS system are due to the edge (binding) effect. Continuous load –penetration depth curve of indentation test shows that TiAlVN/HSS is more fracture–resistant than the pure TiN/HSS.
The addition of aluminium into TiN film decreases the lattice constant while further addition of vanadium into TiAlN films causes the lattice constant to increase. This is discussed in terms of bonding characteristics and atomic size of the elements. SEM analysis shows that columnar structure of TiN films is coarser than that of TiAlN and TiAlVN. TEM studies of microstructure and AFM studies of surface roughness show that the TiAlVN film formed by hot–pressed Ti-6Al-4V on HSS has the densest fibrous structure and smoothest surface, and therefore the best mechanical properties such as hardness and indentation fracture resistance.
(111) and (200) orientations of TiN, TiAlN, and TiAlVN are observed by XRD analysis. The ) phase is found due to small amount of Fe in the films. The oxynitride phase in the nitride films as indicated by the binding energy shifts of the elements may be due to the trace oxygen during deposition. The interlayers of TiAlN/HSS and TiAlVN/HSS have the preferred orientations of and . The texture (columnar) structure of (111) and (200) orientations are observed in TiAlN and TiAlVN films. An orientation relationship of is found between interlayer and tempered martensite in TiAlVN/HSS.
Oxidation test was carried out in oxygen atmosphere at 620-800℃for 3 hours. The TiAlVN film by hot-pressed Ti-6Al-4V on HSS follows the parabolic growth law. It also has the slowest oxidation rate while the TiN/ HSS the fastest one. The activation energy of oxidation of this system is about 184.5 kJ/mole.
The oxidation of TiN/ HSS reveals larger area of spallings, while TiAlN/HSS and TiAlVN/ HSS generally show uniform oxidation with small areas of non-uniform growth. There is a dramatic effect of aluminium in facilitating the uniform oxidation for TiAlN and TiAlVN on HSS. The oxidation products of TiAlN/HSS and TiAlVN/ HSS are oxynitride, amorphous Al2O3, Fe2TiO5 and small amount of TiO2 and Fe2O3.
||Pouyan, Shen - chair|
Edward, Chang - co-chair
Ker-Chang, Hsieh - co-chair
Po-We, Kao - co-chair
Bing-Hwai, Huang - co-chair
Dershin, Gan - advisor
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|Date of Submission