1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2. School of Material Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 3. School of Equipment Engineering, Shenyang Ligong University, Shenyang 110159, China 4. Shenyang National Key Laboratory for Materials Science, Northeastern University, Shenyang 110819, China
Cite this article:
Shasha YANG,Feng YANG,Minghui CHEN,Yunsong NIU,Shenglong ZHU,Fuhui WANG. Effect of Nitrogen Doping on Microstructure and Wear Resistance of Tantalum Coatings Deposited by Direct Current Magnetron Sputtering. Acta Metall Sin, 2019, 55(3): 308-316.
Tantalum coating attracts increasing attention in heat, corrosion and wear resistant applications today because of its high melting point, immunity to chemical attack and high toughness. Recently, tantalum has been considered a desirable candidate to replace electrodeposited (ED) chromium coating which is often used as protective coating against corrosion and wear. However, the wastes associated with ED chromium contain a well-known carcinogen, i.e. hexavalent chromium, which is a hazard to environment. In comparison, thick Ta coating is regarded as a more environmental and beneficial replacement. Tantalum coating is usually obtained by magnetron sputtering. However, tantalum exhibits two distinct crystalline phases. The body-centered cubic α-phase is the common phase in bulk metal and thermodynamically stable. α-Ta with good ductility and excellent mechanical properties is welcomed in most fields. β-Ta is a metastable phase with tetragonal crystalline lattice structure. The properties of β-Ta are not as advantageous as α-Ta because it is hard and brittle. The existence of β-Ta may compromise tantalum coating in adhesion, corrosion and wear resistance, hence, finding appropriate deposition conditions to obtain pure α-phase Ta coating has attracted a lot of interests. In previous work, pure α-phase Ta coating has been deposited by direct current magnetron sputtering when substrates were located in negative glow space. In this work, nitrogen was mixed in sputtering gases to deposit Ta coating with N interstitially dissolved on stainless steel. Effect of N on microstructure, mechanical and tribological performance of Ta coating was studied. Results indicated that when no nitrogen or very low flux of N2 (l mL/s) were introduced in gas mixtures, α-phase Ta coating with coarse grains grew and revealed strong reflections of (211) and (110) diffraction peaks. When N2 flow rate reached to 5 mL/s, Ta coating with N interstitially dissolved was obtained and revealed grain refinement and (110) preferred orientation of TaN0.1 phase. Compared to α-phase Ta coating, N-doped tantalum coatings displayed excellent wear resistance for their high hardness and H 3/E 2 ratio (H—hardness, E—elastic modulus). The wear mechanism for α-Ta coating was abrasive wear, while that of N-doped Ta coating switched to adhesive wear.
Fund: National Key Research and Development Program of China(2017YFB0306100);National Key Research and Development Program of China (No.2017YFB0306100), National Natural Science Foundation of China(51671053);National Key Research and Development Program of China (No.2017YFB0306100), National Natural Science Foundation of China(51701223);Joint Fund of the Equipment Pre-Research and the Ministry of Education(6141A020332-004);Fundamental Research Funds for the Central Universities(N160205001)
Table 1 Deposition parameters of Ta and N-doped Ta coatings
Fig.1 Surface (a, c, e, g, i) and cross-sectional (b, d, f, h, j) SEM images of as-deposited Ta coatings with different Ar and N2 fluxes (Insets in Figs.1a, c, e, g and i show the enlarged views)(a, b) 10Ar (c, d) 10Ar-1N2 (e, f) 10Ar-5N2 (g, h) 15Ar (i, j) 15Ar-5N2
Fig.2 TEM images of as-deposited 15Ar (a) and 15Ar-5N2 (b) coatings in planar view (Insets show the corresponding SAED patterns of the circle regions)
Fig.3 XRD spectra of as-deposited Ta coatings with different Ar and N2 fluxes
Fig.4 XPS result of the Ta4f photoelectron peak for the as-deposited 15Ar-5N2 coating
Coating
Hardness
GPa
Elastic modulus
GPa
H/E
H3/E2
Wear Rate
10-2 mg·N-1·m-1
10Ar
8.159
212.421
0.0384
0.0120
1.333
10Ar-1N2
12.396
237.570
0.0522
0.0337
0.167
10Ar-5N2
19.851
266.452
0.0745
0.1102
0.133
15Ar
7.885
233.845
0.0337
0.0090
1.483
15Ar-5N2
21.569
294.257
0.0733
0.1159
-0.017
Table 2 Mechanical properties and wear rates of as-deposited Ta coatings
Fig.5 Worn surface SEM images of Ta coatings deposited under different Ar and N2 fluxes(a) 10Ar (b) 10Ar-1N2 (c) 10Ar-5N2 (d) 15Ar (e) 15Ar-5N2
Fig.6 Enlarged worn scar of the rectangular region in Fig.5b (a) and the corresponding EDS analyses of elements O (b), Fe (c) and Ta (d)
Fig.7 Schematics of wear mechanisms for Ta coatings (FN—vertical load, V—sliding velocity)(a) none N-doped Ta coating (b) N-doped Ta coating
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