Spontaneous escape behavior of silver from graphite-like carbon coating and its inhibition mechanism

doi:10.1016/j.jmst.2017.07.024

Abstract

Abstract:

A series of silver-doped graphite-like carbon coatings was prepared on the surface of aluminum alloy using the magnetron sputtering method. The spontaneous escape behavior and inhibition mechanism of silver from graphite-like carbon coating were studied. The results showed that when the sample prepared with a 0.01-A current on the silver target was placed in an atmospheric environment for 0.5 h, an apparent silver escape phenomenon could be observed. However, the silver escape phenomenon was not observed for samples prepared with a 0.05-A current on the silver target if the sample was retained in a 10^-1 Pa vacuum environment, even after 48 h. Compared with the sample placed in the atmospheric environment immediately after an ion plating process, the silver escape time lagged for 6 h. Nanometer-thick pure carbon coating coverage could effectively suppress silver escape. When the coating thickness reached 700 nm, permanent retention of silver could be achieved in the silver-doped graphite-like carbon coating. As the silver residue content in the graphite-like carbon coating increased from 2.27 at.% to 5.35 at.%, the interfacial contact resistance of the coating decreased from 51 mΩ cm² to 6 mΩ cm².

Key words: Silver, Graphite-like carbon coating, Spontaneous escape mechanism, Inhibition mechanism, Interfacial contact resistance

Shao Wenting, Zhang Xinyu, Jiang Bailing, Liu Cancan, Li Hongtao. Spontaneous escape behavior of silver from graphite-like carbon coating and its inhibition mechanism[J]. J. Mater. Sci. Technol., 2017, 33(11): 1402-1408.

Figures/Tables 10

Fig. 1. Schematic diagram of a four-target closed-field magnetron sputtering ion plating system.

Table 1 Deposition parameters of silver-doped graphite-like carbon coatings.

Sample group	Coating structure (from substrate to surface, except Cr layer)	Target current of Ag (A)	Deposition time (min)	Retention time (h) and vacuum conditions (Pa) in the vacuum chamber
(1)	C-Ag	0.01, 0.02, 0.03, 0.04, 0.05	180	0
(2)	C-Ag	0.05	180	0, 12, 48/10^-1
(3)	Pure carbon layer	0	60	0
	C-Ag	0.05	60
	Carbon barrier layer	0	0, 30, 120

Fig. 2. After 180-h and 5400-h exposure in an atmospheric environment, surface and cross-sectional morphologies of silver-doped graphite-like carbon coatings obtained under (a, d) a 0.01-A current on the silver target; (b, e) a 0.03-A current on the silver target; and (c, f) a 0.05-A current on the silver target. The upper right inlet of image (c) is the black macro photo for the original appearance of the coating when the sample had just been taken out from the vacuum chamber. The lower right inlet of image (c) is the original appearance of the silver layer after the sample was placed in an atmospheric environment for 0.5 h.

Fig. 3. Relationship between the current on the silver target and silver content escaping from a silver-doped graphite-like carbon coating: A: samples exposed to an atmospheric environment for 180 h; B: samples exposed to an atmospheric environment for 5,400 h.

Fig. 4. Relationship between the current on the silver target and silver residue in a silver-doped graphite-like carbon coating: A: samples exposed to an atmospheric environment for 180 h; B: samples exposed to an atmospheric environment for 5,400 h.

Fig. 5. Effect of the retention time in a 10-1-Pa vacuum environment on the delay of silver escape.

Fig. 6. Surface and cross-sectional morphologies of silver-doped graphite-like carbon coating after 180-h and 2000-h atmospheric environment exposure: (a, d, g) surface carbon coating layer thickness of 0 nm; (b, e, h) Surface carbon coating layer thickness of 200 nm; (c, f, i) surface carbon coating layer thickness of 700 nm.

Fig. 7. Relationship between the silver residue content in the graphite-like coating and surface carbon coating layer thickness, along with the relationship between the silver residue content in the graphite-like carbon coating and the conductivity of the coating.

Fig. 8. Schematic diagram of silver spontaneous escape process from the graphite-like carbon coating: (a) the cross-sectional structure of the carbon cluster interface during the deposition of the silver-doped graphite-like carbon coating and the silver radicals that segregated at the interface of the carbon cluster; (b) when the sample is removed from of the 10-1-Pa vacuum chamber and placed in a 105-Pa atmospheric environment, the gas molecules in the atmosphere rapidly flood in and fill the carbon cluster interface of the coating under a 106-Pa pressure difference; (c) the isolated silver clusters that segregated at the carbon cluster interface will escape under the influx of gas; (d) the metallic layer is formed by silver atoms spreading along the surface drove under reduced surface.

Fig. 9. Effect of silver residue in a graphite-like carbon coating on interfacial contact resistance.

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