Improvement of anti-oxidation capability and tribological property of arc ion plated Ag film by alloying with Cu
Highlights
► Ag–Cu alloy film was deposited by arc ion plating (AIP) and show considerable resistance to atomic oxygen (AO) irradiation. ► The affected thickness of Ag–Cu alloy film by atomic oxygen was greatly reduced and the oxidation product was mainly Ag2O, but AgO and Ag2O in case of pure Ag film. ► As a result, the Ag–Cu alloy film exhibited a stable friction and low wear after the AO irradiation compared with pure Ag film.
Introduction
Ag films have been widely used as solid lubricant for precision components in space technology because of the reliable tribological properties in vacuum [1]. Furthermore, they have also been used for electrically conductive material, reflective medium and sensor material in space technology due to its high electrical/thermal conductivity and high reflectivity [2], [3], [4]. For some applications, they must withstand exposure to some extreme operation conditions in low earth orbit (LEO) space environment, especially the bombardment of atomic oxygen (AO) [5], [6], [7], [8]. Many AO irradiation experiments about Ag film [9], [10], [11], [12], [13], [14] showed that the Ag film could be easily oxidized in AO environment and the flaking and cracking of oxide layer can lead to deterioration of film structure and properties. Therefore, improving oxidation resistance of Ag films is important for its application in AO environment.
Previous studies revealed that film microstructure has significant effect on its oxidation resistance [10] and tribological properties [15]. Generally, a dense microstructure would have better oxidation and wear resistance. Based on the structure zone model [16], [17], [18], [19], the microstructure of single-phase film may be densified by adding another component, which can inhibit grain expansion and hence results in a dense and fine microstructure. These results suggest that Ag alloy films could have better AO resistant and so better tribological behaviors than pure Ag film. However, the oxidation resistance and tribological properties of the Ag alloy films in the AO environment have not been studied very much in detail. Cu element is reasonable for alloying with Ag due to the matched fcc structure, anti-oxidation capability and better electrical/thermal conductivity. The Ag–Cu alloy films have been successfully prepared by vacuum vapor deposition [20], [21]. Early research showed that metal films deposited by AIP have better tribological performance than sputtering due to the enhanced ionization and the improved film-substrate adhesion [22]. Therefore Ag–10 wt.% Cu alloy film was deposited by arc ion plated in present study and the effects of the AO irradiation on its structure, morphology, chemical composition and tribological properties were investigated and compared with the Ag film.
Section snippets
Film deposition
Ag and Ag–Cu alloy films were deposited by AIP shown in Fig. 1. Pure Ag and Ag–10 wt.% Cu alloy targets with 80 mm in diameter were used for the deposition of Ag and Ag–Cu alloy films, respectively. Si (1 0 0) wafer substrates were used for analysis of structure, composition and morphology. AISI 440C steel substrates with a diameter of 45 mm and a thickness of 5 mm were also selected for the analysis of tribological properties. The steel substrates were surface-polished with abrasive paper to Ra ≤ 0.03
Results and discussion
Fig. 2 shows XRD spectra of the Ag and Ag–Cu films before and after the AO irradiation. It can be seen that nonirradiated Ag film showed a polycrystalline fcc structure and the AO irradiation led to a severe oxidation. Oxidation products of the Ag film were mainly AgO, plus minor discernible Ag2O (shoulder aside the peak of AgO at 2θ = 32.2°) [24]. In the XRD spectrum of nonirradiated Ag–Cu film, all the peaks showed small shifts in 2θ with respect to the ones obtained from the pure Ag film, and
Conclusion
The Ag–Cu alloy film deposited by AIP showed a dense microstructure as compared with the Ag film and exhibited a much better oxidation resistance in the AO environment as a result. Due to the improved film microstructure and AO resistant behavior, the irradiated Ag–Cu alloy film also represented a stable friction and low wear. That indicated the Ag–Cu alloy film was a more feasible solid lubricant in space environment than the Ag film. At room temperature, Ag2O was the oxidation product of the
Acknowledgement
The authors gratefully acknowledge the 973 Project of China (no. 2007CB607601) for financial support of this work.
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