Diamond deposition on Fe-Cr-Al alloy substrates: Effect of native oxidation by XPS and XAS investigation
Introduction
Superhard, thermally conductive and corrosion resistant diamond coatings deposited on various heterosubstrates by chemical vapor deposition method are expected to significantly expand their individual properties for advanced industrial applications [1], [2]. However, diamond coating on ferrous alloy substrates has been limited by a major technological barrier of low interfacial adhesion. This is primarily due to the preferential formation of a porous graphite intermediate layer on the substrates, as catalized by the base metals Fe, Co or Ni, prior to diamond nucleation [3]. In addition, the mismatch in thermal expansion coefficients between the diamond coatings and the alloy substrates is usually large, which generates huge internal stress and induces coating cracking and spallation. Moreover, diffusion of carbon into the substrate makes diamond surface nucleation more difficult and a longer incubation period is required [4]. To solve such technological issues, surface modification and applying interalyer strategies have been extensively applied [5], [6], [7], [8].
Recently, we have studied diamond deposition directly on various ferrous alloys and the results reveal that the alloy compositions, especially the types of alloying element presented in the substrates have significant effects on the diamond coating performances [9], [10], [11], [12]. No matter direct deposition on alloy substrates or using interlayers, the quality of diamond coatings in terms of continuity and adhesion should depend on the nucleation behavior at the early stage of deposition, which is in turn closely associated with the interaction between the deposition atmosphere and the substrate surfaces. Therefore, the surface chemistry of the substrates plays a key role during diamond CVD deposition. As known, when ferrous alloys are exposed to ambient atmosphere, a spontaneous oxidation occurs on these alloys surfaces [13]. In this study, the surface electronic structures of native oxides formed on Fe-15Cr, Fe-5Al, and Fe-15Cr-5Al alloys are first characterized, and their effects on the following diamond deposition behavior are investigated to gain better understanding on their correlation.
Section snippets
Experimental
Fe-15Cr, Fe-5Al, and Fe-15Cr-5Al (all in wt%) alloys were prepared by melting appropriate amount of high-purity metal mixtures of pure Fe (99.79 wt%), pure Cr (99.86 wt%) and pure Al (99.82 wt%) in a vacuum arc-induction furnace. The as-prepared alloys were annealed at 800 °C for 20 h to stabilize their microstructures then cut by electrical discharge machining into specimens of 10 mm × 10 mm × 1.5 mm, ground with 1000 # SiC sandpaper, ultrasonically cleaned in acetone followed by distilled
Results and discussion
XPS survey spectra of Fe-15Cr and Fe-15Cr-5Al alloys are presented in Fig. 1. As seen, no detectable impurities (except small amount of Na) are recorded on the surface of these alloys, only Fe LMM, Fe 2p, Cr 2p, O 1s, Na KLL, C 1s and Al 2s signals are observed. The comparison of XPS Fe 2p core level spectra of Fe-15Cr and Fe-15Cr-5Al with those of reference samples (FeO, Fe2O3 and Fe metal) shows the main contribution of Fe2O3 [14], [15]. The contribution of Fe-metal signal is very low for
Conclusion
In this study, the electronic structures of native oxides formed on Fe-15Cr, Fe-5Al and Fe-15Cr-5Al are characterized by means of XPS and XAS analysis. A mixed oxide layer containing Fe oxide forms on Fe-15Cr and Fe-5Al substrates, whereas an exclusive Al-rich oxide layer forms on Fe-15Cr-5Al alloy surface. During diamond deposition in CH4-H2 plasma atmosphere, surface graphitization and substrate carburization occur to Fe-15Cr and Fe-5Al substrates, along with deteriorated diamond coating
Acknowledgements
The authors thank the support from Natural Science and Engineering Research Council of Canada (NSERC), Canadian Light Source and SSSC at the University of Saskatchewan. Pan thanks Jiangsu Province Science and Technology Project (BY2016029-07), China Postdoctoral Science Foundation (2016M600346), the China Scholarship Council Six Talent Peaks Project from Jiangsu, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Kurmaev thanks the support by Russian
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