Synthesis, characterisation and thermal behaviour of Cu-based nano-multilayer
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Synthesis, characterisation and thermal behaviour of Cu-based nano-multilayer M. Czaga´ny1,*, D. Varanasi1, A. Sycheva1,2, D. Janovszky1,2, D. Koncz-Horva´th1, F. Kristaly3, P. Baumli1, and G. Kaptay1,2 1
Institute of Physical Metallurgy, Metal Forming and Nanotechnology, University of Miskolc, Miskolc-Egyetemvaros 3515, Hungary MTA-ME Materials Science Research Group, Miskolc-Egyetemvaros 3515, Hungary 3 Institute of Mineralogy and Geology, University of Miskolc, Miskolc-Egyetemvaros, Hungary 2
Received: 3 September 2020
ABSTRACT
Accepted: 31 October 2020
Cu/AlN–Al2O3 nano-multilayer (NML) was deposited by magnetron sputtering method on 42CrMo4 steel samples, starting with a 15 nm AlN–Al2O3 layer and followed by 200 alternating layers of 5 nm thick Cu and 5 nm thick AlN–Al2O3 layers. The microstructure and thermal behaviour of the as-deposited and heattreated multilayer was studied. Starting from about 400 °C, extensive coarsening of Cu nanocrystallites and the migration of Cu within the multilayer were observed via solid-state diffusion. Part of the initial Cu even formed micronsized reservoirs within the NML. Due to increased temperature and to the different heat expansion coefficients of Cu and the AlN–Al2O3, the latter cracked and Cu appeared on the top surface of the NML at around 250 °C. Below 900 °C, the transport of Cu to the top surface of the NML probably took place as a solidstate flow, leading to faceted copper micro-crystals. However, above 900 °C, the Cu micro-crystals found on the top of the NML have rounded shape, so they were probably formed by pre-melting of nano-layered Cu due to its high specific surface area in the NML. Even if the Cu crystals appear on the top surface of the NML via solid-state flow without pre-melting, the Cu crystals on the top surface of the NML can be potentially used in joining applications at and above 250 °C.
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The Author(s) 2020
Introduction One of the difficulties of today’s modern joining technology is the brazing/joining of heat-sensitive materials [1–6]. Metallic joints with high mechanical Handling Editor: Shen Dillon.
Address correspondence to E-mail: [email protected]
https://doi.org/10.1007/s10853-020-05522-5
strength require high cohesion energy between the atoms of the metallic filler and of the materials to be joined, while the cohesion energy increases monotonously with the melting point of the filler [7, 8]. The most commonly used brazing materials are Cu (Tm: 1085 °C) [9], Ag (Tm: 962 °C) [10] and the eutectic
J Mater Sci
Ag60-Cu40 alloy (Tm: 779 °C) [11], which all require relatively high joining temperatures. In the case of brazing technology, one way to reduce the joining temperature is to reduce the melting point of the braze filler itself, which is conventionally achieved by alloying (eg. with the addition of In, Zn, Sn, P, B). However, alloying may have undesirable disadvantages, such as toxicity (Cd), higher costs (In), and reduced mechanical or corrosion resistance [12, 13]. In recent years, a new approach, the application of n
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