Synthesis, characterisation and thermal behaviour of Cu-based nano-multilayer

Cu/AlN–Al 2 O 3 nano-multilayer (NML) was deposited by magnetron sputtering method on 42CrMo4 steel samples, starting with a 15 nm AlN–Al 2 O 3 layer and followed by 200 alternating layers of 5 nm thick Cu and 5 nm thick AlN–Al 2 O 3 layers. The microstructure and thermal behaviour of the as-deposited and heat-treated 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 micron-sized reservoirs within the NML. Due to increased temperature and to the different heat expansion coefficients of Cu and the AlN–Al 2 O 3 , 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 solid-state 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.