Spontaneous inversion of the submicron ceramic layer deposited on steel and the copper droplet positioned on their top (case of ceramic poorly wetted by liquid Cu)

In this paper, 50 ... 680 nm thick AlN-Al2O3 coatings are deposited by magnetron sputtering on the surface of a steel substrate and a piece of copper is melted on top of the ceramic. Upon heating the ceramic layer is cracked, and the phase inversion of the two top phases from steel/ceramic /copper configuration to the steel/copper/ceramic configuration takes place within 30 s of liquid time of copper. This phase inversion process is accompanied by a Gibbs energy change of about - 1.78 J/m(2), due to good wettability of solid deoxidized steel by liquid copper in contrary to poor wettability of the ceramic by the copper. When copper is melted on AlN-Al2O3 coating with its thicknesses smaller than a critical value of about 170 +/- 60 nm, liquid copper droplets hanging down into the cracks within the ceramic reach the solid steel surface at the bottom of the cracks, thus the flow of Cu down along the cracks is enabled. However, when copper is melted on AlN-Al2O3 with its thickness larger than the critical value of 170 +/- 60 nm, Cu first forms a non-wetting droplet on top of the ceramics, and only after a certain incubation time it starts flowing down the cracks. This incubation time was found to depend linearly on the thickness of the ceramic, as cracks are filled from the bottom upwards by liquid copper via the evaporation- condensation mechanism. By the end of the process, the steel/copper /ceramic configuration is further stabilized by gravity. [GRAPHICS] .

Automated summary

In this paper, AlN-Al2O3 coatings with thicknesses ranging from 50 to 680 nm were deposited on a steel substrate by magnetron sputtering, and the phase inversion process of the steel/ceramic/copper configuration to the steel/copper/ceramic configuration was investigated. The results showed that the wettability of solid deoxidized steel by liquid copper is good, while the wettability of the ceramic by copper is poor. The flow of copper along the cracks in the ceramic depends on the thickness of the ceramic and can be enabled when the coating thickness is below a critical value. The stability of the steel/copper/ceramic configuration is further enhanced by gravity.