Excimer Laser Annealing of Quasicrystalline Coatings
A. Sadhu Kumar and P.A. Molian
Excimer laser annealing (ELA) is well known to cause the transformation of amorphous to crystalline structures in semiconductor materials such as silicon and germanium. However, there exists a disagreement on whether this phase transition involves the formation of a liquid phase or takes place exclusively in the solid-state. In this work, ELA of amorphous 65Al-23 Cu-12Fe quasi-crystalline (QC) coatings was performed to elucidate the mechanism of the phase transition. Magnetron sputtering was used to deposit 10 to 12 µm thick amorphous QC coatings on steel, aluminum and titanium substrates followed by ELA to convert them into crystalline phases. X-ray diffraction data of ELA samples was used to determine the threshold energy fluence for phase transformation. A two-dimensional, finite-element thermal model was applied to estimate the temperature distributions and melt depth as a function of energy fluence. The effect of ELA on the surface roughness was measured and also used to identify the phase change. Results suggest that ELA caused melting of about a 3 µm layer of the coating at the threshold fluence and more at higher energy fluences. The surface roughness (arithmetic average) was increased from an average of 0.06 µm in as magnetron-sputtered to 0.4 µm in laser-crystallized coatings. It is concluded that the presence of thin molten layer is a necessary and sufficient condition for the phase transition to occur and that the formation of a molten layer roughens the laser-modified surface, a chief drawback of ELA.