94 10.3 Result and Discussion In laser spallation, a YAG laser pulse is incident on a thin aluminum layer which vaporizes and induces a large localized temperature increase due to the dissipation within the shock front [19]. In practice, spallation happens in very short time and material goes back to ambient in order of approximately a nano second. Figure 10.3 shows SEM micrograph of spallation point on the Aluminum side for four different laser powers. For 15 and 20 mJ laser energy large circumferential ring cracks were observed around spallation point. When a short and intense laser pulse irradiated on the Aluminum thin film, high pressure plasma is formed on the surface [20]. An elastic–plastic wave propagates trough the Alumina substrate and hits the tin oxide layer. This shock wave has created a plastic strains behind on the surface of alumina. Most of the Aluminum layer has been evaporated but very thin layer aluminum left behind is solidified with wave marks on the surface as labeled with arrows on Fig. 10.3b. The YAG laser beam energy causes the aluminum absorbing layer to vaporize. The thermal expansion of the aluminum causes a compressive stress wave to form that travels longitudinally towards the film/substrate interface. When this compression wave reflects off the free surface of the test film, the wave is reflected back towards the substrate and becomes a tensile wave that loads the film/substrate interface in tension. The tensile stress wave pulls the test film from the substrate. If the tensile stress is stronger than the film/substrate adhesion strength, the film is spalled from the substrate. In order to determine if spallation occurred at a lower power, the test film spots on SnO2 were then viewed with a Scanning Electron Microscope as shown in Fig. 10.4. The power increases as we move from top left to bottom right. The lowest power level at which the film spalled is the point at which the test film to substrate adhesion fails. For the current tests we see that at all powers investigated that some substrate spallation occurred for this SnO2 system. In ongoing studies we are modifying the test specimen to include a metal layer that will simulate the current-collector that is present in these battery systems. Fig. 10.3 Spalled spot of Aluminum coating from an Alumina Substrate (a) 5 mJ, (b) 10 mJ, (c) 15 mJ, (d) 20 mJ L.S. Faraji et al.
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