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The following
is a description of the Laser Spallation Technique used to measure
interfacial strength: |
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In this technique (see image below), a 2.5 nanosecond (ns) long Nd:YAG laser pulse is made to impinge over a 3 mm diameter area on a 0.3 µm-thick aluminum film which is sandwiched between the back surface of a substrate disc and a 50 to 100 µm thick layer of solid waterglass. The melting-induced expansion of aluminum under confinement generates a compressive stress pulse (with 1 nanosecond rise time) directed toward the test coating, which is deposited on the substrate’s front surface. The compressive stress wave reflects into a tensile pulse from the coating’s free surface and leads to its spallation (complete removal) at a sufficiently high amplitude. The critical stress at the interface is calculated by measuring the transient displacement history of the coating’s free surface (induced during pulse reflection) by using an optical interferometer with a resolution of only 0.2 ns in the single shot mode. The measured free surface velocity is related to the local interface stress via a wave mechanics-based simulation.
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