Dominance of mechanical over thermally induced damage during femtosecond laser ablation of monazite
Abstract:Effects of infrared femtosecond laser ablation (800 nm, 60 fs, 5 Hz, 85 μJ/pulse, objective × 15) of a well-characterized monazite on its micro- and nano-structure were investigated. Craters were produced by single and multiple pulses (N = 10, 20, 50, 150 and 300) to follow the evolution of laser-induced damage in monazite using Scanning Electron Microscope (SEM), and Transmission Electron Microscope (TEM) coupled with Focused Ion Beam (FIB) sample preparation, in order to characterize this damage. Voids are observed within craters from the first pulse and cracks appear already after 10 pulses, at the sample surface; radial cracks are well-defined for 50 pulses, and become conchoidal after 150 pulses, indicating high-strain fields in the vicinity of craters. After the first pulse, the monazite lattice is highly strained to depths greater than ∼1 μm with a spotty ring diffraction pattern demonstrating that the damaged monazite is a mosaic crystal. Under this area monazite is moderately strained over 6 μm in depth. Crack formation within the crystal is observed from the first pulse. Cracks formed at the surface and propagated over 2 μm into the crystal. Their number increased notably after 10 pulses, with some cracks propagating 8 μm into the crystal. Increasing lattice defects (mosaic crystal, twins) and fracture intensities demonstrate that a cumulative effect exists. Part of the energy carried by the laser is stored within the crystal and used in the formation of defects. This study highlights the intense damages that are created during a femtosecond laser ablation in monazite. Mechanical effects dominate thermal ones, limited to a thin layer (200 nm–1 pulse) of resolidified monazite, and are induced by high-pressure shock wave from plasma expansion.
Document Type: Research Article
Publication date: March 1, 2010
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