Laser continuous/dispersed quenching techniques were used for surface hardening Cr8 cold work die steel, and its metallographic microstructures and mechanical properties are investigated in this work. Typical mixed structure which composed of carbides in different morphologies and acicular martensite can be obtained by laser quenching. The excellent wear resistance and surface hardness of Cr8 die steel are attributed to the synergistic effect of solid solution strengthening, grain refinement and high dislocation density. Meanwhile, the phase change hardening layer structure and residual tensile stress induced by full area coverage scanning (LCQ) are the main factors to deteriorate the impact toughness and bending performance after laser quenching. The soft substrate-hardened unit structure with laterally spaced distribution (LDQ) obtained by graphical scanning reduces the heat accumulation effect on the surface, which reduces concentration of deformation near the hardened layer. The bending property and fracture mechanism are studied. Compared with the forging die material treated by LCQ, the impact energy and bending resistances of forging die materials treated by LDQ are enhanced significantly. Consequently, Cr8 die steel takes into account the same wear resistance, and scanning strategy should be controlled to retain more interval distribution hardening units in layer structure for superior impact toughness and bending properties.
In view of the fact that the weak laser damage resistance of HfO2 / SiO2 coatings at 355 nm hinders the observation of the fatigue effect, nanosecond single and multiple pulse laser damage studies on Al2O3 / SiO2 high-reflective coatings were performed at 355 nm. Relative to that at the long wavelength, the fatigue effect at 355 nm is very weak and complicated. The damage probability curves and the evolution of the laser-induced damage threshold under multiple irradiations reveal that the fatigue effect is affected by both laser fluence and shot number. As the laser fluence or number of shots increases, the fatigue effect becomes more apparent. The damage morphologies induced by single and multiple irradiations both manifest as micrometer-scale pits without plasma scalding around, with the characteristics of a high defect density and high absorption coefficient. In particular, the accumulation damage mechanism at 355 nm may be reflected not only in the newly created defects but also in the modification of the coating material around the damage precursors. Thus, the coatings at 355 nm “seem to” have no damage growth threshold, no matter what the laser fluence is; once damage occurs, the damage site will grow sharply under subsequent pulses finally resulting in catastrophic damage.
Laser processes of crystalline silicon solar cells become increasingly attractive, because they are fast, accurate and contact-free. Nanosecond and picosecond laser ablations with wavelength of 532nm were performed on the anti-reflection layers deposited on silicon. The laser ablated grooves of AR coatings on monocrystalline and polycrystalline silicon were both characterized to verify the influence on the underlying silicon. And the threshold fluences were specified by contrasting with the corresponding performances under certain laser pulse duration. More importantly, the groove edges were analyzed to further expose the laser ablation mechanism under different laser pulse durations.
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