Effect of pulsed Nd:YAG laser processing parameters on surface properties of polyimide films

As a cost-effective, facile and scalable technology, direct laser writing technology has been widely reported for modifying the surface properties of polymers. In this work, the modification induced in polyimide (PI) films by the pulsed Nd:YAG laser has been investigated. The formation mechanism of surface morphologies of the laser-induced PI films was clarified through laser Mie scattering. Various laser powers, scanning speeds and pulse repetition frequencies were applied to modify surface properties, including surface morphologies, carbonization characteristics and wettability. Analysis of the morphological changes in the PI film surface layer was performed via a three-dimensional optical microscope. The degree of carbonization was measured using the grey-scale extraction tool in the Adobe Photoshop software. It is found that the laser fluence incident on a PI film is closely related to the density of floating carbonized particles generated by the high-temperature carbonization and the high-pressure release of the excited atoms in PI film. Meanwhile, the density of carbonized particle group and the number of cavities in PI film mainly depend on the unscattered static laser fluence regardless of the dynamic laser fluence. And the degree of laser Mie scattering can be inferred by the degree of carbonization of the cavity-free surface and the distribution ratio of the small-sized cavities with diameters of <= 15 mu m. Within the range of the studied processing conditions, the laser power and pulse repetition frequency exhibited a greater sensitivity to the carbonization area rate of cavity defects and the wettability than the scanning speed.