Yttria-stabilized zirconia (YSZ), as a material for thermal barrier coatings, is widely used for the protection of high-temperature components in gas turbines and aero-engines. YSZ coatings prepared by atmospheric plasma spraying have high surface roughness, loose structures, and many pores and cracks. These factors can lead to the frictional wear of the coating, the susceptibility to molten salt corrosion causing the coating to fail and peel off, and the reduction in lifespan of the coating. Laser polishing can reduce the surface roughness, improve the aerodynamic performance of the components, hinder the penetration of molten salts, and reduce the stress concentration. Laser glazing can improve the loose structure, repair the surface defects, enhance the resistance to thermal corrosion, and thereby extend the service life. This paper proposes an integrated laser polishing and glazing processing technology. The coating is treated using the femtosecond laser pulse train mode for integrated polishing and glazing. Preliminary conclusions on the mechanism of femtosecond laser polishing and glazing are drawn, providing valuable reference for femtosecond laser pulse train mode processing.

This paper uses atmospheric plasma spraying technology to prepare YSZ thermal barrier coatings on the substrate. Femtosecond laser technology is utilized for the polishing and glazing integration treatment of the coating. A laser confocal microscopic system is employed to observe and measure the surface roughness and the three-dimensional morphology before and after polishing. Scanning electron microscope and metallographic microscope are used to characterize the cross-sectional morphology of the coating. An X-ray diffractometer is used for phase analysis of the original coating and the coating after experimentation. A mixture of NaCl and Na?SO? molten salt powder is first uniformly spread on the coating surface before and after polishing at a deposition amount of 20 mg/cm2, and then heated to 900 ℃ at a rate of 10 ℃/min and held at temperature for 4 h to test the resistance of the coating to molten salt corrosion.

The surface of the coating forms a flat polished area after undergoing the polishing and glazing integration treatment with a femtosecond laser. In the pulse train mode, the coating surface exhibits vaporization and a “melting peak filling valley” effect (Fig. 3). The change in the number of sub-pulses has little impact on the surface roughness of the coating.The coating roughness can be reduced by more than 74% after femtosecond laser scanning in pulse train mode (Fig. 4). As the number of sub-pulses increases, the width of cracks on the coating surface gradually increases. After the polishing and glazing integration treatment, a dense glazed layer forms on the surface of the coating. In the pulse train mode, as the number of sub-pulses increases, the thickness of the glazed layer gradually decreases, but this leads to an increase in crack width. When the number of sub-pulses is 10, the porosity of the glazed layer is reduced to the minimum of 6.8% (Fig. 6). The main phase component of the coating before and after the polishing and glazing integration treatment is the non-equilibrium tetragonal phase of zirconia (t′-ZrO₂), and the polishing and glazing integration treatment does not lead to the formation of harmful monoclinic phase zirconia (m-ZrO₂) (Fig. 8). After thermal corrosion, the original coating develops distinct horizontal cracks between the thermal barrier coating and the bond coat. In contrast, the coating that has undergone the integrated polishing and glazing treatment maintains its surface integrity after thermal corrosion, the internal structure of the coating is largely preserved, and no horizontal cracks are observed (Fig. 9).

The femtosecond laser polishing and glazing integration process can significantly reduce the surface roughness of YSZ coatings, reducing the original roughness S_a of 7.5 μm to below 2.0 μm. Using the pulse train mode can effectively improve the surface morphology and the quality of the glaze layer, with the porosity of the glaze layer being as low as 6.8%. Compared to the original coating, the coating after the polishing and glazing integration treatment exhibits superior resistance to thermal corrosion. Under the process with 10 sub-pulses, the coating adheres well, and no significant transverse cracks are observed. The femtosecond laser pulse train mode optimizes the proportion of material vaporization and remelting under a laser action through a more gentle heating method and a longer duration of action. By adjusting the number of sub-pulses, precise control over the morphology and thickness of the glaze layer can be achieved.