Yttria-stabilized zirconia (YSZ) thermal barrier coatings have been widely used in the manufacturing of aircraft engines. Reducing the surface roughness of YSZ thermal barrier coatings can significantly improve the aerodynamic performance of materials, reduce stress concentration, and enhance service life of the coatings. This study proposes a grazing incidence laser polishing method to reduce the roughness of YSZ coatings. A femtosecond pulse laser is employed to perform grazing incidence polishing on YSZ coatings to investigate the material removal mechanism, solve the technical problems in the grazing incidence polishing, and expand the laser processing system. The preliminary mechanism of grazing incidence polishing on YSZ coatings is identified, offering valuable insights for the grazing incidence polishing and processing of YSZ thermal barrier coatings.

In this study, YSZ is coated on a 316L substrate using atmospheric plasma spraying (APS) equipment, and grazing incidence laser polishing is performed using a femtosecond laser at different overlap rates. A laser confocal microscope and scanning electron microscope (SEM) are used to observe the surface morphology after polishing. Fourier transform is conducted to analyze the spectrum of the polished surface profile, which contributes to the discovery of the mechanism of grazing incidence laser polishing and material surface ablation.

The key factor affecting the polishing results under the grazing incidence is the overlap rate of the laser spot. When all other factors remain unchanged, the surface roughness (S_a) after polishing initially decreases and then increases with increasing overlap rate, reaching its lowest value, 0.66 μm, when the overlap rate is 20% [Fig. 5(c)]. At both higher and lower overlap rates, a significant ridge-like structure appears at the bottom of the polishing area, indicating that polishing will be affected irrespective of the overlap rates . After conducting profile and spectrum analysis on the polished surface, it is concluded that at 80% overlap rate, two characteristic frequency ripples are generated on the surface. The ripple with frequency f₁=0.010 μm^-1 is caused by pores and defects generated during the spray coating process, which cannot be completely removed. The ripple with frequency f₂=0.025 μm^-1 is caused during the polishing process. Figure 7(b) shows that this type of surface ripple can be smoothed when the overlap rate is 20%. When the overlap rate of the light spot is too high, the focus position of the next light spot is prone to fall into the ablation pit created by the previous light spot. This results in energy dissipation and hinders the accumulation of overlap in the ablation area.

This study conducts grazing incidence polishing experiments on YSZ thermal barrier coatings using femtosecond pulse lasers, investigates the effect of different overlap rates on roughness, and analyzes the mechanism of action between grazing incidence femtosecond lasers and YSZ coatings. Grazing incidence polishing removes elevated structures on rough surfaces by using high-energy laser beams to vaporize and ablate the material surfaces, thus, achieving a smooth surface. This polishing method can significantly reduce the roughness of the polishing area at any overlap rate; however, the best overlap rate is 20%, which will result in effective ablation. The surface morphology will show ridged defects when the overlap rate is either too high or too low.