A novel ultrafast laser ablation fracture polishing method to simultaneously smoothing and improving the surface quality of Al2O3 ceramics with high roughness

•A novel laser polishing method for Al2O3 ceramics with original high surface roughness is proposed.•Picosecond laser is used to form a remelting layer on the surface to eliminate surface void defects.•The remelt layer is then quickly removed using femtosecond laser ablation fracture processing to eliminate the cracks.•Finally, the surface is precision polished using a layer-wise ablation to obtain a submicron smooth surface. Al2O3 ceramics are widely used in industry due to their excellent mechanical and physical properties. However, microcracks, voids and high surface roughness (Sa) on the surface of Al2O3 ceramic parts produced in conventional industrial or additive manufacturing have been one of the technical bottlenecks limiting their high-performance applications. In this paper, a novel method of ultrafast laser ablation fracture polishing (ULAFP) of Al2O3 ceramic parts is proposed for improving the surface quality and enhancing the mechanical properties of the polished surfaces. The ULAFP method is developed with an ultrafast laser, which can be set up as a picosecond (PS) or a femtosecond (FS) laser. The PS laser is used to form a remelting layer on the surface to eliminate surface void defects. Combining laser ablation and laser thermal stress-induced crack extension effects, the FS laser is used to rapidly remove the remelted layer and obtain a surface free of crack defects and finally implement FS laser layer-wise ablation to achieve smoother surface finish. The FS laser energy density is controlled at a critical state close to the ablation threshold of the material to avoid surface defects. This ULAFP can reduce the surface roughness (Sa5.254 μm) polishing of Al2O3 ceramics to 0.80 μm, while controlling the total ablation depth of the polishing process to below 100 μm maintaining the dimensional accuracy of the part and dramatically improving the mechanical properties of the polished surface. Compared with the original surface, the hardness and elastic modulus of the polished surface improved by 104.6% and 63.9%, respectively.