Surface morphology formation and subsurface microstructure evolution of zirconia in ultrasonic vibration-assisted burnishing

Yttria-stabilized zirconia (YSZ) is an outstanding ceramic material with applications in dentistry, biomedical, and mechanical device, where a smooth and durable surface is required. This study employed ultrasonic vibration-assisted burnishing (UVB) to enhance the surface hardness of YSZ while simultaneously reducing its roughness. The surface topography formation, subsurface microstructure evolution, and surface hardness change of YSZ were investigated. It was found that a smooth surface with an average roughness (Ra) of 0.12 μm was achieved using UVB, representing 67.6 % reduction in surface roughness from the original sintered surface, which is attributed to the burnishing-to-cutting phenomenon during the processing. The UVB-induced compression and material densification prevented the formation of the monoclinic phase during the processing of YSZ. Grain refinement and crystal lattice distortion occurring in the subsurface layer led to an increase in surface hardness, reaching up to 19.7 % higher than that of the original sintered surface. Moreover, the UVB-processed surface demonstrated a high resistance to mechanical impacts, effectively inhibiting the tetragonal-to-monoclinic phase transformation in indentation testing. These findings demonstrate that UVB is an effective approach for enhancing the surface and subsurface properties of the material. • Ultrasonic vibration-assisted burnishing (UVB) mechanism of yttria-stabilized zirconia is clarified. • A burnishing-to-cutting phenomenon is observed which contributes to surface smoothing. • Monoclinic volume fraction in yttria-stabilized zirconia surface deceases after UVB. • Burnishing induces a grain refined layer near the surface while vibration causes material densification. • Indentation tests show that the resistance of surface to external loads is improved by UVB.