UV-A and UV-C Irradiation Effects on Zirconia Surface

Objectives: The purpose of this study was to investigate the changes of surface characteristics of zirconia after UV-A and UV-C irradiation at different time points.
Methods: A hundred of polished zirconia disks (Zenostar, Wieland Dental, Pforzheim, Germany) in squared-shape (9mmx9mm) were prepared. Zirconia samples were irradiated by either UV-A (365nm, 550µW/cm²) or UV-C (254nm, 490µW/cm²), respectively, at different time points (15min,3h,24h). Specimens without UV irradiation were served as control group. The surface characteristics were evaluated by colour spectrophotometer, scanning electron microscopy (SEM) with energy dispersive X-ray detector (EDX), water contact angle (WCA) and X-ray diffraction (XRD) measurements. Three zirconia samples for each group and each time point was used. The data were analyzed by ANOVA at α=0.05.
Results: The colour change values (ΔE) L*a*b* after UV-C irradiation for 15min,3h,24h were respectively 1.99,1.92,3.35, which obviously changed from white to light yellow colour. Tiny colour change was detected on UV-A irradiation (respective ΔE: 0.18,0.14,0.57) which is not visually observable. SEM did not reveal any morphological difference after UV-A and UV-C irradiation. EDX result showed C/Zr ratio decreased significantly after UV-C irradiation for different time of 15min(0.33±0.06), 3h(0.35±0.03), and 24h(0.41±0.02) compared to the control group (0.45±0.09) (p<0.05). However, no significant difference of C/Zr ratio was found between UV-A irradiated samples and the control group. WCA of both UV-C/UV-A treated zirconia surface after 15min(49.6°±0.3°/54.4°±2.4°), 3h(52.8°±3.6°/60.1°±2.6), 24h(17.3°±1.1°/58.3°±2.7) were statistically significant less than control group (70.7°±0.3) (p<0.05). XRD showed no tetragonal (T) to monoclinic (M) phase change after any UV irradiation.
Conclusions: Both UV-A and UV-C can functionalize zirconia. Despite UV-C might be more powerful in decreasing adsorbed carbon (C/Zr) and make the zirconia surface more hydrophilic, it could change the colour of zirconia.