Enhancement of zirconia–veneering porcelain bond strength via thermal phosphoric acid etching combined with lithium Silicate Coating and the influence of multiple sintering cycles

Objective To evaluate the effect of thermal phosphoric acid etching combined with a lithium silicate (SiLi) coating on the interfacial bond strength between zirconia and veneering porcelain, and to investigate the influence of multiple sintering cycles on veneering porcelain surface properties and zirconia core mechanical performance. Methods Zirconia specimens were allocated into four groups: untreated control, acid etching, SiLi coating, and acid etching + SiLi coating. Each group was subjected to 1, 3, 5, or 7 low-temperature sintering cycles. Shear bond strength, three-point flexural strength, water contact angle (WCA), and Vickers hardness were measured. Microstructural and compositional changes were analyzed via scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results XRD confirmed the formation of zirconium hydrogen phosphate after acid etching and a SiLi layer after coating. The acid etching + SiLi group showed the highest shear bond strength and a significant reduction in WCA compared with the untreated ( p < 0.05). Multiple sintering cycles produced a parabolic trend in bond strength and veneering porcelain hardness, with peak values after three cycles in the acid etching + SiLi group. No significant differences in three-point flexural strength were observed among groups ( p > 0.05). Conclusion Thermal phosphoric acid etching combined with SiLi coating significantly improves the zirconia–veneering porcelain bond without compromising the flexural strength of zirconia. Clinically, the number of sintering cycles should be limited to three or fewer to preserve optimal interfacial adhesion and veneering porcelain hardness.