Enhancement of Zirconia–Resin Bonding by Water Glass Surface Treatment

Objectives This study evaluated the effect of water glass [(Na₂O)x(SiO₂)y] surface treatment on the bonding performance of zirconia to veneering porcelain and resin composite. Particular consideration was given to the clinical repair of fractured veneering porcelain on zirconia frameworks. Materials and Methods Zirconia specimens were divided into water-glass-treated and untreated groups, followed by sintering and alumina sandblasting. Surface roughness was analyzed by confocal laser microscopy, and elemental composition was examined using X-ray photoelectron spectroscopy (XPS). For porcelain testing, veneered specimens were subjected to shear bond strength (SBS) testing. For resin testing, specimens were bonded to composite resin with a silane-containing primer and divided into two subgroups: 24 h water storage at 37 °C and thermal cycling (5000 cycles, 5–55 °C). and the SBS was measured. Results Water-glass treatment increased surface roughness and produced a silica-rich layer that persisted after sandblasting. Porcelain SBS was significantly higher in the treated group (29.5 ± 7.3 MPa) compared with the untreated group (15.2 ± 4.3 MPa, p < 0.05). For resin bonding, treated zirconia showed higher SBS under both 24 h (21.0 ± 2.3 vs. 15.6 ± 2.6 MPa) and thermal cycling (13.0 ± 2.4 vs. 7.1 ± 1.9 MPa) (p < 0.001). Conclusions Water-glass treatment enhances resin–zirconia and porcelain–zirconia bonding by forming a durable silica-rich interface that resists sandblasting and promotes silane coupling. Clinical Relevance Water-glass pretreatment strengthens porcelain–zirconia adhesion and provides a stable surface for resin bonding, enabling reliable intraoral repair of porcelain-veneered zirconia crowns.