Effect of surface treatment with potassium hydrogen difluoride and ammonium hydrogen difluoride on bond strength between layered veneering porcelain and zirconia

Objectives: The purpose of this study was to show the effect of surface treatment of potassium hydrogen difluoride (KHF2) and ammonium hydrogen difluoride (NH4HF2) on the bonding of veneering porcelain to zirconia frameworks. Material and methods: Two types of zirconia (Katana zirconia (3Y-TZP) and KZR-CAD Zr SHT (6Y-TZP)) were used as the adherend material. Cerabian ZR porcelain was fired to each type of zirconia disc (n = 11). Zirconia discs were prepared using the following surface treatment: no treatment (NT), alumina blasting (AB), KHF2 and NH4HF2. Coefficient of Linear Thermal Expansion (CTE) of each type of zirconia and veneering porcelain were measured. The specimens after surface treatment were analyzed with X-ray diffraction analysis (XRD). Shear bond strength was tested after 24 h water storage. The specimens were evaluated under scanning electron microscopy. Result: Significant differences between CTE values of 6Y-TZP and veneering porcelain occurred as the temperature increased. The XRD analysis of the 3Y-TZP treated with AB, KHF2 or NH4HF2 demonstrated that 3Y-TZP becomes unstable and phase transformation occurs. In 3Y-TZP group, KHF2 and NH4HF2 groups exhibited higher shear bond strengths than AB and NT. In 6Y-TZP group, KHF2 groups showed higher shear bond strengths than NT, AB and NH4HF2. Conclusion: High bond strength between zirconia and veneering porcelain can be achieved by a surface treatment with selected melted fluoride compounds.