Porcelain adherence to dental cast CP titanium: effects of surface modifications

Objectives: A reaction layer forms on cast titanium surfaces due to the reaction of the molten titanium with the investment material. Such a layer prevents strong adhesion between titanium and porcelain. This study characterized the effects of surface modifications on cast titanium surfaces and titanium–ceramic adhesion. Methods: ASTM grade II CP titanium was cast into an MgO-based mold. Castings were devested by sandblasting with alumina particles, and subjected to surface modification by immersion in one of the following solutions: (1) 35% HNO3–5% HF at room temperature for 1 min; (2) 50% NaOH–10% CuSO4•5H2O at 105°C for 10 min; (3) the NaOH–CuSO4 solution followed by the HNO3–HF solution; (4) 50% NaOH–10% NaSO4 at 105°C for 10 min; (5) the NaOH–NaSO4 solution followed by the HNO3–HF solution; and (6) 50% NaOH solution at 105°C for 10 min. Surfaces only sandblasted with alumina were used as controls. Specimen surfaces were characterized by XRD and SEM/EDS, and hardness–depth profiles were determined. All specimens were sandblasted with 110 μm alumina particles before porcelain firing. An ultra-low-fusing porcelain (Vita Titankeramik) was fused on the titanium surfaces. The titanium–ceramic adhesion was characterized by a biaxial flexure test, and area fraction of adherent porcelain (AFAP) was determined by X-ray spectroscopy. Results: EDS analyses revealed a substantial amount (13–17 wt%) of Al on the control, and specimens modified with Methods 2, 4, and 6. XRD revealed residual stress in the titanium surfaces and corundum on the control, and Methods 2, 4, and 6 specimens. A new Ti(Cu, Al)2 phase was identified on the titanium surfaces modified by immersion in 50% NaOH–10% CuSO4•5H2O aqueous solution. Reduced residual stress was observed on Method 1, 3, and 5 specimens. No corundum peaks were detected on these specimens. Compared to the control, significantly lower (P<0.05) hardness values were found for Methods 1–3 and Method 5 specimens at 20 μm below the surfaces and for Method 1–5 specimens at 50 μm below the surfaces. Significantly higher (P<0.05) AFAP values were found for surfaces modified with Methods 2–6 compared to the control and Method 1, and no significant differences were found among Methods 1–6, and between control and Method 1. Significance: Based on the results from the present study, porcelain adherence to cast CP titanium can be improved by the caustic baths used in the study.