Preventing high-temperature oxidation of Co-Cr-based dental alloys by boron doping

Biomedical Co-Cr-based alloys used in dental restorations are usually subjected to high-temperature treatments during manufacturing. Therefore, it is practically essential to characterise and control the oxide films formed on the surfaces of these alloys during heat treatment in terms of material loss, the accuracy of fit, and the aesthetics of dental restorations. In this work, the effects of boron doping on the surface oxide films formed on Ni-free Co-28Cr-9W-1Si (mass%) dental alloys under short-term exposure to high temperatures, which simulate the manufacturing process of porcelain-fused-to-metal (PFM) restorations, were investigated. The surface oxides primarily consisted of Cr₂O₃ in all prepared alloys. The chemical composition of these surface layers varies with the B concentration in the bulk, with the addition of boron stabilising the dense Cr₂O₃ phase in the oxide films. Nanoscale boron enrichment is clearly observed at the interface between the oxide films and the metal substrate, with the oxidation of boron atoms leading to the formation of a B₂O₃ layer. Since B₂O₃ and Cr₂O₃ prevent oxygen diffusion, the surface oxide films on the boron-containing alloys are thinner; however, no additional thinning was observed when increasing the boron content from 0.01 to 0.8 mass%. It was also found that a small amount of boron does not degrade the corrosion properties of the alloys in a 0.9% NaCl solution. The results obtained in this study will aid in the improvement of manufacturing processes, and ultimately, the performance of PFM restorations.