Research On The Modification Of NiCr Dental Alloy By Micro-Arc Oxidation

In this study, Al₂O₃ coatings were prepared on NiCr substrates by micro-arc oxidation (MAO) in the NaAlO₂-containing electrolytes. Surface and cross-section morphologies,phase composition,elemental content of the coatings were observed by SEM, XRD and EDS. The corrosion resistance bonding strength and and were studied by electrochemical test and pull-off test.It is also found that as the NaAlO₂ concentration increase from 14g/L to 16 g/L, the coating becomes denser and thicker, the frequency increase from 60 Hz to 200 Hz, the pore size and the porosity are increased. In NaAlO₂ concentration is 16g / L, treatment frequency is 200Hz, the coating exhibited excellent adhesion ,corrosion resistance , However, the overhigh concentration of NaAlO₂ (18 g/L) is found harmful to the coating's microstructure and properties.The MAO process in NiCr alloy is studied using a solution containing NaAlO₂-KMnO₄.The addition of KMnO₄ to the electrolyte has no significant influence on the formation ofγ-Al₂O₃ crystal phase. With increasing the concentration of KMnO₄, the coating is thinner but denser, the color of the layer turn from gray to deep brown. When the concentration of KMnO₄ is 1.0 g/L, the thickness of coating only 7.86um, but the bond strength is 25.8MPa, compare with the coating formed in the electrolyte without KMnO₄, the corrosion potential of that has increased 387mV and there is no significant change in the corrosion current density.The MAO coatings prepared in NaAlO₂-NaH₂PO₄ electrolytic solutions are thicker and have large roughness, theγ-Al₂O₃ has lower degree of crystallization. There are a lot of amorphous in the coating. The bond strength decreases with increasing the concentration of NaH₂PO₄, whilst the corrosion potential of the coating has increased 383mV and the corrosion resistance decreased two orders.The MAO coatings formed in NaAlO₂-Na₂SiO₃ solution are thinner and denser. The micropores of the coatings are small and compact. The degree of crystallization has no more change. When the concentration of Na₂SiO₃ is up to 1.5 g/L, the bond strength of coating increased to 25.8MPa. Compare with the coating formed in the electrolyte without Na₂SiO₃, the corrosion resistance has no remarkable improvement.