Micro- and nano-scale topographical alterations in dental alloys after exposure to artificial saliva: a combined SEM–AFM study

Abstract

This study aimed to evaluate the surface topography and chemical stability of six commonly used orthodontic archwires: nickel–titanium, gold- and rhodium-coated nickel–titanium, stainless steel, nickel-free stainless steel, cobalt–chromium–nickel, and beta-titanium under simulated intraoral conditions. Two widely applied analytical methods were used: atomic force microscopy (AFM) and scanning electron microscopy with energy-dispersive X-ray spectroscopy, complemented by additional post-processing of the obtained data. Archwires were analyzed in their untreated state and after 28 days of exposure to artificial saliva at two pH values (5.5 and 6.6). Nickel–titanium archwires showed moderate roughness and a stable nickel : titanium ratio, but were not the most resistant under all of the conditions. Coated nickel–titanium archwires showed smoother surfaces and lower element losses, indicating a protective effect of the coating. Stainless steel wires showed moderate corrosion and localized surface cracking at pH 5.5, while Ni-free stainless-steel wires were most affected by acidic conditions and exhibited pronounced degradation and high oxygen content. Cobalt–chromium–nickel archwires maintained their surface integrity better at pH 6.6, but showed selective dissolution at pH 5.5. Beta-titanium archwires exhibited localized oxidation but stable elemental composition, supporting their potential as a hypoallergenic alternative. The inclusion of three-dimensional AFM parameters allows a more comprehensive and nuanced assessment of surface morphology, capturing subtle changes that may not be apparent with conventional two-dimensional roughness analysis alone. These results emphasize the importance of material selection based on corrosion resistance and surface stability, especially for patients with acidic oral environments or metal sensitivities.