The coating of a NiTi alloy has a greater impact on the mechanical properties than the acidity of Saliva

The aim of this study was to evaluate the influence of the acidity of saliva on changes to the surface roughness, friction and microhardness of NiTi alloys with various coatings. Three types of commercially available NiTi archwires: uncoated, rhodium coated and nitrified (dimension 0.508×0.508 mm, 10 cm long) were immersed in 10 mL of artificial saliva with the pH ranging from 4.8 to 6.6 for a period of 28 d. Surface roughness, friction and microhardness were analyzed and compared to the unexposed as-received wires. These mechanical properties were influenced by the wire coating with a moderate-to-high effect size (p£0.005; h=0.132-0.309). The uncoated wire had a lower maximum roughness depth after exposure to pH 6.6 and 5.5 than the unexposed wire (p=0.026; h=0.346). The friction was significantly increased only in the rhodium-coated NiTi at pH 4.8 compared to the lower acidities and the unexposed wire (p=0.005; h=0.437). No correlation was found between pH, surface roughness, friction and microhardness, respectively. The coating of a NiTi alloy has a greater impact on the mechanical proper- ties than the acidity does. A rhodium coating makes the alloy harder, induces a rougher surface and more friction. Nitrification does not alter the alloy as much. The relation between acidity and mechanical properties is not linear. A high acidity of 4.8 induces a high friction, but only in rhodium-coated NiTi. A lower acidity does not change the friction significantly.