Investigating the tribological and corrosion behavior of Co–Cr alloy as an implant material for orthodontic applications

The purpose of this research was to investigate the effectiveness of using wrought cobalt-chromium (Co–Cr) alloy as a dental implant through a set of corrosion, immersion, and tribological behavior analyses. An artificial saliva solution was first prepared with varied pH levels between 4.8 and 9. The corrosive behaviors were analyzed from potentiodynamic tests using saliva samples with varied pH concentrations. The corrosion potential (Ecorr) shifted more negatively and had a slightly higher current density observed with a pH 4.8 acidic medium, demonstrating lower corrosion resistance than neutral (pH 6.9) and basic (pH 9) saliva solutions. The effect of artificial saliva on the surface roughness of the Co–Cr samples was investigated with a continuous immersion in the saliva solution for intervals of 7–28 days. Tribological tests were conducted using ball-on-flat type reciprocating friction and wear testing against an AISI 52100 steel ball. Two different sliding velocities under a constant load of 50 N were used to simulate contact conditions on implants under dry and saliva-lubricated conditions. The wear tracks were analyzed after the experiments using interferometry and microscopy techniques to analyze the wear mechanisms. No significant effects due to changing sliding velocity on dry frictional behavior were observed. Oxide layer generation during dry contact conditions and the formation of P and Ca-enriched tribofilms originating from the artificial saliva played a crucial role in the tribological behavior of the Co–Cr samples. The Co–Cr flat wear volume was significantly higher than the steel counterpart in dry conditions, unlike its performance in lubricated conditions, due to the predominant abrasive wear in the dry contacts. Corrosive wear in lubricated conditions using artificial saliva played an important role, along with abrasive and adhesive wear in both flat and ball samples. The deposition of P and Ca on the flat wear track was more predominant than on the steel ball surface. The ratio of wear volume between the steel balls and the Co–Cr flat samples in the lubricated tests was increased notably compared to dry conditions due to the effect of tribo-corrosion since artificial saliva (pH 4.8) had a higher corrosive impact on the steel ball than the Co–Cr alloy. These findings created new insights into the applicability of using a Co–Cr alloy as a dental implant due to enhanced corrosive and tribological properties. This detailed investigation on wrought Co–Cr alloys can be leveraged as a baseline for future Co–Cr alloy-based dental prosthetics development using metal additive manufacturing techniques.