In-depth hardness profiles of Stainless Steel and Ni-Ti endodontic instrument cross-sections by nano-indentation

Aim: To evaluate the in-depth hardness profiles of Stainless Steel (SS) and nickel titanium (Ni-Ti) endodontic instrument cross-sections using a nano-indentation technique. Methodology: Three SS (Reamer, K and Hedström) and three Ni-Ti (ProFile, NRT and Liberator) instruments were studied. After embedding and metallographic preparation the in-depth hardness profiles of instrument cross-sections were measured starting from the cutting surface towards the centre to a depth of 2000 nm using an MTS XP nanoindenter with a Berkovich diamond indenter. The results of hardness measurements of outer (near to cutting edge) and inner locations were statistically analyzed by two-way anova followed by SNK test (α = 0.05). Results: For all instrument cross-sections the maximum hardness was obtained at the outer surface followed by hardness attenuation towards the centre of the cross section. The statistical analysis of hardness classified the instruments, for both outer and innermost locations, to the following decreasing order: Reamer > K > Hedström > Profile > NRT shank (without thermal treatment) > NRT tip (with thermal treatment) > Liberator. The maximal hardness, at the outer surface of endodontic instruments, can be attributed to the residual stresses developed due to cutting and thermal effects during the manufacturing process. The increased outer layer hardness may have a beneficial effect on the cutting ability and wear resistance of endodontic instruments. Conclusions: All endodontic instruments had a decrease in hardness towards their centre. This implies that the surface hardness of contemporary endodontic instruments is significantly enhanced by the consequences of manufacturing processes. © 2008 International Endodontic Journal.