Enhancement of NiTi superelastic endodontic instruments by TiO 2 coating Diego Pinheiro Aun a , Isabella Faria da Cunha Peixoto a , Manuel Houmard b , Vicente Tadeu Lopes Buono a, ⁎ a Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, Campus Pampulha, 31270-901 Belo Horizonte, MG, Brazil b Department of Materials and Civil Construction Engineering, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, Campus Pampulha, 31270-901 Belo Horizonte, MG, Brazil abstract article info Article history: Received 23 December 2015 Received in revised form 23 April 2016 Accepted 9 June 2016 Available online 11 June 2016 Rotary nickel-titanium (NiTi) endodontic instruments were coated with a nanometric flexible TiO 2 layer through dip-coating sol-gel. Control groups and coated samples of superelastic NiTi instruments model RaCe 25/0.06 (0.25 mm tip-diameter, 6% conicity) were comparatively investigated with respect to the cutting efficiency, fa- tigue life, and corrosion resistance. Results showed an improvement in cutting efficiency for the coated samples and a high resistance to corrosion in NaClO. The coated instruments showed a better performance in fatigue life after corrosion. © 2016 Elsevier B.V. All rights reserved. Keywords: NiTi endodontic instruments TiO 2 ceramic nanolayer Dip-coating sol-gel Cutting efficiency Fatigue Corrosion 1. Introduction Endodontic therapy is based on the complete removal of dental pulp together with all contaminated tissue from the root canal. A proper canal shaping is fundamental for the success of the treatment, removing all the pulp, creating a path for medicament delivery and optimizing the duct for a proper tooth obturation [1]. This preparation and disinfection is accomplished by the mechanical action of a rotary endodontic instru- ment in combination with the chemical action of a lubricant solution, normally sodium hypochlorite (NaClO), which also acts as bactericide and organic tissue solvent [2]. Superelastic (SE) or martensitic NiTi al- loys are the materials of choice for the production of modern rotary end- odontic instruments because of their differential mechanical properties when compared to stainless steel files [1,3]. However, safe clinical use of NiTi files requires an understanding of their mechanical properties and the basic fracture mechanisms, and their correlation to canal anatomy [1]. Fracture of endodontic instruments during the shaping of a curved root canal is one of the main concerns amongst practitioners. The failure of the instrument can sometimes prevent the correct chemo- mechanical preparation and impede the complete disinfection of the root canal [4]. Rotary NiTi files can fail by two mechanisms: excessive torque, and flexural fatigue [5–8]. Torsional overloading can be avoided by limiting the maximum torque applied to the instrument and by con- trolling the apical force used by the practitioner. Failure by fatigue may occur when the material is subjected to cyclic tensile/compressive loading, as is the case when the instrument is rotat- ing inside a curved canal. The basic mechanism of the rupture is the nu- cleation of microcracks at the material surface, followed by inward propagation of these cracks until a reduction in the cross-sectional area reaches a critical value, when the tensile stress becomes high enough to separate the object [9]. NiTi endodontic instruments are sus- ceptible to this kind of failure, and it is a common belief that the high concentration of surface defects generated during machining the instru- ment into its final shape is related to the failure process, with the ma- chining defects acting as crack nucleation sites [10]. With this in mind, some manufacturers have employed thermal treatment [6,7], electro- chemical polishing [11] and/or electrical discharge machining (EDM) [12] to generate a smoother surface, aiming at improving the fatigue life of the file. Improvements up to 700% were achieved for some instru- ments manufactured using the EDM process. Fatigue fracture can also be assisted by corrosion. Corrosion of NiTi endodontic instruments in NaClO had been studied by several authors [13–18]. Sodium hypochlorite is the main irrigant used for root canal treatment because of its excellent properties as a lubricant, bactericide, and an organic tissue solvent [19], and also because it is less corrosive than R-EDTA (ethylenediamine tetraacetic acid) [16]. The main Materials Science and Engineering C 68 (2016) 675–680 ⁎ Corresponding author at: Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, Campus Pampulha, 31270-901 Belo Horizonte, MG, Brazil. E-mail addresses: diegoaun@yahoo.com.br (D.P. Aun), bebelpeixoto@hotmail.com (I.F.C. Peixoto), mhoumard@ufmg.br (M. Houmard), vbuono@demet.ufmg.br (V.T.L. Buono). http://dx.doi.org/10.1016/j.msec.2016.06.031 0928-4931/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec