## 1 Introduction  
Endodontic instruments are commonly used in dental practice to perform endodontic treatments of vital and necrotic teeth, endodontic retreatments, pulpotomies, pulpectomy and specification procedures. Depending on the phase of treatment, endodontic instruments are divided into instruments for probing the endodontic canal for the pre-enlargement, and glidepath for shaping the canal or instruments for the closure and three-dimensional sealing of endodontic canals [1; 2]. Many of these tools are reusable after performing cleaning, disinfection, and sterilization procedures by autoclaving [3]. Endodontic canal disinfection procedures that use sodium hypochlorite [4], and subsequently, heat sterilization procedures, can alter the surface of endodontic instruments, described as corrosion and micropitting, phenomena [5] that can be visualized by SEM and atomic force microscopy analyses. The endodontic instruments used in the probing, pre-enlargement, and shaping phases are made of steel alloy or nickel-titanium alloy (NiTi) and are subject to torsional, flexor, and cyclic fatigue, indeed the reuse of these instruments must be done with the knowledge that these tools are subject to fracture following stress caused during their use [6]. Fracture of the instrument within the canal is an eventuality that can lead to failure of the treatment, and therefore it is important to try to reduce situations that can contribute the fracture. On the surface subject to fatigue, surface alterations can give rise to microcrack which can lead to fracture of the instrument, and also reduce the cutting capacity of the blades on the endodontic files [7]. Therefore, in order to maintain the same cutting efficiency, the endodontist has to exert greater pressure on the instrument with an increase in torsional fatigue stress [6]. An atomic force microscope is an instrument capable of analyzing the surface of instruments. It consists of a cantilever with a pointed tip (tip) mounted on the end, typically composed of silicon or silicon nitride and having a radius of curvature of the \"order of nanometers\". The sample to be scanned, through the Vand Der Waals forces, interacts with the tip of the detector by flexing it. There are several methods to detect any cantilever movement. The majority of atomic force microscopy (AFM) systems use laser beam detection, which is an optical system with position sensitive detectors called photodiodes. The laser light is reflected by the cantilever on the position-sensitive photodiode. Very small forces are produced between the probe and the surface to be scanned, and these are the forces that allow the AFM system to record the deflection of the cantilever. The cantilever deflection is called \"cantilever rigidity\". This rigidity can be measured by Hooke's law. Rigidity is recorded visually and can be viewed on the computer in real time. The surface scan of endodontic instruments is used both in non-contact mode and in contact mode and, in general, the scanned surfaces start at 3 mm from the tip of the instrument up to 6 mm. The parameters which are considered with AFM for the analysis of a surface are the arithmetic mean roughness(AMR) of the maximum height (MH) and root mean square (RMS). The atomic force microscopy, therefore, provides detailed information with measurable parameters of possible alterations and irregularities present on the surface of an instrument [8]. Surface alterations can represent a problem in the use of endodontic instruments. A study by [PERSON], in 2018, identified surface alterations described as surface roughness with statistically significant results for instruments constructed with new M-wire and EDM alloys [9].