## 3 Results and Discussion  
The idea of potentially increased fatigue life of a wire consists of depositing of ALD coating on the surface of the stretched wire (Figure 2). During stretching, it is assumed that many microcracks on the surface become open. (Multiple mechanical load cycles can be applied in advance to increase the microcrack density on the surface). ALD is a promising method that enables uniform coated layer thickness even in microcracks. After the deposition and unloading, it is assumed that compressive stress (\(\sigma<0\)) forms in the vicinity of each microcrack, which has a positive effect for suppressing further growth of the microcracks. Al\({}_{2}\)O\({}_{3}\) coating was chosen because of the possibility of depositing Al\({}_{2}\)O\({}_{3}\) layers at a relatively low temperature of the sample [33], the biocompatibility of Al\({}_{2}\)O\({}_{3}\)[34], wear resistance, and high Young's modulus [35; 36; 37]. However, in our study, it was not possible to reduce the deposition temperature to room temperature. Hence, the potential influence of unintended heat treatment on fatigue resistance cannot be excluded in this study. To assess the effect of this unintended heat treatment during the ALD process, loading-to-rupture experiments were conducted. Given that the deposition temperature exceeds room temperature, we chose to work with as-drawn NiTi wires rather than annealed wires, as the thermomechanical properties of the latter are more sensitive to temperature variations. Figure 1: The schematics of the ALD chamber with the stretched-wire sample placed inside the chamber. The wire surfaces of the as-purchased and Al\({}_{2}\)O\({}_{3}\)-coated NiTi wires are shown in Figure 3. Due to the nanometric thickness of the coating, there is no clear indication of an Al\({}_{2}\)O\({}_{3}\) layer from Figure 3. Even Figure 3d, which is the back-scatter electron image showing a more compositional than surface relief contrast, provides no evidence of an Al\({}_{2}\)O\({}_{3}\) layer composed of the light elements. However, Figure 3 presents a basic characterization of the wire surface. Cracks and signs of delamination in the naturally formed TiO\({}_{2}\) layer are visible. The white arrows in the SEM images indicate microcracks in the exposed alloy region where the TiO\({}_{2}\) layer has detached. The large plastic deformations induced during the cold-drawing process conducted by the wire producer are well accommodated by the NiTi matrix; however, they are detrimental to the brittle, naturally grown TiO\({}_{2}\) ceramic layer. The presence (or absence) of Al\({}_{2}\)O\({}_{3}\) on the surface of the ALD-coated (or uncoated) sample can primarily be determined from the EDS (energy-dispersive X-ray spectroscopy) analysis. The Al K peak is clearly visible in the energy-dispersive X-ray spectrum of the coated-wire sample with the applied Al\({}_{2}\)O\({}_{3}\) (Figure 4b), whereas no aluminum peak is present in the spectrum of the as-purchased NiTi wire (Figure 4a). Figure 3: The SEM micrographs of the surface of the uncoated-NiTi-wire sample using secondary electron (SE) (**a**) and back-scatter electron (BSE) (**b**) detectors and coated-NiTi-wire sample (100 Al\({}_{2}\)O\({}_{3}\) ALD cycles) using SE (**c**) and BSE (**d**) detectors. The magnification and acceleration voltage were 10.0k\(\times\) and 5 kV, respectively. The white arrows in the SEM images indicate microcracks. Figure 2: The schematics for the idea of building compressive stress at the wire surface using ALD to fill microcracks with Al\({}_{2}\)O\({}_{3}\). Table 1 presents the number of mechanical loading cycles (fatigue or tensile cycles) to fracture (Nf) for the as-purchased samples. The average number of cycles, Nf_mean, is 7535 cycles, with a relatively large standard deviation (SD = 6219 cycles). This variability could potentially be explained by differences in the degree of crystallinity of the samples, as well as the random formation of critical microcracks that may arise during the wire drawing process.