Martin Petrenec

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

Monitoring Tensile Fatigue of Superelastic NiTi Wire in Liquids by Electrochemical Potential

Fatigue of superelastic NiTi wires was investigated by cyclic tension in simulated biofluid. The state of the surface of the fatigued NiTi wire was monitored by following the evolution of the electrochemical open circuit potential (OCP) together with macroscopic stresses and strains. The ceramic TiO2 oxide layer on the NiTi wire surface cannot withstand the large transformation strain and fractures in the first cycle. Based on the analysis of the results of in situ OCP experiments and SEM observation of cracks, it is claimed that the cycled wire surface develops mechanochemical reactions at the NiTi/liquid interface leading to cumulative generation of hydrogen, uptake of the hydrogen by the NiTi matrix, local loss of the matrix strength, crack transfer into the NiTi matrix, accelerated crack growth, and ultimately to the brittle fracture of the wire. Fatigue degradation is thus claimed to originate from the mechanochemical processes occurring at the excessively deforming surface not from the accumulation of defects due to energy dissipative bulk deformation processes. Ironically, combination of the two exciting properties of NiTi—superelasticity due to martensitic transformation and biocompatibility due to the protective TiO2 surface oxide layer—leads to excessive fatigue damage during cyclic mechanical loading in biofluids.

In situ study of microstructure and strength of severely predeformed pure Cu in deformation at 573 K

The influence of grain refinement on the strength of pure Cu at the elevated temperature of 573 K was studied in situ in a SEM. Thermomechanical processing by eight passes of equal channel angular pressing at ambient temperature and annealing at test temperature generates grains of size. During deformation in the SEM at a relative elongation rate of further grain coarsening occurs by discontinuous recrystallization in subsequent waves causing oscillations of flow stress. Cracks start to form at the boundaries, but partly become intragranular when boundaries move away. The relaxation of flow stress in the periods where the imposed elongation was interrupted yields information about the deformation kinetics. Two mechanisms are identified. One is thermally activated glide related with defect generation. The second one accompanies the processes of microstructure restoration. Under the present deformation conditions, the grain coarsening by DRX does not soften the material but leads to slight hardening as high-angle boundaries are exchanged for low-angle boundaries. This is discussed in terms of boundary-mediated processes.

Physical Simulation of the Random Failure of Implanted Braided NiTi Stents

A problem of random clinical failures of the braided esophageal NiTi stents has been addressed by performing physical simulation experiments on helical NiTi springs loaded in cyclic tension in air, water, and simulated biological fluid. Strains and stresses involved in spring deformation were analyzed through simulation by FEM implemented SMA model. It was found that the fatigue life of NiTi springs is significantly lower in fluids than in the air pointing toward the corrosion fatigue mechanism. There is, however, a fatigue limit roughly corresponding to the onset of martensitic transformation in the wire, which is not common for corrosion fatigue. It is proposed that surface TiO2 oxide cracking plays major role in that. Once the oxide layer on the NiTi wire surface fractures, typically during the first mechanical cycle, cracks in the oxide layer periodically open and close during subsequent mechanical cycling. This leads to the localization of mechanical and corrosion attacks under the oxide cracked regions. Microcracks within the surface oxide layer crossing over into the NiTi matrix were indeed revealed by scanning electron microscopy of FIB sections of fatigued wires. A corrosion assisted mechanism for fatigue crack nucleation at the interface between the surface oxide and NiTi matrix is proposed based on the available evidence. The approach opens a space for a better assessment of the corrosion fatigue performance of superelastic NiTi and ultimately for estimation of the lifetime of implanted braided NiTi stents.

Corrosion of NiTi Wires with Cracked Oxide Layer

Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO2 surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.

In Situ Study of the Mechanisms of High Temperature Damage in Elastic-Plastic Cyclic Loading of Nickel Superalloy

In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in SEM on flat specimen ofcast Inconel 713LC superalloy. The aim of the investigation was to studymechanisms of the fatigue damage during elastic-plastic cycling by theobservations of the characteristic surface relief evolution and theaccompanying internal dislocation structures. The selected locations on thesurface were systematically studied in-situ and documented by SEM and usingAFM. The surface relief in the first tensile half-cycle was formed by numerousslip steps on the primary slip planes (111). In the following compressionhalf-cycle additional opposite slip were formed. The relief was modified in thenext cycles but without forming additionally new slip traces in the primarysystem. The reorientation of two grains in the gauge area was measured usingEBSD. At the end of cyclic loading the relation between surface persistent slipmarkings and persistent slip bands in the interior of the material wasdocumented by TEM on lamella prepared by FIB. The early stages of extrusion andintrusion formation were documented. The damage mechanism evolution is closelyconnected with the cyclic strain localization to the persistent slip bands thatare also places of fatigue crack initiation.

AFM and FIB Study of Cyclic Strain Localization and Surface Relief Evolution in Fatigued f.c.c. Polycrystals

Atomic force microscopy (AFM) and focused ion beam technique (FIB) were adopted to study the early stages of surface relief evolution in 316L steel and polycrystalline copper fatigued with constant plastic strain amplitudes at different temperatures (316L steel at 93, 173 and 573 K; copper at 83, 173 and 295 K). Qualitative and quantitative data on the morphology and shape of persistent slip markings (PSMs), occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported. They are discussed in relation with recent physically based theories of surface relief formation leading to fatigue crack initiation.

Microstructural Characterization of Slurry Aluminide Diffusion Coatings

Commercially pure aluminium powder and a mixture of aluminium and silicon powders, both in a liquid amyl acetate-based organic binder, were sprayed onto the surface of Inconel 713LC nickel base superalloy. A two-stage heat treatment process in an argon atmosphere flow was designed and applied to produce nickel aluminide diffusion coatings. Two coating systems composed of different layers with gradual changes in chemical composition and phase quantities were formed. Scanning electron microscope, scanning electron microscope/focused ion beam, both equipped with energy dispersive microanalyzers, were utilized to characterize the microstructure and chemical composition of the coatings.