On the opening of a class of fatigue cracks due to thermo-mechanical fatigue testing of thermal barrier coatings

The evolution of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to an accelerated test scheme is investigated via numerical simulations. The TBC system consists of a NiCoCrAlY bond coat and partially yttria stabilized zirconia top coat with a thermally grown oxide (TGO) between these two coatings. The cracks of interest evolve in the bond coat parallel and near the interface with the TGO during thermo-mechanical fatigue testing. In their final stage, the cracks lead to partial spallation of the TBC. This study focuses on why the cracks open to their characteristic shape. To this end, finite element simulations are utilized. The crack surface separation is monitored for a range of material properties and oxidation rates. The simulations show that the inelastic response of the bond coat and the oxidation rate of the TGO govern the crack surface separation. Most interestingly, permanent separation of the crack surfaces is caused by a structural ratcheting in the vicinity of the crack.     

Elasto-plastic impact of a rotating link with a massive surface

This study presents the impact with friction of a rigid or a flexible body against a half-space, each with a small region of contact compliance where the compliance is obtained from the Jackson and Green theory. The model consists of a system of nonlinear differential equations which considers a nonlinear contact force as well as frictional effects at the contacting end, and allows one to predict the motion after the impact. The initial incidence angle, the initial impact velocity, and the contact radius of the link are found to influence the coefficient of restitution with friction. Analytical and experimental results were compared to establish the accuracy of the model.     

Advanced 3D Magnetic Scaffolds for Tumor-Related Bone Defects

The need for bone substitutes is a major challenge as the incidence of serious bone disorders is massively increasing, mainly attributed to modern world problems, such as obesity, aging of the global population, and cancer incidence. Bone cancer represents one of the most significant causes of bone defects, with reserved prognosis regarding the effectiveness of treatments and survival rate. Modern therapies, such as hyperthermia, immunotherapy, targeted therapy, and magnetic therapy, seem to bring hope for cancer treatment in general, and bone cancer in particular. Mimicking the composition of bone to create advanced scaffolds, such as bone substitutes, proved to be insufficient for successful bone regeneration, and a special attention should be given to control the changes in the bone tissue micro-environment. The magnetic manipulation by an external field can be a promising technique to control this micro-environment, and to sustain the proliferation and differentiation of osteoblasts, promoting the expression of some growth factors, and, finally, accelerating new bone formation. By incorporating stimuli responsive nanocarriers in the scaffold’s architecture, such as magnetic nanoparticles functionalized with bioactive molecules, their behavior can be rigorously controlled under external magnetic driving, and stimulates the bone tissue formation.     

Mechanical Behavior of Air Plasma-Sprayed YSZ Functionally Graded Mullite Coatings Investigated via Instrumented Indentation

Yttria-stabilized zirconia (YSZ)-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat are envisioned as solutions to ease their coefficient of thermal expansion mismatch induced stress. In this work, two different types of mullite powder (spray-dried and freeze-granulated) and a mullite-YSZ 75/25 vol.% mixture spray-dried powder were employed. Using instrumented indentation with loads between 10 and 500 mN, the role of the powder characteristics on the mechanical behavior of air plasma-sprayed mullite bond coats deposited on SiC substrates was investigated. Hardness (H) and elastic modulus (E) were measured for the as-sprayed coatings and for coatings heat-treated at 1300 °C, in water vapor environment, for periods up to 500 h. Both H and E values of the coatings are found to be highly dependent on the size distribution of the starting powders. It is aimed the fabrication of an efficient and cost-effective EBC prototype based on YSZ compositionally graded mullite.     

Mechanical and corrosion behaviour of a Ti‐Al‐Nb alloy after deformation at elevated temperatures

The mechanical properties of Ti6Al7Nb alloy deformed and heat treated at elevated temperatures were correlated with its microstructure and corrosion behaviour in Ringer (of different pH values: 2.49, 6.9 and 8.9) and Ringer–Brown solutions. Microstructural analysis revealed a Widmanstatten structure for the alloys deformed at 1100 °C (β field) and structure with α grains at 930 °C (α + β field). The thermo‐mechanical processing improved the electrochemical behaviour of Ti6Al7Nb alloys, especially their passive state. Open circuit potential variations in time reflected more compact, stable, resistant passive films on the surface of the treated alloys. Open circuit potential gradients simulating the non‐uniformities of pH along the implant surface have very low values that cannot generate galvanic corrosion. Corrosion rates and ion release are very much reduced. Impedance spectra were fitted with a two time‐constants equivalent circuit for some alloys and with three time‐constants equivalent circuit for other alloys.     

Self-organization phenomena at semiconductor electrodes

Anodically dissolving semiconductor electrodes such as Si, Ge, GaAs, InP, or GaP exhibit a number of self-organization phenomena such as current oscillations in time and/or in space; some phenomena of this kind are also found during the anodic formation of porous metal oxides. Current oscillations in space are expressed in correlated pore growth and other effects like pore diameter oscillation; this will be introduced and discussed in some detail. Some less well-known effects like self-induced growth mode transitions or pore density oscillations are also included. The paper endeavors to sort through the various self-organization phenomena observed so far and to look for underlying principles that transcend semiconductor-specific dissolution chemistry. Intrinsic time and length scales provide one such principle and this will be discussed with emphasis on the so-called current burst model originally developed for current oscillations in time.     

Interface engineering and characterization at the atomic‐scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin‐film solar cells

In this work, we investigate the p–n junction region for two different buffer/Cu(In,Ga)(Se,S)₂ (CIGSSe) samples having different conversion efficiencies (the cell with pure In₂S₃ buffer shows a lower efficiency than the nano‐ZnS/In₂S₃ buffered one). To explain the better efficiency of the sample with nano‐ZnS/In₂S₃ buffer layer, combined transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies were performed. In the pure In₂S₃ buffered sample, a CuIn₃Se₅ ordered‐defect compound is observed at the CIGSSe surface, whereas in the nano‐ZnS/In₂S₃ buffered sample no such compound is detected. The absence of an ordered‐defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn₃Se₅, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered‐defect compound. In the nano‐ZnS/In₂S₃ sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se₂) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p–n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Empirical exposition of the adsorption’s ionic mechanism on gaseous nitriding

The countless contradictions of practical or theoretical nature associated with the gaseous nitriding process, for a long time, have forced researchers to study in depth the theoretical aspects of the process. The analysis of the main phases of the mass transfer process occurring within the nitriding process led to the conclusion that the phases responsible for the series of noted contradictions are the ones corresponding to the reactions in the environment and at the environment-product interface. Through theoretical studies and experimental verification, it was discovered that, in the reaction space, under the specific conditions for the gaseous nitriding to occur, the ammonia most probably undergoes, in the areas close to the metallic surfaces, at the same time as the thermal catalytic dissociation, an ionization process that is followed as a consequence by the appearance of a greater number of anions and anionic complexes. In this paper are presented the results of the studies and experimental research related to the adsorption’s ionic mechanism on nitriding in a gaseous medium.     

Dynamic vaccination games and hybrid dynamical systems

This paper relates variational inequalities and hybrid dynamical systems to describe the evolution of a class of noncooperative games. To describe such evolution, we define a hybrid system based on a continuous dynamics described by a projected system, and on a finite set of exogenous event times. This hybrid system is then used to track evolution strategies. We apply our results to the evolution of population groups?? strategies playing a noncooperative vaccinating game over a finite time interval.