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We report on the mechanical properties of CH3NH3PbI3 photovoltaic perovskite measured by nanoindentation. The Young's modulus (E) of the pristine sample is 20.0 ± 1.5 GPa, while the hardness (H) is 1.0 ± 0.1 GPa. Upon extended exposure to water vapor, both quantities decrease dramatically and the sample changes color from silver-black to yellow. Calculations based on density functional theory support this trend in the mechanical response. Chemical treatment of the degraded crystal in methylammonium iodide solution recovers the color of the pristine sample and the values of E and H within 50%.

International audience; The present paper focuses on the development of an inverse procedure able to infer the hydrodynamic pressure distribution acting on the material surface and responsible for the creation of the cavitation pit based on the stress in a cavitation pit and its geometrical features. To achieve this goal, experimental pitting and nanoindentation measurement techniques were used together with a model of pit formation based on a Gaussian distribution of the hydrodynamic impact pressure pulse. The pitting tests were performed at four different operating pressures on aluminum alloy samples and the geometrical characteristics of the pits were measured. Then the strain, stress, and load in a cavitation pit were quantified by coupling the pitting test analysis with the material information obtained via the indentation tests. Finally, a Gaussian distribution of the hydrodynamic impact pressure on the material surface has been hypothesized and the peak of this distribution as well as its width have been inferred. This procedure, allowing the evaluation of the hydrodynamic impact pressure and load responsible for the material erosion and the deduction of insightful information on the flow aggressiveness at different operating conditions, could certainly represent a significant step in developing a technique able to evaluate the cavitation intensity from pitting tests.

International audience; This article focuses on the use of spherical nanoindentation measurements to estimate the pressure of cavitation impacts and its statistical distribution. Indeed, nanoindentation techniques are supposed to represent an effective tool in this field due to the similarities between substrate deformation under liquid impact and indentation testing. First, nanoindentation experiments were used to extract the mechanical parameters of a Nickel-Aluminum-Bronze alloy; second, pitting tests were performed at different operating pressures, and the geometrical characteristics of the pits were measured; and finally, the spectra of impact pressure and loads responsible for material erosion were obtained by coupling the findings of indentation tests with the analysis of pitting tests. Results assessed the capability of the proposed methodology to quantify the hydrodynamic aggressiveness of the cavitating flow. This procedure, which assumes the material itself as a sensor that is able to detect the impact loads, could represent an alternative solution to pressure transducers in estimating the cavitation intensity.

Reduction of macroparticles embedded within the cathodic arc deposited coatings is important for achieving high quality coatings with superior tribological properties, and it remains a major challenge in cathodic arc depositions. In the present study, the authors aim to understand the nature of macroparticles formed from TiNb alloy cathodes and the role of TiNb cathode nitridation in reducing the amount of generated macroparticles. The microstructure of the macroparticles was investigated using scanning transmission electron microscopy-energy dispersive spectroscopy, scanning electron microscopy, and x-ray diffraction techniques and revealed compositionally different macroparticles, such as macroparticles with a Nb-rich core, Ti-rich macroparticles, and macroparticles with a composition similar to that of the coatings. The examination of the macroparticles indicated the compositional segregation of niobium and titanium. With increasing N2 gas pressure during deposition, the macroparticles showed increased...

Cavitation erosion during the incubation period was investigated via pitting tests conducted on three different materials: an Aluminum alloy, a Nickel Aluminum Bronze alloy and a Duplex Stainless Steel. Pitting tests were conducted in a cavitation tunnel in the velocity range 45-90 m/s at a constant cavitation number. The test section was made of a straight nozzle 16 mm in diameter discharged into the radial 2.5 mm space between two flat walls. Cavitation appears in the form of a toroidal cavity attached to the nozzle exit and damage on the samples facing the nozzle is concentrated in a circular ring centered in the cavity closure region. The exposure time was adjusted to avoid pit overlapping. The material surface was examined using a conventional contact profilometer which allowed us to identify the pits, count them, and measure their main characteristics such as depth, surface area, and volume. From these the pitting rate, the coverage rate, and the depth of deformation rate were defined. Pits were classified according to their diameter. For all materials and operating conditions, pitting rate appears to follow an exponential law in relation to the pit diameter. This law depends upon two parameters only, which were identified as the coverage time tau (i.e. the time required for the surface to be covered by erosion pits) and a characteristic pit diameter delta, which corresponds to the pits whose contribution to the coverage process is the highest. Scaling laws for pitting were derived accounting for both material properties and flow velocity, and a procedure to make pitting test results non-dimensional is proposed. The influence of the material on pitting test results was analyzed. It is shown that the damage is not correlated in simple terms with the elastic limit determined from conventional tensile tests and it is conjectured that other parameters such as the strain rate might play a significant role and should be included in the analysis. The effect of flow velocity on both parameters tau and delta was analyzed and a classical power law was found for the influence of the flow velocity on pitting rate for all three materials. Finally, some analysis and discussion is given concerning distributions of pit volume and pit depth. (C) 2011 Elsevier B.V. All rights reserved.

Nanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@silica core-shell nanoparticles in a photocurable polymeric matrix. The results reveal a complex interplay between graded morphology, indentation depth, and calculated modulus and hardness values, which was elucidated through numerical simulations. First, it was experimentally shown how for small (1 μm) indentations, large increases in modulus (up to +40%) and hardness (up to +93%) were obtained for graded composites with respect to their homogeneous counterparts, whereas at a larger indentation depth (20 μm), the modulus and hardness of the graded and homogeneous composites did not substantially differ from each other and from those of the pure polymer. Then, through a material point method approach, experimental nanoindentation tests were successfully simulated, confirming the importance of the indentation depth and of the associated plastic zone as key factors for a more accurate design of graded polymer nanocomposites whose mechanical properties are able to fulfill the requirements encountered during operational life.

In order to mimic the complex architecture of many bio materials and synthesize composites characterized by continuously graded composition and mechanical properties an innovative synthetic strategy making use of magnetic field gradients and based on the motion of superparamagnetic Fe 3O4SiO2 core shell nanoparticles is adopted. It is demonstrated that by lowering the viscosity of the system through particle functionalization and increasing the magnetic force acting on the nanoparticles upon optimization of a simple set up composed of two permanent magnets in repulsion configuration the magnephoretic process can be considerably accelerated. Thus owing to the magnetic responsiveness of the Fe3O4 core and the remarkable mechanical properties of the SiO2 shell approximately 150 µm thick polymeric films with continuous gradients in composition and characterized by considerable increments in elastic modulus (up to ˜70) and hardness (up to ˜150) when going from particle depleted to particle enriched regions can be synthesized even in times as short as 1 hour. The present methods are highly promising for a more efficient magnetic force based synthesis of inhomogeneous soft materials whose composition is required to be locally tuned to meet the specific mechanical demands arising from non uniform external loads. This journal is © The Royal Society of Chemistry 2014.

Cavitation erosion prediction and characterization of cavitation field strength are of interest to industries suffering from cavitation erosion detrimental effects. One means to evaluate cavitation fields and materials is to examine pitting rates during the incubation period, where the test sample undergoes localized permanent deformations shaped as individual pits. In this study, samples from three metallic materials, an Aluminum alloy (Al 7075), a Nickel Aluminum Bronze (NAB) and a Duplex Stainless Steel (SS A2205) were subjected to a vast range of cavitation intensities generated by cavitating jets at different driving pressures and by an ultrasonic horn. The resulting pitted sample surfaces were examined and characterized with a non-contact 3D optical scanner and the resulting damage computer-analyzed. A statistical analysis of the pit population and its characteristics was then carried out. It was found that the various cavitation field strengths can be correlated to the measured pit distributions and that two characteristic quantities: a characteristic number of pits per unit surface area and unit time, and a characteristic pit diameter or a characteristic pit depth can be attributed to a given "cavitation intensity level". This characterization concept can be used in the future to study the cavitation intensity of the full scale and to develop methods of full scale predictions based on model scale erosion data. (C) 2012 Elsevier B.V. All rights reserved.