Microstructural and Tribological Characteristics of Composites Obtained by Detonation Spraying of Iron-Based Alloy-Carbide Powder Mixtures.

iron-based coatings have exhibited good mechanical properties, such as high hardness and good wear resistance, which are desirable properties in applications such as automobile brake rotors. iron-based coatings are also good replacements for Co- and Ni-based coatings, which are costly and could have health and environmental concerns due to their toxicity. In this research, three different iron-based coatings were deposited using the Detonation Gun Spraying (DGS) technology onto aluminum substrates, including the steel powders alone (unreinforced), and steel powders mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings and mechanical properties, such as hardness and wear resistance, were examined. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. While it was expected that steel particles reinforced with hard ceramic particles would result in increased hardness, instead, the unreinforced steel coating had the highest hardness, possibly due to a higher degree of amorphization in the coating than the other two. The microstructural observation confirmed the formation of dense coatings with good adhesion between layers. All samples were subjected to ball-on-disk wear tests at room temperature (23 °C) and at 200 °C. Similar wear resistances of the three samples were obtained at room temperature. At 200 °C, however, both ceramic reinforced composite samples exhibited higher wear rates in line with the reduction in their hardness values. This work explains, from the microstructural point of view, why adding hard particles to steel powers may not always lead to coatings with higher hardness and better wear resistance.

Effect of Heat Treatment on Residual Stress of Cold Sprayed Nickel-based Superalloys.

Residual stress formation during cold spraying process may result in deteriorative effects on the performance of coating materials. The objective of this investigation is to characterize residual stress built-up in two well-known nickel-based superalloys (Inconel 625 and Inconel 718) deposited using cold spraying technique. To this end, the residual stress was precisely measured using x-ray diffraction method. Here, residual stress in the subsurface regions was only studied because the surface properties may alter during sample preparation. The average residual stress was slightly higher in Inconel 625 compared to the Inconel 718 sample. Heat treatment at 800 °C helped in the reduction of porosities which exerted tensile stress in subsurface regions of both coatings. Stresses with opposite signs could cancel each other and result in reduction of residual stress after heat treatment. However, the recovery of residual stress was higher for Inconel 718 coating. In the next step as-sprayed and heat-treated coating samples were subjected to microindentation test to measure their hardness and study the crack formation in the samples. The as-sprayed Inconel 625 exhibited higher hardness than Inconel 718, but the hardness of Inconel 625 decreased more drastically after heat treatment. While the cracks were formed on both as-sprayed samples around indents, no cracks were found in the heat-treated samples. The results from this study will contribute to better understanding the performance of cold spray deposited superalloys under service conditions and the effect of stress relaxation heat treatment on elimination of residual stress.

Computational biomechanics of human brain with and without the inclusion of the body under different blast orientation.

Three different human head models in a free space are exposed to blast waves coming from four different directions. The four head-neck-body models composed of model a, with the neck free in space; model b, with neck fixed at the bottom; and model c, with the neck attached to the body. The results show that the effect of the body can be ignored for the first milliseconds of the head-blast wave interactions. Also one can see that although most biomechanical responses of the brain have similar patterns in all models, the shear stresses are heavily increased after a few milliseconds in model b in which the head motion is obstructed by the fixed-neck boundary conditions. The free-floating head model results are closer to the attached-body model.

A computational study of influence of helmet padding materials on the human brain under ballistic impacts.

The results of a computational study of a helmeted human head are presented in this paper. The focus of the work is to study the effects of helmet pad materials on the level of acceleration, inflicted pressure and shear stress in a human brain model subjected to a ballistic impact. Four different closed cell foam materials, made of expanded polystyrene and expanded polypropylene, are examined for the padding material. It is assumed that bullets cannot penetrate the helmet shell. Finite element modelling of the helmet, padding system, head and head components is used for this dynamic nonlinear analysis. Appropriate contacts and conditions are applied between the different components of the head, as well as between the head and the pads, and the pads and the helmet. Based on the results of simulations in this work, it is concluded that the stiffness of the foam has a prominent role in reducing the level of the transferred load to the brain. A pad that is less stiff is more efficient in absorbing the impact energy and reducing the sudden acceleration of the head and consequently lowers the brain injury level. Using the pad with the least stiffness, the influence of the angle of impacts as well as the locations of the ballistic strike is studied.