Magnesium alloys are attractive materials for reducing the weight of the engineering components. Despite their high strength to weight ratio, poor corrosion and wear performances limit their extensive usage. In this respect, improvement of their wear and corrosion resistance by coating processes is a necessity for magnesium alloys. In this study, the wear and corrosion performances of AZ91D magnesium alloy have been examined after magnesia and alumina coatings. While micro arc oxidation (MAO) process was applied to generate a magnesia layer, cold spraying (CS) and MAO processes were combined to obtain novel alumina layer. CS was conducted to cover the substrate by depositing an aluminum layer (monolithic or composite). Afterwards, application of the MAO process produced an alumina layer on the deposited aluminum layer (monolithic or composite) forming a multi-layered coating on the examined magnesium alloy. Results of the experiments revealed that the alumina layer formed on the alumina reinforced aluminum matrix composite layer ensured superior protection for AZ91D alloy against mechanical and chemical degradations as compared to the magnesia layer.

New fracture toughness (K-C) data for cobalt boride (CoB and CO2B) coatings were obtained using the Vickers depth-sensing microindentation technique. The coatings were developed on the surface of a CoCrMo alloy using the powder-pack bonding process at temperatures between 1223 and 1273 K using various exposure times for each temperature. The mechanical characterization of the boride coatings was divided into two procedures: first, Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For the entire set of experimental conditions, the behavior of the indentation load as a function of the diagonal length (d(m)) was examined on the CoB and CO2B coatings. Second, the crack lengths emanated on the corners of the indentations marks (with applied loads above 250 mN) were measured on both coatings using a scanning electron microscope (SEM).Based on the indentation results, the K-C values of the cobalt boride coatings were estimated using the universal crack equation, which is applicable independent of the cracking mode. The results indicated that the CoB and CO2B coatings exhibited two types of cracking modes (intermediate and radial-median, respectively), and that the fracture toughness of the CO2B coating was ten-fold greater than that of the CoB coating. (C) 2014 Elsevier B.V. All rights reserved.

In this study, microstructural evolutions, mechanical properties and corrosion performance of coatings made of 7075 Al matrix with B4C or SiC reinforcement deposited on T6 6061 Al alloy using the cold gas dynamic spraying process were investigated. Microstructural surveys have shown that coatings with no discontinuity at the interface as well as with fine grains were obtained and the addition of ceramic particles enhanced the coating density for a prescribed set of spray parameters and nozzle configuration. The presence of ceramic particles in the 7075 Al matrix improved the coatings hardness and wear resistance when compared to unreinforced 7075 Al coatings. Although coatings hardness increased with increasing ceramic particle content, the effect on the coatings wear performance is not that significant. B4C reinforced composite coatings exhibited slightly better wear performance compared to SiC reinforced composite coatings. The cold sprayed coatings showed more noble corrosion potentials but higher corrosion current densities than those of the T6 6061 Al substrate. The addition of ceramic particles into 7075 Al matrix led to increased corrosion current densities when compared to that of unreinforced 7075 Al coating.

This study aims at improving mechanical properties of electrical contacts through copper and copper matrix silicon carbide reinforced composite coatings produced by cold gas dynamic spraying coating and heat treatment. Characterization of coatings was made by microstructural investigations, electrical conductivity measurements and mechanical tests. Microstructural evaluations have shown that coatings adhere to the substrate without any discontinuity at the interface and ceramic particles distribute uniformly. XRD analyses have revealed that there are no other phases rather than constituents of feedstock powder mixtures. Cold sprayed coatings exhibited much higher hardness and lower electrical conductivity when compared with those of Cu substrate. In general cold sprayed coatings showed better wear performance when compared with Cu substrate. Composite coatings unexpectedly exhibited poor wear performance when compared with that of unreinforced Cu coating. Heat treatment significantly reduced the hardness and wear rate of the coatings as well as increased the electrical conductivity of the coatings.

Submitted for the SHOCK11 Meeting of The American Physical Society Enhanced reactivity of mechanically-activated nano-scale gasless reactive materials consolidated via the cold-spray technique ANTOINE BACCIOCHINI, MATEI RADULESCU, ONUR MEYDANOGLU, YANNICK CHARRON-TOUSIGNANT, JASON VAN DYKE, BERTRAND JODOIN, MICHEL NGANBE, MOHAMED YANDOUZI, University of Ottawa, JULIAN J. LEE, DRDC-Suffield — It has been speculated that gasless reactive systems can sustain supersonic detonations waves, provided the local decomposition rate is sufficiently fast and the initial density is sufficiently close to the theoretical maximal density. The present study presents a novel method to prepare nano-scale energetic materials with high reactivity, vanishing porosity, structural integrity and arbitrary shape. The experiments have focused on the Ni-Al system. To increase the reactivity, an initial mechanical activation was achieved by the technique of ball milling. The consolidation of the materials used the supersonic cold gas spray technique, where the particles are accelerated to high speeds and consolidated via plastic deformation upon impact, forming activated nano-composites in arbitrary shapes with close to zero porosity. This technique permits to retain the micro-structures in the powders and prevents any reactions during the consolidation phase. Deflagration tests of the obtained samples showed an increase in the deflagration rate by up to two orders of magnitude. Matei Radulescu University of Ottawa Date submitted: 23 Mar 2011 Electronic form version 1.4

In this study, microstructures and mechanical properties of Al matrix B4C reinforced composite coatings deposited on a structural steel by cold spraying were investigated. Feedstocks were prepared as the mixtures of Al and B4C powders, where the B4C content varied between 0 to 60 vol. %. The average sizes of the Al and B4C powders were similar to 10 mu m and similar to 60 mu m, respectively. Air at room temperature was used as a propellant gas under a pressure of 6 bar. Hardness measurements and wear tests were performed to characterize the mechanical performances of the coatings. Hardness measurements were made by using Vickers micro hardness tester. Wear tests were conducted under dry sliding conditions on a reciprocating wear tester by rubbing alumina and stainless steel balls to the coatings. Presence of B4C improved the hardness and the wear resistance when compared to those of B4C free Al coating. Hardness of the coatings increased with increasing B4C content; while the wear resistances did not show any significant change with the volume fraction of the B4C powders. In this respect, the optimum composition of the feedstock was determined as 80 vol. % Al + 20 vol. % B4C.

Wear and biological performances of a thermally oxidized Ti6Al7Nb alloy were investigated. Thermal oxidation (TO) performed at 600 °C for 60 h in air formed a 0.6 μm thick and relatively rough (having an average surface roughness of 1.1 μm) oxide layer (OL) on the surface. The OL was identified as the rutile form of TiO 2 and there was an oxygen diffusion zone (ODZ) with an average thickness of 5 μm just beneath it. The applied TO process resulted in more than ten-fold increase in wear resistance in a simulated body fluid (SBF) solution. Additionally, the biological performance was also enhanced as revealed by SBF immersion and cell culture tests.

The production of nano-calcium phosphate, such as HA (hydroxyapatite), materials from synthetic chemicals could sometimes lead to a costly and tedious work. Sea creatures could be an alternative way to produce very fine and even nano-structured calcium phosphate materials. Nacres vastly consist of rich calcium carbonate and/or aragonite mater. With simple conversion methods, like hotplate stirring, various bioceramic structures could be produced suitable for thin film coatings with various methods, like pulsed laser deposition (MAPLE). This study is part of a bigger project which eventually and ultimately aims to produce nano-phases of calcium phosphate biocompatible bioceramics, which can be used for biomedical coatings. In this particular study, we focus at transforming chemically, using hotplate stirring method, local sea snail shells rapana thomasiana. Cleaned sea snail samples were provided from local markets in Istanbul. The shells were smashed down, ball-milled and the powder was sieved (<100 µm powder particles). Differential thermal analysis (DTA/TG) was employed to evaluate the exact CaCO3 content of the shells. According to these results, the required volume of H3PO4 was added in order to set the molar ratio of Ca/P (during hotplate stirring) either 10/6 or 3/2 (these ratios correspond to HA and TCP, respectively). SEM and X-ray diffraction analyses were conducted. The SEM observations showed brick-like particles were formed with sizes <5 µm. From the X-ray diffraction analysis, predominantly monetite, which can be considered as a precursor of HA and TCP, was detected. The results of this study showed that to produce HA and other bioceramic phases, hot-plate stirring method is a reliable, fast, rapid and economic method when compared to other tedious HA production methods. Moreover, sea snail shells are very good candidate materials to produce fine powders with hotplate stirring method for various tissue engineering applications.

In this study, microstructures and mechanical properties of sheep hydroxyapatite (SHA) and commercial synthetic hydroxyapatite (CSHA)-MgO composites were investigated. The production of hydroxyapatite (HA) from natural sources is preferred due to economical and time saving reasons. The goal of development of SHA and CSHA based MgO composites is to improve mechanical properties of HA. SHA and CSHA composites were prepared with the addition of different amounts of MgO and sintered at the temperature range of 1000-1300 °C. The physical and mechanical properties were determined by measuring density, compression strength and Vickers microhardness (HV). Structural characterization was carried out with X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies. In all composites, mean density values and mechanical properties increased with increasing sintering temperature. The increase of MgO content in SHA-MgO composites showed better mechanical properties in contrast to CSHA-MgO composites. Although the highest hardness and compression strength values were obtained at the SHA-10wt% MgO composite sintered at 1300°C, higher hardness and compression strength values were achieved with 5 wt% MgO addition at the CSHA-MgO composites when compared to SHA-MgO composites sintered between 1000-1200°C.

The regeneration potential of human bone is limited in the cases of repairing large bone defects, such as those associated with comminuted fractures or bone tumor resection. In most cases, autogenous and allogenic bones are used as bone grafts. However, the amount of both of them is severely limited. Nowadays, natural biomaterials are in question, like corals, cuttlefish, and various nacre species, or hydroxyapatite (HA) made from egg shells. The present work aims at preparing inexpensive nano-sized HA and whitlockite particles from various raw materials of natural-biological origin. Razor shells (ensis ensis) were collected from beaches of Thessaloniki in Greece. Each sample was reduced to particle size <100 µm and DTA/TG was employed to determine their exact CaCO3content. The suspended raw powders were put on a hotplate. The temperature was set to 80°C for 15 min. Then, equivalent amount of H3PO4was added, drop by drop, into the solution. Different Ca/P ratios were tried. The reaction was ultrasonically assisted and continued for 8h. Then, to evaporate the liquid part, the mixture was put into an incubator at 100°C for 24 h and the resulting dried sediment was collected. The morphology of the produced powders was examined by SEM and revealed nano-sized particles. X-ray diffraction analysis indicated various Ca-phosphate phases, i.e. monetite and calcium phosphate hydrate. Thus, razor shells could be an alternative source for calcium phosphate ceramics production. In this study, long nacre shells were converted to various bioceramic structures with simple ultrasonic method without using hydrothermal method, which is carried out in a close vessel heated in a furnace and could cause accident if the vessel is worn. Chemical ultrasonic method is very safe and reliable method for bioceramic production from aragonite structures.

Air oxidation behavior of a Ti6Al7Nb alloy was examined over the temperature range of 873 –1173 K for different time intervals ranging in between 12 and 72 h. The rate of oxidation evaluated according to the weight gain measurements, fitted parabolic kinetics by yielding oxidation activation energy of 226 kJ/mol. Rutile and anatase modifications of TiO2 formed on the surface as the result of air oxidation. Oxidation temperatures higher than 923 K encouraged rutile formation rather than anatase. As temperature of oxidation was increased, the thickness of the oxide layer increased. Thicker oxide layer provided higher surface hardness and better protection against a corrosive media (5 M HCl solution) was provided.

In this study titanium carbide reinforced titanium matrix composites were produced by conventional powder metallurgy method. Titanium powders were mixed with graphite powders at different compositions. Composites produced directly from titanium and graphite powder mixtures provided high amount of porosity in the microstructure, while involving high hardness values. In the present study porosities were eliminated by a two step process. As the first step bulk titanium carbide was produced from titanium and graphite powder mixture. In the second step porosity free titanium matrix composites were produced by utilizing the mixture of titanium carbide and titanium powders. Microstructural examinations, hardness measurements and wear tests were conducted on composites. Although relative densities of composites decreased, higher hardness values and wear resistances were obtained as titanium carbide content increased.

Composites of calcinated bovine bone derived hydroxyapatite (HA) with 5, 7.5 and 10 wt % B2O3 were prepared by sintering. The production of HA from natural sources is preferred due to economical and time saving reasons. In this study scanning electron microscopy (SEM) investigations, microhardness and compression strength measurements were performed on composites. The experimental results indicated that compression strength and microhardness of HA-boron-oxide composites decrease when the content of boric acid and sintering temperature increase. The best mechanical properties achieved for 5 wt % addition of dehydrated boric acid. It was seen that at higher sintering temperatures, the compression strength and the microhardness decrease due to the very intensive pore formation. The results agree fairly well with microstructure analysis.

In this study, the effect of thermal oxidation on fatigue behavior of Ti6Al4V and Ti6Al7Nb alloys has been examined. Thermal oxidation was carried out by holding the alloys at 600 degrees C for 60 h in normal atmospheric condition. Upon thermal oxidation the surfaces of the alloys were covered with a 1-2 mu m thick oxide layer supported by a 6-7 mu m thick oxygen diffusion zone. Thermal oxidation caused considerable increase in surface hardness without altering the tensile properties (with the exception of yield strength) significantly. When compared to the as-received state, thermal oxidation caused a reduction in rotating bending endurance limit almost about 30% for Ti6Al4V alloy and about 10% for Ti6Al7Nb alloy.

Composites of calcinated bovine bone derived hydroxyapatite (HA) with 0.5 and 1 wt% Y2O3 were prepared by sintering. Money and time saving feature the production of HA from natural sources. In this study, results of scanning electron microscopy (SEM) and X-ray diffraction analysis aimed to interpret the results of measurements of densification, microhardness, and compression strength of the produced composites. The best mechanical properties were achieved after sintering at 1200°C for compressive strength and 1300°C for microhardness. The results are in a fair agreement with densification measurements and microstructure analysis.

In this study, the effect of thermal oxidation on the high cycle rotating bending fatigue behavior of Ti6Al4V alloy was investigated. Oxidation, which was performed at 600°C for 60 h in air, considerably improved the surface hardness and particularly the yield strength of the alloy without scarifying the tensile ductility. Unfortunately, the rotating bending fatigue strength at 5x106 cycles decreased from about 610 MPa to about 400 MPa upon oxidation. Thus, thermal oxidation leaded a reduction in the fatigue strength of around 34%, while improving the surface hardness (HV0.1) and yield strength 85 % and 36 %, respectively.

In this study, producibilify of B4C particle reinforced Ad alloy matrix composites from 7075 Al alloy machining chips has been investigated. After blending 7075 Al alloy with 10 wt % B4C particles, mixtures were pressed at 350 degrees C for two hours under a pressure of 625 MPa. Characterization of the composites was made by structural examinations and mechanical tests (hardness and dry sliding wear tests). When compared to conventionally extruded 7075 Al alloy, 10 wt.% B4C particles reinforced 7075 aluminum alloy matrix composite exhibited superior hardness and wear resistance especially after T6 tempering.

Composites of bovine bone derived HA with 5 wt% and 10 wt% of TiO2 were sintered at different temperatures. Their characterization comprised measurements of density, microhardness, and compression strength together with SEM observations and X-ray diffraction analysis. Better densification behaviour was achieved at higher sintering temperatures. The highest microhardness value was measured in the samples sintered at 1300°C. The best compressive strengths of the samples containing 5% and 10% TiO2 were obtained after sintering at 1300°C and 1200°C, respectively.

Composites of calcinated bovine bone derived hydroxyapatite (HA) with 0.25, 0.5, 1, and 2 wt % La2O3 were prepared by sintering. The experimental results indicated that compression strength and microhardness of HA-La2O3 composites increase when the content of La2O3 and sintering temperature increase. The best mechanical properties were achieved after sintering at 1300°C. The results are in agreement with densification measurements and microstructure analysis.