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In this study, nanostructured titania (TiO2; n-TO) and nanostructured alumina-titania (Al2O3-13 wt.%TiO2; n-ATO) coatings were successfully deposited on the 6061 aluminum alloy by applying the high-velocity oxy-fuel process. The n-TO coating showed limited pores or microcracks accompanied by both partially and fully melted areas within splats. The n-TO coating exhibited about 25% higher microhardness compared with the n-ATO coating. The agglomerated non-molten nanostructured TiO2 particles that are randomly dispersed and embedded within the n-TO coating microstructure act as arresters of cracks through either branching or blunting crack tips, resulting in about 17% higher bonding strength, as compared to the n-ATO coating with a looser microstructure. The n-TO coating demonstrated superior hardness, higher wear resistance, and smoother wear scar than the n-ATO and uncoated Al substrate. The electrochemical test indicated that the n-ATO coating had about a 50% higher corrosion rate than the n-TO coating because of its looser structure. The immersion test revealed that the n-ATO coating had been severely attacked by blistering and deep cracks corrosions, whereas limited attacks could be distinguished using the n-TO coating in the substrate interface. Based on the results, the n-TO coating can effectively protect the Al alloy substrate against 3.5 wt.% NaCl solution.

In this paper, two coating techniques, the high velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques, were used to deposit a bond coat of NiCoCrAlYTa on the Inconel 625 substrate, followed by applying a topcoat of yttria-stabilized zirconia (YSZ). The samples were preoxidized in an argon-controlled furnace at a temperature of 1000 °C for 12 and 24 h to characterize the microstructure of a thermally grown oxide (TGO) using the two coating techniques. The most suitable preoxidized samples were further tested for isothermal oxidation at 1000 °C for up to 120 h, and a hot corrosion test was performed at 1000 °C for up to 52 h or until spalling occurred. As-sprayed and oxidized samples prepared with different coating techniques were evaluated in terms of their microstructure using different characterization methods, such as field emission scanning electron microscopy (FESEM), variable pressure scanning electron microscopy (VPSEM), energy dispersive X-ray spectroscopy (EDS) equipped with energy dispersive X-ray and X-ray diffraction (XRD) analyses. In addition, the mechanical properties of these samples were evaluated using adhesion tests. The results show that the YSZ/NiCoCrAlYTa coating applied with the HVOF technique forms a more thin and continuous layer of TGO than that obtained when applying a YSZ/NiCoCrAlYTa coating using the APS technique, indicating that a severe brittle oxidation interface exists between the two layers. The results also indicate that the mechanical strength obtained from the adhesion test of the coated samples is observably affected by the oxidation behaviors obtained with the different deposition techniques chosen.

Cobalt chromium alloys constitutes the base of one important group of biometallic with excellent mechanical properties. However, due to its non-bioactive and non-antimicrobial surface, cobalt chromium alloys are vulnerable to wear corrosive attack and bacterial infection. Hence, coating cobalt chromium alloys surface with superior biomaterials is the best proven techniques. In this work, nanostructure silver adopted titanium dioxide coating was deposited on cobalt chromium alloys using air plasma spray (APS) and high velocity oxy-fuel (HVOF) techniques. Nanostructure TiO2–Ag coating demonstrated superior hardness, wear resistance and lower coefficient of friction when compared to bare CoCr alloys. Reduced plastic deformation detected by microstructural analysis indicate that the interference of nano-structure coating in metallic matrix help lower the wear rate of the sample. Meanwhile, low porosity of the coating produced by HVOF technique shown a better wear resistance result than APS technique. Keywords: Cobalt–chromium alloy, Nanostructure TiO2–Ag coating, Thermal spray coating method, Surface properties, Wear performance

Since the use of the most stable Pb-based materials in the electronic industry has been banned due to human health concerns, numerous research studies have focused on Pb-free materials such as pure tin and its alloys for electronic applications. Pure tin, however, suffers from tin whiskers’ formation, which tends to endanger the efficiency of electronic circuits, and even worse, may cause short circuits to the electronic components. This research aims to investigate the effects of stress on tin whiskers’ formation and growth and the mitigation method for the immersion of the tin surface’s finish deposited on a copper substrate. The coated surface was subjected to external stress by micro-hardness indenters with a 2N load in order to simulate external stress applied to the coating layer, prior to storage in the humidity chamber with environmental conditions of 30 °C/60% RH up to 52 weeks. A nickel underlayer was deposited between the tin surface finish and copper substrate to mitigate the formation and growth of tin whiskers. FESEM was used to observe the whiskers and EDX was used for measuring the chemical composition of the surface finish, tin whiskers, and oxides formed after a certain period of storage. An image analyzer was used to measure the whiskers’ length using the JEDEC Standard (JESD22-A121A). The results showed that the tin whiskers increased directly proportional to the storage time, and they formed and grew longer on the thicker tin coating (2.3 μm) than the thin coating (1.5 μm). This is due to greater internal stress being generated by the thicker intermetallic compounds identified as the Cu5Sn6 phase, formed on a thicker tin coating. In addition, the formation and growth of CuO flowers on the 1.5 μm-thick tin coating suppressed the growth of tin whiskers. However, the addition of external stress by an indentation on the tin coating surface showed that the tin whiskers’ growth discontinued after week 4 in the indented area. Instead, the whiskers that formed were greater and longer at a distance farther from the indented area due to Sn atom migration from a high stress concentration to a lower stress concentration. Nonetheless, the length of the whisker for the indented surface was shorter than the non-indented surface because the whiskers’ growth was suppressed by the formation of CuO flowers. On the other hand, a nickel underlayer successfully mitigated the formation of tin whiskers upon the immersion of a tin surface finish.

Titania nanoparticles were utilized as suspension in alkaline aluminate electrolyte to form nanocomposite coatings on magnesium alloy containing 1 wt% calcium by plasma electrolytic oxidation process. Microhardness, wettability, potentiodynamic polarization, wettability, electrochemical impedance spectroscopy (EIS), and immersion tests were performed in 3.5% NaCl solution to study corrosion behavior of the coated samples in reference to the uncoated Mg alloy. The coating with 4 g/L concentration of nanoparticles showed the highest microhardness, lowest hydrophilic properties and highest corrosion resistance. This coating substantially diminishes the Mg dissolution in the in 3.5% NaCl solution and increases the charge transfer resistance of the Mg alloy. Although higher concentrations of nanoparticles enhanced the defect density in the coating, they resulted in lower corrosion resistance. © 2016 Elsevier B.V.

Bone defects are very challenging in orthopedic practice. The ideal bone grafts should provide mechanical support and enhance the bone healing. Biodegradable magnesium (Mg)–based alloys demonstrate good biocompatibility and osteoconductive properties, which are promising biomaterials for bone substitutes. However, the high rate of their biodegradation in human body environment is still challenging. For this scope, synthesis Mg-based composites with bioceramic additives such as HA and titania (TiO2) is a routine to solve this problem. The aim of this study was to evaluate the effect of addition TiO2 nanopowders on the corrosion behavior and mechanical properties of Mg/HA-based nanocomposites fabricated using a milling-pressing-sintering technique for medical applications. The microstructure of Mg/HA/TiO2 nanocomposites, in vitro degradation and biological properties including in vitro cytocompatibility were investigated. The corrosion resistance of Mg/HA-based nanocomposites was significantly improved by addition 15 wt% of TiO2 and decrease HA amount to 5 wt% this was inferred from the lower corrosion current; 4.8 µA/cm2 versus 285.3 µA/cm2 for the Mg/27.5 wt%HA, the higher corrosion potential; −1255.7 versus −1487.3 mVSCE, the larger polarization resistance; 11.86 versus 0.25 kΩ cm2 and the significantly lower corrosion rate; 0.1 versus 4.28 mm/yr. Compressive failure strain significantly increased from 1.7% in Mg/27.5HA to 8.1% in Mg/5HA/15TiO2 (wt%). The Mg/5HA/15TiO2 (wt%) nanocomposite possessed high corrosion resistance, cytocompatibility and mechanical properties and can be considered as a promising material for implant applications.

Nanostructured composite ceramic coatings consisting of Al2O3–13%TiO2 and Al2O3–13%TiO2/TiO2 on NiCoCrAlYTa-coated Mg alloys were sprayed using the atmospheric plasma spraying method. The composition and microstructure of the coated samples were investigated by XRD and FESEM equipped with EDS. Corrosion and wear behaviours of the coated samples were also evaluated. The results showed that Al2O3–13%TiO2/TiO2 coating is able to reduce the anodic dissolution of the Mg alloy in chloride solutions compared to other samples. The surface of the Al2O3–13%TiO2/TiO2 coating with the lowest wear rate exhibited a typical mild wear mode with narrow wear tracks along with very few small transferred particles. This observation was mainly related to the dense structure of the TiO2 coating, which could reduce severe spalling within the splats during the wear test.

Micro arc oxidation (MAO) layer (as bond coat) was successfully deposited on Mg–1%Ca alloy in KF + NaAlO 2 electrolyte. In order to improve the corrosion resistance of the conventional MAO coated Mg–1%Ca alloy, air plasma sprayed (APS) nanostructure TiO 2 coating (as top coat with bimodal structure) was successfully applied on the aforementioned bond coat. The results indicated that the porous MAO layer was effectively sealed by the (APS) nanostructure TiO 2 top coat which showed compact and dense structure along with nano regions. In fact, MAO coating acted as interlayer to provide inert rough surface and necessary hardness for the Mg alloy to bond with air plasma sprayed TiO 2 top layer. The positive effect of the air plasma sprayed nanostructure TiO 2 coating was attributed to its mechanical isolation against aggressive medium to the MAO coating and Mg matrix. So, novel duplex ceramic composite coating (MAO/nanoAPSTiO 2 ) could significantly improve the corrosion resistance of the Mg–1%Ca alloy in the chloride solutions.

In this study, plasma sprayed multilayered coatings were successfully applied on the Mg alloys. On the other hand, corrosion behaviors of the coated magnesium alloy substrates with three different plasma sprayed coatings consisting of NiCrAlY, NiCrAlY/nano Al 2 O 3 ·13%TiO 2 and NiCrAlY/nano Al 2 O 3 ·13%TiO 2 /nano TiO 2 coatings were elaborated in 3.5 wt% NaCl solution. Electrochemical studies depicted that the novel coating (NiCrAlY/nano Al 2 O 3 ·13%TiO 2 /nano TiO 2 ) is able to reduce the corrosion rate of the Mg alloy and increase impedance values compared to the other samples. In fact, TiO 2 sealing layer could fill some micro-pores in the Al 2 O 3 –13 wt%TiO 2 coating and could prevent the concentration of the electrolyte in the Al 2 O 3 –13 wt%TiO 2 coating.

Al-AlCr was coated on Mg-Ca and Mg-Zn-Ce-La alloys using physical vapor deposition method. The surface morphology of the specimens was characterized by x-ray diffraction, scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy, and atomic force microscopy (AFM). The AFM results indicated that the average surface roughness of Al-AlCr coating on the Mg-Ca alloy is much lower than that of Al-AlCr coating on the Mg-Zn-Ce-La alloy. However, Al-AlCr coating on the Mg-Ca alloy presented a more compact structure with fewer pores, pinholes, and cracks than Al-AlCr coating on the Mg-Zn-Ce-La alloy. Electrochemical studies revealed that the novel coating (Al-AlCr) can remarkably reduce the corrosion rate of the Mg-Ca alloy in 3.5 wt.% NaCl solution. It was seen that the anodic current density of the Al-AlCr-coated Mg-Ca alloy was very small when compared to the Al-AlCr-coated Mg-Zn-Ce-La and uncoated alloys. Impedance modulus (Z) of the Al-AlCr-coated samples was higher than that of the bare Mg alloys. Z of Al-AlCr-coated Mg-Ca alloy was higher than that of the Al-AlCr-coated Mg-Zn-Ce-La alloy at low frequency.

Al and Al/AlCr were coated on Mg–Zn–Ce–La alloy by physical vapour deposition (PVD). Microstructural characterization depicted that the average surface roughness of the nano-Al/AlCr coating is higher than that of the mono-layered Al coating, which was mainly related to the surface morphology of the Mg–Zn–RE alloy. Nano Al coating (1 1 1) presented a more compact structure with fewer pinholes and cracks than the Al/AlCr coating (1 1 0). Electrochemical studies reveal that Al coated Mg alloy has better corrosion resistance than the other samples. The lowest anodic current density and the highest charge transfer resistance were observed in the Al coated Mg alloy.

A continuous and thin Al 2 O 3 layer would act as an infiltration barrier to diminish the growth of spinels in thermal barrier coating systems. Therefore, isothermal oxidation of normal NiCrAlY/YSZ, nano NiCrAlY/YSZ and nano NiCrAlY/YSZ/nano Al 2 O 3 coatings was investigated at 1000 °C. These samples were then subjected to thermal cycles at 1150 °C. The microstructural characterisation showed that the spinel growth in the nano NiCrAlY/YSZ/nano Al 2 O 3 coating was much lower than the other coatings. This observation can be related to the formation of a fully continuous Al 2 O 3 layer on the nano NiCrAlY coating during pre-oxidation at 1000 °C and also to the presence of a nano Al 2 O 3 layer over the YSZ coating.

Microstructural consideration is implemented in the calculation using commercial finite element analysis to study the effect of thermal treatment for engineering applications. As a typical example, decarburization problem has been studied in the paper. The modeling analysis is then to be compared with the experimental results. The simulation of phases such as single γ-Fe, mixture of γ-Fe and α-Fe, and the mixture of α-Fe and Fe3C is performed by altering the heat treatment temperature. The simulated models have good agreement with the experimental results. The decarburization rate is the lowest at the temperature range between the two phases of γ-Fe and α-Fe coexist.

This research uses a continuum mechanics approach, combined with metallurgical analysis, to understand microscopically the effect of the phases existing within the austenite microstructure. The approach used is a classical continuum mechanics at a microscopic level to understand the stress distribution among the constituents within the austenite grains. Further, by employing simple failure theories, the research predicts the preferential cracking path numerically.

YVO4crystals and monoclinic ZrO2are known as hot corrosion products that can considerably reduce the lifetime of thermal barrier coatings during service. The hot corrosion resistance of two types of air plasma sprayed thermal barrier coating systems was investigated: an Inconel 738/NiCrAlY/YSZ (yttria-stabilized zirconia) and an Inconel 738/NiCrAlY/YSZ/nano-Al2O3as an outer layer. Hot corrosion test was accomplished on the outer surface of coatings in molten salts (45% Na2SO4+ 55% V2O5) at 1000°C for 52 hour. It was found that nanostructured alumina as outer layer of YSZ/nano-Al2O3coating had significantly reduced the infiltration of molten salts into the YSZ layer and resulted in lower reaction of fused corrosive salts with YSZ, as the hot corrosion products had been substantially decreased in YSZ/nano-Al2O3coating in comparison with normal YSZ coating after hot corrosion process.

Oxidation has been considered as one of the principal disruptive factors in thermal barrier coating systems during service. So, oxidation behavior of thermal barrier coating (TBC) systems with nanostructured and microstructured YSZ coatings was investigated at for 24 h, 48 h, and 120 h. Air plasma sprayed nano-YSZ coating exhibited a trimodal structure. Microstructural characterization also demonstrated an improved thermally grown oxide scale containing lower spinels in nano-TBC system after 120 h of oxidation. This phenomenon is mainly related to the unique structure of the nano-YSZ coating, which acted as a strong barrier for oxygen diffusion into the TBC system at elevated temperatures. Nearly continues but thinner Al 2O 3 layer formation at the NiCrAlY/nano-YSZ interface was seen, due to lower oxygen infiltration into the system. Under this condition, spinels formation and growth on the Al 2O 3 oxide scale were diminished in nano-TBC system compared to normal TBC system.

We examine chemical mapping of reaction phases in a Cu-Al multilayer system using low-loss electron energy loss spectroscopy spectrum imaging and image spectroscopy techniques. The sensitivity of the plasmon peak position and shape to various crystal structures and phases is exploited using postprocessing of spectra into second derivative plasmon maps and line scans. Analytical transmission electron microscopy is complemented by studies of the orientation relationship of the multilayer system using high-resolution electron microscopy of interfaces and selected area diffraction. The techniques have been applied to the Cu-Al multilayer sample and sharply bound epitaxial phases are found, before and after heat treatment.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Metallic multilayers have been used in cross-sectional TEM specimen geometry to explore nanostructuring routes using ion and electron beams (top-down) as well as self-organised nanopatterning (bottom-up). Intermetallic alloy formation during heat treatment is studied for binary and ternary layer compositions. A choice of pillars (0D), 1D and 2D layer geometries are used to explore dimensionality effects.

We report an electron beam (e-beam) fabrication method to produce sub-5 nm structures, e.g. nanohole-arrays, nanojunctions and nanotips inside a TEM. The method is demonstrated using ferromagnetic nickel cross-sectional TEM specimens as well as electrochemically etched nickel tips. Different e-beam shapes and electron guns are compared, including point versus line focus and field-emission versus LaB6 guns. As an extension of this 2-D patterning of nanostructures, a 3-D nanofabrication technique has been introduced using a high tilt tomographic holder. An electrochemically etched nickel tip of initially 18 nm in radius is sculpted down to sub-5 nm diameter. In some cases, mostly in 3-D nanofabrication, a protrusion during hole drilling was observed and confirmed by EELS as pure nickel.

The microstructure and phase stability of a CuAl-on-sapphire thin film material is examined using in-situ and ex-situ heating experiments and TEM, SAD, HREM, and EELS as analysis technique. An in-situ heating sequence has revealed formation and decomposition of the metastable tetragonal θ-alloy phase CuAl2 at two consecutive reaction temperatures. Pockets of unreacted Al remain at the interface to the substrate. The epitaxial orientation relationship of the θ α-Al2O3 interface is examined by HREM, while composition mapping across the metal-alloy-oxide regions is demonstrated using plasmon (low-loss) spectrum imaging and EDX mapping. A 2nd derivative technique is shown to be powerful for enhancing phase changes which exhibit similar plasmon resonance energies.