Muhamad Azizi Mat Yajid

In-vitro degradation behavior of Mg alloy coated by fluorine doped hydroxyapatite and calcium deficient hydroxyapatite

Fluorine-doped hydroxyapatite (FHA) and calcium deficient hydroxyapatite (CDHA) were coated on the surface of biodegradable magnesium alloy using electrochemical deposition (ED) technique. Coating characterization was investigated by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The result shows that nano-FHA coated sample presents nano needle-like structure, which is oriented perpendicular to the surface of the substrate with denser and more uniform layers compared to the nano-CDHA coated sample. The nano-FHA coating shows smaller crystallite size (65 nm) compared to the nano-CDHA coating (95 nm); however, CDHA presents thicker layer (19 μm in thickness) compared to the nano-FHA (15 μm in thickness). The corrosion behaviour determined by polarization, immersion and hydrogen evolution tests indicates that the nano-FHA and nano-CDHA coatings significantly decrease corrosion rate and induce passivation. The nano-FHA and nano-CDHA coatings can accelerate the formation of bone-like apatite layer and significantly decrease the dissolution rate as compared to the uncoated Mg alloy. The nano-FHA coating provides effective protection to Mg alloy and presents the highest corrosion resistance. Therefore, the nano-FHA coating on Mg alloy is suggested as a great candidate for orthopaedic applications.

Investigation of three steps of hot corrosion process in Y2O3 stabilized ZrO2 coatings including nano zones

Abstract Phase transformation of tetragonal ZrO2 to monoclinic phase and also increment of bond coat oxidation kinetic (TGO thickening) can substantially restrict the life time of thermal barrier coating systems (TBCs). So, nanostructured and conventional Y2O3 stabilized ZrO2 coatings were evaluated in fused V2O5-Na2SO4 salts during thermal exposure in air. Microstructural characterization showed lower hot corrosion products (monoclinic zirconia, YVO4 crystals) formation and reduction of TGO thickness in thermal barrier coating system consisting of nanostructured Y2O3 stabilized ZrO2 (YSZ) top coat. It was found that inhomogeneities, pores and micro-cracks played a principal role in the molten salts infiltration into the YSZ coating during three steps of hot corrosion process. In the nanostructured YSZ coating with tri-model structure, nano zones which surrounded by fully molten parts could fill the aforementioned defects and could act as barrier for the oxygen and corrosive molten salts penetration into the TBC.

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

Abstract A thermally grown oxide (TGO) layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al2O3 coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni(Cr,Al)2O4 mixed oxides (as spinel) and NiO onto the Al2O3 (TGO) layer. The formation of detrimental mixed oxides (spinels) on the Al2O3(TGO) layer of nano NiCrAlY/YSZ/nano Al2O3 coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.

Effect of Y2O3 stabilized ZrO2 coating with tri-model structure on bi-layered thermally grown oxide evolution in nano thermal barrier coating systems at elevated temperatures

Abstract Bi-layered thermally grown oxide (TGO) layer plays a major role in the spallation of Y2O3 stabilized ZrO2 (YSZ) layer form the bond coat in the thermal barrier coating (TBC) systems during oxidation. On the other hand, bi-layered TGO formation and growth in the TBC systems with nanostructured YSZ have not been deeply investigated during cyclic oxidation. Hence, Inconel 738/NiCrAlY/normal YSZ and Inconel 738/NiCrAlY/nano YSZ systems were pre-oxidized at 1000 °C and then subjected to cyclic oxidation at 1150 °C. According to microstructural observations, nanostructured YSZ layer over the bond coat should have less micro-cracks and pinholes, due to the compactness of the nanostructure and the presence of nano zones that resulted in lower O infiltration into the nanothermal barrier coating system, formation of thinner and nearly continuous mono-layered thermally grown oxide on the bond coat during pre-oxidation, lower spinels formation at the Al2O3/YSZ interface and finally, reduction of bi-layered thermally grown oxide thickness during cyclic oxidation. It was found that pre-heat treatment and particularly coating microstructure could influence microstructural evolution (bi-layered TGO thickness) and durability of thermal barrier coating systems during cyclic oxidation.

The Role of Nanostructured Al2O3 Layer in Reduction of Hot Corrosion Products in Normal YSZ Layer

YVO4 crystals and monoclinic ZrO2 are 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-Al2O3 as 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-Al2O3 coating 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-Al2O3 coating in comparison with normal YSZ coating after hot corrosion process.

Synthesis and Characterization of Rice Husk Ash Derived - Silica Aerogel Beads Prepared by Ambient Pressure Drying

Low density silica aerogel beads were synthesized from rice husk ash via sodium silicate route. The gel beads were prepared by the ball dropping method and dried at ambient pressure after surface modification (silylation) with trimethylchlorosilane (TMCS) solution. The characteristics of porous structure of silica aerogel beads were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption and desorption method. The silica aerogel beads showed a specific surface area as high as 773 m2/g. They also displayed water-repellant effect (hydrophobic) due to silylation as revealed by Fourier transform infrared spectroscopy (FTIR).

Effects of silica aerogel particle sizes on the thermal–mechanical properties of silica aerogel – unsaturated polyester composites

ABSTRACT Silica aerogels with a surface area as high as 773 m2 g−1 and a density of 0.077 g cm−3 were produced from rice husk via sol–gel process and ambient pressure drying. A particulate composite material was prepared by adding silica aerogel particles of three different particle sizes (powder, granules and bead) to unsaturated polyester resin with a fixed volume fraction of 30%. Thermogravimetric and thermal conductivity studies revealed that silica aerogel composites were having higher thermal stability and thermal insulation than the neat resin. It was suggested that the preservation of aerogel pores from resin intrusion is important for better thermal properties. Larger silica aerogel particles have more porous area (unwetted region) which results in a lower degradation rate and lower thermal conductivity of the base polymer. However, the addition of silica aerogel into resin has reduced the tensile modulus of the polymer matrix where smaller particle size displayed higher toughness than those with bigger particle size.

In situ and ex situ transmission electron microscopy investigation of Cu–Al–Cu–Ti reactive metallic multilayer coatings

Metallic multilayers of Cu/Al/Ti composition were studied by transmission electron microscopy (TEM) and plasmon energy-loss mapping as prototypes of nanoscale reactive multilayer systems with exothermic alloy formation in oxygen-free conditions. The selection and arrangement of alloy phases by the system during ex situ and in situ heating experiments were found to depend not only on temperature but strongly on the initial volume ratios of metals, and to a lesser degree on the dimensionality of the reactive sample. Here, a two-dimensional sample was represented by ex situ heating of the full multilayer structure, a one-dimensional sample refers to in situ heating of thin cross-sectional TEM specimens, while a zero-dimensional sample (or metallic dot-array) was obtained after cutting thin pillars using focused ion beams. Lamellar self-organized alternation between Heusler phase and Cu9Al4 was found.

Heating and EELS experiments of CuAl reactive multilayers

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.

Evaluation of normal and nanolayer composite thermal barrier coatings in fused vanadate-sulfate salts at 1000°C

Hot corrosion behavior of yttria stabilized zirconia (YSZ), YSZ/normal Al2O3, and YSZ/nano-Al2O3 coatings was investigated in the presence of molten mixture of Na2SO4