Al Microparticles Research Articles

Effect of CeO2 on the Isothermal Oxidation of Electrode-posited Ni-Al Composite Coatings at 950 °C

Abstract Electrodeposited Ni-Al and three sorts of Ni-Al-CeO 2 composite coatings with different contents of Al were prepared by co-deposition of Ni matrix with Al microparticles and/or CeO 2 nanoparticles. The Al microparticles were in a size range of 1∼5 μm, and the CeO 2 nanoparticles in an average size of 10 nm. SEM/EDAX and TEM results indicate that CeO 2 addition can refine the Ni-Al composite. Oxidation at 950 °C indicates that the addition of CeO 2 improve the oxidation resistance at a suitable amount of Al particles in such composites, no matter whether the Ni-Al composite forms NiO or Al 2 O 3 scale. The effect of CeO 2 on the oxidation behavior of the electrodeposited Ni-Al composite coatings was discussed.

Rheological Behaviour, Synthesis and Performance of Smart Thermal Barrier Coating Systems Based on Hollow Alumina

The stability of three water-based slurries containing different loads of Al microparticles was measured by laser scattering. The slurries displayed a Newtonian behaviour and the evolution of viscosity over nine days was found to be constant. Ground surfaces of nickel were also wetted similarly irrespective of the Al content in the slurries (30, 40 or 50 wt%) thereby demonstrating that the 1/10 PVA/H2O water based slurries are adequate for spraying. After deposition and annealing of the as-sprayed coatings, a thermal barrier coating system was formed with a top coat of hollow alumina spheres, an intermediate thermally grown oxide and a bottom aluminized bond coat. The coating system is shown to display very low thermal conductivity and remarkable oxidation resistance at high temperatures.

Effect of Annealing Treatment on the Isothermal and Cyclic Oxidation Behavior of an Electrodeposited Alumina-Forming Ni-Al Microcomposite

In this study, an electrodeposited alumina-forming Ni-Al composite having a nanocrystalline Ni matrix dispersing Al microparticles was produced and annealed in vacuum at 500°C for different time (3hr, 5hr and 8hr, respectively). Effect of annealing time on the microstructure, isothermal and cyclic oxidation behavior of the Ni-Al composite at 1000°C was comparably investigated. The results indicated that Al microparticles slowly dissolved into nickel grain to form an ultrafine-grained Ni3Al/NiAl composite with micro-sized pores dispersion due to volume shrinkage. The increased distribution uniformity of Al with the increasing of annealing time favors the formation of a protective alumina layer in a short time, and then significantly reduced the isothermal oxidation rate. Besides, the reduction of pores size with the increasing of annealing time also helps the ultrafine-grained Ni3Al/NiAl composite to grow an adherent alumina scale with increased cyclic oxidation resistance.

Effect of Subatmospheric Pressure on Deposited Energy for Electrical Wire Explosion in Air

Pressure is an important factor for the effect of the medium in the process of the electrical explosion of wire in air. In this paper, the electrical explosions of copper and aluminum wires at different air pressures in the range of subatmospheric pressure were investigated with pulsed voltage in a submicrosecond time scale. Based on the measurements of the current and voltage in wire explosion with a current monitor and a high-voltage probe, respectively, the deposited energy in the stages of melting, liquid state, and vaporization was calculated by mathematical methods. The effect of air pressure on deposited energy in the three stages mentioned above was analyzed by experiments and calculation. The results show that the breakdown field strength varied with air pressure, and plays a vital role in energy deposition and the time of duration in the vaporization stage. In addition, the exothermic reaction of Al microparticles and surrounding air also has an important effect on the energy deposition in the vaporization stage. The deposited energy in the electrical explosion of copper and aluminum wire is higher at the higher air pressure, particularly after the beginning of vaporization.

Electron microscope investigation of aluminium-comprising nanoparticles

A new approach was suggested for creating nano-dimensional materials based on water-soluble polymers. Nano-seized Al-contained structures were synthesized; such structures were screened by a cover of sodium-carboxymethylcellulose by heating up to 70-80 degrees of Celsius of a sodium-carboxymethylcellulose's polymere with Al microparticles with dimension less than 20 mkm. Morphological features of created composites were researched. It was established that agglomerates of received particles were reached values of 300-600 nm and consisted of tubular types' structures with dimensions 80-100 nm.

Laser-assisted fragmentation of Al particles suspended in liquid

Laser-assisted fragmentation of Al micro- and nano-particles in liquid is studied both experimentally and theoretically. Modeling of the evolution of size distribution function of particles is performed on the basis of numerical solution of kinetic equation for distribution function which takes into account the ablation and splitting of particles into fragments of different sizes. It is assumed that depending on the size of particles, the fragmentation scenario may vary from monoatomic evaporation to detachment of relatively large fragments. The first mechanism of fragmentation is typical for nanoparticles while the second one is characteristic for large (micrometer-sized) particles. Laser-assisted fragmentation of Al micro-particles suspended in liquid ethanol to Al nanoparticles is experimentally studied using a Nd:YAG laser with pulse duration of 10ps and repetition rate of 300kHz. The evolution of the size distribution function with laser exposure time is monitored and compared to modeling results.

Influences of Al particles on the microstructure and property of electrodeposited Ni–Al composite coatings

Ni–Al composite coatings with different contents of Al microparticles were prepared from a conventional Watt bath. The influences of Al particle loadings in the bath on the surface morphology, composition, texture, grain size, microstrain, residual stress and anti-corrosion of the Ni–Al composite coating were investigated. The friction coefficients of the coatings at 200°C were also evaluated by a pin-on-disctribometer. The results showed that the surface morphology of the coatings changed from pyramid+colonied structure to colonied structure with increasing Al particle loadings. The (200) preferred orientation for pure Ni coating evolved to random orientation with increasing Al particle loadings. The grain size obtained the minimum value of 72.28nm at Al particle loading of 100g/L and the microstrain of the coating increased with increasing the Al particle loadings. The incorporation of Al particles decreased the residual stress of the electro-deposited coating and all the coatings deposited at different Al particle loadings possessed low residual stress. As the Al particle loading increased, the anti-corrosion of the Ni–Al coatings increased owing to the combined effect of increasing Al content in the coatings and the texture evolution from (200) plane to (111) plane. The wear result suggested that the increasing Al particle content did not improve the wear performance of the Ni–Al composite coatings.

Regularities of formation of the structure of Al-containing nanocomposites upon interaction of ASD-6 powder with polymer suspension

A new approach toward development of nanosize materials based on water-soluble polymers is suggested. Nanosize aluminum-containing structures screened by a polymer sodium carboxymethyl cellulose shell are obtained at temperatures of 70–80°C by introducing Al microparticles with a size of up to 10 μm into an aqueous solution of sodium carboxymethyl cellulose. The methods of XRD and IR spectroscopy were used to establish that metallic aluminum is present in nanoparticles and the polymer Na CMC matrix is preserved without fundamental changes. Electron microscopy is used to characterize the morphological regularities and elemental composition of nanoparticles. Agglomerates of the obtained particles reach a size of about 300–500 nm and consist of tubular structures with a size of up to 150 nm with a total thickness of 3–5 nm. This approach may form the basis for a comparatively simple and cheap method of obtaining nanosize particles that can serve as modifying agents in protective corrosion-resistant coatings.

Slurry aluminizing mechanisms of Ni-based superalloys incorporating an electrosynthesized ceria diffusion barrier

This study investigates the effect of an electrosynthesized ceria interlayer on the growth mechanisms of a full TBC system sintered from a slurry containing spherical Al micro-particles. A potential Selective Diffusion Barrier (SDB) ability of the ceria interlayer appears. It is demonstrated that ceria allows the Al enrichment of the substrate while limiting the Cr upward diffusion and segregation of AlxCry. It is found that the ceria interlayer partially hinders the flow of solid and molten Al into the substrate at 700 °C from the spheres. Simultaneously, the flow of Al among the spheres results in the establishment of bridges and in the loss of sphericity all by maintaining a metallic core. In contrast, emptying of the particles and quick formation of δ-Ni2Al3 rapidly occur in the absence of ceria interlayer. Increasing the annealing temperature to 1100 °C for 2 h brings about the formation of a β-NiAl coating thicker than the one obtained without an interlayer and promotes thicker crusts and a greater sintering of the Al2O3 shell of the particles. This is proposed to result both from delayed Al inward diffusion and enhanced Ni outward diffusion promoted by the heat release accompanying the exothermic formation of the NixAly phases.

Influence of water vapour on the oxidation behaviour of a conventional aluminide and a new thermal barrier coating system sintered from a slurry

In this work, the effect of water vapour on a new thermal barrier coating system synthesised from a slurry containing Al microparticles was studied under isothermal regime at 1100 °C for short oxidation times (50 h). Both dry air and argon gas atmospheres were investigated with increasing water vapour contents (0, 5, 10% RH, relative humidity) and the oxidation mechanisms and growth kinetics of the oxide scales were compared to conventional aluminide coatings up to 50% RH. The phase transformations occurring in the coating were characterised by local Raman micro‐spectrometry with complementary analytical techniques (X‐ray diffraction and FEG‐ESEM/EDS). The results demonstrated that water vapour does not exert a great influence on the oxidation of this type of coatings once an α‐Al2O3 scale is stabilised after an appropriate sintering of the coating. In contrast the combined effect of water vapour and oxidation onto oxide scales grown from raw aluminide coatings was shown to trigger delayed transformation of θ into α‐Al2O3, the appearance of pores and spallation, in particular in air + x %RH.

Reducing minimum flash ignition energy of Al microparticles by addition of WO3 nanoparticles

Aluminum (Al) is an attractive solid fuel for rocket propulsion and energy conversion systems due to its large volumetric energy density, earth abundance, and low cost. Nonintrusive optical flash ignition is attractive for many applications due to its simplicity and flexibility in controlling the area exposed to the flash. However, flash ignition of Al microparticles (MPs) is challenging due to their higher minimum flash ignition energy, which originates from weaker light absorption and higher ignition temperature compared to Al nanoparticles (NPs). Herein, the minimum flash ignition energy of Al MPs is reduced by the addition of WO3 NPs.

Comparative degradation of nickel aluminized by slurry and by pack cementation under isothermal conditions

The degradation mechanisms of pure nickel aluminized by a new slurry containing Al microparticles and by pack cementation were studied isothermally between 900 and 1100°C in air for 100h. The parabolic rate constants indicated faster oxidation kinetics in the slurry coated samples with respect the pack cemented ones. The change in microstructures and compositions of the coatings and of the thermal oxides were more marked in the slurry coatings. This was related to both Ni and Al interdiffusion. It is concluded that the remains of Al in the spheres contributed to an increased oxidation compared to the pack cemented coatings.

Potential thermal barrier coating systems from Al microparticles. Mechanisms of coating formation on pure nickel

The mechanisms of formation of a full thermal barrier coating system (thermal barrier, thermally grown oxide and aluminium diffusion coating) from a slurry is presented in this work. Al microparticles were mixed to a water-based binder and deposited onto the surface of pure nickel. A single step or two steps heat treatment in Ar (g) atmosphere brought about the formation of the coating system. The foam-like top coating based on Al2O3 hollow particles is expected to provide the thermal insulation. Underneath, an aluminium diffusion coating expected to provide the high temperature oxidation resistance was formed. Simultaneously, an alumina thermally grown oxide was developed between the top coat and the diffusion coating.It was found that Al began to diffuse into the Ni substrate at 650 °C following the Ni–Al equilibrium diagram. Thickening of the aluminized layer followed parabolic growth with time whether at 650°, 700° or a combination of 700 °C (2 h−1) + 1100 °C. The presence of molten Al seemed to account for the very rapid aluminisation of the substrate via δ-Ni2Al3. The flow of solid and molten Al could be regulated by cracking and shrinking of the Al2O3 shells surrounding the Al particles. In a combined 700 °C (2 h−1) + 1100 °C treatment, β-NiAl grew at the expense of δ-Ni2Al3 formation. Simultaneously, thickening of the Al2O3 crusts and phase transformations occurred, which provided enhanced sintering of the particles. At 650 and 700 °C, γ-Al2O3 formed whereas exposure at 1100 °C resulted in the appearance of α-Al2O3 even for short very short times.

Initial Aluminizing Steps of Pure Nickel from Al Micro-Particles

Novel, unconventional type of high temperature coating systems can be elaborated by depositing Al micro-particles on nickel base substrates, using an appropriate binder, and converting them into a thermal barrier type coating by a two-step heat treatment under argon. Final result is a coating structure consisting of a quasi-foam top coat, constituted by spherical hollow alumina particles, surmounting a β-NiAl diffusion layer able to form during high-temperature oxidation a protective alumina scale. In this work, pure nickel was employed as a model material to evaluate the effects of moderate temperatures (550-700°C), dwelling times and Al particle size on the final characteristics of the coatings. Almost no diffusion occurred below 600°C. In contrast, a Ni2Al3 layer very quickly formed at 650 or 700°C. The rapidity of coating formation was attributed to the appearance of a liquid phase at the coating/substrate interface. The increase of dwelling time did not provide any significant thickness increase as the Al particles got practically emptied after 2h. In addition, the use of different micro-sized particles resulted in similar Al diffusion coatings under the investigated conditions.

On the Influence of a Heat Treat for an Aluminizing Progress Based on Al Microparticles Slurry for Model Ni and Ni20Cr. Experimental and Theoretical Approaches.

The use of thermal barrier coating systems allows superalloys to withstand higher operating temperatures in aeroengine turbines. Aiming at providing oxidation protection to such substrates, an aluminum-rich layer is deposited to form the α-Al2O3 scale over which a ceramic layer (i.e. YSZ layer) is applied to provide thermal insulation. A new approach is now being investigated within the FP7 European project « PARTICOAT », in which a single step process is employed by applying micro-sized aluminum particles. The particles are mixed in a binder and deposited by brushing or spraying on the substrate surface. During a heat treatment, the particles sinter and oxidize to form a top coat composed of hollow con-joint alumina spheres and simultaneously, an Al-rich diffusion zone is formed in the substrate. For a better understanding of the diffusion / growth processes, preliminary tests were carried out on pure nickel and Ni20Cr model alloys prior to further application on commercial superalloys. The effect of the heat treatment on the coating characteristics (number of layers, thickness, composition, homogeneity, etc.) was particularly investigated to emphasize the mechanisms of diffusion governing the growth of the coatings. The establishment of the diffused layers occurred very readily even at intermediate temperatures (650 and 700°C). However, the layers formed did not match perfectly with the thermodynamic modeling because of the quick incorporation of Ni into molten Al at intermediate temperatures (650°C). In contrast, at higher temperatures (700 and 1100°C) the phases predicted by Thermocalc are in good agreement with the observed thickness of the diffused layers. The incorporation of Cr as an alloying element restrained Al ingress by segregation of Cr even at very low temperatures aluminizing temperatures (625°C).

Effect of CeO 2 on the microstructure and isothermal oxidation of Ni–Al alloy coatings transformed from electrodeposited Ni–Al films at 800 °C

The electrodeposited two-phase γ-Ni + γ′-Ni 3Al and single-phase γ′-Ni 3Al coatings with and without CeO 2 particles were developed by the conversion of electrodeposited Ni–Al or Ni–Al–CeO 2 films with dispersed Al microparticles in Ni matrix into γ′-Ni 3Al by vacuum annealing at 800 °C for 3 h. SEM/EDAX and TEM characterizations showed that the CeO 2-dispersed γ-Ni + γ′-Ni 3Al or γ′-Ni 3Al coatings exhibited finer grains compared to the CeO 2-free Ni–Al alloy coatings. The oxidation at 1000 °C for 20 h showed that for a given Al content the addition of CeO 2 significantly improved the oxidation resistance of the electrodeposited Ni–Al alloy coatings. The effect of CeO 2 particles on the microstructure and oxidation behavior of the electrodeposited Ni–Al alloy coating is discussed in detail.

Electrochemical Assembly of Ni – xCr – yAl Nanocomposites with Excellent High-Temperature Oxidation Resistance

Various amounts of Cr and Al nanoparticles were electrochemically assembled into a nanocrystalline Ni matrix template to form nanocomposites (, in weight percent). By characterization of the steady-state oxide scales formed at in air, an oxide map correlating the scale types with the composition ranges of the nanocomposites was established, showing that these nanocomposites can transform from NiO formers to chromia or alumina formers, by increasing the amount of to a critical, particle-size-dependent value and by changing the value of the ratio . The nanocomposites, in comparison to ternary composites assembled using Cr and Al microparticles instead of the nanoparticles and with conventionally coarse-grained alloys of similar composition, exhibited a greatly increased ability of thermally developing chromia or alumina scales. A model for the establishment of various oxide scales on the nanocomposites is proposed, correlating the nature of the scales with their specific microstructures and with the values of and .

Effect of Y 2O 3 on microstructure and oxidation of γ -Ni+γ′ -Ni 3Al coatings transformed from electrodeposited Ni-Al films at 1 000 °C

The electrodeposited Y 2O 3-dispersed γ-Ni+γ ′-Ni 3Al coatings on Ni substrates were developed by the conversion of electrodeposited Ni-Al-Y 2O 3 films with dispersed Al microparticles in Ni matrix into Ni 3Al by vacuum annealing at 800 °C for 3 h. For comparison, Y 2O 3-free γ-Ni+γ ′-Ni 3Al coatings with a similar Al content were also prepared by vacuum annealing the electrodeposited microparticle-dispersed composite coatings of Ni-Al under the same condition. SEM and TEM characterizations show that the electrodeposited Y 2O 3-dispersed γ+γ ′ coatings exhibit finer grains, a more homogeneous distribution of γ ′, and a narrowed γ ′ phase spacing compared with the electrodeposited Y 2O 3-free γ+γ ′ coatings. The oxidation at 1 000 °C shows that the addition of Y 2O 3 significantly improves the oxidation resistance of the electrodeposited γ+γ ′coatings. The effect of Y 2O 3 particles on the microstructure and oxidation behavior of the electrodeposited γ+γ ′ coatings was discussed in detail.

Atomic force microscopy analysis of morphology of low density polyethylene influenced by Al nano‐ and microparticles

AbstractSpherulites are important morphological forms in semicrystalline polymers. Polarized optical microscope (POM) is a powerful tool for observing the spherulite morphology of semicrystalline polymers and their composites, but it can not resolve spherulites with average size smaller than 5 μm and also not be used to study the opaque nanocomposites film samples. Atomic force microscopy (AFM) is an excellent tool for observing spherulites comparatively small in size and not restricted by the transparency of the samples. In this work, AFM was used to investigate the impact of Al nano‐ and microparticles on spherulitic textures, lamellar thickness, and thickness distribution of low density polyethylene (LDPE). It was found that the spherulitic texture was seriously disordered by the nanoparticles, and that the LDPE in nanocomposites was found only to have poorly developed spherulite structure. The incorporation of microparticles can facilitate the growth of the lamellae but decrease the size of the spherulites. Furthermore, the lamellar thickness and its distribution did not alter so much with the addition of nano‐ and microparticles. Wide‐angle X‐ray diffraction (WXRD) was used to determine the crystal structure of LDPE, and the results show that both Al nanoparticles and microparticles do not change the unit cell parameters. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Nonisothermal crystallization behavior and nucleation of LDPE/Al nano‐ and microcomposites

AbstractThe nonisothermal crystallization and melting behavior of LDPE and LDPE/Al nano‐ and microcomposites prepared from melt compounding were studied using differential scanning calorimetry (DSC). The DSC results show that the Al nanoparticles can either facilitate or hinder the crystallization of LDPE, depending on the dispersion of the nanoparticles in LDPE. The well‐dispersed Al nanoparticles do not have nucleating effects and mainly act as obstacles in the crystallization process, but the agglomerates of Al nanoparticles can act as nucleating agents and slightly accelerate the crystallization process of LDPE. The Al microparticles have nucleating effect and facilitate the crystallization process of LDPE. The combined Avrami–Ozawa equation was used to describe the nonisothermal crystallization process. It was found that the combined Avrami–Ozawa method can successfully describe the nonisothermal crystallization process. The melting behavior indicates that the lamellar thickness distribution of the nanocomposites and microcomposites is not significantly changed in comparison with the neat LDPE. POLYM. ENG. SCI., 47:1052–1061, 2007. © 2007 Society of Plastics Engineers