Nano-phenomena during exposure of plasma-sprayed ceria stabilised zirconia coatings to oxygen rich environments

Plasma-sprayed zirconia coatings are widely used for oxidation protection. Up-to-date, microstructural stabilisation of such coatings is mainly achieved with yttrium oxide; however recent scientific attempts indicated that ceria stabilised zirconia coatings could be a very promising alternative. In the present work, a coating of this kind has been deposited onto a Ni-based superalloy with the interference of a NiCrAlY bond coating by plasma spraying. Its oxidation resistance was estimated with thermogravimetric analysis with exposure at 1100 °C in air. The microstructure of the as-sprayed coating was studied with X-ray diffraction and electron microscopy before and after oxidation. From this examination it was deduced that ceria stabilised zirconia (CSZ) coating is rather stable at the temperature under question. However reduction of ceria takes place at larger exposure periods.     

Preparation and characterization of mullite whiskers from silica fume by using a low temperature molten salt method

Mullite whiskers were prepared from silica fume in molten Al₂(SO₄)₃-Na₂SO₄ mixture salts at low temperatures. The resulting mullite whiskers, as well as the nucleation and growth mechanism in the molten environment, have been investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TG-DTA) techniques. XRD studies showed that the materials obtained were orthorhombic mullite. SEM, TEM and HRTEM results revealed that the mullite whiskers were single crystal fibers with diameters ranging from 30 to 150 nm and lengths of over several microns. According to thermodynamic analysis, mullite phase might be spontaneously formed in molten salts as the temperature reached the decomposition temperature of aluminum sulfate (1023 K). Moreover, the mullite crystals grew along [1 1 1] crystal plane firstly and developed into fibrous microstructure finally.     

MOCVD of YSZ coatings using β-diketonate precursors

Metallorganic chemical vapor deposition (MOCVD) was investigated as a more efficient means to fabricate yttria-stabilized zirconia (YSZ) for thermal barrier coating. The MOCVD precursors were Y(tmhd)₃ and Zr(tmhd)₄ (tmhd, 2,2,6,6-tetramethyl-3,5-heptanedianato) and delivered via aerosol-assisted liquid delivery (AALD). The maximum YSZ coating rate was 14.2 ± 1.3 μm h⁻¹ at 827 °C yielding a layered coating microstructure. The growth was first-order with temperature below 827 °C with an apparent activation energy of 50.9 ± 4.3 kJ mol⁻¹. Coating efficiency was a maximum of approximately 10% at the highest growth rate. While homogeneous nucleation remained a problem, the deposition of YSZ with only minor carbon content was achieved.     

Synthesis and characterization of cuprous oxide dendrites: New simplified green hydrothermal route

Cuprous oxide (Cu₂O) dendrites were prepared by simple hydrothermal route at two different temperatures using starch as reducing and stabilizing agent. Scanning Electron Microscopy (SEM) revealed the alterations in morphology with reaction temperature and time. The spherical nanoparticles obtained at lower reaction temperature self-assembled into distinct dendritic nanostructures at high temperature. The mechanism of formation of dendrite over the polysaccharide template has been discussed. Transmission Electron Microscopy (TEM) revealed that the crystalline size of these dendrites in one dimension is about 50 nm. The nanoparticles were characterized by UV-vis and photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), FT-IR and Thermal Gravimetry Analyzer (TGA). Impedance analysis of the nanostructures showed conductivity to be a function of temperature.     

Microstructure evolution and thermal stability of an Fe-based amorphous alloy powder and thermally sprayed coatings

High velocity oxy-fuel (HVOF) thermal spraying has been used to produce coatings of an Fe–18.9%Cr–16.1%B–4.0%C–2.8%Si–2.4%Mo–1.9%Mn–1.7%W (in at.%) alloy from a commercially available powder (Nanosteel SHS7170). X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to investigate the powder, as-sprayed coatings and annealed coatings which had been heated to temperatures in the range of 550–925 °C for times ranging from 60 to 3900 min. Microhardness changes of the coatings were also measured as a function of annealing time and temperature. The powder was found to comprise amorphous and crystalline particles; the former had a maximum diameter of around 22 μm. The coating was composed of splat like regions, arising from rapid solidification of fully molten powder, and near-spherical regions from partially melted powder which had a largely retained its microstructure. The amorphous fraction of the coating was around 50% compared with 18% for the powder. The enthalpies and activation energies for crystallization of the amorphous phase were determined. Crystallization occurred in a two stage process leading to the formation of α-Fe (bcc), Fe_1.1Cr_0.9B_0.9 and M₂₃C₆ phases. DSC measurements showed that the first stage occurred at 650 °C. Annealing the coating gave a hardening response which depended on temperature and time. The as-sprayed coating had a hardness of 9.2 GPa and peak hardnesses of 12.5 and 11.8 GPa were obtained at 650 and 750 °C, respectively. With longer annealing times hardness decreased rapidly from the peak.     

Effects of C particle size on the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system

C particle size plays an important role in the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system. When coarse C particles (38 and 75 μm) are used, the SHS reactions consist of two different combustion stages with different brightness intensity of the combustion wave; XRD results indicate that the first and second combustion stages mainly correspond to the formation of Ni–Ti compounds and TiC ceramics, respectively. However, the final reaction is incomplete with a few Ni–Ti compounds and unreacted C. In contrast, when the fine C particle (1 μm) is used, the SHS reaction consists of only one combustion stage with high brightness intensity of the combustion wave; XRD result indicates that final products consist of TiC and Ni, without any intermediate phase. With the decrease of C particle size, the wave velocities increase, and the ignition time becomes shorter. In addition, the morphology of TiC particulate changes to near-spherical, as C particle size decreases.     

Microstructural formations and phase transformation pathways in hot isostatically pressed tantalum carbides

A series of XTa:(1 − X)C (0.5 < X < 1) compositions have been fabricated by hot isostatic pressing (HIP) of Ta and TaC powder blends. Depending upon the targeted stoichiometry, single- or multiple-phase microstructures formed. The single-phase microstructures of both TaC and Ta₂C had equiaxed grain morphologies. The multiphase microstructures had either equiaxed TaC grains with a crisscross pattern of Ta₄C₃ laths or acicular grain morphologies with rafts of TaC, Ta₄C₃ and Ta₂C laths running parallel to the major axis of the grains. The effect of phase transformations on the microstructure of these specimens is discussed and compared to those microstructures seen in a reaction diffusion couple formed between Ta and TaC powders processed under the same HIP conditions. This couple revealed the depletion of carbon from the TaC phase and its reaction with the tantalum metal to form the various Ta-rich carbide phases. The precipitation sequence was found to be paramount in controlling the grain morphology. A close-packed plane and direction orientation relationship was seen between all the phases. The crisscross pattern of Ta₄C₃ precipitation in TaC was a consequence of TaC’s multiple variant {1 1 1} orientations and had little or no effect on the grain morphology. In contrast, the single variant close-packed plane {0 0 0 1} in Ta₂C resulted in the parallel alignment of the precipitated phases within its grain and an anisotropic growth direction that facilitated the acicular grain morphology.     

Electrocatalytic behaviour of NiBi coatings for hydrogen evolution reaction in alkaline medium

The nickel-bismuth binary coatings with various chemical compositions were galvanostatically deposited on the copper electrode in view of their possible applications as electrocatalytic materials for the hydrogen evolution reaction (HER) in alkaline solution. The HER activity of coatings was tested with the help of potentiodynamic measurements and electrochemical impedance spectroscopy (EIS) technique. The electrochemical characterization was achieved by the means of cyclic voltammetry (CV). The surface morphology and surface composition of coatings were determined with scanning electron microscopy (SEM), and energy dispersive X-ray (EDX). The potentiodynamic measurements show that, the binary coatings decrease the hydrogen over potential and increase the current density values for HER. The EIS analysis confirms, the charge transfer resistances decrease and the double layer capacitance values increase for binary coatings. The EDX results in sign that the composition of binary coating changes by using coating bath. The Cu/NiBi-2 coating (Ni²⁺/Bi³⁺ is 99.71:0.29 molar ratio) is the best suitable cathode composition for the HER in alkaline media under these experimental conditions.     

Microstructural characterization of a die-cast magnesium-rare earth alloy

Microstructural characterization of high-pressure die-cast alloy MEZ (Mg–2.5RE–0.35Zn–0.3Mn) reveals equiaxed dendrites of -Mg with a partially divorced interdendritic eutectic. Detailed diffraction studies coupled with WDS analysis reveal the presence of a continuous Mg₁₂RE intermetallic phase in the eutectic aggregate along with fine Mg particles.     

Improvement of the magnetic properties of nanocomposite permanent magnetic alloys with Dy and Ga substitutions

The Dy and Ga substituted NdFeB nanocomposite permanent magnetic alloys with high magnetic properties have been prepared by appropriate wheel speed of melt-spinning and post-annealing treatment. Under optimal conditions, compared with the best magnetic properties of ternary NdFeB alloy of J_r=1.18 T, H_ci=379.5 kA/m and (BH)_max=119.5 kJ/m³, the best magnetic properties of the alloy with Dy and Ga substitutions are J_r=1.16 T, H_ci=580.5 kA/m, and (BH)_max=162.7 kJ/m³. The XRD and TEM results showed that each of two alloys consists of hard magnetic 2:14:1 phase and soft magnetic α-Fe phase. The grain size of the 2:14:1 phase is about equal in the two alloys. The grain size and content of α-Fe phase in Dy and Ga substituted alloy are finer and lower, respectively.     

Synthesis and characterization of proton conducting Sr(Ce1−xZrx)0.95Yb0.05O3−δ by the citrate method

The citrate method was used to synthesize Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ (x = 0.1, 0.2, 0.3, 0.4) and to avoid the drawbacks of the conventional solid state reaction method. The products were characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe X-ray microanalyzer (EPMA). The results indicate that the citrate method is an advantageous route in producing Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ materials. Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ powders are composed of nanoscaled crystallites with the average grain size in the range of 60–70 nm. Single phase is confirmed over the whole x range. In addition, chemical stability against CO₂ and electrical conduction behavior of the sintered Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ ceramics were investigated. The chemical stability of the ceramics against CO₂ is certified to increase with the increase in zirconium content. Impedance spectroscopy was used to study the electrical conduction behavior of Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ ceramic.     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.     

Microstructural evaluation of tungsten carbide-cobalt coatings

Tungsten carbide-12 wt.% cobalt coatings were deposited using optimized high-energy plasma (HEP) and high-velocity oxygen fuel (HVOF) thermal spray techniques. The coatings were evaluated using transmission electron microscopy, differential thermal analysis, X-ray diffraction, and subjected to wear tests to relate the coating structure to wear performance. Coatings were evaluated in the assprayed condition, as well as after heat treatments in inert atmosphere. The results indicate that a substantial amount of amorphous matrix material is created during the thermal spray process. Carbon and tungsten, liberated through the dissociation of the WC, combine with cobalt present in the starting powder to form amorphous material on solidification. Differential thermal analysis revealed an exothermic reaction for both the HVOF and HEP coatings at approximately 853 and 860 °C, respectively, which did not occur for the powder. Post-coating heat treatment in an inert atmosphere resulted in the recrystallization of the amorphous material into Co₆W₆C and Co₂W₄C, which was dependent on the time and temperature of the heat treatment. Wear testing showed improvement in the wear performance for coatings that were subjected to the heat treatment. This was related to the recrystallization of the amorphous matrix into eta phase carbides. Editor’s Note: This paper was presented at the 4th National Thermal Spray Conference, Pittsburgh, 6-10 May, 1991. The proceedings of this conference will be published by ASM International. Dr. T.F. Bernecki is the Editor of these proceedings.     

Formation and thermal stability of new Zr–Cu-based bulk glassy alloys with unusual glass-forming ability

The simultaneous addition of Al and Ag to Zr–Cu binary alloys increased in the stabilization of supercooled liquid, the reduced glass transition temperature and γ value, leading to greatly enhance the glass-forming ability (GFA). The Zr–Cu–Ag–Al glassy alloy samples with diameters above 15 mm were obtained in the wide composition range of 42–50 at% Zr, 32–42 at% Cu, 5–10 at% Ag, and 5–12 at% Al. The best GFA was obtained for Zr₄₈Cu₃₆Ag₈Al₈ alloy, and the glassy samples with diameters up to 25 mm were fabricated by an injection copper mold casting. The Zr₄₈Cu₃₆Ag₈Al₈ glassy alloy exhibited high tensile and compressive fracture strength of over 1800 MPa.     

A convenient thermal reduction–nitridation route to nanocrystalline molybdenum nitride (Mo2N)

Nanocrystalline molybdenum nitride (γ-Mo₂N) was synthesized via a thermal reduction–nitridation route by the reaction of metallic sodium with anhydrous molybdenum pentachloride and ammonium chloride in an autoclave at 550 °C. X-ray powder diffraction pattern indicated that the product was cubic Mo₂N, and the cell constant was a = 4.161 Å. Scanning electron microscopy image showed that it consisted of particles with an average size of about 30 nm. The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 400 °C in air.     

Microstructures and mechanical behaviors of Mg58Cu31Gd11 and Mg65Cu25Gd10 amorphous alloys synthesized by injection casting and melt spinning

Mg₅₈Cu₃₁Gd₁₁ and Mg₆₅Cu₂₅Gd₁₀ alloys were synthesized via two processing routes, injection casting and melt spinning. The diameter of the injection-cast bars was 4 mm in diameter. The XRD results obtained for the Mg₅₈Cu₃₁Gd₁₁ are nearly identical to those for the Mg₆₅Cu₂₅Gd₁₀, showing amorphous-like broad characteristic peaks. All the four characteristic temperatures, T_g, T_x, T_m and T_l, of the Mg₆₅Cu₂₅Gd₁₀ are essentially lower than those of Mg₅₈Cu₃₁Gd₁₁, for both injection-cast rods and melt-spun ribbons. The glass forming abilities of the Mg₆₅Cu₂₅Gd₁₀ are similar to those of Mg₅₈Cu₃₁Gd₁₁, for both injection-cast rods and melt-spun ribbons, indicated by T_rg = 0.60 and γ = 0.42. The average microhardness of the Mg₆₅Cu₂₅Gd₁₀ is 2.41 GPa and 2.27 GPa for injection-cast bars and melt-spun ribbons, respectively, which are significantly lower than 2.84 GPa and 2.49 GPa of the Mg₅₈Cu₃₁Gd₁₁. The nanohardness at the maximum load from the multiple loading is 3.5 GPa for Mg₆₅Cu₂₅Gd₁₀, which is lower than 3.9 GPa for Mg₅₈Cu₃₁Gd₁₁. The curves of load vs. the depth obtained from the nanoindentation tests all show stepwise behavior due to the pop-in events, and the step width increases as the indentation rate decreases. The modulus at the maximum load from the multiple loading obtained from the nanoindentation tests is 64.9 GPa for Mg₆₅Cu₂₅Gd₁₀, which is lower than 70.7 GPa for Mg₅₈Cu₃₁Gd₁₁. The fracture stress and strain of the Mg₆₅Cu₂₅Gd₁₀ BMG rod at room temperature are 490 MPa and 3%, respectively, smaller than those of the Mg₅₈Cu₃₁Gd₁₁ BMG rod, 548 MPa and 3.2%, respectively. The Mg₅₈Cu₃₁Gd₁₁ BMG rod is stronger at room temperature, and also shows higher yield stress and less deformable at elevated temperature, than the Mg₆₅Cu₂₅Gd₁₀ BMG rod.     

Preparation of nanosized sodium-aluminum tungstate, NaAl(WO4)2

Investigations are carried out for preparing nanosized pure phase of NaAl(WO₄)₂ by means of solid state synthesis with mechanical activation, applying the sol-gel method (Pechini) and by co-precipitation. It is shown that it is not possible to obtain pure phase when the initial substances are in stoichiometric amounts due to the simultaneous formation of a number of accompanying tungstate phases. The reasons for their origin are discussed. A method is demonstrated for obtaining a pure phase of NaAl(WO₄)₂ by co-precipitation of aqueous Na₂WO₄ and Al(NO₃)₃ solutions with considerable excess of Na₂WO₄. It is proved that NaAl(WO₄)₂ with particle size 40-80 nm is obtained with final synthesis of the powders at temperature 600-650 °C and duration of thermal treatment of 1-2 h.     

Simple hydrothermal synthesis and sintering of Na0.5Bi0.5TiO3 nanowires

Single-crystalline Na_0.5Bi_0.5TiO₃ (NBT) nanowires, with diameters of 100 nm and lengths of about 4 μm, were synthesized by using a simple hydrothermal method. Phase composition, morphology and microstructure of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The effects of reaction temperature and reaction time on precipitation of the NBT nanowires were investigated. It was found that reaction time significantly influenced the growth behavior of the powders in the hydrothermal system. Based on the experimental results, the one-dimensional (1D) growth mechanism of the NBT was governed by a dissolution-recrystallization mechanism. NBT ceramics derived from the nanowires showed typical characteristics of relaxor ferroelectrics, with diffuseness exponent γ of as high as 1.73.