1,720,978 research outputs found
Modified aluminide coatings with reactive element for turbine blade protection
Full text linkIncreasing the maximum temperature of the gases introduced into the turbine increases both the efficiency and performance of a turbo-gas system; Switching from an input temperature of 900 ° C to 1250 ° C can result in a 30% increase in output power, leaving unchanged consumption. In order to ensure the structural integrity of components working in the hottest areas of the engine, research over the last decades has focused on the development of increasingly innovative technologies that aim to increase the efficiency of the system while at the same time increasing its operating life. In this thesis, it was studied a multiple modification approach of a standard aluminide coating in order to increase the oxidation and hot corrosion resistance. The first part of the study has involved the manufacturing, characterization and high temperature tests of a standard aluminide coating obtained by pack cementation. The optimized parameters were selected as deposition parameters for the modified coating: diffusion aluminide were doped with Zr and the characterization test were carried out in order to compare the standard with the modified samples. The second part of the study has involved the introduction of a modified process in order to optimize the manufacturing process of the coating. Electroless pure nickel plating were selected as pre-aluminization process in order to introduce a Ni reservoir onto the substrate surface. Otherwise, n-Al2O3 was used as nano-reinforcement in the Ni layer to improve the oxidation resistance by modifying the kinetic of oxidation. The final part of the work has involved the test of a different aluminization process. In collaboration with the University of La Rochelle, France, slurry-aluminization (low temperature) was carried out onto the standard and the electroless pure nickel samples. Oxidation results were compared with the first part of the work and an innovative production way of diffusion aluminide coating modified with Zr and electroless Ni was proposed and studied
High-temperature resistivity of aluminum–carbon nanotube composites
No full textThe resistivity and thermal coefficient of resistivity (TCR), of metallic matrix composites, MMCs, aluminum–carbon nanotube, Al-CNT, were studied under
high vacuum in the temperature interval from RT to 800 K. The samples shaped as small cylinders and containing single-walled CNTs or multi-walled CNTs were sintered at 625 C. The resistivity of sintered samples of pure Al was found three orders of magnitude higher with respect to bulk, having the former a density value equal to 98.8 % of bulk Al. The explored range of the CNT concentration was within 5 wt%. At the highest CNT concentrations, the trend of resistivity against temperature was found negative being more pronounced for composites with MWCNTs. For Al-SWCNT composites, at around 3.3 wt%(4.2 vol%), TCR is practically independent from temperature; for Al-MWCNT, the TCR zero-crossing occurs at different compositions depending on temperature. Higher is the temperature, lower is the TCR zero-crossing composition. Resistivity data were discussed in the framework of the Matthiessen’s rule and sound evidences were shown that no Al4C3 formation was detected at working temperatures
Nanostructured nickel film deposition on carbon fibers for improving reinforcement-matrix interface in metal matrix composites
Full text linkThe issues in dispersing any form of carbon in metal matrix is the major problem in the field of metal matrix
composites with carbon reinforcement (MMCcr). The low wettability of carbon in molten metals and the
difference in density are some of the difficulties to obtain a good dispersion of carbon fibers in the matrix and,
as a consequence, an improvement of some critical properties for metals in a wide range of application
(mechanical properties, electrical properties, optical properties). For this reason, the aim of this work is to
obtain a metallic coated carbon fiber to enhance the interaction between the reinforcement and the matrix.
Moreover, also the density of carbon fibers could be adjusted depending on the thickness of the coating.
Electroless Nickel-Phosphorus Plating (ENP) is one of the candidate to be a coating technique to improve the
interaction between the carbon fibers and the metal matrix. Despite of its versatility in terms of complex
geometry of the substrate and homogeneity and adhesion of the coating, the presence of the phosphorus in
the alloy could create some problems with the metal matrix such as the formation of metal-phosphorus
products that can drastically decrease the mechanical properties of the composite. For this reason, in this
work, is presented a new way of Electroless Pure Nickel Plating (EPP) without any introduction of phosphorus
in the nickel coating. The dependence of the coating thickness and the density of the coated fibers were
studied under different plating parameters (temperature of the plating solution, deposition rate and plating
solution composition). All the samples were characterized with SEM and XRD and the thickness, density and
homogeneity were compared for all the samples obtained
SYNTHESIS AND CHARACTERIZATION OF ELECTROLESS Ni-P ALLOYS FOR ANTI-WEAR AND ANTI-FOULING COATINGS
No full textIntroduction: Fouling is a typical problem in compressors used for processing
of hydrocarbon gasses and it is caused by solids adsorption onto the internal
surfaces of the machinery. This phenomenon can influence the performance of the facility and, in some
cases, it causes the stop or the failure of the system. Surface coating can be a
suitable solution to this kind of problem.
Anti-fouling and anti-wear coatings can be obtained by electroless-nickel
plating technique. It allows for depositing very uniform and dense Ni-P alloy
coatings on irregular shaped surface and on different kind of substrate.
Materials and methods: Ni-P amorphous alloy was deposited on a steel
substrate. The electroless plating bath is an acid solution and consists of
a nickel source, a reducing agent (P compounds), stabilizing agents and a
buffer. The temperature and the pH were continuously monitored during
the deposition and kept constant. Plating parameters were optimized and
several coatings were deposited. Nano-particles were added in order to improve
the performance of the coating, in particular particles of polytetrafluoroethylene
(PTFE) were chosen to improve anti-fouling behavior while
nano-zirconia particles were added to improve wear resistance. In order
to prevent agglomeration of nano-ZrO2, nano-particles were previously
surface modified with methacrylic acid. Ni-P coatings were characterized
through SEM imaging and EDS analysis. Coating thickness and roughness
were measured. Top view and cross section Vickers hardness was evaluated.
The morphology of the samples was compared through a surface
profilometry analysis. The wettability was evaluated considering the drop
contact angle for different fluids.
Results: All deposited coatings (Ni-P, Ni-P with ZrO2, Ni-P with PTFE, Ni-P
with ZrO2 and PTFE) are dense layers with a very good adhesion with the
substrate. As expected the morphology of the coating follows the morphology
of the substrate. SEM micrographies show an optimal dispersion and
distribution of nano-particles. Nano-zirconia addiction allows for obtaining
small values of roughness and high values of Vickers hardness. Nano-ZrO2
and PTFE particles cause a reduction of drop contact angle for different
fluid. In particular PTFE results to be more beneficial for a decrease of the
wettability.
Discussion: Electroless Nickel Plating technique was well optimized resulting
in dense coatings and good rate of deposition. The results show improvements
of the desired coating properties in particular when PTFE and ZrO2
are both added
Diffusion aluminide coatings for hot corrosion and oxidation protection of nickel-based superalloys. Effect of fluoride-based activator salts
Full text linkThe influence of two different fluoride-based activator salts (NH4F and AlF3) was studied for diffusion aluminide coatings obtained via pack cementation on a Ni-based superalloy (René
108DS). The resistance to oxidation and hot corrosion was assessed as a function of the concentration of activator salts used during the synthesis process by means of pack cementation. Two different concentrations were selected for activator salts (respecting the equimolarity of fluoride in the synthesis) and the obtained diffusion coatings were compared in terms of morphology, thickness and composition, as well as in terms of microstructural evolution after high temperature exposure. Isothermal oxidation tests were conducted at 1050 C in air for 100 h in a tubular furnace. The oxidation kinetics were evaluated by measuring the weight variation with exposure time. The microstructural evolution induced by the high temperature exposure was investigated by SEM microscopy, EDS
analysis and X-ray diffraction. Results showed that the coatings obtained with AlF3 activator salt are thicker than those obtained using NH4F as a consequence of different growth mechanism during
pack-cementation. Despite this evidence, it was found that the NH4F coatings show a better oxidation resistance, both in terms of total mass gain and of quality of the microstructure of the thermally grown oxide. On the other hand, coatings produced with high concentration of AlF3 exhibited a better resistance in hot corrosion conditions, showing negligible mass variations after 200 h of high
temperature exposure to aggressive NaCl and Na2SO4 salts
Ni-fluoropolymers composite coatings obtained via electroless plating for anti-icing application
Full text linkElectroless pure nickel plating was used as a manufacturing technique to produce Ni-fluoropolymers composite
coatings on Ti6Al4V substrates. To evaluate the influence of the surface roughness on the water drop contact
angle, four different coating architectures (Ni, Ni+FEP, Ni polished and Ni+FEP polished) have been proposed
and implemented. Firstly, as a coating reference, a single layer of pure Ni coating was deposited (thickness
around 10 μm); then, Ni+FEP single layer was deposited with the help of a surfactant added to the nanoparticles
solution to avoid the agglomeration of the filler. Water drop contact angles as a function of time were studied for
the different samples and it turns out that the presence of the fluoropolymer fillers inside the matrix deeply modify
the wettability of the Ni matrix surface. These results show how coatings obtained by this synthesis technique
can be used as possible candidates for passive anti-icing system
EFFECT OF ZIRCONIUM CONCENTRATION FOR MODIFIED DIFFUSION ALUMINIDE COATING IN HOT CORROSION AND OXIDATION TESTS
No full textIntroduction: Diffusion aluminide coatings are widely used in hot components of aero- and land-base gas turbines to protect from oxidation and hot
corrosion. The effect of doping diffusion aluminide coating with reactive element (zirconium) was studied under different concentrations.
Materials and methods: Nickel-based super alloy were coated with diffusion aluminide by means of the Vapour Phase Aluminizing (VPA). Firstly the effects
of different activators salts during the coating process was studied: for this reason several concentrations of two different fluorides activator salts
(NH4F and AlF3) were investigated and the process parameters of the conventional (undoped) aluminide were optimized.
In a second step a Zr-doped coating was developed introducing ZrF4 in the activator salts mix with the process parameters obtained from the first step
of experimental procedure. The oxidation kinetics was evaluated by measuring the mass gain at different exposure times. The microstructural evolution
induced by the high temperature treatments was investigated by SEM, EDS and XRD analysis. The zirconium concentrations were assessed by means of
GDOES.
Results: The results of different activator salts in the VPA process are a thicker layer of β-NiAl and a consequent interdiffusion zone (IDZ) for AlF3 depositions.
This increase can be explained by considering the different quantities of aluminum available on the super- alloy surface as diffusion occurs.
The effect of the reactive element addition, in terms of oxidation and hot corrosion resistance and microstructural evolution, was assessed by comparing
the performance of the modified coatings with the standard diffusion aluminide systems.
Discussion: The results show that modified coatings exhibits an enhanced oxidation and hot corrosion resistance in comparison to standard diffusion aluminide coatings. The improvement of oxidation resistance for the doped samples can be explained by EDS mapping results. After 100h of oxidation tests, Zr prevents the path of oxygen that penetrate through the Al2O3 scale blocking the degradation phenomena typical fot the standard aluminide
coating (spallation, depletion layer of aluminum with consequent change of concentration in β-NiAl). Furthermore the diffusivity of Zr can counteract against the porosity made by the different diffusivity of Al and Ni during oxidation (Kirkendall effect)
PRODUCTION AND CHARACTERIZATION OF CARBON-PHENOLIC ABLATIVE MATERIALS MODIFIED WITH NANO-FILLERS
No full textConcept: Ablative materials represent the traditional approach to protect
space vehicles from the severe heating during hypersonic flight through a
planet’s or the Earth’s atmosphere. The Materials and Surface Engineering
Lab (LIMS) of Sapienza-University of Rome has developed carbon-phenolic
ablative materials with several densities and compositions. This work presents
a new methodology for their manufacture adding functionalized nanofillers.
In particular acetic, decanoic and methacrylic acid were chosen as
functionalizing group of nano-zirconia. Mechanical (bending and compression
tests) and thermal characterization (TGA/DSC) are carried out in order to
understand the effect of nano-fillers on the material properties.
Motivations and objectives: Ablative materials consist of a rigid felt impregnated
with phenolic resin. During the process the resin is diluted and nanoparticles
are added to this solution. Nano-filler are introduced in order to
improve mechanical and thermo-physical properties of virgin and charred
material, but their efficiency is strictly connected to the homogenous dispersion
of the particles. An acid surface modification were conducted to avoid
the agglomeration of particles according to Chiang et al. The efficiency of the
method was proved by FE-SEM photographs, mechanical and thermal tests
will enlighten the properties improvement.
Results and discussion: The FE-SEM photograph (Fig. 1) of a cured carbonphenolic
ablative material shows that addition of nano-fillers does not decrease
the goodness of the distribution of the resin in the sample without
density variation. Figure 2 shows that the surface modification and the impregnation
procedure does not change the dimension of the nanoparticles
(as shown in the inset). Although the particles are well dispersed in the resin.
Bending and compression tests will provide a comparison between samples
manufactured with and without nano-zirconia
High temperatures gas-solid reactivity of aluminum-carbon nanotubes composites
No full textThe gas-solid reactivity of metal-carbon matrix composites such as aluminum-carbon nanotubes (Al-CNT) sintered samples was studied at temperatures below and above the melting point of Al in O2, synthetic air,CO2, H2-Ar (5% v/v) and Ar. Small cylindrical samples of different composition with “single-walled” CNTs (SWCNTs) or “multi-walled” (MWCNTs) were sintered in Ar at 625 °C and the resulting materials showeddensities ranging from 92.2 to 99.0% of the theoretical density of bulk Al. Thermogravimetric analysis(TG) with simultaneous differential thermal analysis (DTA) up to 1200 °C shows that the Al-CNT composites do not behave as a two independent phases system. This is mainly demonstrated by the following phenomena: i. The lowering of the melting point of Al, the magnitude of which cannot be explainedby the expected very low solubility of C in Al at this temperature; ii. The amount of Al2O3 grown in oxidizing atmospheres and at the highest temperatures depends from the CNT content in the compos-ite; iii. The formation of Al4C3 occurs only in presence of molten Al as shown by environmental X-ray diffraction “XRD”. Field emission scanning electron microscope “FESEM” and high-resolution analytical transmission electron microscopy “HRTEM” investigations confirm that at the sintering temperature no detectable chemical interaction exists between Al and CNT
Rivestimenti diffusivi a base di alluminio modificati con zirconio per la protezione da corrosione e ossidazione”
No full textI materiali utilizzati nei componenti nelle turbine a gas aeronautiche e di terra devono
sostenere condizioni operative gravose e possono subire fenomeni di corrosione a caldo e
ossidazione ad alta temperatura; per questa ragione i rivestimenti diffusivi a base di
alluminio sono ampiamente utilizzati per la protezione di componenti in superlega che
lavorano in ambiente aggressivo, tipico delle turbine a gas. Nel presente lavoro viene
discussa e valutata la resistenza alla corrosione a caldo e all’ossidazione ad alta
temperatura da parte dei rivestimenti diffusivi a base di alluminio modificati con elementi
reattivi. La prima fase di questo studio prevede l’ottimizzazione della procedura di
alluminizzazione attraverso la tecnica di pack cementation; in particolar modo viene
analizzata l’influenza dei parametri di deposizione sul processo, inclusa l’analisi di
differenti sali attivatori a base di fluoruri. Nella seconda fase viene sviluppato un
rivestimento a base di alluminio drogato con zirconio introducendo fluoruro di zirconio
(IV) nella miscela di sali attivatori. Sono presentati i risultati ottenuti dall’ossidazione
isoterma e dai test di corrosione a caldo per il rivestimento standard e per il modificato. La
cinetica di ossidazione è valutata misurando l’aumento in massa per differenti tempi di
esposizione e viene osservata l’evoluzione microstrutturale indotta dall’alta temperatura.
L’effetto dell’elemento reattivo, in termini di aumento di resistenza alla corrosione a caldo
e all’ossidazione ad alta temperatura è stato investigato per confronto tra i risultati ottenuti
per il rivestimento standard e per quello modificato
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