Papers by Author: Begoña Ferrari

Abstract: This work is part of a project that aims to carry out industrial research on recovery of metallic Ti6Al4V waste material through the generation of recycled powders for their incorporation into the feedstock (in granular form) of extrusion based additive manufacturing processes. Preliminary printing tests show that commercially available 3D printing extrusion-based systems are ineffective at printing granules with particle sizes below 100 μm. Thus, a novel material extrusion screw-based 3D printing extrusion head is designed and tested with granular feedstock of different particle size distributions (1-100, 1-400, 100-475 and 475-550 μm). By means of this novel system, granular feedstock with sizes between 1 and 475 μm was successfully employed for the printing of green Ti6Al4V highly complex geometries. Critical printing parameters were investigated and optimized, demonstrating the feasibility of the incorporation of recycled Ti6Al4V powders into thermoplastic feedstocks for material extrusion additive manufacturing processes.

Abstract: Aimed by reducing the total cost of products, powder metallurgy (PM) processing of Ti is a subject of high interest. However, using of conventional PM techniques presents difficulties due to the intrinsic characteristics of Ti, like low strain ability, and high reactivity, which lead to low compressibility. Moreover, Ti powders with small particle size are difficult to process by conventional PM techniques as they present a lower compressibility and also a poor flowability. On the other hand, the colloidal processing has been used for long in ceramics to achieve green bodies with high densities, complex shapes and homogeneous microstructures, but they are rarely used to shape metal powders because of its high density and high surface reactivity. However, the possibility to process fine particles makes these techniques interesting for metals with low density like Ti.The colloid-chemistry control of metallic powders in aqueous slurries is proposed as a way to prepare Ti porous parts with small particle size, throughout the formulation of aqueous slurries with solid contents as high as 50 vol.%. The chemical and chemical-physic stability of Ti powders 10 μm in size was determined by measuring the zeta potential as a function of pH, and dispersant concentration, while the later optimization of Ti slurries and their adequation for the use of different colloidal techniques, were studied in terms of rheology and the addition of the processing additives, such as gel or foaming agents. Techniques such as thermal gelling, foaming, and impregnation of exo-templates or robocasting were used to build Ti parts with random and/or tailored macroporosity. The shaped pieces made on Ti were sintered in vacuum at 1100 oC for 30 minutes, and their microstructure and mechanical properties were determined and compared with dense materials shaped by combining PM and colloidal techniques in previous works

Abstract: This work focuses in the corrosion and wear properties of titanium reinforced with 1% wt. alumina particles, produced by a combination of colloidal techniques and powder metallurgy. The alumina particles were added to control the grain growth of titanium during sintering, and simultaneously to increase hardness and wear resistance. Colloidal techniques permitted a homogeneous dispersion of alumina particles on the surface of fine Ti particles by the formulation of stable aqueous suspensions that were further processed by spray-dry to obtain spherical granules with improved compressibility. Ti-alumina samples were produced by uniaxial pressing of granules and vacuum sintering leading to materials with homogeneous microstructure, a reduction of grain size higher than 50 % with respect to pure titanium, and a sensible increase in hardness. But the addition of ceramic particles can also have an influence on the corrosion behavior that is one of the most interesting properties of titanium alloys, and on wear resistance, that is one of the drawbacks of Ti. Moreover, the study of simultaneous action of wear and corrosion (tribocorrosion) is an area of highest interest in applications like biomedical or automotive. The corrosion behavior was evaluated by Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PP) in NaCl at two concentrations (0.9 % and 3.5 %) and temperatures (37 oC, and room temperature). Tribocorrosion tests were performed using a reciprocating ball-on-plate tribometer where a 10 mm diameter alumina ball was used as counter material, and 10 N normal load was applied during 30 min in the same concentrations and temperatures of NaCl as in the static corrosion tests. The results showed a clear improvement of wear resistance on the composite without reducing the corrosion behavior in both conditions.

Abstract: Biodegradable polymers and bioactive ceramics are being combined in a variety of novel materials for tissue engineering scaffolds. These composite systems, which combine the useful mechanical properties of polymers with the bioactivity of ceramics, seem to be a promising choice for bone tissue engineering. In recent years, the use of biopolymers that gelate on cooling has received a lot of attention with regards to the production of laminates and coatings. In this work, we report the incorporation of hydroxyapatite (HA) into a gelatin coating on stainless steel substrate using colloidal processing technology. A titania (Ti) buffer layer prepared by dip coating was inserted to improve the bonding strength between the HA/gelatin layer and stainless steel substrate. The suspensions, composed of 1 vol% of HA and three different additions of gelatin, were formulated with a focus on rheological properties for codeposition of both phases by electrophoretic deposition (EPD). The composite coatings performed by EPD were investigated in terms of deposition efficiency and kinetics over different deposition times. The EPD process was performed at both ambient temperature and the gelling temperature of the suspension. While at room temperature no electrophoretic growth of the layers was observed, the thermal gelation of gelatin promotes the growth of a homogeneous, well-adherent coating.

Abstract: Nickel oxide/Ni foam composite was studied as an electrode for electrochemical capacitors. The material has been processed by a simple, green and cost-effective electrophoretic deposition (EPD) method directly from the post-reaction medium in an one-pot synthesis and its later heat treatment.NiO films were obtained after calcination (325 and 450ᵒC) of β-Ni(OH)₂ deposits. β-Ni(OH)₂ nanoplatelets were produced by chemical precipitation of Ni precursors and the aid of ultrasound, free of modifiers. The stabilization of the hydroxide powder in suspension in different mixtures of Ethanol:Water (4:1, 9:1 and 19:1) and the optimization of the EPD parameters allowed coating the commercial Ni foams via colloidal processing.Electrochemical properties of the electrodes were evaluated by cyclic voltammetry (CV) showing significant differences in terms of specific capacitance of 192 and 79 F/g for the electrodes calcined at 325 and 450ᵒC, respectively.

Abstract: In this work the stability of TiN and TiC nanopowders in isopropyl alcohol as well as the fabrication of dense and well adhered thin coatings based on TiN and TiC by electrophoretic deposition (EPD) have been evaluated in terms of zeta potential and mass deposited when hydrazine is added. The surface of TiN and TiC nanoparticles has been modified to improve the dispersion in isopropyl alcohol adding a cationic polymer (polyethylenimine) as dispersant with two different molecular weights. The influence of acidic/basic character of the solvent also has been evaluated in order to reach the most efficient EPD process. It was found that the adsorption of polyethylenimine with higher molecular weight can preserve the homogeneity of TiC coatings. The surface of TiN nanoparticles can be tuned in order to achieve a similar polyethylenimine adsorption which also improves the deposition in basic media.

Abstract: The colloid-chemistry control of metallic powders in aqueous slurries is proposed as a way to prepare Ti powders with small particle size for a better pressing behavior through the spray dry process. The chemical-physic behavior of titanium powders with two different particle size distributions dispersed in water has been studied by measuring the zeta potential as a function of pH, and dispersant concentration. The employment of poly-acrylic dispersants allowed the fabrication of stable slurries with solid contents up to 50 vol% that have been sprayed under different conditions to form agglomerates ranging between 50 and 200 µm. Conditions were selected to achieve spherical agglomerates formed by a broad distribution of particle sizes that shown excellent flowability. Agglomerates were pressed in a uniaxial die to measure the compressibility, showing an improvement in pressing behavior with respect to powders with bigger particle size. The sintering behavior is also improved, as values of 96 % of the theoretical density were obtained for compacts sintered in vacuum at 1100 °C for 30 minutes.

Abstract: In this study, coatings with irregular nanotopography prepared by electrophoretic deposition (EPD) in electro-polished surfaces are characterized. Films are composed by Yttria Stabilized Zirconia (YSZ) particles, 5-8 nm in size and spherical morphology, synthesized under hydrothermal conditions and re-dispersed in the post reaction medium. Growth behaviour of coatings with time, including array morphology, aggregation origin and activity, are explored by means of atomic force microscopy (AFM) and its different measurement modes. Arrangement of electrically driven particles at the nanoscale can be assessed through the topographic description of films prepared with increasing deposition times. Moreover, topography can be associated to the electrokinetic behaviour of particles and agglomeration degree of the suspension. The evaluation of particle junctions and then the quality of particle cohesion within the film can also be discerned by AFM characterization.

Abstract: Electrophoretic deposition (EPD) is a powerful method for obtaining particulate layers in a broad range of thicknesses if an adequate control of the growing kinetics is reached. Existing models of EPD kinetics consider that the growth of the deposit increases linearly with deposition time and deviations are due to a reduction of powder concentration and/or a decrease of electric field when EPD is performed at constant voltage conditions. Experimental observations show that long time tests lead to a S-shaped growing kinetics. This work presents a resistivity model that predicts a S-shape variation of mass per unit area with deposition time, with a first step in which the deposition rate increases, as a consequence of resistivity changes, followed by a decreasing slope associated to the lose of powder concentration. Currently available EPD models, such as the Hamaker and Sarkar & Nicholson models are particular cases of the generalized resistivity model proposed in this work.

Abstract: There is a growing interest to develop reliable, economic and environmentally-friendly methods for manufacturing thick coatings and layered systems. For thick films, the adhesion to the substrate and the shape retention become important problems that lead to low density, cracking, and heterogeneous surfaces. To overcome these problems, a processing route is proposed consisting in the formation of a thick deposit by EPD in water and its immediate consolidation by thermogelation of polysaccharides. The process is based on the use of aqueous suspensions containing low concentration of biopolymers (i.e. carrageenan) and the formation of the film by dipping and/or EPD on suspensions heated at 60°C. On withdrawal at constant rate a film is obtained after cooling at room conditions. Maintaining the stability of the hot suspensions and during gelation is a key parameter to produce controlled deposits with good adhesion to the substrate and uniform microstructure. In this work, ceramic suspensions or Ni-containing suspensions have been stabilized with acrylic-based polyelectrolytes at controlled pH. Self-supported films of Al2O3/Y-TZP have been produced by dipping and EPD on graphite substrates that are burnt out on heating. On the other hand, Ni/YSZ composites have been deposited on graphite to produce either monolithic or laminated films. The optimization of the rheological behaviour of the suspensions and the EPD kinetics allows manufacturing a variety of complex structures with layers having controlled thickness ranging from 10 to 200 6m.