Antonio Mario Locci

Simultaneous spark plasma synthesis and consolidation of WC/Co composites

Antonio Mario Locci Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), and Unità di Ricerca del Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali (INSTM), Università degli Studi di Cagliari, 09123 Cagliari, Italy Roberto Orrù and Giacomo Cao Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), and Unità di Ricerca del Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali (INSTM), Università degli Studi di Cagliari, 09123 Cagliari, Italy; and PROMEA Scarl, c/o Dipartimento di Fisica, Cittadella Universitaria di Monserrato, 09042 Monserrato (CA), Italy

A methodology to investigate the intrinsic effect of the pulsed electric current during the spark plasma sintering of electrically conductive powders

Abstract A novel methodology is proposed for investigating the effect of the pulsed electric current during the spark plasma sintering (SPS) of electrically conductive powders without potential misinterpretation of experimental results. First, ensemble configurations (geometry, size and material of the powder sample, die, plunger and spacers) are identified where the electric current is forced to flow only through either the sample or the die, so that the sample is heated either through the Joule effect or by thermal conduction, respectively. These ensemble configurations are selected using a recently proposed mathematical model of an SPS apparatus, which, once suitably modified, makes it possible to carry out detailed electrical and thermal analysis. Next, SPS experiments are conducted using the ensemble configurations theoretically identified. Using aluminum powders as a case study, we find that the temporal profiles of sample shrinkage, which indicate densification behavior, as well as the final density of the sample are clearly different when the electric current flows only through the sample or through the die containing it, whereas the temperature cycle and mechanical load are the same in both cases.

Combustion synthesis of metal carbides: Part I. Model development

The definition of a rigorous theoretical framework for the appropriate physico-chemical description of self-propagating high-temperature synthesis (SHS) processes represents the main goal of this work which is presented in two sequential articles. In this article, a novel mathematical model to simulate SHS processes is proposed. By adopting a heterogeneous approach for the description of mass transfer phenomena, the model is based on appropriate mass and energy conservation equations for each phase present during the system evolution. In particular, it takes microstructural evolution into account using suitable population balances and properly evaluating the different driving forces from the relevant phase diagram. The occurrence of phase transitions is treated on the basis of the so-called enthalpy approach, while a conventional nucleation-and-growth mechanistic scenario is adopted to describe quantitatively the formation of reaction products. The proposed mathematical model may be applied to the case of combustion synthesis processes involving a low melting point reactant and a refractory one, as for the synthesis of transition metal carbides from pure metal and graphite. Thus, the model can be profitably used to gain a deeper insight into the microscopic elementary phenomena involved in combustion synthesis processes through a suitable combination of experimental and modeling investigations, as it may be seen in Part II of this work [J. Mater. Res. 20, 1269 (2005)].

Spark plasma sintering of ZrB2- and HfB2-based Ultra High Temperature Ceramics prepared by SHS

The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication of fully dense MB2-SiC and MB2-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B4C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities ≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods.

Combustion synthesis of metal carbides: Part II. Numerical simulation and comparison with experimental data

Based on the general theoretical model proposed in Part I of this work [J. Mater. Res. 20, 1257 (2005)], a series of numerical simulations related to the self-propagating high-temperature synthesis in the Ti–C system is presented. A detailed and quantitative description of the various physical and chemical processes that take place during combustion synthesis processes is provided in Part II of this work. In particular, the proposed mathematical description of the system has been discussed by highlighting the relation between system macroscopic behavior obtained experimentally with the modeled phenomena taking place at the microscopic scale. Model reliability is tested by comparison with suitable experimental data being nucleation parameters adopted for the fitting procedure. The complex picture emerging as a result of the model sophistication indicates that the rate of conversion is essentially determined by the rate of nucleation and growth. In addition, comparison between model results and experimental data seems to confirm the occurrence of heterogeneous nucleation in product crystallization.

Combustion synthesis of TIC-metal composites and related plasma spraying deposition

The synthesis of titanium carbide/metal (i.e. Fe, Ti) composites from elemental powders by means of combustion reactions is investigated. A combustion booster (i.e. Teflon) to guarantee the self-propagating character of the combustion reaction is taken into account in the case of the TiC/Ti system. The combustion temperature and velocity of propagating front are found to decrease as the amount of metal in the starting mixture increase while both parameters increase as the amount of booster added to the mixture is augmented. The final products result constituted by titanium carbide and metal, being the latter one found as binder distributed around the carbide grains. In particular, a strong dependence of the grains size of the obtained titanium carbide on the iron content is observed. Moreover, the amount of titanium in the final product decreases as the Teflon content is increased, thus demonstrating that the latter one directly participates to the carburisation process. Subsequently, the TiC/Fe composite, once reduced in powder form, is tested as promising candidate for thermal spraying coatings. Results obtained by vacuum plasma spraying coatings are also reported.

Modeling of Point Defects Annihilation in Multilayered Cu/Nb Composites under Irradiation

This work focuses on a mathematical modeling of the response to irradiation of a multilayer composite material. Nonstationary balance equations are utilized to account for production, recombination, transport, and annihilation, or removal, of vacancies and interstitials at interfaces. Although the model developed has general validity, Cu/Nb multilayers are used as case study. Layer thickness, temperature, radiation intensity, and surface recombination coefficients were varied systematically to investigate their effect on point defect annihilation processes at interfaces. It is shown that point defect annihilation at interfaces mostly depends on point defect diffusion. The ability of interfaces to remove point defects can be described by a simple map constructed using only two dimensionless parameters, which provides a general tool to estimate the efficiency of vacancy and interstitial removal in multilayer composite materials.

Self-propagating combustion synthesis of intermetallic matrix composites in the ISS

Combustion Synthesis experiments have been performed on the ISS (International Space Station) during the Belgian taxi-flight mission ODISSEA in November 2002, in the framework of the ESA-coordinated project COSMIC (Combustion Synthesis under Microgravity Conditions). The main objective of the experiments was to investigate the general physico-chemical mechanisms of combustion synthesis processes and the formation of products microstructure. Within the combustion zone, a number of gravity-dependent phenomena occur, while other phenomena are masked by gravity. Under certain conditions, gravity-dependent secondary processes may also occur in the heat-affected zone after combustion. To study the influence of gravity, a specially dedicated reactor ensemble was designed and used in the Microgravity Science Glovebox (MSG) onboard the ISS. In this work, the experiment design is first discussed in terms of the experimental functionality and reactor ensemble integration in the MSG. To investigate microstructure formation, a sample constituted by a cylindrical portion followed by a conical one, the latter being inserted inside a massive copper block, is used. The experiment focused on the synthesis of intermetallic matrix composites (IMCs) based on the Al-Ti-B system. Depending on the composition, different intermetallic compounds (TiAl and TiAl3) can be formed as matrix phase while TiB2 represents the reinforcing particulate phase. During the ISS mission, six samples with a relatively high green density of 65%TD have successfully been processed. The influence of the composition on the combustion process will be examined.

Conventional and SPS sintering of a nanocrystalline alumina: a comparative study

In this paper, five powdered samples, containing different amounts of nanosized α-alumina, were obtained by controlled thermal pre-treatments of a transition alumina, and then densified by both free-sintering in air at 1500°C for 3 h and by Spark Plasma Sintering in the temperature range 1150°C - 1400°C with different soaking times at the maximum temperature. A comparative study of the influence of the phase composition in the starting alumina powders on both sintering behavior and microstructural features of the densified bodies is presented, showing a relevant influence of powder thermal pre-treatment on the SPS process, in a strict analogy to natural sintering.

Role of Interface in Multilayered Composites under Irradiation: A Mathematical Investigation

A continuum model of point-defects evolution during irradiation of a multilayer composite material is presented in this work. Nonstationary balance equations are used to describe production, recombination, transport, and annihilation, or removal, of vacancies and interstitials in a β-α-β three-layer system (α = Cu and β = Nb, V, or Ni). In addition, transport and trapping of point-defects at interfaces are taken into account. Numerical investigation on similarities and differences between Cu/Nb, Cu/V, and Cu/Ni systems is also performed. A general comparison of model results reveals that average vacancy concentration is typically higher than SIA one in both layers for all the systems investigated. This is a consequence of the higher diffusion rate of SIAs with respect to vacancies. Stationary state is reached without saturating interface point-defect traps by all systems but Cu/Ni for the case of SIAs. It can be also seen that Cu/Nb and Cu/V systems have a very similar behavior regarding point-defect temporal evolution in copper (layer α), while higher SIA concentration at steady state is shown therein by the Cu/Ni structure. Moreover, Cu/V system displays the lower stationary vacancy concentration in layer β.