Effects of Boron Addition on the Microstructure and Mechanical Properties of (Ti,Ta)(C,N)-Co Based Cermets

In this work, a titanium–tantalum carbonitride based cermet, with cobalt as the binder phase and boron as a sintering additive, was developed by a mechanically induced self-sustaining reaction process using two different methodologies. The boron additive was added to prevent the formation of brittle intermetallic compounds generally formed during the liquid phase sintering step due to the excessive ceramic dissolution into the molten binder phase. A systematic study was carried out to understand the effects of boron addition on the nature of the phases, microstructure, and mechanical properties of cermets. With the boron addition, the formation of two different boride solid solutions, i.e., (Ti,Ta)B2 and (Ti,Ta)3B4, was observed. Moreover, the nature of the binder was also modified, from the (Ti,Ta)Co2 brittle intermetallic compound (for cermets without boron addition) to ductile and tough (Ti,Ta)Co3 and -Co phases (for cermets with boron addition). These modifications caused, as a general trend, the increase of hardness and toughness in cermets.Fondo Europeo de Desarrollo Regional MAT2014-52407-

Effects of the speed ratio on the efficiency of planetary mills

The ignition time (tig) of the mechanically induced self-sustaining reaction (MSR) process involving the formation of TiB2 from Ti/2B elemental mixtures was used to study the influence of the ratio (k = -ωv/ωd) between the rotational speed of the supporting disc (ωd) and vials (ωv) on the milling efficiency of a Pulverisette 4 planetary mill. The variation of the inverse of the ignition time (1/tig), which is directly related to the milling power provided by the planetary mill, with the process conditions has shown that it is not possible to find a single k value as optimal independently of the experimental conditions used (ωd and the ball-to-powder ratio, BPR). Moreover, it was observed that the lowest milling efficiency (longer tig values) was found for k = 1, which is the usual value employed in routine laboratory works. The best efficiencies were found for the larger k values (2.5 or 3). At lower ωd, the shortest tig was obtained for k = 2.5 and at higher ωd for k = 3, independently of BPR.Secretaria de Estado de Universidades, Investigacion, Desarrollo e Innovacion, Spain MAT2014-52407-REuropean Regional Development Fund, European Unio

High temperature oxidation resistance of (Ti,Ta)(C,N)-based cermets

Cermets based on titanium–tantalum carbonitride were oxidized in static air between 800 °C and 1100 °C for 48 h. The thermogravimetric and microstructural study showed an outstanding reduction in the oxidation of more than 90% when the Ta content was increased. In cermets with low Ta content, the formation of a thin CoO/Co3O4 outer layer tends to disappear by reacting with the underlying rutile phase, which emerges at the surface. However, in cermets with higher Ta content, the formation of an external titanate layer, observed even at a low temperature, appears to prevent the oxygen diffusion and the oxidation progressionGobierno de España European Regional Development MAT2011- 2298

Development of multicomponent-multiphase materials based on (Ti, Ta, Nb)CxN1-x carbonitride solid solutions

A set of powdered cermets based on (Ti,Ta,Nb)C xN 1-x carbonitride solid solutions were synthesized from mixtures of elemental powders by a mechanically induced self-sustaining reaction (MSR) method and subsequently sintered using a pressureless method. Differing nominal compositions of the hard phase were used, and the nature of the metallic-binder phase (Co, Ni, or Co-Ni) was varied. For comparative purposes, the design of the material was performed using two different synthesis pathways. The composition and microstructure of the ceramic and binder phases before and after sintering were analyzed and related to the microhardness of the material, which was found to increase with increasing contiguity of the hard phase and with decreasing particle size.The samples synthesized in one step (SERIES 2) showed higher microhardness and a more homogeneous microstructure with smaller particle size of the hard phase due to the presence of Ti, Ta, and Nb in the molten binder that hindered ceramic growth during liquid phase sinteringGobierno de España No. MAT2010- 1704

Liquid-phase sintering of Ti(C,N)-based cermets. the effects of binder nature and content on the solubility and wettability of hard ceramic phases

Different commercial TiC-TiN/Co/Ni mixtures were used as raw materials for Ti(C,N) cermets, and the effects of the sintering parameters (binder content, binder nature, sintering time and additives) on the final hard ceramic phase were studied at the sintering temperature of 1400 °C. When Co is used as the binder medium, it is possible to completely convert the starting commercial TiC-TiN mixture into TiCxN1-x. When Ni is used, which exhibits lower solubilising capacity than Co, the total conversion can never be reached and the metallurgical reactions between TiC and TiN during the liquid-phase sintering are more dependent on the sintering time than on the binder content. However, the use of Co-Ni mixtures, showing a synergic effect between the wettability capacity of Ni and the solubilising capacity of Co, enhances the metallurgical reactions at short sintering times. © 2013 Elsevier B.V. All rights reserved.Gobierno de España No. MAT2011- 2298

Kinetics of high-temperature oxidation of (Ti,Ta)(CN)-based cermets

The kinetics of the high-temperature oxidation of titanium–tantalum carbonitride-based cermets with different Ti/Ta ratios was studied. Isothermal oxidation tests were conducted under static air for 48 h at temperatures between 700 °C and 1200 °C. The oxidation satisfied the parabolic kinetics, characteristic of the existence of a protective oxide layer. The apparent activation energy suggests the rate-controlling process during oxidation is the simultaneous inward and outward diffusion of oxygen and titanium, respectively, through the formed protective layer, consisting mainly of a rutile phase. A higher Ta(V) content in the rutile decreased the oxygen diffusivity due to the reduction of oxygen vacancy concentration.Gobierno de España European Regional Development MAT2011- 2298

Development by Mechanochemistry of La0.8Sr0.2Ga0.8Mg0.2O2.8 Electrolyte for SOFCs

In this work, a mechanochemical process using high-energy milling conditions was employed to synthesize La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) powders from the corresponding stoichiometric amounts of La2O3, SrO, Ga2O3, and MgO in a short time. After 60 min of milling, the desired final product was obtained without the need for any subsequent annealing treatment. A half solid oxide fuel cell (SOFC) was then developed using LSGM as an electrolyte and La0.8Sr0.2MnO3 (LSM) as an electrode, both obtained by mechanochemistry. The characterization by X-ray diffraction of as-prepared powders showed that LSGM and LSM present a perovskite structure and pseudo-cubic symmetry. The thermal and chemical stability between the electrolyte (LSGM) and the electrode (LSM) were analyzed by dynamic X-ray diffraction as a function of temperature. The electrolyte (LSGM) is thermally stable up to 800 and from 900 °C, where the secondary phases of LaSrGa3O7 and LaSrGaO4 appear. The best sintering temperature for the electrolyte is 1400 °C, since at this temperature, LaSrGaO4 disappears and the percentage of LaSrGa3O7 is minimized. The electrolyte is chemically compatible with the electrode up to 800 °C. The powder sample of the electrolyte (LSGM) at 1400 °C observed by HRTEM indicates that the cubic symmetry Pm-3m is preserved. The SOFC was constructed using the brush-painting technique; the electrode-electrolyte interface characterized by SEM presented good adhesion at 800 °C. The electrical properties of the electrolyte and the half-cell were analyzed by complex impedance spectroscopy. It was found that LSGM is a good candidate to be used as an electrolyte in SOFC, with an Ea value of 0.9 eV, and the LSM sample is a good candidate to be used as cathode

Chemical and electrical properties of LSM cathodes prepared by mechanosynthesis

Mechanosynthesis of La1-xSrxMnO3 (x = 0, 0.25, 0.5, 0.75 and 1) was carried out at room temperature from stoichiometric mixtures of La2O3, Mn2O3 and SrO, obtaining monophasic powders with the perovskite structure. Physical properties of these materials and their chemical compatibility with the electrolyte yttria stabilized zirconia (YSZ), which depend strongly on the La/Sr ratio, were evaluated to corroborate availability to be implemented as cathode material in solid oxide fuel cells (SOFCs). Electrical conductivity values in air ranged between 100 and 400 S cm-1 in the temperature range of 25-850 C. Samples presented low reactivity with YSZ in the working temperature range (600-1000 C) maintaining the grain size small enough to preserve the catalytic activity for oxygen reduction.Gobierno de España MAT2010-1704

In-situ synthesis of a ZrB2–based composite powder using a mechanochemical reaction for the zircon/magnesium/boron oxide/graphite system

A ZrSiO4/B2O3/Mg/C system was used to synthesize a ZrB2-based composite through a high-energy ball milling process. As a result of the milling process, a mechanically induced self-sustaining reaction (MSR) was achieved in this system. A composite powder of ZrB2–SiC–ZrC was prepared in situ by a magnesiothermic reduction with an ignition time of approximately 6 min. The mechanism for the formation of the product was investigated by studying the relevant subreactions, the stoichiometric amount of B2O3, and thermal analysis.Gobierno de España No. MAT2011-2298

Mechanosynthesis of nanocrystalline ZrB2-based powders by mechanically induced self-sustaining reaction method

Preparation of nanocrystalline ZrB2-based powder by aluminothermic and magnesiothermic reductions in M/ZrO2/B 2O3 (M = Al or Mg) systems was investigated. In this research, high energy ball milling was employed to persuade necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). The course of MSR reactions were recorded by a noticeable pressure rise in the system during milling. Ignition times for ZrB2 formation by aluminothermic and magnesiothermic reductions were found to be 13 and 6 min, respectively. Zirconium diboride formation mechanism in both systems was explained through the analysis of the relevant sub-reactions. © 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.Gobierno de España No. MAT2011- 2298