Search

Three WC-Ti powder mixtures with 5, 10 and 15 wt% titanium were sintered by the spark plasma sintering technique. The microstructures and phase compositions of the samples were investigated by SEM, STEM, EBSD and XRD. The samples consisted of WC, W2C and a (W1-xTx)C phases when the starting amounts of titanium were 5 and 10 wt%. At the titanium content of 15 wt% the microstructure of the samples included W2C, (W1-xTx)C phases and elemental tungsten. The solubility of WC in TiC with the appearance of the (W1-xTx)C phase depended on the stoichiometry of the starting powder composition and sintering temperature. The results of EBSD phase mapping and the XRD investigation are in good agreement with the molar analysis. The best combination of hardness and fracture toughness was achieved with 5 wt% titanium. The appearance of elemental tungsten after sintering the WC-15Ti composition led to a significant reduction in hardness.

Dense Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO-NCO-BCCO) nanocomposites were produced from sol-gel derived Ca2.25Na0.3Bi0.35Tb0.1Co4O9 powder by four methods: Hot-pressing (HP), spark plasma sintering (SPS) and pressureless sintering in air or O2 atmosphere. Nanocomposites from HP and SPS revealed nanosized grains and showed a thermoelectric power factor of 4.8 and 6.6 mW cm−1 K−2, respectively, at 1073 K in air. A dense 2D nanocomposite with structures on multiple length scales and enhanced thermoelectric properties was obtained from pressureless sintering in O2 atmosphere. The resulting 2D nanocomposite enabled the simultaneous increase in isothermal electrical conductivity s and Seebeck coefficient a, and showed a thermoelectric power factor of 8.2 mW cm−1 K−2 at 1073 K in air. The impact of materials with enhanced electrical conductivity and power factor on the electrical power output of thermoelectric generators was verified in prototypes. A high electrical power output and power density of 22.7 mW and 113.5 mW cm−2, respectively, were obtained, when a hot-side temperature of 1073 K and a temperature difference of 251 K were applied. Different p- and n-type materials were used to verify the effect of the thermoelectric figure-of-merit and power factor on the performance of thermoelectric generators.

The membrane microstructure and phase composition of Ruddlesden–Popper-type La 2 NiO 4+ δ ceramics, which were prepared by field-assisted sintering technique/spark plasma sintering (FAST/SPS) process or by conventional pressing and pressureless sintering were investigated. As starting material, a La 2 NiO 4+ δ powder, with an average particle size of 0.2 μm was used. The grain-size distribution of the resulting membranes varied from 0.015 μm 2 for FAST/SPS sintered ceramic to 6.11 μm 2 for pressureless sintered membrane. The microstructure analysis of membranes was performed by transmission and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Phase transition from orthorhombic to tetragonal crystallographic structure in FAST/SPS material was investigated by temperature-dependent X-ray powder diffraction. The effect of sintering technique and grain size on the oxygen permeation performance of the membranes is discussed with respect to impurities in the material.

Field‐assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure‐assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non‐equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the microstructure and electrical properties of such materials. In this work different materials produced by SPS from ‘turbostratic’ and well-crystallised powders are compared with commercial materials in terms of densification, microstructure and electrical properties. The turbostratic powders could be densified at temperatures as low as 1500–1600 °C, but recrystallisation of the grains took place at much higher temperatures (1800–1900 °C). The electrical resistivity of the investigated materials reached values of up to 1015 Ω cm and strongly depended on the microstructure. The main factor influencing the resistivity was the amount and nature of the grain boundary phase.

Binderless TiC–WC–Cr 3 C 2 hard materials with different compositions were prepared by FAST (field-assisted sintering technique) and by SPS from mixtures of TiC, WC and Cr 3 C 2 . The effect of the microstructure on the properties was evaluated in as-densified materials and materials heat treated at 1900 °C. The mechanical properties were determined and the microstructures were characterised using X-ray diffraction and scanning electron microscopy. A cubic (Ti, W, Cr)C phase was found to form during sintering. Hardness values of up to 23 GPa were observed. For the TiC-rich materials annealing did not have a significant effect on the hardness, but in the case of the WC-rich materials it resulted in a reduction in the hardness due to pore formation.

Particle reinforced metal matrix composites (MMCs) based on high-alloyed CrMnNi steels and partially stabilized zirconia (Mg-PSZ) are produced using the hot pressing technique. The resulting materials differ in the chemical composition of the high-alloyed steel matrix and volume fraction of Mg-PSZ. The change of chemical composition has a substantial influence on the austenite stability and stacking fault energy, triggering different deformation mechanisms in terms of the deformation-induced transformation of austenite into ϵ- and/or α′-martensite and/or deformation-induced twinning. MMCs with 5 and 10 vol% Mg-PSZ as well as unreinforced steels are investigated in total strain controlled fatigue tests. The focus of this work is to clarify the effect of the matrix strength and Mg-PSZ volume fraction on the cyclic deformation behavior. This includes the discussion of cyclic hardening and/or softening during fatigue tests, consequences on the fatigue life times and deformation-induced microstructures.

B 6 O/TiB 2 composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B 6 O material showed that the addition of TiB 2 in a non-reactive sintering route promotes the B 6 O densification. Further improvement was obtained by sintering reactive B–TiO 2 mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m 1/2 (SEVNB) and 2.6–5.0 MPa m 1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV 0.4 ) and 28 GPa (HV 5 ) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10 −6 /K, respectively.

During Field Assisted Sintering Technology (FAST) the temperature differences at two different positions were investigated using two pyrometers, an internal and an external one. Two substances, an electrically conductive (tungsten carbide) and a non-conductive material (96 wt.% silicon nitride with 2 wt.% alumina and yttria) were used to monitor the temperature differences between both pyrometers during heating, sintering shrinkage and dwell time by varying die geometry and heating rate. It was shown that the temperature distribution is strongly influenced by the electrical conductivity of the material as well as by tool design and setup. The alpha–beta transformation of silicon nitride was analyzed to predict the radial temperature distribution within the sample. For comparison and for visualization a dynamical FE model including piston movement for simulating sintering shrinkage was introduced. With this, a complete time dependent FAST run could be simulated. The modeled differences in temperature distribution are in good agreement with real temperature measurements as well as phase analyses.

Effect of electric current on sintering behavior and microstructure evolution of zirconium diboride (ZrB2) was investigated using three different configurations of Field Assisted Sintering Technology/Spark Plasma Sintering. The current flow through the ZrB2 compact was controlled by modifying the interface between the graphite punches and the electrical conductive powder. Boron nitride discs, graphite foils or direct contact with the graphite punches were the three different interfaces used in order to deflect, conduct or promote, respectively, the current during the sintering process of the electrically conductive ZrB2 ceramics. The current flow during the sintering process triggered the elimination/reduction in B2O3, leading to faster diffusion rates at high temperatures and limiting the formation of B4C secondary phase. This allows to control the final density, grain size (from 19.6 to 43.2 μm) and secondary phase formation (from 5.95 to 11.61 vol%) as well as the electrical resistivity (from 7.7 to 9.4 μΩ·cm) of the specimens.

This research work presents the fatigue behaviour of a high-alloyed austenitic TWIP steel as well as metal matrix composites based on this steel grade as matrix material and MgO partially stabilized zirconia (Mg-PSZ) as reinforcing phase. The specimens were produced using the hot pressing technique. The unreinforced steel and MMCs containing 5 and 10 vol.% Mg-PSZ were investigated under total strain control in order to clarify the influence of the ceramic reinforcement. The cyclic deformation behaviour in terms of cyclic softening and/or cyclic hardening as well as fatigue lives of the different materials are discussed. Microstructures were examined to study the deformation and dislocation structures as well as phase transformations using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD), respectively.

Hexagonal boron nitride/previous termtitaniumnext term diboride composites are widely used as evaporation boats for aluminium deposition to produce functional and decorative layers on different target materials. The lifetime of such a material is limited mainly by the previous terminteractionnext term of the metal with the ceramic substrate, but the corrosion mechanism has still not yet been thoroughly investigated and understood. In this article the corrosion mechanism for the evaporation boats used was investigated using thermodynamic calculations, FESEM, EDX and XRD phase analysis. The analysis showed that hexagonal boron nitride (hBN), which is thermodynamically less stable than TiB2, is passivated during the application process through the formation of AlN surface layers, whereas the thermodynamically more stable TiB2 phase dissolves and Ti-rich components precipitate in cooler regions of the evaporation boats.