Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 μm diamond dispersed Si₃N₄ matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si₃N₄ phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si₃N₄ with 4 wt% Al₂O₃ and 5 wt% Y₂O₃ sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to β-Si₃N₄ transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si₃N₄ matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si₃N₄ matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si₃N₄ matrix.     

Development of ZrO2–WC composites by pulsed electric current sintering

ZrO₂–WC ceramic composites with 40 vol% WC were consolidated by pulsed electric current sintering (PECS) for 4 min at 1450 °C under a pressure of 60 MPa. The effect of ZrO₂ stabilizers and the source of WC powder on the densification, phase constitution, microstructure and mechanical properties of the ZrO₂–WC composites were investigated and analyzed. The experimental results revealed that the amount and type of ZrO₂ stabilizers played a primary role on the phase constitution and mechanical properties of the composites in comparison to the morphology and size of the WC powder. The 2 mol% Y₂O₃-stabilized composites exhibited much better mechanical properties than that of 1.75 mol% Y₂O₃-stabilized or 1 mol% Y₂O₃ + 6 or 8 mol% CeO₂ co-stabilized composites. A Vickers hardness of 16.2 GPa, fracture toughness of 6.9 MPa m^1/2, and flexural strength of 1982 MPa were obtained for the composites PECS from a mixture of nanometer sized WC and 2 mol% Y₂O₃-stabilized ZrO₂ powder.     

In situ synthesis and densification of submicrometer-grained B4C-TiB2 composites by pulsed electric current sintering

B₄C composites with 15 and 30 vol% TiB₂ were pulsed electric current sintered from B₄C-TiO₂-carbon black mixtures in vacuum at 2000 °C. Full densification could be realised when applying an optimized loading cycle in which the maximum load is applied after completion of the B₄C-TiB₂ powder synthesis, allowing degassing of volatile species. The influence of the sintering temperature on the phase constitution and microstructure during synthesis and densification was assessed from interrupted sintering cycles. The in situ conversion of TiO₂ to TiB₂ was a complex process in which TiO₂ is initially converted to TiB₂ with B₂O₃ as intermediate product at 1400-1700 °C. At 1900-2000 °C, B₂O₃ reacted with C forming B₄C and CO. The B₄C and TiB₂ grain size in the fully densified 30 vol% TiB₂ composite was 0.97 and 0.63 μm, combining a Vickers hardness of 39.3 GPa, an excellent flexural strength of 865 MPa, and modest fracture toughness of 3.0 MPa m^1/2.     

Fabrication of hydroxyapatite/diopside/alumina composites by hot-press sintering process

Hydroxyapatite/alumina/diopside ceramic composites were fabricated by hot-pressing. The hardness, fracture toughness and bending strength of the new fabricated composites were measured. The compositions of hydroxyapatite matrix ceramic composites were discussed by XRD and FT-IR analysis. Microstructures of the composites were studied on fracture surfaces. The bending strength and fracture toughness of 58 vol.% hydroxyapatite, 40 vol.% alumina and 2 vol.% diopside sample, were 200 MPa and 2.80 MPa m^1/2, respectively.     

Flash sintering of alumina/reduced graphene oxide composites

The flash sintering behavior of Al₂O₃/reduced graphene oxide (rGO) composites was investigated. rGO was used as a composite component and a conductive additive. Under the electric fields of 250–400 V cm⁻¹, the flash event occurred at extremely low temperatures of 236–249 °C. The current density limit played a significant role in the degree of densification. A larger current density resulted in a higher density of the sample. However, current densities larger than 33.33 A cm⁻² resulted in broken samples because of the localization of high current density coupled with the formation of hot spots. Flash sintering at a furnace temperature of 800 °C, electric field of 300 V cm⁻¹ and current density limit of 33.33 A cm⁻² produced nearly completely dense Al₂O₃/rGO composites. In addition to the current limit, the furnace temperature is also a key parameter that controls the degree of densification to achieve “safe” flash sintering.     

Insight into tribological and corrosion behaviour of binderless TiCxNy ceramic composites processed via pulsed electric current sintering technique

The study presents the corrosion behaviour and wear response of pulsed electric current sintered binderless TiC₅₀N₅₀, TiC₇₀N₃₀, and TiC₉₀N₁₀ based ceramic composites, consolidated by spark plasma sintering (SPS), and the relative densities were evaluated using the Archimedes principle. The microstructural evolutions of the sintered samples were examined through various microscopy techniques, and their susceptibility to corrosion in aggressive chloride environment was assessed using open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The microstructural examination of the specimens showed the presence of different phases within the titanium carbonitride (TiCN) cermets. The wear resistance evaluated using the frictional coefficient (COF) and calculated wear rate showed that the specimens exhibited an improved resistance. The specimens further showed enhanced resistance to corrosion in the test electrolyte, as the TiC₅₀N₅₀ cermet displayed enhanced resistance to the aggressive chloride ions in comparison to the other specimens.     

Creep behaviour of alumina/YAG composites prepared by different sintering routes

Al₂O₃-5 vol.% Y₃Al₅O₁₂ (YAG) composite powders have been prepared by surface doping of α-alumina powders by an yttrium chloride aqueous solution. Two commercial, one submicron-sized, the other ultra-fine, alumina powders were compared as matrix materials. YAG phase was yielded by an in situ reaction promoted by the subsequent thermal treatment of the doped powders. In particular, a flash soaking into a tubular furnace kept at a fixed temperature in the range 1050-1150 °C was employed, for inducing the crystallization of yttrium-aluminates on the alumina particles surface, but avoiding a relevant crystallites growth. After that, aqueous suspensions of the calcined powders were dispersed by ball-milling and cast into porous moulds or simply dried in a oven. Slip cast green bodies were densified by pressure-less sintering, while powdered samples were consolidated by hot pressing or spark plasma sintering. The low- and high-temperature mechanical performances of the sintered materials were investigated and related to monolithic aluminas behaviour as well as to the composites microstructures. It is shown that the hot-pressed and spark plasma sintered composites present a significantly lower creep rate as compared to reference, monolithic alumina samples.     

Dense mullite zirconia composites obtained from the reaction sintering of milled stoichiometric alumina zircon mixtures by SPS

The main objective of this article is to obtain dense (porosity under 0.5%) polyphasic ceramics belonging to the Al₂O₃–SiO₂–ZrO₂ system by SPS sintering of high energy powders milled drily; the stoichiometric (54.45:45.54 zircon–alumina, weight basis) mixture was explored in this work. Particularly the principal sintering variables: sintering temperature and dwell time were investigated. The textural, structural and microstructural changes were evaluated together with the hardness and toughness of the obtained ceramics and their microstructure. The effect of the mechanical pre-treatment was carried out by X-ray diffraction and particle distribution evaluation. Due to the rapid heating process an incomplete reaction was achieved in several cases, as a consequence multiphasic ceramics with different alumina, mullite, zircon and zirconia contents were obtained.     

Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

Strong and tough HfC-HfB2 solid-solution composites obtained by reactive sintering with a SiB6 additive

HfC-HfB₂ composite ceramics were successfully reactively spark plasma sintered with a unique SiB₆ additive. The incorporation of the SiB₆ not only promotes the densification of HfC (up to ~99%), but also significantly enhances the toughness from 4.3 ± 0.5 MPa m^1/2 to 14.2 ± 1.4 MPa m^1/2. The flexural strength of the HfC-HfB₂ composite ceramics was simultaneously improved to 529 ± 48 MPa, which is about 1.4 times higher than that of HfC. This improvement is attributed to the dense composite microstructure comprising an in-situ formed HfB₂ and a solid solution of Si and O in HfC and HfB₂ grains.     

Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

ZrB₂/x·vol%B₄C (x = 30–90) composites were fabricated from ZrB₂ and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 10² s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B₄C phase was formed at 1600 °C, the relative density (D_r) was evaluated; the composites sintered at 1900 °C attained the highest D_r. Their D_r values increased gradually from 99.35% to 99.99% with increasing B₄C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (H_v), fracture toughness (K_IC) and three-point bending strength (σ_b) were measured. H_v exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, K_IC and σ_b revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m^1/2 for K_IC and 870 MPa for σ_b, in the 60 vol%B₄C sample, and then the K_IC decreased gradually to 9.73 MPa m^1/2, and σ_b dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σ_b in the 90 vol% B₄C sample. Next, the high-temperature σ_b values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σ_b (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B₄C composites at 1600 °C, and then the σ_b of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.     

Synthesis and characterization of B13C2 boron carbide ceramic by pulsed electric current sintering

Approximately 400 nm grain sized boron-carbon ceramic was synthesized by the pulsed electric current sintering (PECS) method using boron and carbon powders. Relative density of up to 95% was achieved at sintering temperature of 1900 °C. This ceramic was composed with B₁₃C₂ as major phase and few B₄C and C, which were characterized by X-ray diffraction (XRD) and Rietveld refinement quantitative analysis and chemical analysis (CA) and electron probe microanalysis (EPMA). The microstructure was also observed via transmission electron microscope (TEM).     

Effect of compaction on the sintering of borosilicate glass/alumina composites

The effect of initial compaction on the sintering of borosilicate glass matrix composites reinforced with 25 vol.% alumina (Al₂O₃) particles has been studied using powder compacts that were uniaxially pressed at 74, 200 and 370 MPa. The sintering behaviour of the samples heated in the temperature range 850–1150 °C was investigated by density measurement, axial and radial shrinkage measurement and microstructural observation. The density of the sintered composites increased continuously with temperature for compacts pressed at 74 MPa, while for compacts pressed at 200 and 370 MPa it reached the maximum value at 1050 °C and at higher temperatures it decreased slightly due to swelling. The results showed anisotropic shrinkage behaviour for all the samples, which exhibited an axial shrinkage higher than the radial shrinkage, and the anisotropic character increased with the initial compaction pressure.     

Morphological and mechanical properties of polyamide 6/nanodiamond composites prepared by melt mixing: effect of surface functionality of nanodiamond

Surface chemistry of as‐received nanodiamond (ND) was first tailored by dry thermal oxidation to obtain carboxylated ND (ND‐COOH) and by wet chemistry to obtain ethylenediamine‐functionalized ND (ND‐EDA). Then, the surface‐functionalized ND particles were dispersed in polyamide 6 (PA6) using the melt‐mixing method. Transmission optical and scanning electron microscopies indicated a fine dispersion at low nanodiamond concentrations, e.g. 0.25 wt%, particularly with ND‐EDA. Differential scanning calorimetry revealed that ND‐EDA favoured the α‐phase crystal and enhanced the degree of crystallinity of PA6. Experimental data indicated that ND‐EDA had considerably improved tensile properties at low concentration of 0.25 wt% compared to ND‐COOH, which was correlated to the fine dispersion and stronger and thicker interphase in the case of ND‐EDA. It was also found that the toughness of PA6 was improved on incorporation of ND‐EDA due to development of microcracking and crazing. © 2016 Society of Chemical Industry     

Influence of sintering temperature on the properties of pulsed electric current sintered hybrid coreshell powders

The influence of pulsed electric current sintering (PECS) temperature on the properties of bulk materials consolidated from three different types of hybrid powders have been studied. These powders consisted of iron oxide–silica coreshell structure, silver doped iron oxide–silica coreshell structure and, silver doped silica. The powders were prepared using a modified Stöber method. The sintering temperature was varied from 873 K up to 1273 K and sintering pressure and time were 50 MPa and 15 min respectively. Porous structures were obtained with relative densities from about 58 to 68%. Sintering temperature induced the growth of silver nanoparticles on the silica surfaces. Oxidation of the iron oxide during the compaction was affected by thermal decomposition of silver oxides. Sintering temperature changed the magnetic properties of iron oxide compacts via crystallite growth and oxide transformation. At temperature higher than 1173 K, iron oxide was reduced into pure iron (α-Fe).     

Carbon nanotube/boehmite-derived alumina ceramics obtained by hydrothermal synthesis and spark plasma sintering (SPS)

Preparation, structure and properties of hydrothermally treated carbon nanotube/boehmite (CNT/γ-AlOOH) and densification with spark plasma sintering of Al₂O₃ and CNT/Al₂O₃ nanocomposites were investigated. Hydrothermal synthesis was employed to produce CNT/boehmite from an aluminum acetate (Al(OH)(C₂H₃O₂)₂) and multiwall-CNTs mixture (200 °C/2 h.). TEM observations revealed that the size of the cubic shape boehmite particles lies around 40 nm and the presence of the interaction between surface functionalized CNTs and boehmite particles acts to form ‘nanocomposite particles’. Al₂O₃ and CNT/Al₂O₃ compact bodies were formed by means of spark plasma sintering (SPS) at 1600 °C for 5 min using an applied pressure of 50MPa resulting in the formation of stable α-Al₂O₃ phase and CNT–alumina compacts with nearly full density. It was also found that CNTs tend to locate along the alumina grain boundaries and therefore inhibit the grain coarsening and cause inter-granular fracture mode. The DC conductivity measurements reveal that the DC conductivity of CNT/Al₂O₃ is 10^?4 S/m which indicate that there is a 4 orders of magnitude increase in conductivity compared to monolithic Al₂O₃. The results of the microhardness tests indicate a slight increase in hardness for CNT/Al₂O₃ (28.35 GPa for Al₂O₃ and 28.57 GPa for CNT/Al₂O₃).     

High-strength ZrB2-based ceramics prepared by reactive pulsed electric current sintering of ZrB2–ZrH2 powders

Fully densified ZrB₂-based ceramic composites were produced by reactive pulsed electric current sintering (PECS) of ZrB₂–ZrH₂ powders within a total thermal cycle time of only 35 min. The composition of the final composite was directly influenced by the initial ZrH₂ content in the starting powder batch. With increasing ZrH₂ content, ZrB₂–ZrO₂, ZrB₂–ZrB–ZrO₂ and ZrB₂–ZrB–Zr₃O composites were obtained. The ZrB₂–ZrB–ZrO₂ composite derived from a 9.8 wt% ZrH₂ starting powder exhibited an excellent flexural strength of 1382 MPa combined with a Vickers hardness of 17.1 GPa and a fracture toughness of 5.0 MPa m^1/2. The high strength was attributed to a fine grain size and the removal of B₂O₃ through reaction with Zr. Higher ZrH₂ content starting powders were densified through solution-reprecipitation resulting in the formation of coarser angular ZrB₂–ZrB composites with a Zr₃O grain boundary phase with a fracture toughness of 5.0 MPa m^1/2 and an acceptable strength in the 852–939 MPa range.     

Evaluation of alumina reinforced oil fly ash composites prepared by spark plasma sintering

The thermomechanical behavior of micro/nano-alumina (Al₂O₃) ceramics reinforced with 1-5 wt.% of acid-treated oil fly ash (OFA) was investigated. Composites were sintered using spark plasma sintering (SPS) technique at a temperature of 1400°C by applying a constant uniaxial pressure of 50 MPa. It was evaluated that the fracture toughness of micro- and nanosized composites improved in contrast with the monolithic alumina. Highest fracture toughness value of 4.85 MPam^1/2 was measured for the nanosized composite reinforced with 5 wt.% OFA. The thermal conductivity of the composites (nano-/microsized) decreased with the increase in temperature. However, the addition of OFA (1-5 wt.%) in nanosized alumina enhanced the thermal conductivity at an evaluated temperature. Furthermore, a minimum thermal expansion value of 6.17 ppm*K⁻¹ was measured for nanosized Al₂O₃/5 wt.% OFA composite. Microstructural characterization of Al₂O₃-OFA composites was done by x-ray diffraction and Raman spectroscopy. Oil fly ash particles were seen to be well dispersed within the alumina matrix. Moreover, the comparative analysis of the nano-/microsized Al₂O₃/OFA composites shows that the mechanical and thermal properties were improved in nanosized alumina composites.     

Novel iron oxide–silica coreshell powders compacted by using pulsed electric current sintering: Optical and magnetic properties

The properties of the bulk materials consolidated of silica coreshell powders with iron oxide core have been studied. Iron oxide nanoparticles smaller than 20 nm in size were synthesized by a reverse co-precipitation process in ambient atmosphere. Coreshell structures with various amounts of iron oxide were prepared via a modified Stöber method. The powders were compacted by using pulsed electric current sintering (PECS) at 1373 K. The morphologies, microstructures, phases, optical, and magnetic properties of the samples were studied by using transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), UV–visible spectroscopy (UV–Vis), and vibrating sample magnetometer (VSM). Transmittance values in the 250–800 nm range varied with the amount of iron oxide. Sample with the lower content was transparent while the sample with the highest content was opaque with microporosity. The compact with the highest iron oxide content showed the ferromagnetic behaviour at 300 K. The phase transformations in the coreshell powders during the sintering process are discussed.     

LZS/Al2O3 nanostructured composites obtained by colloidal processing and spark plasma sintering

Li₂O-SiO₂-ZrO₂ (LZS) glass-ceramics have high mechanical strength, hardness, resistance to abrasion and chemical attack, but also a high coefficient of thermal expansion (CTE), which can be reduced adding alumina nanoparticles. The conventional glass-ceramic production is relatively complex and energy consuming, since it requires the melting of the raw materials to form a glass frit and a two-step milling process to obtain particle sizes adequate for compaction. This study describes the preparation of LZS glass-ceramics through a colloidal processing approach from mixtures of SiO₂ and ZrO₂ nanopowders and a Li precursor (lithium acetate obtained by reaction of the carbonate with acetic acid). Concentrated suspensions were freeze-dried to obtain homogeneous mixtures of powders that were pressed (100 MPa) and sintered conventionally and by spark plasma sintering. The effect of the alumina nanoparticles additions on suspensions rheology, sintering behavior and properties such as thermal expansion, thermal conductivity, hardness and Young’s modulus were evaluated.