Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Development and evaluation of TiB2–AlN ceramic composites sintered by spark plasma

Titanium diboride (TiB₂) is a ceramic material with high mechanical resistance, chemical stability, and hardness at high temperatures. Sintering this material requires high temperatures and long sintering times. Non-conventional sintering techniques such as spark plasma sintering (SPS) can densify materials considered difficult to sinter. In this study, TiB₂–AIN (aluminum nitride) composites were sintered by using the SPS technique at different sintering temperatures (1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C). x-ray diffraction was used to identify the phases in the composites. mechanical properties such as hardness and indentation fracture toughness was obtained using a vickers indenter. Different toughening mechanisms were identified, and good densification results were obtained using shorter times and lower temperatures than those previously reported.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Spark plasma sintering and hot pressing of ZTA-NbC materials – A comparison of mechanical and electrical properties

Zirconia toughened alumina can be made electrically conductive and thus electric discharge machinable by addition of a percolating dispersion of niobium carbide. In order to boost the productivity of the sintering process spark plasma sintering was tested at identical temperature and pressure but shorter dwell than in hot pressing. SPS sintering parameters for ZTA-NbC are developed and spark plasma sintered ceramics are compared to the hot pressed benchmark.During SPS a percolating NbC backbone of niobium carbide grains is formed which enhances electrical conductivity but impedes densification. Identical strength at however higher sintering temperature is achieved by SPS but the fracture resistance and hardness were always superior in hot pressed samples. The monoclinic content of zirconia grains in as fired SPS samples is higher despite smaller average grain size and the transformation toughening effect is less pronounced. SPS promises economic benefits due to shorter dwell and cooling cycles.     

Properties of spark plasma sintered pseudocubic BiFeO3–BaTiO3 ceramics

Perovskite solid solution materials, namely, 0.67BiFeO₃-0.33BaTiO₃, were synthesized by spark plasma sintering method. The effects of the spark plasma sintering temperature on phase purity, microstructure, and electric properties of the as-prepared materials were investigated. The materials could be referred as pseudocubic phases based on the X-ray diffraction patterns. The bulk density first increased and then decreased. The 880 °C-sintered-ceramics had the maximal density and a compact microstructure with grain size of 0.77 ± 0.34 μm. The dielectric constant as a function of temperature exhibited a broad peak. At the optimal spark-plasma-sintering temperature, enhanced ferroelectric properties were observed with a value of P_r ～ 21 μC/cm². This investigation on the spark plasma sintering process confirms it as an efficient approach to prepare outstanding performance BiFeO₃–BaTiO₃ ceramics.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Effect of processing on the stability and electrical properties of pressureless sintered graphene oxide–alumina composites

This paper explores the processing of an alumina matrix composite with a percolating network of graphene oxide (GPO), which exhibits a moderate electric resistivity and a near zero temperature coefficient of resistance. Different formulations of GPO–alumina composites were processed using a water–base blending, and, the pellets were densified by pressureless sintering under Argon flow. Electrical conduction at room temperature was achieved in the 2 wt % GPO–alumina composite sintered at 1400 °C, and, the 3 wt % GPO–alumina composites sintered at 1400, 1550 and 1700 °C. An investigation of the degradation of electrical conductivity was used to identify potential stable operating regimes in which these materials could be used as heaters. Thermogravimetric analysis using the Ozawa–Flynn–Wall method, was used to determine the kinetic parameters of a 3 wt % GPO composite sintered at 1400 °C which, had an activation energy for GPO degradation of 195 ± 68 kJ/mol and, an estimated thermal lifetime of 8.7 ± 0.8 years for a conversion of 0.5 wt % (failure criterion) at an application temperature of 340 °C.     

Effect of heating rate on microstructure and properties of spark plasma sintered Li-α-sialon

Li-α-sialon ceramics with low oxygen content were prepared by spark plasma sintering at 1750 °C, using three heating rates of 100 °C/min, 200 °C/min and 300 °C/min. In all cases, the densification of Li-α-sialon ceramics is effectively promoted. The rapidly anisotropic growth of grains, either α-sialon or β-sialon, is significantly enhanced with a heating rate above 200 °C/min. This could be attributed to a large amount of low viscosity oxygen-rich liquid formed at elevated temperatures, which gives rise to a so-called dynamic ripening mechanism at the early stage of sintering. Furthermore, for the composition near α-sialon boundary, the oxygen-rich liquid results in the formation of β-sialon together with a large amount of intergranular glassy phase. Only the Li-doped sialon with low lithium and oxygen content possesses relatively high infrared transmittance when sintered at a rate of 100 °C/min.     

Synthesis and spark plasma sintering of Si3N4–ZrN self-healing composites

A Si₃N₄–ZrN wear-resistant self-healing composite material was developed. Si₃N₄–ZrN composite ultrafine powders were synthesized at a temperature of 1200 °С via solid-state reactions without milling and densified by spark plasma sintering at 1650 °C to a relative density of 97 ± 0.5%. Balls 13.494 mm in diameter for ball bearings manufactured by spark plasma sintering had a fine-grained structure with a grain size of 200–500 nm, Vickers hardness of 22.5 ± 1.8 GPa, and indentation fracture toughness of 6.2 ± 0.4 MPa. The tribological properties of the composite were investigated under static and dynamic loading. The self-healing capability of the Si₃N₄–ZrN composite was evaluated in the temperature range 500–550 °С. High-temperature three-point bending tests of notched specimens showed a bending strength of 383 ± 21 MPa at room temperature and 413 ± 30 MPa at 500 °С, which confirmed the self-healing of the composite.     

Microstructure,mechanical properties and machining performance of spark plasma sintered Al2O3–ZrO2–TiCN nanocomposites

The effect of addition of nanocrystalline ZrO₂ and TiCN to ultrafine Al₂O₃ on mechanical properties and microstructure of the composites developed by spark plasma sintering (SPS) was investigated. The distribution of the nanoparticles was dependent on their overall concentration. Maximum hardness (21 GPa) and indentation toughness (5.5 MPa m^1/2) was obtained with 23 vol% nanoparticles, which was considered as the optimum composition. The Zener pinning criteria were also satisfied at this composition with grain size of the restraining nanoparticles ～63–65 nm. Hardness of the composites follows the rule of mixtures; crack deflection and crack arrest by nanoparticles at grain boundaries along with mixed fracture mode led to high toughness in the nanocomposites. Cutting tool inserts were developed by SPS with the optimized composition and their machining performance was compared with commercial alumina based inserts. Increased toughness in the nanocomposite inserts reflects in the machining performance as the tool life improves drastically compared to that of the commercial inserts at high cutting speeds ≥500 m min^?1. This was attributed to differences in their failure modes; the commercial inserts fail catastrophically by fracture due to their low toughness whereas the nanocomposite inserts reach the tool failure criteria by crater wear at all machining conditions.     

Nanoindentation and TEM investigation of spark plasma sintered TiB2–SiC composite

The impact of adding 20 vol% SiC on the properties of TiB₂ was studied in this research. The spark plasma sintering (SPS) process was used as the preparation technique at 1850 °C, the resulted composite was characterized using X-ray diffraction (XRD), field emission electron probe micro analyzer, transmission electron microscopy (TEM), field emission scanning electron microscopy, energy dispersive X-ray analysis, and nanoindentation. The prepared composite presented a relative density of ～98.5%. XRD and TEM results confirmed the in-situ formation of graphite; no in-situ TiC could be detected in the final microstructure of the composite. Forming a low melting point compound between SiO₂ and B₂O₃ oxides lead to the creation of wet interfaces between the ingredients. In terms of mechanical properties, the composite possessed Vickers hardness of 21.6 ± 2.2 GPa, flexural strength of 616 ± 28 MPa, fracture toughness of 5.3 ± 1.2 MPa m^1/2, and elastic modulus of 498 ± 12 GPa. According to the microstructural images, crack deflection, crack branching, crack arresting, crack bridging, and grain breaking events were found to be the main toughening mechanisms in this ceramic. In addition, the nanoindentation investigation indicated the role of SiC addition in improving the elastic modulus, hardness, and wear resistance of the prepared composite.     

Effects of density on the mechanical properties of spark plasma sintered β-SiC

The influence of density on the hardness value of β-SiC samples was studied based on Knoop and Vickers indentation tests. Hardness measurements were performed on additive-free spark plasma sintered SiC samples in the [80%–95%] density range and on highly dense samples (>99%) sintered with very low content of sintering aids. Results revealed that the density has a strong influence on the hardness value, which increases of about 7 GPa between samples presenting densities of 80% and 95% and even reaches 21 GPa under 2 kg with Knoop indenter for the densest samples sintered with very low content of sintering aids. These results allowed us to give a comprehensive model-supported analysis of the mechanical properties of spark plasma sintered β-SiC with controlled porosity that currently does not exist in the literature. The calculation of Young modulus and toughness further resulted in encouraging properties for our samples with regards to mechanical and ballistic performances.     

Thermal conductivity and dielectric property of hot-pressing sintered AlN–BN ceramic composites

A study was conducted of the effects of sintering temperature and CaF₂ additives on densification, microstructure, dielectric property and thermal conductivity of AlN–BN composites. Increasing sintering temperature and CaF₂ contents help to improve the densification, thermal conductivity, and purification of the grain boundaries. Thermal conductivity value reached 110 W m⁻¹ K⁻¹ for AlN–BN composites with 3 wt.% CaF₂ and sintered at 1850 °C. Increasing sintering temperature decreases relative dielectric constant and tan δ. The increase in CaF₂ content increases relative dielectric constant and decreases tan δ. Relative dielectric constants values were between 7.29 and 7.64 and dielectric loss tangent values ranged from 6.36 to 7.83 × 10⁻⁴ at 1 MHz.     

Pressureless spark plasma–sintered Bioglass® 45S5 with enhanced mechanical properties and stress–induced new phase formation

Commercial Bioglass^® 45S5 powder was sintered using spark plasma sintering (SPS) technique without the assistance of mechanical pressure with heating and cooling rate of 100 °C/min, dwell temperature of 1050 °C and dwell time of 30 min. Such route enabled the production of samples exhibiting superior mechanical properties in comparison with Bioglass^® sintered in furnace. In particular, flexural strength and fracture toughness reached values close to those of apatite-wollastonite bioceramics, already widely used in clinical applications. The residual stresses implemented by indentation promoted the formation of a new phase in samples sintered by SPS. Complementary use of Raman and energy dispersive spectroscopy (EDS) indicated the phase as sodium carbide and a formation mechanism was proposed.     

Dielectric and piezoelectric properties of 1–3 non-lead barium zirconate titanate-Portland cement composites

Non-lead barium zirconate titanate (BZT)-Portland cement (PC) composites have been seen as promising new non-lead composites. This paper reports research work on the dielectric and piezoelectric properties of 1–3 non-lead barium zirconate titanate (BZT)-Portland cement (PC) composites. The 1–3 non-lead composites with different BZT contents at 40–70% by volume were fabricated by the dice-and-fill method. The results show that the dielectric loss of the 1–3 non-lead composite was lowest at 0.08 at 70% BZT composites. The models are applied for the calculation with the dielectric constant, piezoelectric coefficient and piezoelectric voltage constant and the results were found to fit closest to that of the parallel model. At 70% BZT content or higher, piezoelectric coefficient was found to have values higher than 130 pC/N. In addition, the new 1–3 non-lead composites can be tuned to an ideal compatible value that match the requirement of concrete structure.     

MoSi2–Si3N4 composites: Influence of starting materials and fabrication route on electrical and mechanical properties

The fabrication method and the mechanical and electrical properties of different MoSi₂–Si₃N₄ composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi₂–Si₃N₄ and submicron MoSi₂ powders were used as starting materials, followed by hot pressing. It was found that the sintering atmosphere used, nitrogen or argon, had a significant effect on the phase composition, mechanical and electrical properties of the final materials. It was shown that in some cases partial nitridation of MoSi₂ occurred with the formation of MoSi₂–Mo₅Si₃–Si₃N₄ ternary composites. The electrical conductivity of the composites depends also on the microstructure of materials. It was shown that the composites fabricated using combustion synthesized MoSi₂ powders (500 nm) are characterized by higher flexural strength at room temperature compared to those from commercial powders. On the other hand, the composites fabricated from the commercial powders had higher strength and fracture toughness at elevated temperatures (up to 1200 °C). For all composites, the strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Enhanced mechanical and tribological properties of oscillatory pressure sintered WC–GNPs composites

It remains as a challenge to develop binderless WC ceramics that integrate high mechanical properties and low friction wear. Here, we report the preparation of strong and tough WC ceramics with low wear rate by adding graphene nanoplatelets (GNPs) and using oscillatory pressure sintering (OPS) process. The introduced GNPs lead to the formation of nearly fully dense composites with the aid of an oscillatory pressure. The OPS-prepared WC–0.3-wt% GNPs composites reached a high flexural strength, hardness, and fracture toughness, being up to 1420 MPa, 24.9 GPa, and 6.89 MPa m^1/2, respectively. Moreover, a low friction wear rate of 3.17 × 10⁻⁷ mm³ N⁻¹ m⁻¹ is achieved for such composites, which can be ascribed to the formation of a friction lubrication film during dry sliding friction process and their higher mechanical properties.     

Effect of annealing atmosphere on the structural and electrical properties of BiFeO3 multiferroic ceramics prepared by sol–gel and spark plasma sintering techniques

Phase-pure BiFeO₃ powders were synthesized by sol–gel technique. Based on these powders, high-density BiFeO₃ ceramics were prepared by spark plasma sintering (SPS) at 700 °C along with annealing for 2 and 4 h, respectively, at 650 °C under atmospheres of air and oxygen. X-ray diffraction analysis revealed that the 4 h-oxygen-annealed sample contained a single rhombohedral perovskite phase while the samples annealed in the other conditions contained small quantities of impurity phases besides the rhombohedral perovskite phase. The relative density of the 4 h-oxygen-annealed sample was about 96%, being apparently higher than that of the other samples. In comparison with the 4 h-air-annealed sample, the dielectric constant of the 4 h-oxygen-annealed sample was relatively higher. The activation energy for electrical conduction was about 1.17 eV for the 4 h-oxygen-annealed sample while it was about 0.98 eV for the 4 h-air-annealed sample, showing that the former would have a lower room-temperature conductivity (~2.6×10⁻¹⁴ S cm⁻¹) than the latter (~2.1×10⁻¹³ S cm⁻¹). It is therefore anticipated that the oxygen-annealed sample could possess better ferroelectric properties as compared to the air-annealed sample.     

Impedance study of spark–plasma–sintered lutetium titanate ceramics: Effect of post–annealing

After a post–annealing treatment, spark-plasma-sintered (SPSed) black lutetium titanate (Lu₂Ti₂O₇) became colorless and the transmittance at 550 nm increased from zero to 57%. To investigate the effect of post-annealing, transmission electron microscopy, X–ray photoelectron spectroscopy, and impedance spectroscopy were performed on the SPSed Lu₂Ti₂O₇ body before and after annealing. Clean grain boundaries and triple junctions with high crystallinity were observed in both samples. X–ray photoelectron spectra revealed that both carbon contamination and lattice defects existed in the sample before annealing. The post-annealing process could help to eliminate the carbon contamination and compensate for defects. The Lu₂Ti₂O₇ body exhibited higher bulk conductivity at relatively low temperatures before annealing than that after annealing and the possible cause was analyzed.