Densification of SiO2–cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder

Cubic boron nitride (cBN) powder was coated with Ni nanoparticle and SiO₂ nanolayer (abbreviated as cBN/Ni and cBN/SiO₂, respectively) by rotary chemical vapor deposition (RCVD), and compacted with SiO₂ powder by spark plasma sintering at 1473–1973 K for 0.6 ks. The effects of Ni and SiO₂ coatings on the densification, phase transformation of cBN and hardness of SiO₂–cBN composites were compared. The phase transformation of cBN to hBN was identified at 1973 K in SiO₂–cBN/SiO₂ composites, 300 K higher than that in SiO₂–cBN/Ni composites, indicating that SiO₂ retarded the transformation of cBN. The relative density of SiO₂–cBN/SiO₂ with 50 vol% cBN sintered at 1873 K was 99% with a hardness of 14.5 GPa.     

Spark plasma sintering of TiN–TiB2 composites

TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB₂ content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB₂ composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB₂. Shrinkage of the TiN–70 vol% TiB₂ composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB₂ composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB₂ became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB₂ composite containing 50–60 vol% TiB₂ above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB₂ composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m^1/2, respectively.     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

Toughened carbon nanotube–iron–mullite composites prepared by spark plasma sintering

Carbon nanotube–iron–mullite nanocomposite powders were prepared by a direct method involving a reduction in H₂–CH₄ and without any mechanical mixing step. The carbon nanotubes are mostly double- and few-walled (3–6 walls). Some carbon nanofibers are also observed. The materials were consolidated by spark plasma sintering. Their electrical conductivity is 2.4 S/cm whereas pure mullite is insulating. There is no increase in fracture strength, but the SENB toughness is twice than the one for unreinforced mullite (3.3 vs. 1.6 MPa m^1/2). The mechanisms of carbon nanotube bundle pullout and large-scale crack-bridging have been evidenced.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Wear resistance of αSiC–TiB2 composites prepared by reactive sintering

The relative wear resistance of αSiC–TiB₂ composites prepared by reactive sintering was investigated on a pin on flat tribometer, in air and in presence of water. Experimental results show that the composite materials are less worn than monolithic SiC. The wear mechanisms in air and water are identified.In air, a protective oxidised debris layer is formed on the composites, whereas roller formation was observed with SiC. In water, the surface of the composites is polished, whereas SiC is worn by fragile ruptures (cleavages).     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

Microstructure and mechanical properties of B4C–TiB2 ceramic composites prepared via a two-step method

B₄C–TiB₂ ceramic composites were fabricated by a two-step method. First, B₄C–TiB₂ composite powders were synthesized from TiC–B powder mixtures at 1400 ℃, then mixed with commercial B₄C powders by ball milling and the B₄C–TiB₂ ceramic composites were prepared by hot pressing at 1950 ℃. This two-step method not only effectively refined TiB₂ grains, but also allowed the composition of the composites to be freely designed. The microstructure and mechanical properties of the composites were investigated. The results showed that the B₄C–TiB₂ ceramic composite with a 10 wt% TiB₂ content obtained the ideal comprehensive performance, with a volume density, Vickers hardness, bending strength, and fracture toughness of 2.61 g/cm³, 35.3 GPa, 708 MPa, and 5.82 MPa m^1/2, respectively. The advantages of the in-situ reaction process were fully exerted by the two-step method, which made a remarkable contribution to the excellent properties of B₄C–TiB₂ ceramic composites.     

Solid solution formation during spark plasma sintering of ZrB2–TiC–graphite composites

Densification and mechanical behavior of graphite-free and graphite-doped ZrB₂–TiC composites were investigated. Spark plasma sintering was used to achieve near fully-dense composites. Microstructural and phase analysis were carried out via scanning electron microscopy and X-ray diffraction spectroscopy, to illustrate the sintering and toughening mechanisms in the fabricated samples. Results indicated that 1 wt% graphite nano-flakes can improve the hardness of the composite. However, 3% drop in relative density and ~6% decrease in indentation fracture toughness were observed. The formation of TiB₂ and ZrC was verified in both TiC-contained composites, although B₄C was recognized as the byproduct of reactive sintering in graphite-doped composite. Moreover, the microstructural analysis and the peak shifts in XRD pattern indicated the formation of a solid solution between the ZrB₂ and TiB₂ phases. Higher hardness of the graphite-doped sample was also attributed to the formation of B₄C as a superhard interfacial phase. Toughening mechanisms as well as possible chemical reactions which result in the in-situ formed reinforcement phases were also discussed.     

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 and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

Structural,thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sintering

Highly dense AlN–SiC composites with various SiC additions (0–50?wt-%) were fabricated at 1800°C by plasma activated sintering. The effect of SiC addition on structural, thermal and dielectric properties as well as microwave absorbing performance of the composites was investigated. The thermal conductivity decreases with increasing SiC addition, from 68.7 W (m?K)^?1 for 0?wt-% SiC to 19.38?W (m?K)^?1 for 50?wt-% SiC. On the contrary, the permittivity and dielectric loss increase gradually, from 7.6–8.5 to 22–26.7 and from 0.02–0.1 to 0.2–0.53, respectively. AlN–SiC composite with better thermal and dielectric properties in 30?wt-% SiC, whose thermal conductivity and dielectric loss are found to be 24.88?W (m?K)^?1 and 0.15–0.74, respectively. Furthermore, the composite exhibits microwave absorbing performance with the minimum reflection loss (RL) of ?16.5 dB at 15.5 GHz and the frequency range of 2.6 GHz for RL below ?10 dB (90% absorption).     

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.     

Properties of Si3N4–TiN composites fabricated by spark plasma sintering by using a mixture of Si3N4 and Ti powders

Si₃N₄–TiN composites were successfully fabricated via planetary ball milling of 70 mass% Si₃N₄ and 30 mass% Ti powders, followed by spark plasma sintering (SPS) at 1250–1350 °C. The sintering mechanism for SPS was a hybrid of dissolution–reprecipitation and viscous flow. The electrical resistivity decreased with increasing sintering temperature up to a minimum at 1250 °C and then increased with the increasing sintering temperature. The composites prepared by SPS at 1250–1350 °C could be easily machined by electrical discharge machining. Composite prepared by SPS at 1300 °C showed a high hardness (17.78 GPa) and a good machinability.     

cBN–TiN,cBN–TiC composites: chemical equilibria,microstructure and hardness mechanical investigations

This paper summarizes theoretical and experimental studies of cBN–TiN and cBN–TiC of cBN:TiN/TiC molar ratio 1:1 and 2:1. Theoretical calculations show that, at temperatures between 1000 and 1400°C, TiN reacts with BN forming one new phase, TiB₂, and that TiC reacts with cBN forming two new phases, TiB₂ and TiC_0.8N_0.2.. Experimental cBN–TiC/TiN composites were prepared by high pressure hot pressing and the samples were subsequently heat treated.After heat treatment, sinters of cBN–TiN/TiC were characterized using transmission electron microscopy and X-ray diffraction. The samples exhibited a dense polycrystalline structure, and a thin layer of fine TiB₂ was visible at the BN–binder interface. It was found that hardness decreased significantly after heat treatment.     

Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

In situ synthesis of PcBN composites from Sialon–TiN–TiB2 binder and cBN under high temperature and pressure

Polycrystalline cubic boron nitride (PcBN) composites were synthesized in situ with TiB₂, TiN, and Sialon as binders under ultrahigh temperature (1550°C) and high pressure (5.5 GPa). X-ray diffractometry, universal testing machine, field emission scanning electron microscopy, and transmission electron microscopy were used to study the effect of Sialon content (0%, 20%, 40%, 60%, and 80% by mass of binder) on the phase composition, microstructure, and mechanical properties of PcBN composites. The results show that the main phases in the system are lamellar TiN, needle-rodlike TiB₂, and irregular long-rodlike Sialon. Some TiB₂ grows along the (1 0 −1 0) face, and Sialon grows along the (0 0 0 1) face. The density and mechanical properties of the sintered product are significantly improved due to the formation of Sialon. When the content of Sialon is 60%, the binder is uniformly distributed, the cBN interface is well bonded, and the density is the highest. At this time, the strength reaches the maximum 34.57 GPa, the fracture toughness is 6.82 MPa m^1/2, and the flexural strength reaches the maximum 870.79 MPa. On the whole, cBN composites have excellent prospects for future applications in cutting inserts due to their excellent mechanical properties.