Densification and mechanical properties of TiC by SPS-effects of holding time,sintering temperature and pressure condition

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness (H_V) increase with sintering temperature and holding time, but fracture toughness (K_IC) was not significantly influenced by sintering parameters. The H_V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H_V values of ～24–30 GPa and K_IC of ～3.7–5 MPa m^1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.     

Dense bulk silicon oxycarbide glasses obtained by spark plasma sintering

Dense silicon oxycarbide glasses (SiOC) have been produced by spark plasma sintering (SPS) of SiOC powders. Raw powders were obtained by pyrolysis under nitrogen at 1100 °C of tetraethylorthosilcate/polydimethylsiloxane (TEOS/PDMS) hybrids. SPS experiments were carried out at 1300 and 1500 °C at 10 and 80 MPa and then were studied by chemical analysis, ²⁹Si and ¹³C MAS NMR, ATR, Raman, XRD, FE-SEM, density, porosity, microhardness (H_v) and thermal conductivity (K). The SiOC materials are formed by Si_xOC_4?x units within a silica matrix where silicon carbide and graphite nanodomains are also present. After the SPS treatment the silicon carbide crystallite size is close to 2.5 nm. At 1300 °C and 1500 °C the carbon nanodomain size is close to 3 nm and 2 nm, respectively. H_v values vary from 3.4 to 9.15 GPa, for 30% and 1% of porosity, respectively. Finally, K is always close to 1.38 W m^?1 K^?1.     

Effect of spark plasma sintering parameters on microstructure and room-temperature hardness and toughness of fine-grained boron carbide (B4C)

Spark plasma sintering (SPS) parameters are reported to have a remarkable effect on microstructure and fracture toughness of boron carbide. Fully dense fine-grained boron carbide samples have been sintered by SPS at 1700 °C for 3 min as optimal conditions. Both temperature and the current density applied during sintering seem to be important parameters for fully dense output and can induce significant changes on the final microstructure. The initial grain size of the powder is a crucial factor to perform fine-grained fully dense specimens. The improvement on room-temperature hardness and toughness is discussed.     

Carbothermal reduction synthesis of TiB2 ultrafine powders

Submicrometric TiB₂ powders were synthesized by carbothermal reduction process using titanium dioxide, boron carbide and carbon black as the starting materials. The influence of different amount of boron carbide (22.0–26.8 wt%), calcination temperature (1400–1900 °C) and holding time (15–90 min) on the composition and microstructure of the product was investigated. The resultant powders were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results showed that hexagonal impurity-free TiB₂ crystalline powders with the grain size below 1.0 μm could be successfully prepared at 1600 °C for 30 min in Ar atmosphere when the amount of boron carbide was 25.3 wt%. The increase in temperature contributed to reaction completion and grain growth, but the abnormal grain growth and oversintering took place above 1800 °C.     

Densification and ablation behavior of ZrB2 ceramic with SiC and/or Fe additives fabricated at 1600 and 1800 °C

The effects of Fe and SiC additions on the densification, microstructure, and ablation properties of ZrB₂-based ceramics were investigated in this study. The sample powders were conventionally mixed by cemented carbide ball then sintered by spark plasma sintering. The ablation rates and behavior of the ceramics were investigated under an oxyacetylene torch environment at about 3000 °C. A sample with high relative density (96.3%), high flexural strength (415.6 MPa), and low linear ablation rate (−0.4 µm/s) was obtained via SPS at 1600 °C. Adding 4 vol% Fe was more beneficial to the density of ZrB₂ sintered at 1600 °C as compared to ZrB₂ sintered at 1800 °C. The ablation behavior and rates were similar among samples sintered at 1600 °C and 1800 °C.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Pressureless sintering of boron carbide with Cr3C2 as sintering additive

In this study, chromium carbide (Cr₃C₂) was selected as the sintering additive for the densification of boron carbide (B₄C). Cr₃C₂ can react with B₄C and form graphite and CrB₂ in situ, which is considered to be effective for the sintering of B₄C composites. The sintering behavior, microstructure development and mechanical properties of B₄C composites were studied. The density of B₄C composite increased with the increase of Cr₃C₂ content and sintering temperature. The formation of liquid phase could effectively improve the densification of B₄C composites. The abnormal grains began to appear at 2080 °C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr₃C₂ samples after sintering at 2070 °C.     

Effect of La2O3 addition on long-term oxidation kinetics of ZrB2–SiC and HfB2–SiC ultra-high temperature ceramics

Long-term oxidation kinetics of SiC-reinforced UHTCs and La₂O₃-doped UHTCs over an intermediate temperature range (1400–1600 °C) reveal partially protective behavior for the former characterized by an oxidation kinetic exponent 1 < n < 2. In addition, unstable oxidation behavior was observed in HfB₂-based UHTCs due to the presence of SiC agglomerates. On the other hand, La₂O₃-doped UHTCs were found to be protective over the whole temperature range studied (n = 2), in particular at 1600 °C, where oxidation kinetic exponents as high as 8 were observed as a consequence of formation of new oxidation protective particles, MeO_xC_y, where Me is Zr, Hf or Si. Adsorption of oxygen-containing species formed protective MeO_xC_y phases, which enhanced the thermal stability of the oxide scale as well as providing protection against oxidation for long exposure times at 1600 °C.     

Increased high temperature strength and oxidation resistance of Al4SiC4 ceramics

Al₄SiC₄ bulk ceramics were synthesized by reaction hot-pressing using Al, graphite powders and polycarbosilane (PCS) as starting materials. The present work confirmed that this process was an effective method for the preparation of Al₄SiC₄ ceramics having high relative density and well-developed plate-like grains. The mechanical, thermal properties and oxidation behaviors of the Al₄SiC₄ ceramics were also investigated. The flexural strength, fracture toughness (K_IC) and Vickers hardness at room temperature were 297.1 ± 22 MPa, 3.98 ± 0.05 MPa m^1/2, 10.6 ± 1.8 GPa, respectively. The high-temperature bending strength showed an increasing trend with increasing test temperatures, with the value of 449.7 ± 26 MPa at 1300 °C. The thermal expansion coefficient was 6.2 × 10⁻⁶ °C⁻¹ in the temperature range from 200 °C to 1450 °C. The isothermal oxidation of Al₄SiC₄ ceramics at 1200–1600 °C for 10–20 h revealed that it had excellent oxidation resistance.     

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B₅₀N₂ secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.     

Hard polycrystalline eutectic composite prepared by spark plasma sintering

A polycrystalline eutectic B₄C–TiB₂ composite was prepared by spark plasma sintering. The starting eutectic powder was obtained by mechanical grinding of the directionally solidified eutectic B₄C–TiB₂ alloy. The microstructure of the polycrystalline composite exhibited randomly oriented eutectic grains with an average size of about 50–100 μm. Eutectic grains consisted of boron carbide matrix reinforced by titanium diboride inclusions. The secondary eutectic structure in the grain boundary is formed at sintering temperature higher than 1700 °C. XRD analysis revealed that the eutectic B₄C–TiB₂ composite consist mainly of B₄C and TiB₂ phases. The measured Vickers hardness was in the range of 32.35–54.18 GPa and the average fracture toughness of the samples was as high as 4.81 MPa m^1/2. The bending strengths of the composite evaluated at room temperature and at 1600 °C were 230 and 190 MPa, respectively.     

Nitrate–citrate combustion synthesis of Ce1−xGdxO2−x/2 powder and its characterization

The structure, thermal expansion coefficients, and electric conductivity of Ce_1−xGd_xO_2−x/2 (x = 0–0.6) solid solution, prepared by the gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering into highly dense ceramic pellets at 1300 °C, a significantly lower temperature compared to that of 1600–1650 °C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions formed in all the investigated range. The maximum conductivity, σ_600 °C = 5.26 × 10⁻³ S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data, was 8.125 × 10⁻⁶ K⁻¹ at x = 0.2.     

B4C/metal boride composites derived from B4C/metal oxide mixtures

This study examines the reactions occurring from room temperature to 2180 °C during the heating under argon of mixtures of B₄C and metal oxides, as well as the properties of the ceramic composites prepared by these reactions. The cations of the oxides investigated, belonged to the transition metal and to the lanthanide groups. The mixtures underwent solid-state reactions in the range between 1100 and 1900 °C. These reactions resulted in composites in which the metal borides and B₄C are the majority phases. The boron carbide/metal boride(s) mixtures resulted from these reactions exhibited a sintering aptitude significantly higher than that of pure boron carbide. The improvement in the sintering aptitude was proportional to the oxide content present in the initial mixture, up to an upper limit. B₄C/boride(s)-type composites, exhibiting bulk densities ≥97% TD, could be prepared for certain compositions by pressureless heating at 2180 °C. The ceramic parts prepared under these conditions are characterized by strength and hardness values similar to those determined for pure boron carbide.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

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.     

Chemical corrosion of basic refractories by cement kiln materials

The study of basic refractories corrosion by cement kiln materials was carried out with powder tests and coating tests at the typical temperature range of the sintered material in the transition zone and the sintering zone (1200–1450 °C). The tested basic refractories were magnesia-spinel (MSp) and magnesia–zirconia (MZ) refractory bricks. The industry cement kiln materials were rich of sulphur and chlorine. The microstructures of the as-delivered and tested samples were researched by XRD and SEM–EDS techniques. The new phases detected in the samples with MSp (with and without ZrO₂) bricks after tests at the temperature of 1200 °C were binary (C₁₂A₇ t_mp ? 1392 °C, CaZrO₃: t_mp ? 2345 °C)¹: and ternary (C₂AS: t_mp ? 1593 °C or C₃MS₂: t_dp ? 1573 °C) phases. After tests at the temperature of 1300 °C and higher, ternary phases of C₇A₃Z (t_mp ? 1550 °C) and CaZrO₃ and quaternary phases Q-C₂₀A₁₃M₃S₃ or C₆A₄(M,f)S (t_dp ? 1380 °C) were detected. The C₃A₃·CaSO₄ phase was formed in the samples after the corrosion tests performed up to the temperature of 1300 °C. The new phases formed in the sample with the MZ bricks were clinker phases (β-C₂S, C₃A and C₂(A,F)/C₄AF) and the C₇A₃Z phase.     

Exciton and piezoelectricity in insulating 2-dimensional boron carbide

Using first-principles density functional theory, we predict a hexagonal structure of boron carbide with two shells, which consists of the sp² hybridized boron and carbon in (001) plane and the pz–pz (σ) bonding carbon along [001] direction. The calculated results show that the structure is thermodynamically stable and possesses lower formation energy than other candidates. In addition, the quasiparticle calculations within the GW approximation reveal that the boron carbide, which is a two dimensional insulator, exhibits the indirect band gap of 2.4 eV and large exciton bonding energy of 1.35 eV. In optical absorption spectra, a bright Frenkel class bound exciton has been discovered at about 2.98 eV, which is desirable for light emitting applications. Besides, the piezoelectric coefficient (e₂₂) of −2.38×10⁻¹⁰ Cm⁻¹ is predicted for monolayer boron carbide, which indicates that the monolayer boron carbide is a potential candidate for piezoelectric applications in the nanoelectromechanical systems.     

Microstructure and mechanical properties of Al2O3–YSZ and Al2O3–YAG directionally solidified eutectic plates

Plates of Al₂O₃–YSZ and Al₂O₃–YAG eutectic composition with a thickness from 0.1 to 1 mm were prepared by directional solidification using a diode laser stack. The melt processed regions of plates exhibited colony microstructure consisting of finely dispersed phases. Due to the curved shape of the melted pool, the growth rate depends on the distance to the surface plate, decreasing from top to bottom. In this way, the microstructure characteristic length changes as a function of the distance to the plate surface. Vickers indentations and piezo-spectroscopy measurements were done on longitudinal and transverse cross-sections of the samples at different depths. From these measurements, we concluded that the Vickers hardness (H_V), indentation fracture toughness (K_IC) and residual stresses (σ_h) of the plates were mainly independent from the distance to the surface. The mean values that we obtained in the Al₂O₃–YSZ plates were H_V = 16 GPa, K_IC = 4.2 MPa m^1/2 and σ_h = −0.33 GPa, and in the Al₂O₃–YAG plates were H_V = 16 GPa, K_IC = 2.0 MPa m^1/2, and σ_h = −0.1 GPa. These values are similar to those found in directionally solidified eutectic rods.     

Processing factors influencing the free carbon contents in boron carbide powder by rapid carbothermal reduction

High quality boron carbide powder without free carbon is desired for many applications. In this study, the factors that influence free carbon content in boron carbide powders synthesized by rapid carbothermal reduction reaction were evaluated. The dominant factors affecting free carbon contents in boron carbide powder were reaction temperature, precursor homogeneity, the particle size of reactants, and excess boron reactant amount. The reaction temperature at 1850 °C was sufficient to synthesize boron carbide with low free carbon content. Depending on process conditions, precursor homogeneity was also affected by the calcination temperature and time. Smaller particle size of reactants contributed to less carbon content and more uniformity in synthesized boron carbide. Excess boric acid effectively compensated for B₂O₃ volatilization. In the optimal sample, using 80 mol% excess nano boric acid and calcined at 500 °C, the free carbon in the synthesized boron carbide was negligible (0.048 wt.%).     

Investigation of fracture toughness of modified (KxNa1 − x)NbO3 lead-free piezoelectric ceramics

While most of the previous studies have focused on the processing and electrical properties of KNN-based ceramics, very little research has been carried out to evaluate their mechanical behavior. This work presents for the first time an examination of the fracture toughness, K_IC, of the most widely studied (K_xNa_1 ? x)NbO₃ (KNN)-based lead-free ceramics modified with lithium, tantalum and antimony. The samples were produced through the conventional mixed-oxide route and the K_IC values were measured using the single edge V-notched beam (SEVNB) method under four-point bending. The mean K_IC values were determined to be 0.48 ± 0.18 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)NbO₃, 0.8 ± 0.18 MPa m^1/2 for (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃, 0.86 ± 0.04 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.9Ta_0.1)O₃ and 1.06 ± 0.21 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ compositions. The microstructure, phase structure and dielectric constant values of the samples have been used to correlate the results of the K_IC values.