Mechanical,electrical and thermal properties of ZrC-ZrB2-SiC ternary eutectic composites prepared by arc melting

ZrC-ZrB₂-SiC composites were prepared by arc-melting in Ar atmosphere using ZrC, ZrB₂ and SiC as starting materials. The ternary eutectic composition of 20ZrC-30ZrB₂-50SiC (mol%) was first identified. SiC about 7?μm in length and 500?nm in diameter, ZrC about 4 μm in length and 1 μm in diameter, in rod-like microstructure, were uniformly dispersed in ZrB₂ matrix of eutectic composite. The eutectic temperature of ZrC-ZrB₂-SiC composite was approximately 2550?K. The Vickers Hardness and fracture toughness of eutectic composite was 23?GPa and 6.2?MPa?m^1/2, respectively. The electrical conductivity decreased from 7.2?×?10⁷ to 1.75?×?10⁶?S?m^?1 with the temperature increasing from 287 to 800?K. The thermal conductivity decreased from 85 to 61?W?K^?1?m^?1 with increasing temperature from 287 to 973?K.     

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.     

Grain-growth-induced high electrical conductivity in SiC–BN composites

SiC–BN composites were fabricated by conventional hot-pressing from β-SiC and h-BN nanopowders with 2?vol% yttria as a sintering additive. Electrical and thermal properties of the composites were investigated as a function of initial BN content. Owing to the nanosize of the starting powders, the grain-growth-assisted N-doping of the SiC lattice was significantly enhanced during liquid-phase sintering, yielding the highest-reported electrical conductivity of ~124 (Ω?cm)^?1 for a SiC–4-vol% BN composite. The typical values of electrical resistivity and thermal conductivity of the SiC–4-vol% BN composite at room temperature were 8.1?×?10^?3 Ω?cm and 92.4?W?m^?1 K^?1, respectively.     

Pressureless sintering of SiC ceramics with improved specific stiffness

The effects of B₄C content on the specific stiffness and mechanical and thermal properties of pressureless-sintered SiC ceramics were investigated. SiC ceramics containing 2.5 wt% C and 0.7–20 wt% B₄C as sintering aids could be sintered to ≥ 99.4% of the theoretical density at 2150 °C for 1 h in Ar. The specific stiffness of SiC ceramics increased from 136.1 × 10⁶ to 144.4 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 to 20 wt%. The flexural strength and fracture toughness of the SiC ceramics were maximal with the incorporation of 10 wt% B₄C (558 MPa and 3.69 MPa‧m^1/2, respectively), while the thermal conductivity decreased from ∼154 to ∼83 W‧m⁻¹‧K⁻¹ when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 20 wt% B₄C were ∼346 MPa and ∼105 W‧m⁻¹‧K⁻¹, respectively.     

Microstructure refinement and mechanical properties improvement of HfB2–SiC composites with the incorporation of HfC

HfB₂ and HfB₂–10 vol% HfC fine powders were synthesized by carbo/boronthermal reduction of HfO₂, which showed high sinterability. Using the as-synthesized powders and commercially available SiC as starting powders, nearly full dense HfB₂–20 vol% SiC (HS) and HfB₂–8 vol% HfC–20 vol% SiC (HHS) ceramics were obtained by hot pressing at 2000 °C/30 MPa. With the incorporation of HfC, the grain size of HHS was much finer than HS. As well, the fracture toughness and bending strength of HHS (5.09 MPa m^1/2, 863 MPa) increased significantly compared with HS (3.95 MPa m^1/2, 654 MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB₂–SiC ceramics.     

Magnesiothermic synthesis of HfB2–HfC-SiC nanocomposite by spark plasma technique

The HfB₂-HfC-SiC nanocomposite was produced using H₃BO₃, HfO₂, Si, C, and Mg as starting materials by spark plasma for the first time. The reactions during synthesis indicate that the synthesis process progressed in self-propagating mode. The reaction mechanisms were investigated by the displacement-temperature-time (DTT) diagram, which was obtained during spark plasma cycles. The synthesis process of the composite was completed at the temperature of 400 °C in less than 30 min. The tendency to form the composite was investigated by thermodynamic calculations, and the formation of HfB₂, HfC, and SiC phases was observed by X-ray diffraction. Finally, using the Rietveld method, the mean crystallites sizes of about 54, 26, and 43 nm were calculated for HfB₂, SiC, and HfC phases, respectively.     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Hot Pressed HfB2 and HfB2–20 vol%SiC Ceramics Based on HfB2 Powder Synthesized by Borothermal Reduction of HfO2*

Phase pure hafnium diboride (HfB₂) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB₂ powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as-synthesized HfB₂ powder and a commercial SiC, HfB₂ monolithic, and HfB₂–20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993±90 MPa for the composite. Fracture toughness of the composite was 6.29±0.65 MPa m^1/2, which was significantly higher than that of the HfB₂ monolithic and the reported values in literature.     

Mechanical,Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB₂, HfC_0·98, HfC_0·67, and HfN_0·92 ceramics. The oxidation resistance of ceramics in the ZrB₂–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0·98 to 1100°C for HfC_0·67 ceramics. The transition temperature of HfB₂ was 1100°C. The ZrB₂/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB₄, and C using displacement reactions. The ceramics with ZrB₂ as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB₂ and ZrC ceramics. The ceramics with ZrB₂/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.     

Harmonized toughening and strengthening in pressurelessly reactive-sintered Ta0.8Hf0.2C-SiC composite

Ta_0.8Hf_0.2C-27?vol%SiC (99.0% in relative density) composite was toughened and strengthened via pressurelessly in-situ reactive sintering process. HfC and β-SiC particles were formed after reaction of HfSi₂ and carbon black at 1650?°C. Ta_0.8Hf_0.2C was obtained from solid solutioning of HfC and commercial TaC. The β-α phase transformation of SiC proceeded below 2200?°C. High aspect ratio, platelet-like α-SiC grains formed and interconnected as interlocking structures. Toughness and flexural strength values of 5.4?±?1.2?MPa?m^1/2 and 443?±?22?MPa were measured respectively. The toughening mechanisms by highly directional growth of discontinuous α-SiC grains were crack branching, bridging and deflection behaviors.     

Mechanical and tribological properties of SiC whisker-reinforced SiC composites via oscillatory pressure sintering

SiC whisker (SiCw)-reinforced SiC composites were prepared by an oscillatory pressure sintering (OPS) process, and the effects of SiCw content on the microstructure and mechanical and tribological properties of such composites were investigated. The addition of SiCw could promote the formation of long columnar α-SiC, and the aspect ratio of α-SiC grains first increased and then decreased with the increase of SiCw content. When the SiCw content was 5.42 wt%, the relative density of the SiC–SiCw composite reached up to 99.45%. The SiC–5.42 wt% SiCw composite possessed the highest Vickers hardness, fracture toughness, and flexural strength of 30.68 GPa, 6.66 MPa·m^1/2, and 733 MPa, respectively. In addition, the SiC–5.42 wt% SiCw composite exhibited the excellent wear resistance when rubbed with GCr15 steel balls, with a friction coefficient of .76 and a wear rate of 4.12 × 10⁻⁷ mm³·N⁻¹·m⁻¹. This could be ascribed to the improved mechanical properties of SiC–SiCw composites, which enhanced the ability to resist peeling and micro-cutting, thereby enhancing the tribological properties of the composites.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Single-source-precursor synthesis and high-temperature evolution of a boron-containing SiC/HfC ceramic nano/micro composite

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 °C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp²-hybridized boron-containing carbon (i.e. B_yC). High temperature annealing of B_yC/SiHfC(N,O) leads to the precipitation of HfC_xN_1-x nanoparticles as well as to β-SiC crystallization. After annealing at temperatures beyond 1900 °C, HfB₂ formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 ± 5.3 and 344.7 ± 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfC_xN_1-x/HfB₂/C ceramic nano/micro composite.     

Synthesis,Characterization, and Microstructure of Hafnium Boride‐Based Composite Ceramics Via Preceramic Method

The ceramic precursor for HfB₂/HfC/SiC/C was prepared via solution‐based processing of polyhafnoxanesal, linear phenolic resin, boric acid and poly[(methylsilylene)acetylene)]. The obtained precursor could be cured at 250°C and subsequently heat treated at relative lower temperature (1500°C) to form HfB₂/HfC/SiC/C ceramic powders. The ceramic powders were characterized by element analysis, thermal gravimetric analysis, X‐ray diffraction, Raman spectroscopy, and Scanning electron microscopy. The results indicated that the ceramic powders with particle size of 200~500 nm were consisted of pure phase HfB₂, HfC, and SiC along with free carbon as fourth phase with low crystallinity.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Thermal,mechanical and electrical properties of hard B4C,BCN, ZrBC and ZrBCN ceramics

We study the thermal, mechanical and electrical properties of B₄C, BCN, ZrBC and ZrBCN ceramics prepared in the form of thin films by magnetron sputtering. We focus on the effect of Zr_x(B₄C)_1−x sputter target composition, the N₂+Ar discharge gas mixture composition, the deposition temperature and the annealing temperature after the deposition. The thermal properties of interest include thermal conductivity (observed in the range 1.3–7.3 W m⁻¹ K⁻¹), heat capacity (0.37–1.6×10³ J kg⁻¹ K⁻¹ or 1.9–4.1×10⁶ Jm⁻³ K⁻¹), thermal effusivity (1.6–4.5×10³ J m⁻² s^−1/2 K⁻¹) and thermal diffusivity (0.38–2.6×10⁻⁶ m² s⁻¹). We discuss the relationships between materials composition, preparation conditions, structure, thermal properties, temperature dependence of the thermal properties and other (mechanical and electrical) properties. We find that the materials structure (amorphous×crystalline hexagonal ZrB₂-like×nanocrystalline cubic ZrN-like), more than the composition, is the crucial factor determining the thermal conductivity and other properties. The results are particularly important for the design of future ceramic materials combining tailored thermal properties, mechanical properties, electrical conductivity and oxidation resistance.     

Erosion mechanism of ternary-phase SiC/ZrB2-MoSi2-SiC ultra-high temperature multilayer coating under supersonic flame at 90° angle with speed of 1400 m/s (Mach 4)

A ternary-phase SiC/ZrB₂-MoSi₂-SiC multilayer coating was prepared on graphite by two-step reactive melt infiltration (RMI) method. The formation mechanism of the coating was studied by HSC chemistry software 6.0. The erosion resistance of the coating was investigated by supersonic flame erosion test at 90° angle, temperature of 2173 K and speed of 1400 m/s (Mach 4) for 120 s. Erosion test results revealed that the SiC/ZrB₂-MoSi₂-SiC multilayer coating had very good erosion resistance. Weight change percentage, mass erosion rate and linear erosion rate of the coating were −0.18 %, −0.027 × 10⁻³g cm⁻² s⁻¹ and 0.33 μm s⁻¹, respectively. Microstructural characterization demonstrated that interesting structures such as rod-like, flake-like, spherical, worm-like and fibrous structures were formed during erosion test. The erosion mechanism of ZrB₂-MoSi₂-SiC coatings is controlled both chemically and mechanically. The reduction of chemical degradation can be attributed to the presence of MoSi₂ particles and the reduction of mechanical degradation can be related to the presence of ZrB₂ particles.     

Electrical,thermal, and mechanical properties of porous SiC-nitride composites

The effects of nitride (AlN, BN, TiN) addition on the electrical, thermal, and mechanical properties of porous SiC-nitride composites were investigated within a porosity range of 40–74 %. The electrical conductivity was predominantly controlled by chemistry rather than porosity, whereas the thermal conductivity was more susceptible to changes in porosity. These results suggest that the electrical conductivity of porous SiC ceramics can be tuned independently from the thermal conductivity by nitride addition. At constant thermal conductivity (∼5 Wm⁻¹ K^-1), the electrical conductivity of the baseline specimen (6.3 × 10^-3 Ω^-1 cm^-1) could be: (1) increased by an order of magnitude (8.3 × 10^-2 Ω^-1 cm^-1) by adding AlN and (2) decreased by an order of magnitude (7.0 × 10^-4 Ω^-1 cm^-1) by adding BN. Typical electrical conductivity and thermal conductivity values of the porous SiC-10 vol% TiN composite were 5.3 × 10^-1 Ω^-1 cm^-1 and ∼14.0 Wm⁻¹ K^-1, respectively, at 51 % porosity.     

Plastic deformation-induced improved mechanical and thermal properties in hot-forged SiC-TiC composite

A novel SiC-20 vol% TiC composite prepared via a two-step sintering technique using 6.5 vol% Y₂O₃-Sc₂O₃-MgO exhibited high deformation (60 %) on hot forging attributed to the high-temperature plasticity of TiC (ductile to brittle transition temperature ～800 °C) and fine-grained microstructure (～276 nm). The newly developed SiC-TiC composite exhibited a ～2-fold increase in nominal strain as compared to that of monolithic SiC. The plastic deformation caused by grain-boundary sliding in monolithic SiC was supplemented by the plastic deformation of TiC in the SiC-TiC composite. The hot-forged composite exhibited anisotropy in its microstructure and mechanical and thermal properties due to the preferred alignment of α-SiC platelets formed in situ. The relative density, flexural strength, fracture toughness, and thermal conductivity of the composite increased from 98.4 %, 608 MPa, 5.1 MPa?m^1/2, and 34.6 Wm^?1 K^?1 in the as-sintered specimen to 99.9 %, 718–777 MPa, 6.9–7.8 MPa?m^1/2, and 54.8–74.7 Wm^?1 K^?1, respectively, on hot forging.     

Influence of sintering atmosphere and BN additives on microstructure and properties of porous SiC ceramics

The effects of the boron nitride (BN) content on the electrical, thermal, and mechanical properties of porous SiC ceramics were investigated in N₂ and Ar atmospheres. The electrical resistivity was predominantly controlled by the sintering atmosphere and secondarily by the BN concentration, whereas the thermal conductivity and flexural strength were more susceptible to changes in the porosity and necking area between the SiC grains. The electrical resistivities of argon-sintered porous SiC ceramics (6.3 × 10⁵ – 1.6 × 10⁶ Ω·cm) were seven orders of magnitude higher than those of nitrogen-sintered porous SiC ceramics (1.5 × 10⁻¹ – 6.0 × 10⁻¹ Ω·cm). The thermal conductivity and flexural strength of the argon-sintered porous SiC ceramics increased from 8.4–11.6 W·m⁻¹ K⁻¹ and from 9.3–28.2 MPa, respectively, with an increase in the BN content from 0 to 1.5 vol%, which was attributed to the increase in necking area and the decrease in porosity.