Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Effect of BN nanoparticle content in SiC matrix on microstructure and mechanical properties of SiC/SiC composites

BN-nanoparticle-containing SiC-matrix-based composites comprising SiC fibers and lacking a fiber/matrix interface (SiC/BN + SiC composites) were fabricated by spark plasma sintering (SPS) at 1800°C for 10 min under 50 MPa in Ar. The content of added BN nanoparticles was varied from 0 to 50 vol.%. The mechanical properties of the SiC/BN + SiC composites were investigated thoroughly. The SiC/BN + SiC composites with a BN nanoparticle content of 50 vol.%, which had a bulk density of 2.73 g/cm³ and an open porosity of 5.8%, exhibited quasiductile fracture behavior, as indicated by a short nonlinear region and significantly shorter fiber pullouts owing to the relatively high modulus. The composites also exhibited high strength as well as bending, proportional limit stress, and ultimate tensile strength values of 496 ± 13, 251 ± 30, and 301 MPa ± 56 MPa, respectively, under ambient conditions. The SiC fibers with contents of BN nanoparticles above 30 vol.% were not severely damaged during SPS and adhered to the matrix to form a relatively weak fiber/matrix interface.     

Energetics and structure of SiC(N)(O) polymer-derived ceramics

This study presents new experimental data on the thermodynamic stability of SiC(O) and SCN(O) ceramics derived from the pyrolysis of polymeric precursors: SMP-10 (polycarbosilane), PSZ-20 (polysilazane), and Durazane-1800 (polysilazane) at 1200°C. There are close similarities in the structure of the polysilazanes, but they differ in crosslinking temperature. High-resolution X-ray photoelectron spectroscopy shows notable differences in the microstructure of all polymer-derived ceramics (PDCs). The enthalpies of formation (∆H°_{f, elem}) of SiC(O) (from SMP-10), SCN(O) (from PSZ-20), and SCN(O) (from Durazane-1800) are −20 ± 4.63, −78.55 ± 2.32, and −85.09 ± 2.18 kJ/mol, respectively. The PDC derived from Durazane-1800 displays greatest thermodynamic stability. The results point to increased thermodynamic stabilization with addition of nitrogen to the microstructure of PDCs. Thermodynamic analysis suggests increased thermodynamic drive for forming SiCN(O) microstructures with an increase in the relative amount of SiN_xC_4−x mixed bonds and a decrease in silica. Overall, enthalpies of formation suggest superior stabilizing effect of SiN_xC_4−x compared to SiO_xC_4−x mixed bonds. The results indicate systematic stabilization of SiCN(O) structures with decrease in silicon and oxygen content. The destabilization of PDCs resulting from higher silicon content may reach a plateau at higher concentrations.     

Effects of SiC whisker addition on mechanical,thermal, and permeability properties of porous silica-bonded SiC ceramics

The effects of SiC whisker addition into nano-SiC powder-carbon black template mixture on flexural strength, thermal conductivity, and specific flow rate of porous silica-bonded SiC ceramics were investigated. The flexural strength of 1200°C-sintered porous silica-bonded SiC ceramics increased from 9.5 MPa to 12.8 MPa with the addition of 33 wt% SiC whisker because the SiC whiskers acted as a reinforcement in porous silica-bonded SiC ceramics. The thermal conductivity of 1200°C-sintered porous silica-bonded SiC ceramics monotonically increased from 0.360 Wm^–1K^–1 to 1.415 Wm^–1K^–1 as the SiC whisker content increased from 0 to 100 wt% because of the easy heat conduction path provided by SiC whiskers with a high aspect ratio. The specific flow rate of 1200°C-sintered porous SiC ceramics increased by two orders of magnitude as the SiC whisker content increased from 0 to 100 wt%. These results were primarily attributed to an increase in pore size from 125 nm to 565 nm and secondarily an increase in porosity from 49.9% to 63.6%. In summary, the addition of 33 wt% SiC whisker increased the flexural strength, thermal conductivity, and specific flow rate of porous silica-bonded SiC ceramics by 35%, 133%, and 266%, respectively.     

Brazing of SiC ceramics pretreated by chromium coating using inactive AgCu filler metal

In this work, chromium coating conducted by magnetron sputtering was introduced to braze SiC ceramics using inactive AgCu filler metal. The results showed that reliable metallurgical bonding of SiC ceramics was obtained at 900°C for 10 minutes. SEM, XRD, and TEM were used to identify the reaction phase in the joint, and the typical interfacial microstructure was SiC/mixed CrSi₂ + Cr₂₃C₆ layer/CrSi₂/Ag(s,s)+Cu(s,s)/CrSi₂/mixed CrSi₂ + Cr₂₃C₆ layer/SiC. The shear strength of SiC joint using chromium coating brazed with inactive AgCu filler metal was 29.6 MPa and the joint fractured at the SiC substrate entirely after shear test. The proposed active element coating method provides a feasible way to achieve the brazing of ceramics.     

Effects of hydrolysis of precursor on the structures and properties of polymer-derived SiBN ceramic fibers

In this paper, polyborosilazane precursor was synthesied from HMDZ, HSiCl₃, BCl₃ and CH₃NH₂ using a multistep method. By controlling the storage conditions, parts of the polyborosilazane fibers were hydrolyzed. FT-IR, NMR, XRD, TEM and monofilament tensile strength test were employed to study the effects of hydrolysis of precursor on the structures and properties of polymer-derived SiBN ceramic fibers. FT-IR and NMR results indicate that Si-N group in PBSZ reacts with H₂O to form Si-O-Si group. After pyrolysis reaction at 1400℃, Si-O-Si group will finally transformed into highly ordered cristobalite and β-quartz, resulting in formation of the wrinkled surface of the obtained SiBN ceramic fiber. The strip-like defects on fiber surface, according to monofilament tensile strength test, had a significant effect on mechanical property of the obtained SiBN ceramic fiber and caused no increase in fiber tensile strength of hydrolytic polyborosilazane fiber before and after pyrolytic process.     

Additive manufacturing of polymer-derived ceramic matrix composites

This study presents a fabrication method and identifies processing bounds for additively manufacturing (AM) ceramic matrix composites (CMCs), comprising a silicon oxycarbide (SiOC) ceramic matrix. A digital light projection printer was used to photopolymerize a siloxane-based preceramic resin containing inert ceramic reinforcement. A subsequent pyrolysis converted the preceramic polymer to SiOC. Particle reinforcements of 0 to 40% by volume in the green state were uniformly dispersed in the printed samples to study their effects on pyrolysis mass loss and shrinkage, and CMC notch sensitivity and strength. Both particle and whisker reinforcements toughened the glassy SiOC matrix (1 MPa m^1/2), reaching values >3 MPa m^1/2. Bending strengths of >300 MPa (>150 MPa (g cm⁻³)⁻¹) and a Weibull modulus of 10 were measured on AM samples without surface finish. We identified two pore formation mechanisms that placed processing bounds on sample size and reinforcement volume fraction. Methods for increasing these bounds are discussed. With properties commensurate to traditionally processed technical ceramics, the presented process allows for free-form fabrication of high-performance AM CMC components.     

Microwave Processing of Actively Seeded Precursor for Fabrication of Polymer Derived Ceramics

An innovative, faster and nonconventional processing technique is demonstrated for preparing silicon carbide‐based ceramics derived from polymer precursor. The technique is based on microwave‐induced pyrolysis of an actively seeded, high‐purity preceramic polymer that leads to rapid fabrication of silicon carbide components. It is successfully demonstrated that it is feasible to carry out microwave‐induced pyrolysis by seeding the polymer precursor with very low volume fractions of micrometer and nanometer‐sized metal and dielectric fillers. This process allows for rapid, net‐shape, and potentially low‐cost fabrication of silicon carbide‐based materials. Mechanical properties and microstructure of the silicon carbide‐based composites fabricated using this process are characterized.     

纳米SiO2对先驱体浸渍裂解法制备SiCf/SiC复合材料力学性能的影响

以纳米SiO2为填料,采用先驱体浸渍裂解法制备2.5D-SiCf/SiC（D为维数,SiCf为SiC纤维）复合材料,研究了前驱液中纳米SiO2含量对复合材料力学性能的影响。结果表明,纳米SiO2的添加能有效抑制先驱体裂解过程中的体积收缩,提高致密度,但过量引入易导致浸渍液黏度过高,浸渍效率降低。纳米SiO2含量对材料力学性能有较大影响,添加纳米SiO2后材料的抗弯强度和断裂韧性均高于没有添加的样品,材料抗弯强度随纳米SiO2含量的增加先增大后降低。当浸渍液中纳米SiO2含量为6%时,复合材料具有优异的力学性能,抗弯强度达到211.1MPa。     

Making lightweight SiC(rGO,xMoSi2) bulk ceramics via polymeric precursor route

Unique properties of MoSi₂ open new opportunities for preparing bulk polymer-derived ceramics (PDCs) displaying favorable structural-functional capabilities. Herein, an ingenious production route via re-pyrolysis process of ball-milling-induced rigid SiC(rGO, xMoSi₂)_p fillers/flexible polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO, PVG) precursors blends is proposed to obtain in situ formed SiC(rGO, xMoSi₂) bulk PDCs. Interestingly, the possible dense β-SiC/SiOxCy/C_free(rGO, xMoSi₂) framework suffers load and tiny microsized pores relaxes stress, which is beneficial to providing optimized hardness and fracture toughness, ceramic yield, and linear shrinkage. Attractively, MoSi₂ prominently enhances thermal and electrical conductivities of the products owing to increased continuity and compactness. To the best of our knowledge, lightweight SiC(rGO, 20%MoSi₂) bulk PDCs own brilliant ceramic yield (92.13%), liner shrinkage (6.69%), hardness (10.34 GPa), fracture toughness (4.35 Mpa·m^1/2), and thermal conductivity (8.57 W·m^–1·K^–1), opening potential emerging uses in aerospace fields.     

Complex impedance spectra of polymer-derived SiC annealed at ultrahigh temperature

This study reports the complex impedance and alternative current conductivity of polymer-derived ceramic SiC (PDC-SiC) annealed at ultrahigh temperatures. The PDC-SiC shows an inductive response when annealed at temperatures of 1700°C-1900°C due to the percolation of turbostratic carbon. The material returns to a capacitive response at an annealing temperature of 2000°C due to the dissolution of carbon into the SiC lattice. The electrical resistance of the carbon phase decreases with the increase in annealing temperature. These results provide new insights into the effects of processing temperature on microstructure evolution and electrical and dielectric property development of the PDC-SiC ceramic system.     

Effect of anionic substitution on the high temperature stability of polymer-derived SiOC glasses

In this study the high temperature stability (crystallization and decomposition) of two silicon oxycarbide glasses with a similar amount of free carbon (8.3 vs 9.6 wt%) but different content of Si-C bonds (SiC_0.22O_1.57 vs SiC_0.07O_1.86) is presented. The two SiOC glasses are obtained from the same precursor (2 µm methyl-silsesquioxane spheres) via pyrolysis at 1100°C in inert (Ar) or reactive (CO₂) atmospheres. Further annealing in Ar flow at temperatures above 1100°C and up to 1500°C is performed and the samples are characterized by Fourier Transformed Infrared Spectroscopy (FT-IR) and X-ray diffraction (XRD). For comparison purposes the same precursor was annealed in air flow to obtain SiO₂ and its high temperature evolution is also studied. Results suggest that the onset for the carbothermal reduction is not dependent on the amount of Si-C bonds. Moreover, contrary to what is usually reported in the scientific literature, silica phase present in the SiOC glasses does not show, in the same experimental conditions, superior crystallization resistance compared to pure silica glass.     

Effects of aggregate/matrix-phase ratio on the in-situ synthesis of SiC whiskers and properties of reaction-bonded SiC

Reaction-bonded silicon carbide (RBSC) materials were synthesized by a carbon-buried firing method, using α-SiC powders with different sizes as aggregates, while silicon and graphite powders as matrix-phase materials for in-situ synthesis of β-SiC. The effect of the mass ratio of α-SiC aggregate to β-SiC matrix-phase on the in-situ synthesis of β-SiC whiskers was investigated and the optimum mass ratio was determined according to the properties of RBSC materials. The phase compositions, microstructure, and properties of the RBSC materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM), respectively. The results showed that the decreasing aggregate/matrix-phase ratio could increase the yield and aspect ratio of β-SiC whiskers by improving the amount of pores and SiO₂ content. The β-SiC whiskers with stacking faults grew along [111] direction. The in-situ formed β-SiC whiskers and SiO₂ in the matrix-phase enhanced the hot modulus of rupture (HMOR) of the RBSC materials, and the sample with the aggregate/matrix-phase ratio of 70:30 showed the highest cold modulus of rupture (CMOR) of 53.3 MPa, high HMOR of 43.3 MPa, and medium water absorption of 13.8%.     

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.     

Anisotropic compressive properties of porous CNT/SiC composites produced by direct matrix infiltration of CNT aerogel

Carbon nanotube‐reinforced silicon carbide composites (CNT/SiC) produced by direct infiltration of matrix into a porous CNT arrays have been demonstrated to possess a unique microstructure and excellent micro‐mechanical properties. However, the thickness of the array preforms is usually very small, typically less than 2 mm. Therefore, fabrication of macroscopic CNT/SiC composites by chemical vapor infiltration (CVI) process requires that the nanoscale fillers could form macroscopic architectures with an open pore network. Here, this study reports an experimental strategy for the fabrication of SiC matrix composites reinforced by CNT based on an ice‐segregation‐induced self‐assembly (ISISA) technique. Macroscopic CNT aerogel with well‐defined macroporous network was produced by ISISA technique and was subsequently infiltrated by SiC in a CVI reactor. After five CVI cycles, the porosity of as‐fabricated composites was 11.6±0.3% and the machined specimens exhibited lamellar structure with parallel lamellaes intersected at discrete angles. By observed, there are in fact five different representative anisotropic macrostructures, the compressive strengths of these five different loading modes with respect to lamella orientation were 933±55, 619±34, 200±45, 199±21, and 297±41 MPa, respectively, and the failure mechanisms were attributed to the anisotropic nature of the macrostructures. Energy dissipation toughening mechanism at the nanoscale such as CNT pull‐out was observed and the phase composition of the fabricated materials included β‐SiC, CNT, and SiO₂.     

Mechanical and Electrical Properties of Silicon Nitride–Silicon Carbide Nanocomposite Material

Mechanical and electrical properties of nanocomposite materials composed of a Si₃N₄ matrix and nanometer-sized SiC particles are described. Composites containing less than 10 vol% SiC particles have the same order of resistivity and dielectricity as the non-SiC material as well as highly improved mechanical properties. The composites are promising materials for use under harsh conditions.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Processing of polymer-derived,aerogel-filled,SiC foams for high-temperature insulation

Porous polymer-derived ceramics (PDCs) are outperforming materials when low-density and thermal inertia are required. In this frame, thermal insulating foams such as silicon carbide (SiC) ones possess intriguing requisites for aerospace applications, but their thermal conductivity is affected by gas phase heat transfer and, in the high temperature region, by radiative mechanisms. Owing to the versatility of the PDC route, we present a synthesis pathway to embed PDC SiC aerogels within the open cells of a SiC foam, thus sensibly decreasing the thermal conductivity at 1000°C from 0.371 W·m⁻¹K⁻¹ to 0.243 W·m⁻¹K⁻¹. In this way, it was possible to couple the mechanical properties of the foam with the insulating ability of the aerogels. The presented synthesis was optimized by selecting, among acetone, n-hexane, and cyclohexane, the proper solvent for the gelation step of the aerogel formation to obtain a proper mesoporous colloidal structure that, after ceramization at 1000°C, presents a specific surface area of 193 m²·g⁻¹. The so-obtained ceramic composites present a lowest density of 0.18 g·cm⁻³, a porosity of 90% and a compressive strength of 0.76 MPa.     

Synthesis and properties of a new, branched polyhydridocarbosilane as a precursor for silicon carbide

Polymerization of Cl₂Si(CH₃)CH₂Cl with Mg in THF, followed by reduction with LiAlH₄, gave a polycarbosilane with Si-H groups and branches at the Si atoms. The polymer could be cross-linked thermally at 150°C. Pyrolysis of the cross-linked material gave SiC with a yield of 70%.Presented at the XXVIth Silicon-Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.