SiC BN Composites Research Articles

Influence of temperature, impact angle and h-BN content on the erosive wear behavior of hot-pressed SiC-BN composites

Owing to the unique combination of high strength, high hardness, and excellent thermal shock resistance, SiC-BN composites can be a potential candidate for components of exhaust systems where material loss by erosion is a major concern. In view of the foregoing, the present investigation deals with the solid particle erosion behavior of hot-pressed SiC composites with varying h-BN content: 0, 4, and 10 vol%. Erosion tests were conducted using SiC particles (size of 50–70 μm) at different impingement angles: 30°, 60°, and 90° and temperature: room temperature (25 °C) and high temperature (800 °C). The dominant erosive wear mechanisms were determined using SEM-EDS, XRD, Raman spectroscopy and XPS analyses. Results indicated that the erosion rate of the investigated composites decreased with the addition of h-BN and at shallow angles of impingement and high temperature. Among the investigated composites, SiC-4 vol% BN composites exhibited the least erosive wear at a given temperature and impingement angle, while the maximum erosive wear obtained for monolithic SiC ceramics at 25 °C and 800 °C. Fracture and deformation of SiC grains along with the pull-out of BN platelets are identified as the major mechanisms of material removal for the investigated SiC-BN composites. The presence of SiO2, B2O3 or SiO2·B2O3 phases is observed on the surface of composites eroded in high temperature conditions.

Tuning the electrical, thermal, and mechanical properties of SiC-BN composites using sintering additives

ABSTRACT SiC-4 vol% BN composites were hot-pressed at 2050°C for 4 h at 40 MPa in a N2 atmosphere using micron-sized β-SiC and h-BN starting powders with 1 vol% sintering additives. Four batches were prepared using four different types of additive systems, i.e. Y2O3-Sc2O3, Yb2O3-CaO, Yb2O3-MgO, and Al2O3-AlN-Y2O3. The electrical, thermal, and mechanical properties of the SiC-4 vol% BN composites, which are primarily limited by the intrinsic weakness of h-BN and by the point defects (BC and AlSi) created by the dissolution of B and Al in the SiC lattice, were successfully tuned with the use of different additive systems. The electrical conductivity of SiC-4 vol% BN composites improved ~fivefold, i.e. from 2.6 (Ω·cm)−1 for the Al2O3-AlN-Y2O3-containing specimen to 13.9 (Ω·cm)−1 for the Y2O3-Sc2O3-containing specimen, owing to grain-growth-assisted N-doping and the elimination of Al-derived acceptors. Thermal conductivity was altered by 16% with the use of different additive systems. Fracture toughness dramatically increased from 4.3 MPa∙m1/2 in Yb2O3-CaO-containing specimens to 7.3 MPa∙m1/2 in Y2O3-Sc2O3-containing specimens. The electrical conductivity, thermal conductivity, flexural strength, and fracture toughness of SiC-4 vol% BN composite sintered with 1 vol% Y2O3-Sc2O3 were 13.9 (Ω·cm)−1, 82.0 W·m−1 K−1, 505 MPa, and 7.3 MPa∙m1/2, respectively.

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.

Electrically conductive SiC-BN composites

SiC-BN composites were fabricated by conventional hot-pressing from β-SiC and h-BN powders with 2vol% Y2O3 as a sintering additive. Electrical, mechanical, and thermal properties of SiC-BN composites were investigated as a function of the BN content in the starting materials. The addition of BN suppressed the grain growth of the SiC and triggered the β→α phase transformation of the SiC in SiC-BN composites. A minimum electrical resistivity of 3.7×10−2Ωcm was obtained for the SiC-4vol% BN composite (SBN4). This low electrical resistivity was ascribed to N-doping in the SiC lattice, which acts as a donor for supplying electrons to the conduction band. Typical values for the electrical conductivity, fracture toughness, flexural strength, and thermal conductivity of the SBN4 at room temperature were 27Ω−1cm−1, 4.1MPam1/2, 566MPa, and 87W/mK, respectively. The SiC-4vol% BN composite can be electrical discharge machined to form complex shapes.

Electrical and thermal properties of silicon carbide–boron nitride composites prepared without sintering additives

Composites of SiC–BN consisting of uniformly distributed plate-shaped hexagonal BN grains and nitrogen-doped α-SiC grains were fabricated by conventional hot-pressing in an argon atmosphere without sintering additives. The effects of the BN content on the electrical, structural, thermal, and mechanical properties of the SiC–BN composites were investigated. The electrical resistivity showed a minimum at a BN content of 10vol%, which is attributable to N substitution for C sites in the 6H–SiC lattice, which acts as donors for supplying carriers to the conduction band. The thermal conductivity of the composites showed a decreasing trend with increasing BN content, indicating that the presence of the SiC/BN interface discourages phonon conduction in the composites. The electrical resistivity, thermal conductivity, flexural strength, fracture toughness, and hardness of the SiC–10vol% BN composite were 9.3×105Ω×cm, 74.4W/mK, 559MPa, 3.6MPam1/2, and 23GPa, respectively.

Microstructure, Thermal Conductivity, and Electrical Properties of In Situ Pressureless Densified SiC–BN Composites

The microstructure, thermal conductivity, and electrical properties of pressureless densified SiC–BN composites prepared from in situ reaction of Si3N4, B4C, and C were systematically investigated, to achieve outstanding performance as substrate materials in electronic devices. The increasing BN content (0.25–8 wt%) in the composites resulted in finer microstructure, higher electrical resistivity, and lower dielectric constant and loss, at the expense of only slight degradation of thermal conductivity. The subsequently annealed composites showed more homogeneous microstructures with less crystal defects, further enhanced thermal conductivities and electrical resistivities, and reduced dielectric constants and losses, compared with the unannealed ones. The enhanced insulating performance, the weakened interface polarization, and the reduced current conduction loss were explained by the gradual equalization of dissolved B and N contents in SiC crystals and the consequent impurity compensation effect. The schottky contact between graphite and p‐type SiC grains presumably played a critical role in the formation of grain‐boundary barriers. The annealed composites doped with 8 wt% BN exhibited considerably high electrical resistivity (4.11 × 1011 Ω·cm) at 100 V/cm, low dielectric constant (16.50), and dielectric loss (0.127) at 1 MHz, good thermal conductivity [66.06 W·(m·K)−1] and relatively high strength (343 MPa) at room temperature.

Enhanced electrical resistivity in SiC–BN composites with highly-active BN nanoparticles synthesized via chemical route

In order to fabricate SiC–BN composites with enhanced electrical resistivity, boric acid and urea were used as reactants for synthesizing nano-sized BN particles on the surfaces of SiC powders in flowing ammonia atmosphere. The composites containing 10 wt% BN were pressureless densified at 2150°C for 1h with B4C and C as sintering additives. The chemical route was advantageous for obtaining finer BN dispersoids and microstructures of higher homogeneity, which significantly enlarged the SiC/BN interdiffusion area. Consequently, the concentrations of dissolved B and N in SiC grains became more equivalent and the carrier concentration was further decreased through impurity compensation mechanism. The chemically processed composite exhibited a considerably high electrical resistivity (1.03×1012Ωcm), which was ∼15 times higher than that of the composite prepared by conventional powder mixing method. Besides, considerably low dielectric constant (13.88) and loss tangent (0.128) at 1MHz, as well as good thermal conductivity (70.08Wm−1K−1) were also achieved.

A top-down approach to densify ZrB2–SiC–BN composites with deeper homogeneity and improved reliability

A novel top down approach was developed to fabricate dense ZrB2–SiC–BN (ZSN) composites with a fine-grained (<1 μm) microstructure using coarse-grained (∼10 μm) ZrN and Si precursors that reacted with fine (∼0.5 μm) B4C powders at 1850 °C. The results show that the “reaction” and “densification” mechanisms acting during sintering could be separated or happen simultaneously by changing the pressure loading strategy. Loading cycles not only have a substantial influence on the microstructure homogeneity and the strength reliability of as sintered composites, but might alter the electric current path during the Spark Plasma Sintering process as well. A residual compressive stress of 361 MPa, generated upon the SiC grains in the ZSN composites, was measured by Raman spectroscopy. The fine SiC grains tended to form larger clusters in the dense microstructure if the loading cycle is inappropriate, which was further employed to interpret the observed scattering of strength values in one batch of ZSN. The 48ZrB2–24SiC–28BN (number in vol%) composite has an average strength of 473 MPa, which was 94% of that of in situ densified ZrB2–SiC ceramics (ZS). Considering the corresponding Younǵs modulus was only half of that for ZS, the in situ ZSN composite owns a better mechanical strain tolerance.

High electrical resistivity of pressureless sintered in situ SiC–BN composites

SiC–BN composites with a BN content of 6 wt.% were pressureless sintered to ∼99.4% theoretical density at 2200 °C for 1 h and found to have a high electrical resistivity of 1.24 × 10 10 Ω cm. Starting B 4 C and C were added to react partially with Si 3 N 4 for in situ synthesis of BN and also to serve as sintering aids. Interface diffusion of B and N from BN into SiC was crucial to improving the insulating and dielectric properties through carrier compensation without causing any obvious decrease in the thermal conductivity .

Machinable ZrB2–SiC–BN composites fabricated by reactive spark plasma sintering

A solid state reaction of ZrH2, Si3N4 and B4C has been proposed to synthesize machinable ZrB2–SiC–BN composites. Dense ZrB2–SiC–BN composites with fine grain size and homogeneous microstructure were fabricated via spark plasma sintering at 1900°C in vacuum. The kinetics of the reaction process was studied with XRD results. SEM and TEM results showed that the in-situ formed BN phase was composed of micro-sized intergranular and nano-sized intragranular h-BN particles. The influences of the BN content on the mechanical properties as well as the thermal conductivities of the composites were studied. The composites that contained >20vol% BN exhibited excellent machinability and high strength.

Microstructure and mechanical properties of ZrB2–SiC–BN composites fabricated by reactive hot pressing and reactive spark plasma sintering

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process.

Effect of BN grain size on microstructure and mechanical properties of the ZrB 2–SiC–BN composites

ZrB 2–20 vol.%SiC composites containing 10 vol.% h-BN particles (ZSB) with average grain sizes ranging from 1 μm to 10 μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB 2 (2.3–3.5 MPa m 1/2) and SiC particle reinforced ZrB 2 composites (4.0–4.5 MPa m 1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB 2–SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The Δ T c of ZSB5 material is 400 °C which is higher than ZrB 2–20%SiC and ZrB 2–15%SiC–5%AlN.

Fabrication and properties of ZrB 2–SiC–BN machinable ceramics

ZrB 2–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB 2–SiC–BN (40 vol%ZrB 2–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m 1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB 2–SiC (80 vol%ZrB 2–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB 2–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB 2–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB 2–20 vol%SiC and 43.1 vol%ZrB 2–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm 2, respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB 2–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB 2–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB 2–SiC–BN.

Thermal shock resistance of in situ formed SiC–BN composites

SiC–BN ceramic matrix composites (SiC–BN CMCs) were in situ synthesized, using Si 3N 4, B 4C and carbon as reactants. Thermal shock resistance of in situ SiC–BN composites was investigated by measuring strength retention after varying temperature difference (Δ T) up to 1100 °C and compared with that of monolithic SiC. Monolithic SiC showed rapidly drop in flexural strength above Δ T of 900 °C, while SiC–BN composites showed an increase or negligible reduction in flexural strength till Δ T equal to 1100 °C, showing a great improvement of thermal shock resistance.

Oxidation resistance of hot-pressed SiC–BN composites

The oxidation resistance of SiC–BN composites with different BN content hot-pressed from Si 3N 4, B 4C and C was investigated. The oxidized products of SiC and BN were identified to be SiO 2, C and B 2O 3, N 2. SiO 2 and B 2O 3 could further form a borosilicate glass which covered the surfaces of the samples and withstood oxidation because of its flowability and self-healing. The oxidation resistance of the SiC–BN composites in static air atmosphere deteriorated with the increase of temperature as well as of the BN content.

Microstructure and Mechanical Properties of SiC-BN Composites with Oxynitride Glass

By using an oxynitride glass as a sintering additive, the effects of BN content on microstructure and mechanical properties of the hot-pressed and subsequently annealed SiC-BN composites were investigated. The microstructures developed were analyzed by image analysis. The morphology of SiC grains was strongly dependent on BN content in the starting composition. The aspect ratio of SiC decreases with increasing BN content and the average diameter of SiC shows a maximum at 5 wt% BN and decreases with increasing BN content in the starting powder. The fracture toughness increased with increasing BN content while the strength decreased with increasing BN content. The strength and fracture toughness of SiC or SiC-TiC composites were strongly dependent on the morphology of SiC grains, but the strength and fracture toughness of SiC-BN composites were strongly dependent on BN content rather than morphology of SiC grains. These results suggest that fracture toughness of SiC ceramics can be tailored by manipulating BN content in the starting composition. Typical fracture toughness and strength of SiC-10 wt% BN composites were 8 MPa·m 1/2 and 445 MPa, respectively.

In situ Si 3N 4–SiC–BN composites: preparation, microstructures and properties

Boron nitride composites in Si–B–C–N system have been widely studied and applied in industry in these decades. In the present investigation, in situ reaction process was used to fabricate a new kind of silicon nitride-silicon carbide-boron nitride (Si 3N 4–SiC–BN) composites according to a previously proposed reaction of Si 3N 4, B 4C, and C with extra addition of Si 3N 4 in the starting powder mixture. The composites were produced by hot pressing at 1800 °C in a nitrogen atmosphere. The obtained composites demonstrated relatively high density and high bending strength over those of conventionally-processed ones, although with increasing BN content the densification behavior became poorer especially for composites with BN contents higher than 25 vol.%. The growth of rodlike β-Si 3N 4 grains was suppressed in the composites. Different from the previously reported SiC–BN in situ composites in which the strength almost kept unchanged when the BN content was lower than 35 vol.%, the strength of the present Si 3N 4–SiC–BN composites decreased monotonously with increase in BN content due to the decreased relative density and limited grain growth of rodlike β-Si 3N 4 grains.

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composites

SiC–BN composites were prepared via the proposed in situ reaction, which used Si3N4, B4C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC–BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC–25‐vol%‐BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These in situ SiC–BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.

Reaction mechanism and microstructure development of strain tolerant in situ SiC–BN composites

The reaction mechanism and microstructure development of strain tolerant in situ SiC–BN composites fabricated from the in situ reaction of Si 3N 4, B 4C and C were investigated. This exothermic reaction took place at about 1400°C in an argon atmosphere according to the results of X-ray diffraction analysis and differential thermal analysis. The reaction finished after hot pressing at 1700°C for 60 min, and densification occurred mainly in the temperature range of 1700°C to 2000°C. In spite of poor sinterability of BN, composites with rather high density were obtained. Chemical composition analysis of the composites obtained showed that there was no obvious change in the composition after hot pressing. The in situ formed SiC was of the β-type with a quasi-spherical shape, whereas the in situ BN was graphitic hexagonal with a flake shape, and was located at the grain boundaries of SiC. The composite obtained showed a very fine and homogeneous microstructure. The bending strength of the composite was high, while the elastic modulus decreased substantially.

Phase and Property Studies of SiC-BN Composites

Phase composition and solid solubility in SiC-BN composites were studied using cubic SiC, and cubic and graphitic BN with consolidation by uniaxial and isostatic hot-pressing techniques. Results revealed that SiC and BN were the only phases present. When BN is the major phase, cubic SiC is the major SiC phase, and some hexagonal polytypes are sometimes found. For samples hot isostatically pressed to 2,000{degree}C, cubic BN transformed to graphitic BN, and a solubility of 0.5 to 1 mol% BN in cubic SiC was indicated. Electron microscopy revealed that the cubic and highly cubic polytypes (i.e., those with a large fraction of ABCABC-type stacking arrangements) appeared adjacent to the BN phase and to be stabilized by it. Young's modulus of hot-pressed composites decreased nonlinearly with increasing BN content.