Boron nitride–silicon carbide interphase boundaries in silicon nitride–silicon carbide particulate composites

Interphase boundaries between SiC and h-BN grains in hot isostatically pressed Si₃N₄–SiC particulate composites made from both as-received powders and deoxidised powders, in which sub-micron size h-BN particles occur as a contaminant, have been characterised using transmission electron microscopy techniques. Most of the h-BN grains observed were aligned with respect to SiC grains so that (111) 3C SiC and (0001) α-SiC planes were parallel to (0001) h-BN planes. The h-BN–SiC interphase boundaries in the composites made from as-received powders were covered with thin silica-rich intergranular films, in contrast to the interphase boundaries in the composites made from deoxidised powders. These observations are discussed in the light of models for the formation of intergranular amorphous films in ceramic materials, geometric considerations for low interfacial energies and the possible bonding at h-BN–SiC interphase boundaries free of intergranular films.     

Tensile creep behavior of a ytterbium silicon oxynitride–silicon nitride ceramic

Tensile creep behavior of hot pressed silicon nitride on the Si₃N₄–Yb₄Si₂O₇N₂ tie line was investigated at temperatures of 1300 and 1400 °C under an applied stress of 125 to 200 MPa. During the tests, the creep strain increased with time and the creep rate monotonically decreased both with time and strain. On the basis of minimum strain rates, the stress exponents for 1300 and 1400 °C were determined to be 3.1 and 1.7, respectively. All the specimens tested at 1400 °C lead to failure while exhibiting a large scatter in the time-to-failure data. The activation energy was determined to be 879 kJ/mol from a comparison between creep rates at different temperatures. The creep mechanism is discussed on the basis of the creep parameters and creep damage observation.     

Dispersion,slip casting and reaction nitridation of silicon–silicon carbide mixtures

The dispersing behaviour of silicon, silicon carbide and their mixtures in aqueous media were monitored by particle size, sedimentation, viscosity and zeta potential analyses as a function of pH of the slurry. The pH values for optimum dispersion were found to be 4 and 8 for silicon, 10 for SiC and 9 for Si+SiC mixtures. Optimum slips of Si+SiC mixtures were slip cast to obtain green compacts which were nitrided once at 1450°C for 2 or 4 h or successively and cumulatively for 8 (2+6) and 10 (4+6) h in a resistively heated graphite furnace. The binding phases in the nitrided products were found to be fibrous/needle like α-Si₃N₄, flaky grains of β-Si₃N₄ and Si₂ON₂. The products containing 19–47% of silicon nitride as bond/matrix possessed flexural strength (three-point bending) values of 50–85 MPa. ©     

Growth of epitaxial β-SiC films on silicon using solid graphite and silicon sources

Epitaxial β-SiC films have been grown on mirror-polished Si(111) substrates using bias-assisted hot filament chemical vapor deposition at a substrate temperature of 1000°C. A graphite plate was used as the carbon source, and the silicon source came from the silicon substrate itself. The gas phase in the system is hydrogen only. Atomic hydrogen produced by hot filaments reacted with the graphite to form hydrocarbon radicals, which further reacted with the silicon substrate and deposited as β-SiC. The effect of negative bias applied to the substrate is the key factor for epitaxial growth. Under the growth conditions without the negative bias applied, only polycrystalline β-SiC was obtained.     

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si₃N₄ and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si₂N₂O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si₃N₄ matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si₂N₂O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si₂N₂O grains, the texture formation, and the coarsening of β-Si₃N₄. Fracture toughness, however, was still low (∼5.2 MPa m^1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si₂N₂O-matrix interfacial bond and nearly equiaxed β-Si₃N₄ with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.     

Novel polysilazanes as precursors for silicon nitride/silicon carbide composites without “free” carbon

Polysilazanes with chemical composition Si₄N₄CH_x (x = 12–14) were obtained on two different reaction pathways. In the first route three equivalents of dichlorosilane, H₂SiCl₂, were mixed with one equivalent of methyldichlorosilane, H₃CSiHCl₂, and reacted with ammonia to yield [(SiH₂–NH)₃(H₃CSiH–NH)]_n (1) after appropriate work-up. In the second route, dichlorosilane was reacted with methylamine and ammonia in a 3:1 ratio to deliver [(SiH₂–NH)₃(SiH₂–NCH₃)]_n (2). The raw products were further cross-linked by base-catalyzed dehydrocoupling reactions involving SiH and NH units to yield products 1a and 2a, respectively. The molecular structure of 1 and 2 was investigated by means of high resolution ¹H, ¹³C{¹H}, and ²⁹Si{¹H} NMR spectroscopy in C₆D₆ solution as well as by IR spectroscopy. Because of the insolubility of the cross-linked products 1a and 2a solid state ¹H, ¹³C, and ²⁹Si CP-MAS NMR in combination with IR spectroscopy were applied for their spectroscopic characterization. Chemical compositions were determined using elemental analysis.Thermolysis in an argon atmosphere up to 1400 °C of the cross-linked products delivered Si₃N₄/SiC ceramics in ∼94% yield. The absence of “free” carbon was revealed by mass spectra-coupled thermogravimetric analysis (TGA). These investigations indicated a one-step decomposition in the 250–700 °C range with predominant evaporation of hydrogen. In contrast, elimination of methane and ammonia (1) or methane and methylamine (2) was below 0.5 mass%. Additionally, neutron wide angle scattering proved the absence of a “free” carbon phase. High-temperature properties were investigated using high temperature TGA as well as XRD, EDX and TEM of annealed ceramic samples. The onset of crystallization in both materials was around 1500 and 1300 °C (1 atm N₂), respectively, and α- as well as β-Si₃N₄ formed besides α/β-SiC. Above 1900 °C ceramics derived from both 1a and 2a decomposed to α/β-SiC/β-Si₃N₄/α-Si composites.     

Characterization of copper–silicon nitride composite electrocoatings

Silicon nitride particles were incorporated to electrolytic copper by co-electrodeposition in acidic sulfate bath, aiming the improvement of its mechanical resistance. Smooth deposits containing well-distributed silicon nitride particles were obtained. The current density did not show significant influence on incorporated particle volume fraction, whereas the variation of particle concentration in the bath had a more pronounced effect. The microhardness of the composite layers was higher than that of pure copper deposits obtained under the same conditions and increased with the increase of incorporated particle volume fraction. The microhardness of composites also increased with the increase of current density due to copper matrix grain refining. The composite coatings were slightly more corrosion resistant than pure copper deposits in 3.5% NaCl solutions.     

Effect of silicon carbide dispersion on the microwave absorbing properties of silicon carbide-epoxy composites in 2–40 GHz

In this study, silicon carbide powders were manufactured successfully by the method of preheating combustion synthesis in nitrogen atmosphere where it was introduced into an epoxy resin to produce a microwave absorber. The structure of the silicon carbide was characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Composite based on the various loadings of silicon carbide and epoxy resin specimens were prepared and the reflection losses of these composite samples were studied using the free space method. Based on the microwave measurements, microwave absorber specimens of silicon carbide with thermal plastic resin at frequencies between 2 and 18 and 18–40 GHz could be obtained from a matching thickness of 2.0 mm by controlling the content of silicon carbide.     

Dispersants for silicon carbide in water—A comparative study

The present work describes a comparative study on a pool of 12 dispersants for the de-agglomeration and stabilization of silicon carbide in aqueous suspensions with solids loading relevant for dip coating applications. As silicon carbide slurries may include sintering aids, different functional groups, molecular weight, and stabilization mechanisms were considered for the dispersants to be able to stabilize both slurry components. Additionally, pH effect, toxicity, additive compatibility, and foaming properties were considered, giving all the necessary information for developing new aqueous formulation of SiC suspensions, including advantages and disadvantages of the different candidates. Different de-agglomeration procedures, powder surface area, and calcination temperature were also considered to study the effect of the SiC surface properties. The outcome is that slurry stabilization provided by an alkaline environment at pH larger than 8‒9 is significantly more effective than slurry stabilization by any of the tested dispersants. Alkaline environments facilitate a negative surface charge on SiC particles and provide a stable electrostatic stabilization mechanism not observed in neutral or acidic environments. One among the dispersant candidates (FA 4404) seems to broaden slightly the range of stability toward the acidic regime. Anionic surfactants or block co-polymers tested exhibited no significant interaction with the SiC particles.     

Microstructure and mechanical strength of diffusion-bonded silicon nitride–molybdenum joints

Solid-state bonding of reactive systems, such as Si₃N₄–Mo often results in the formation of excessively thick intermetallic layers that can be detrimental to the final strength of the joint. The objective of this work was to study the microstructural evolution of Si₃N₄–Mo interfaces, aiming at maximum joint strength via a balance between the fraction of bonded material and the amount of interfacial reaction. Joining was carried out under vacuum or nitrogen atmosphere for temperatures between 1100 and 1800°C. Microstructural analyses of the interfaces revealed the presence of Mo₃Si and Mo₅Si₃ along with residual pores. The results from shear strength tests revealed a strong relationship between the microstructure of the interface and the mechanical strength of the joint. ©     

Mechanical properties of silicon carbide—in situ zirconium carbonitride composites

SiC–Zr₂CN composites were fabricated by conventional hot pressing from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃–Sc₂O₃ as a sintering additive. The effects of the ZrN addition on the room-temperature (RT) mechanical properties and high-temperature flexural strength of the SiC–Zr₂CN composites were investigated. The fracture toughness gradually increased from 4.2 ± 0.3 MPa·m^1/2 for monolithic SiC to 6.3 ± 0.2 MPa·m^1/2 for a SiC–20 vol% ZrN composite, whereas the RT flexural strength (546 ± 32 MPa for the monolithic SiC) reached its maximum of 644 ± 87 MPa for the SiC–10 vol% ZrN composite. The monolithic SiC had improved strength at 1200°C, whereas the SiC–Zr₂CN composites could not retain their RT strengths at 1200°C. The typical flexural strength values of the SiC–0, 10, and 20 vol% ZrN composites at 1200°C were 650 ± 53, 448 ± 31, and 386 ± 19 MPa, whereas their RT strength values were 546 ± 32, 644 ± 87, and 528 ± 117 MPa, respectively.     

Silicon nitride–silicon carbide micro/nanocomposites: A review

This paper reviews investigations of silicon nitride–silicon carbide micro–nanocomposites from the original work of Niihara, who proposed the concept of structural ceramic nanocomposites, to more recent work on strength and creep resistance of these unique materials. Various different raw materials are described that lead to the formation of nanosized SiC within the Si₃N₄ grains (intragranular) and at grain boundaries (intergranular). The latter exert a pinning effect on the amorphous grain boundary phases in the silicon nitride and also act as nucleation sites for β-Si₃N₄, which limits grain growth during sintering. This finer microstructure results in strengths higher than for the monolithic silicon nitride. Intragranular SiC particles enhance strength and fracture toughness as a result of residual compressive thermal stresses within the nanocomposites. High temperature strength and creep resistance are also much higher than for monolithic silicon nitride and a few investigations of these topics are briefly reviewed and the proposed mechanisms are described. Within the context of other studies cited, work on Si₃N₄–SiC micro–nanocomposites by the current authors describes an aqueous processing route for better dispersion of commercial powders prior to sintering.     

The wetting behaviour and reaction kinetics in diamond–silicon carbide systems

The wetting behaviour of silicon on diamond and the interaction of diamond with molten silicon were investigated. It was found that diamond is well wetted by molten silicon reaching a contact angle of about 20° after melting. The wetting is caused by the rapid formation of a SiC interlayer by nucleation of silicon carbide grains on the surface of the diamond. Investigations of the interaction of silicon with CVD diamond, using SEM, showed that the initial rate of SiC formation is very fast and is significantly reduced after the formation of a 4–6 μm thick dense SiC interlayer. At that stage further growth is likely to be controlled by the diffusion of Si and C through the grain boundaries of the silicon carbide interlayer. The results were compared with the interaction of silicon with glassy carbon.     

Oxidation,mechanical and thermal properties of hafnia–silicon carbide nanocomposites

Dense nanocomposites constituted from 70/30 vol% of hafnia–silicon carbide and were prepared by spark plasma sintering. Silicon carbide suppresses grain growth. The fracture strength of as prepared composites is 400–600 MPa. Oxidation up to 1600 °C in air for 10 h has minor influence on the mechanical strength, which is ascribed to the dense nature of the oxidation scale. The high density of the oxidation scale is attributed to a volume increase when silicon carbide oxidizes and reacts with hafnia to form hafnium silicate. The composite has a thermal conductivity of 14 W m⁻¹ K⁻¹ at room temperature. Design approaches for further enhancement of ultrahigh temperature properties of oxide/non-oxide composites are discussed.     

Corrosion behaviour of silicon carbide–diamond composite materials in aqueous solutions

The corrosion behaviour of the relatively new silicon carbide bonded diamond materials (ScD) was investigated in NaOH, H₂SO₄ and hydrothermal conditions and compared with that of conventional SSiC and SiSiC-materials. The corrosion resistance increases with decreasing diamond grain size. In H₂SO₄ all investigated materials show a very high corrosion resistance, whereas in NaOH and under hydrothermal conditions above 100 °C some leaching of residual silicon takes place. Nevertheless the fine grained ScD material exhibits a residual strength of 400 MPa after 200 h corrosion in NaOH at 90 °C. Under the same conditions the strength of the SiSiC-material reduces to 50 MPa. The silicon carbide-diamond composites demonstrate corrosion resistance superior to SiSiC and wear properties analogous to that of conventional superhard materials. This material would therefore be suitable for use in demanding corrosive wear applications.     

Additive manufacturing of continuous carbon fiber–reinforced silicon carbide ceramic composites

A material extrusion (MEX) technology has been developed for the additive manufacturing of continuous carbon fiber–reinforced silicon carbide ceramic (C_f/SiC) composites. By comparing and analyzing the rheological properties of the slurries with different compositions, a slurry with a high solid loading of 48.1 vol% and high viscosity was proposed. Furthermore, several complex structures of C_f/SiC ceramic composites were printed by this MEX additive manufacturing technique. Phenolic resin impregnation–carbonization process reduces the apparent porosity of the green body and protects the C_f. Finally, the reactive melting infiltration (RMI) process was used to prepare samples with different C_f contents from 0 to 2 K (a bundle of carbon fibers consisting of 1000 fibers). Samples with C_f content of 1 K show the highest bending strength (161.6 ± 10.5 MPa) and fracture toughness (3.72 ± 0.11 MPa·m^1/2) while the thermal conductivity of the samples with the C_f content of 1 K reached 11.0 W/(m·K). This study provides a strategy to prepare C_f/SiC composites via MEX additive manufacturing and RMI.     

Solid-state pressureless sintering of silicon carbide below 2000 °C

To date, solid-state pressureless sintering of silicon carbide powder requires sintering aids and high sintering temperature (>2100 °C) in order to achieve high sintered density (>95% T.D.). Two-step sintering (TSS) method can allow to set sintering temperature lower than that conventionally required. So, pressureless two-step sintering process was successfully applied for solid-state sintering (boron carbide and carbon as sintering additives) of commercial SiC powder at 1980 °C. Microstructure and mechanical properties of TSS-SiC were evaluated and compared to those obtained with the conventional sintering (SSiC) process performed at 2130 °C. TSS-SiC showed finer microstructure and higher flexural strength than SSiC with very similar density (98.4% T.D. for TSS-SiC and 98.6% T.D. for SSiC).     

Microstructural development of silicon nitride with aligned β-Si3N4 whiskers

Silicon nitride was sintered with 3 wt.% silicon nitride whiskers that were aligned using tape casting. Sintering was carried out at temperatures between 1550 and 1850°C for 1 h. The α to β phase transformation was complete at 1750°C. XRD results also showed that the amount of β-phase grains in the matrix increased faster than growth rate of the whisker grains at the early stage of sintering. The intensities of the peaks diffracted from the whisker grains increased faster than those diffracted from the matrix β-phase grains after the α to β phase transformation was complete. Both XRD results and the etched microstructures indicated that the whisker grains grew preferentially in the length direction.     

Thermal properties of silicon carbide composites fabricated with chopped Tyranno® SiAlC fibres

Thermal diffusivity, a, and thermal conductivity, κ, between room temperature and 600 K were investigated for SiC composites containing 0–50 mass% of Tyranno^® SiAlC (SA) fibre (mean length: 394 μm) hot-pressed at 1800 °C for 30 min under a pressure of 31 MPa. The monolithic SiC specimen possessed κ of 32.1 W m⁻¹ K⁻¹ at room temperature; no significant changes were found for the SiC composite containing ≤20 mass% of SA fibre addition. However, further increases in the amount of SA fibre to 50 mass% improved κ to a maximum of 56.3 W m⁻¹ K⁻¹. The value of a for the SiC composite containing 40 mass% of SA fibre was 0.185 cm² s⁻¹ at room temperature and decreased to 0.120 cm² s⁻¹ at 600 K. In addition, SiC composites using 40 mass% of SA fibre with a carbon interface of approximately 100 nm were fabricated. The effect of this interface on a and κ was marginal.