Silicon carbide matrix composites reinforced with two-dimensional titanium carbide – Manufacturing and properties

Within this paper, it is explored how two-dimensional Ti₂C sheets addition affects silicon carbide matrix composites in terms of the microstructure and mechanical properties. In order to consolidate the powder mixtures, powder metallurgy processing followed by Spark Plasma Sintering was performed to prepare the sinters. According to our knowledge, this is the first attempt to apply delaminated MXene phases as a reinforcing phases of ceramic matrix composites. The delaminated MXene phases were characterized using a high-resolution transmission microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS). Significant improvement of the fracture toughness and hardness for the composites reinforced with 1.5?wt% 2D Ti₂C compared to the reference sample were observed. It is expected that the applied reinforcing phase will have an influence on the fracture mechanism, and so this has also been investigated. Two of the main mechanisms of crack propagations (crack deflection and bridging) were observed.     

Improved ablation resistance of C–C composites using zirconium diboride and boron carbide

Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon–carbon (C–C) composites at high temperature (1500 °C). Our approach combines using a precursor to ZrB₂ and processing them with B₄C particles as filler material within the C–C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B₄C, and ZrB₂–B₄C filled C–C composites from 800 to 1500 °C. Ablation rates decreased by 30% when C–C composites were filled with a combination of ZrB₂–B₄C particles over carbon black and B₄C filled C–C composites. We also investigated using a sol–gel precursor method as an alternative processing route to incorporate ZrB₂ particles within C–C composites. We successfully converted ZrB₂ particles within C–C composites at relatively low temperatures (1200 °C). Our ablation results suggest that a combination of ZrB₂–B₄C particles is effective in inhibiting the oxidation of C–C composites at temperatures greater than 1500 °C.     

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.     

Creep behavior of a zirconium diboride–silicon carbide composite

Flexural creep studies of ZrB₂–20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB₂ grain or ZrB₂–SiC interphase boundary and ZrB₂ lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB₂–ZrB₂ and/or ZrB₂–SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 < n < 2.2. In addition, cavitation is found in all specimens above 1600 °C where strain-rate contributions agree with a stress exponent of n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.     

Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Sintering behaviour of alumina–niobium carbide composites

Ceramic cutting tools have been developed as a technological alternative to cemented carbides in order to improve cutting speeds and productivity. Al₂O₃ reinforced with refractory carbides improve fracture toughness and hardness to values appropriate for cutting applications. Al₂O₃–NbC composites were either pressureless sintered or hot-pressed without sintering additives. NbC contents ranged from 5 to 30 wt%. Particle dispersion limited the grain growth of Al₂O₃ as a result of the pinning effect. Pressureless sintering resulted in hardness values of approximately 13 GPa and fracture toughness around 3.6 MPa m^1/2. Hot-pressing improved both hardness and fracture toughness of the material to 19.7 GPa and 4.5 MPa m^1/2, respectively.     

Preparation of interpenetrating ceramic–metal composites

Ceramic reinforced metals are attractive because of their enhanced elastic modulus, high strength, tribological properties and low thermal expansion. Most work in this sector has focused on particle- or fiber-reinforced composites where the ceramic phase is not continuous. This work presents aluminium–alumina composites where both phases are interpenetrating throughout the microstructure. Ceramic preforms were produced with sacrificial pore forming agents leading to porosities between 50% and 67%. Pore wall microstructure was varied by changing the sintering temperature. Permeability and strength was measured for the porous preforms and infiltration results were compared with theoretical predictions based on capillary law and Darcian flow. A direct squeeze-casting process was used to infiltrate the preforms with aluminium resulting in an interpenetrating microstructure on both macropore and micropore scale.     

ZrB2–SiC laminated ceramic composites

The oxidation behavior for ZrB₂–20 vol% SiC (ZS20) and ZrB₂–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB₂–30 vol% SiC (ZS30)/ZrB₂–25 vol% SiC (ZS25)/ZrB₂–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m^1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics.     

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.     

Temperature-dependent mechanical and long crack behavior of zirconium diboride–silicon carbide composite

Hot pressed ZrB2–20 vol.% SiC ultra-high temperature ceramic composites have been prepared for strength and fracture investigations. Two composites fabricated under differing hot pressing temperatures with (ZSB) and without (ZS) B4C sintering aids were selected for room temperature modulus of rupture (MOR) strength and single-edge-notch bend (SENB) fracture toughness experiments. Structure property relationships were examined for both composites. MOR and stiffness temperature dependence was also investigated up to 1500 °C. Long crack propagation studies were conducted up to 1400 °C using the double cantilevered beam geometry with half-chevron-notch initiation zones. Residual Boron-rich carbide maximum particle sizes were found to be strength limiting in ZSB billets while SiC controlled strength in ZS billets. Flexure strength decreased linearly with temperature from 1000 to 1500 °C with no visible plastic deformation prior to fracture. Similar stiffness decreases were observed with a transition temperature range of 1100–1200 °C. Long crack studies produced R-curves that show no significant toughening behavior at room temperature with some modest rising R-curve behavior appearing at higher temperatures. These studies also show the plateau toughness increases with temperature up to 1200 °C. This is supported by an observed transition from primarily transgranular fracture at room temperature to primarily intergranular fracture at high temperatures. Wake zone toughening is evident up to 1000 °C with K_R rise from 0.1 to 0.5 MPa√m. Beyond 1000 °C fracture mechanism transitions to include creep zone development ahead of crack tip with wake zone toughening vanishing.     

Preparation of boron carbide–aluminum composites by non-aqueous gelcasting

Gelcasting is an attractive forming process to fabricate ceramic parts with near-net-shape. In the present work, non-aqueous gelcasting of boron carbide (B₄C)–aluminum (Al) composites was studied. A stable B₄C–Al slurry with solids loading up to 55 vol.% for gelcasting was prepared. The slurry was solidified in situ to green body with the mean value of relative density of 64% and flexural strength of 21 MPa. The SEM images showed that powders in green body compact closely by the connection of polymer networks. B₄C–Al samples were also obtained by the process of gelcasting and sintering at 1300 °C for 1 h in 0.1 MPa Ar atmosphere. The average bulk density of sintered body was 2.05 g cm⁻³.     

Crack healing in liquid-phase-pressureless-sintered silicon carbide–aluminum nitride composites

Crack healing in liquid-phase-pressureless-sintered SiC–AlN composites was investigated by introducing cracks into specimens and subsequently heat-treating the specimens. It was observed that cracks were healed and the strength was recovered. Cracks were filled with silica or mullite produced by the oxidation of the composites. It was shown that the healing temperature could be fixed in the range 1100–1300 °C and that large cracks up to about 300 μm could be healed completely. Our results imply that a simple oxidation heat-treatment can improve the reliability of silicon carbide–aluminum nitride components.     

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.     

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB₂ containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B₄C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB₂, 29.5 vol% SiC, and 2.0 vol% B₄C using image analysis. The average ZrB₂ grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ～360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m^½ at room temperature, increased to 4.8 MPa·m^½ at 800 °C, and then decreased linearly to 3.3 MPa·m^½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.     

Room temperature fatigue of ZrB2–SiC ceramic composites

Room temperature time dependent properties of ZrB₂–30 wt%SiC ceramic composite have been studied. Both static slow crack growth and cyclic fatigue deformation have been investigated. While static slow crack growth has been evaluated only in air, three different environments, water, air, and dry air, have been used to study the cyclic fatigue. It was established that under cyclic fatigue the environment plays an important role and humidity significantly facilitate crack growth in ZrB₂–30wt%SiC. The fractography of selected ZrB₂–30wt%SiC samples was performed and it was established that both defects introduced during machining as well as larger defects introduced during the processing served as fracture origins of ceramic composites.     

Methylene-bridged carbosilanes and polycarbosilanes as precursors to silicon carbide—from ceramic composites to SiC nanomaterials

Polymeric and oligomeric carbosilanes having Si atoms linked by methylene (CH₂) groups were used to prepare nano-sized tubules and bamboo-like SiC structures by both CVD and liquid precursor infiltration and pyrolysis inside of nanoporous alumina filter disks, followed by dissolution of the alumina template in HF_(aq). These initially amorphous SiC structures were characterized by SEM, EMPA, TEM, and XRD. Typical outer diameters of the SiC nanotubes (NTs) were 200–300 nm with 20–40 nm wall thicknesses and lengths up to the thickness of the original alumina templates, ca. 60 μm. In the case of the CVD-derived SiC NTs, annealing these structures up to 1600 °C in an Ar atmosphere yielded a nanocrystalline β-SiC or β-SiC/C composite in the shape of the original NTs, while in the case of the liquid precursor-derived nanostructures, conversion to a collection of single crystal SiC nanofibers and other small particles was observed.     

Microstructural characterization of ceramic–intermetallic composites using TEM related techniques

TiC_xN_y/Ti–Ni and TiC_xN_y/Ti–Co composites formed by ceramic and intermetallic binder phases were produced by pressureless sintering at 1400 °C from powders synthesized by a mechanically induced self-sustaining reaction (MSR) process. Four different composites were characterized using high-resolution electron microscopic techniques, in both scanning (SEM, HRSEM) and transmission (TEM, HRTEM, ED, EDS and EELS) modes and using an energy filtered technique (EFTEM) associated with electron energy loss spectroscopy (EELS). The microcharacterization showed that the ceramic phase with an fcc-cubic structure displayed a short-range order in many crystals detected by diffuse scattering in the ED patterns. This was possibly due to a sequence of C, N, and vacancies of both atoms along certain directions in the structure. On the other hand, even though the binder phase was introduced as metal in the reaction process, it was formed by Ni–Ti or Co–Ti known intermetallic compounds (NiTi₂, Ni₃Ti, and Co₃Ti). An unknown Ni–Ti intermetallic structure with a Ni:Ti ratio close to 2:1 was only found in one of the synthesized composites and displayed a cubic structure with a lattice parameter, a, of about 8.7 Å.     

Multilayered ceramic composites – a review

With the aim of improving the toughness of ceramic materials, laminated composites have been successfully developed since Clegg et al. (1990) inserted weak interfaces using very thin graphite layers between silicon carbide sheets and obtained a composite that exhibited non-catastrophic fracture characteristics. The weak interface must allow the crack to deviate either by deflection or delamination; in other words, the interface must exhibit a fracture resistance that is lower than that of the matrix layer. In parallel, ceramic laminated composites with strong interfaces were developed in which the residual tensile and compressive stresses appeared in alternate layers during cooling after sintering. These composites are prepared by stacking ceramic sheets produced by lamination or tape casting or by the sequential formation of layers by slip casting, centrifugation or electrophoretic deposition. The techniques may be combined to obtain a composite with the most adequate configuration. This work presents a review about the obtainment of multilayered ceramic composites as a toughening mechanism of ceramic plates.     

Sol–gel method and reactive SPS for novel alumina–graphene ceramic composites

Reinforced ceramic matrix composites of alumina and graphene oxide have been widely researched, but there are still unresolved issues such as the optimum distribution of the graphene or the presence of efficient bonds between filler and matrix. This work introduces a novel fabrication procedure based on the sol–gel method, using boehmite as an alumina precursor, and graphene oxide nanoplatelets as the reinforcing phase. Full densification of the samples was done through reactive spark plasma sintering under milder conditions than usual. Structural characterization was done by XRD, SEM and micro-Raman among other techniques, and the presence of Al-O-C bonds was studied by XPS. Mechanical characterization was performed by Vickers microindentation and nanoindentation. No significant change was observed concerning the Young’s modulus, hardness or fracture toughness, though improvements in the homogeneity of the distribution of the graphene and the chemical bonds between the matrix and the reinforcing phase were confirmed.