Fabrication of laminated SiCw/SiC ceramic composites by CVI

Laminated SiC_w/SiC ceramic composites were prepared by chemical vapor infiltration (CVI) and tape casting, and the advantages of this method were investigated. The results showed that this method can increase strength of the composites by reducing the damage of SiC whiskers and increasing their volume fraction, and increase toughness of the composites by controlling the interfacial bonding between whiskers and matrix and inter-laminar bonding between layers. The volume fraction of whiskers reached 40 vol.%, and the flexural strength, tensile strength and fracture toughness were 315 MPa, 158 MPa and 8.02 MPa m^1/2, respectively. Interfacial and interlaminar crack deflection and bridging were observed.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Strengthening and stiffening carbon fiber epoxy composites by halloysite nanotubes,carbon nanotubes and silicon carbide whiskers

Low-cost natural halloysite (Hal) nanotubes (0.1 μm diameter) were effective for strengthening and stiffening continuous fiber epoxy composites, as shown for cross-ply carbon fiber (5 μm diameter, ~ 59 vol.%) epoxy nanocomposites under flexure, giving 17% increase in strength, 11% increase in modulus and 21% decrease in ductility. They were less effective than expensive multiwalled carbon nanotubes (0.02 μm diameter), which gave 25% increase in strength, 11% increase in modulus and 14% decrease in ductility. However, they were more effective than expensive silicon carbide whiskers (1 μm diameter), which gave 15% increase in strength, 9% increase in modulus and 20% decrease in ductility. Each filler, at ~ 2 vol.%, was incorporated in the composite at every interlaminar interface (interface between adjacent fiber laminae) by fiber prepreg surface modification. The flexural strength increase due to Hal nanotubes incorporation corroborated with the interlaminar shear strength increase. The measured values of the composite modulus agreed roughly with the calculated values based on the Rule of Mixtures. The interlaminar interface thickness was higher for the SiC whiskers case than the carbon nanotubes or Hal nanotubes case. The lamina thickness was not affected by the fillers. The composite density was 2% higher for the Hal nanotubes and SiC whiskers cases than the carbon nanotubes case.     

SiC whisker reinforced multi-carbides composites prepared from B4C and pyrolyzed rice husks via reactive infiltration

SiC whisker reinfored carbide-based composites were fabricated by a reactive infiltration method by using Si as the infiltrate. Rice husks (RHs) were pyrolyzed to SiC whiskers, particles and amorphous carbon, and were then mixed with different contents of B₄C as well as Mo powders. The mixtures were molded to porous preforms for the infiltration. The SiC whiskers and particles in the preform remained in the composite. Molten Si reacted with the amorphous carbon, B₄C as well as Mo in the preform during the infiltration, forming newly SiC, B₁₂(C,Si,B)₃ as well as MoSi₂. The upper values of elastic modulus, hardness and fracture toughness of the composites are 297.8 GPa, 16.8 ± 0.8 GPa, and 3.8 ± 0.2 MPa m^1/2, respectively. The influence of the phase composition of the composites on the mechanical properties and the fracture mechanism are discussed.     

Toughened ZrB2-based ceramics through SiC whisker or SiC chopped fiber additions

In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB₂ matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730 °C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcement.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks and SiC powder

A reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks (RHs) and SiC powder in different ratios is reported, in which FeSi₂ alloy was used as infiltrant. The preforms were heat-treated at 1550 °C for 6 h prior to the infiltration. The coked RHs, which are composed of SiO₂ and C, were converted to SiC and poorly crystallized C by carbothermal reduction during the heat treatment. The study of the microstructure and mechanical properties of the composites shows that molten Fe–Si alloy had good wetting of the heat-treated preforms and adequate infiltration properties. Free carbon in the preform reacted with Si in the molten FeSi₂ during infiltration forming new SiC, the composition of the intermetallic liquid being moved towards that of FeSi. As a result, the infiltrated composites are composed of SiC, FeSi₂ and FeSi phases. Vickers hardness, elastic modulus, three-point flexural strength and indentation fracture toughness of the composites are found to increase with SiC additions up to 30% w/w in the preforms, reaching the values of 18.2 GPa, 290 GPa, 213 MPa and 4.9 MPa m^1/2, respectively. With the SiC addition further raised to 45% w/w, the elastic modulus, flexural strength and fracture toughness of the composite turned down probably due to high residual stress and hence the more intense induction of microcracks in the composite. De-bonding of SiC particles pulled out of the Fe–Si matrix, transgranular fracture of part of the SiC particles and in the Fe–Si matrix, and crack bridging all exist in the fracture process of the composites.     

Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite

Hot pressed monolithic ZrB₂ ceramic (Z), ZrB₂–20 vol% SiC composite (ZS₂₀) and ZrB₂–20 vol% SiC–10 vol% nano-graphite composite (ZS₂₀G_n10) were investigated to determine the influence of graphite nano-flakes on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB₂–SiC composites. Hot pressing at 1850 °C for 60 min under 20 MPa resulted in a fully dense ZS₂₀G_n10 composite (relative density: 99.6%). The results disclosed that the grain growth of ZrB₂ matrix was efficiently hindered by SiC particles as well as graphite nano-flakes. The fracture toughness of ZS₂₀G_n10 composite (7.1 MPa m^1/2) was essentially improved by incorporating the reinforcements into the ZrB₂ matrix, which was greater than that of Z ceramic (1.8 MPa m^1/2) and ZS₂₀ composite (3.8 MPa m^1/2). The fractographical observations revealed that some graphite nano-flakes were kept in the ZS₂₀G_n10 microstructure, besides SiC grains, which led to toughening of the composite through graphite nano-flakes pull out. Other toughening mechanisms such as crack deflection and branching as well as crack bridging, due to the thermal residual stresses in the interfaces, were also observed in the polished surface.     

Effect of graphene oxide on the interlaminar fracture toughness of carbon fiber/epoxy composites

Graphene oxide (GO) nanoparticles were introduced in the interlaminar region of carbon fiber–epoxy composites by dispersing it in a thermoplastic polymer carrier such as polyvinylpyrrolidone (PVP). Mode‐I fracture toughness (G_IC) was investigated using double cantilever beam testing to evaluate the effect of the GO on the delamination behavior of the composite. The GO content was varied from 0% to 7% by weight as a function of the PVP content. Improvement of ～100% in the Mode I fracture toughness (G_IC) was observed compared to composites with no GO. The optimum amount of nanoparticles for improving the interlaminar fracture toughness was found to be ～0.007% by weight of the composite. The increase in the value of flexural strength value was also observed. Scanning electron microscopy of fracture surfaces, X‐ray diffraction, and transmission electron microscopy, and reflectance Fourier transform infrared spectra, as well as Raman spectroscopy results, are presented to support the conclusions. POLYM. ENG. SCI., 59:1199–1208 2019. © 2019 Society of Plastics Engineers     

Establishing microstructure-mechanical property correlation in ZrB2-based ultra-high temperature ceramic composites

The current work focuses on enhancing the flexural strength and fracture toughness of zirconium diboride (ZrB₂) reinforced with silicon carbide (SiC) and carbon nanotubes (CNT). The flexural strength has shown to increase by ~ 1.2 times from 322.8 MPa (for ZrB₂) to 390.7 MPa and fracture toughness up to 3 times from 3.2 MPam^0.5 (for ZrB₂) to 9.5 MPam^0.5 with the synergistic addition of both SiC and CNT in ZrB₂ matrix through energy dissipating mechanisms such as deflection, branching and strong interfacial bonding evidenced from the transmission electron microscopy (TEM). A modified fractal model is used to evaluate the fracture toughness and delineate the contribution of residual stresses, and reinforcements (SiC and CNT) in enhancing the fracture toughness. Interfacial bonding, in terms of a debonding factor, was also evaluated by theoretically predicting the elastic modulus and then correlated with the microstructure along with other mechanical properties of ZrB₂-SiC-CNT composites.     

Mechanical and physical behaviors of short pitch-based carbon fiber-reinforced HfB2–SiC matrix composites

Short Pitch-based carbon fiber-reinforced HfB₂ matrix composites containing 20 vol% SiC, with fiber volume fractions in the range of 20–50%, were manufactured by hot-press process. Highly dense composite compacts were obtained at 2100 °C and 20 MPa for 60 min. The flexural strength of the composites was measured at room temperature and 1600 °C. The fracture toughness, thermal and electrical conductivities of the composites were evaluated at room temperature. The effects of fiber volume fractions on these properties were assessed. The flexural strength of the composites depended on the fiber volume fraction. In addition, the flexural strength was significantly greater at 1600 °C than at room temperature. The fracture toughness was improved due to the incorporation of fibers. The thermal and electrical conductivities decreased with the increase of fiber volume fraction, however.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

Effect of physical aging on the toughness of carbon fiber-reinforced poly(ether ether ketone) and poly(phenylene sulfide) composites. I

The effect of physical aging on the penetration impact toughness and Mode I interlaminar fracture toughness of continuous carbon fiber (C.F.) reinforced poly(ether ether ketone) (PEEK) and poly(phenylene sulfide) (PPS) composites has been investigated by using an instrumented falling weight impact (IFWI) technique and a double cantilever beam (DCB) test. Composite materials studied are aged below their glass transition temperature (T_g) at various periods. Initiation force and energy of damage, failure propagation energy, impact energy and ductility index (D.I.) are reported. The Mode I critical value of strain energy release rate (G_IC) of the unidirectional carbon fiber-reinforced PEEK (APC-2) composites is obtained. Results show that aging has a significant effect on the toughness of both composite materials. Energy absorbed during impact decreases with the increase of aging temperature and period. The PEEK/C.F. composites exhibit a higher retention of impact toughness than that of the PPS/C.F. composites after aging; however, the PPS/C.F. composites show a much higher ductility index. The Mode I fracture mechanism of the APC-2 composite is a combination of stable and unstable failure and shows a “stick-slip” behavior. Owing to the formation of a relative rigid structure, the fracture toughness (G_IC) of APC-2 decreased with the increase of aging temperature and period.     

Densification,microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

Fabrication and mechanical properties of SiC composites toughened by buckypaper and carbon fiber fabrics alternately laminated

SiC ceramics, for the first time, were toughened with nano scale carbon nanotubes (CNTs) buckypapers and micro scale carbon fibers within this work. The CNTs buckypapers were alternately laminated with carbon fiber fabrics (C_fb) to a preform by needle punched in Z-direction. Afterwards, the buckypaper-C_fb/SiC composites were obtained by infiltrating of SiC into the as-laminated preform via chemical vapor infiltration (CVI). Some effects of different lamination thickness and CVI times on the mechanical properties of the composites were investigated. Results showed that the maximum flexural strength and work of fracture of the buckypaper-C_fb/SiC composites reached 262.4 MPa and 4.15 kJ m⁻², respectively, when the thickness reached about 3.50 mm. Compared to C_fb/SiC composites without buckypapers, the strength and work of fracture of the buckypaper-C_fb/SiC composites increased by 19.8% and 111.7%, respectively. Densified composites can be obtained after CVI for 8 times. A main factor affecting the mechanical properties of buckypaper-C_fb/SiC composites is the degree of densification. Introducing nano scale CNTs and micro scale carbon fibers reaches a multiscale co-toughening effect. Meanwhile, a sandwich structure ceramic matrix composite with high-CNT concentration was obtained in this work.     

Synthesis and mechanical properties of (Mo0.94Nb0.06)(Si0.97Al0.03)2−x vol% SiC composites fabricated via SHS-HP

(Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂–SiC composites were consolidated via vacuum HP sintering from the mixture of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder and SiC nanoparticles. The (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder was prepared by self propagating synthesis (SHS). The effects of SiC content on the ambient temperature mechanical properties and microstructure of the SHS-HP composites were investigated. The results show that the indentation fracture toughness, bending strength and Vickers hardness of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−x vol% SiC composites increase gradually with the increase of SiC content. The grain size and relative density of the composites decrease gradually with increase of SiC content. (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−15 vol% SiC composite exhibits excellent mechanical properties: Vickers hardness 12.62 GPa, fracture toughness 5.01 MPa m^1/2, bending strength 472 MPa. With the addition of SiC, the fracture mode transforms from a mixed mode of transcrystalline and intercrystalline fracture to mainly transcrystalline fracture.     

Fracture characteristics of SiC/graphene platelet composites

Silicon carbide/graphene platelet (SiC/GPLs) composites were prepared using different weight percent of GPLs filler by hot pressing (HP) technology at 2100 °C in argon. The influence of the GPLs addition on bending strength, fracture toughness and related fracture characteristics was investigated. Both the bending strength and fracture toughness increased with increasing GPLs additives. The main fracture origins – strength degrading defects were pores at the low content of platelets and combination of pores and GPLs or clusters of GPLs particles in systems with a higher content of platelets. The fracture toughness increased due to the activated toughening mechanisms mainly in the form of crack bridging and crack branching, while the crack deflection was limited. The highest fracture toughness of 4.4 MPa m^1/2 was achieved at 6 wt.% of GPLs addition, which was ∼30% higher than the K_IC value of the reference material.     

Spark plasma sintering of TiC-based composites toughened by submicron SiC particles

TiC-based composites toughened by submicron SiC particles with improved fracture toughness were fabricated and fracture mechanism has been investigated. It has been found that the improvement in fracture toughness of TiC–SiC composites is due to both crack paths propagating through uniformly distributed SiC particles and the fracture mode transition from intergranular type to transgranular type caused by the change of residual stresses originating from the addition of SiC particles. The optimum of fracture toughness (5.2 MPa m^1/2) was achieved at 14.6 vol% SiC, whereas the toughness decreased with increasing amount of SiC beyond 30 vol%.