Effect of multi-walled carbon nanotubes on microstructure and fracture properties of carbon fiber-reinforced ZrB2-based ceramic composite

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB₂-based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB₂ grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C_f/ZrB₂-based composite with MWCNTs were 7.0±0.4 MPa m^1/2 and 379±34 J/m², respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.     

ZrB2-based composites toughened by as-received and heat-treated short carbon fibers

In order to improve the fracture toughness of ZrB₂ ceramics, as-received and heat treated short carbon fiber reinforced ZrB₂-based composites were fabricated by hot pressing. The toughening effects of the fibers were studied by investigating the relative density, phase composition, microstructure and mechanical properties of the composites. It was found that the densification behavior, microstructure and mechanical properties of the composites were influenced by the fibers’ surface condition. The heat treated fiber was more appropriate to toughen the ZrB₂-based composites, due to the high graphitization degree, low surface activity and weak interfacial bonding. As a result, the fracture toughness of the composites with heat-treated fiber is 7.62 ± 0.12 MPa m^1/2, which increased by 10% as compared to the composites with as-received fiber (6.89 ± 0.16 MPa m^1/2).     

High-performance ZrB2-SiC-Cf composite prepared by low-temperature hot pressing using nanosized ZrB2 powder

High-performance ZrB₂-SiC-C_f composite was successfully prepared by low temperature (1450 °C) hot pressing using nanosized ZrB₂ powder. Such material exhibited a non-brittle fracture feature, high work of fracture (321 J/m²) and excellent thermal shock resistance as well as good oxidation resistance. Composite incorporating carbon fibers in which the degradation of the carbon fiber was effectively inhibited through low-temperature sintering displayed remarkably improved thermal shock resistance with a critical temperature difference of 754 °C, almost twice those of the reported ZrB₂-based ultra-high temperature ceramics. The thermal and chemical stability of the carbon fiber and ceramic matrix were further analyzed by thermodynamic calculation and HR-TEM analysis.     

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.     

Influence of sol-gel derived ZrB2 additions on microstructure and mechanical properties of SiBCN composites

A simple method for introducing ZrB₂ using sol-gel processing into a SiBCN matrix is presented in this paper. Zirconium n-propoxide (ZNP), boric acid and furfuryl alcohol (C₅H₆O₂) (FA) were added as the precursors of zirconia, boron oxide and carbon forming ZrB₂ dispersed in a SiBCN matrix. SiBCN/ZrB₂ composites with different contents of ZrB₂ (5, 10, 15, and 20 wt%) were formed at 2000 °C for 5 min by spark plasma sintering (SPS). The microstructures were carefully studied. TEM analysis showed that the as formed ZrB₂ grains were typically 100–500 nm in size and had uniform distribution. HRTEM revealed clean grain boundaries between ZrB₂ and SiC, however, a separation of C near the SiC boundary was observed. The flexural strength, fracture toughness, Young's modulus and Vicker's hardness of composites all improved with the ZrB₂ contents and SiBCN matrix containing 20 wt% of ZrB₂ could reach 351±18 MPa, 4.5±0.2 MPa m^1/2, 172±8 GPa and 7.2±0.2 GPa, respectively. The improvement in fracture toughness can be attributed to the tortuous crack paths due to the presence of reinforcing particles.     

Influence of milling time on the synthesis,microstructure and mechanical properties of ZrB2SiCZrO2f ceramic composites

Zirconium diboride toughened by silicon carbide and zirconia fiber (ZrB₂SiCZrO_2f) was prepared by using planetary ball mill and the effect of milling time was investigated. The results showed that both the length of fiber and particle size of ZrB₂SiC-matrix were reduced as the ball milling time increased. When milling time varied from 8 h to 12 h, the accumulated fibers and agglomerated particles were observed. The production of a homogeneous ceramic could be successfully achieved by using a combination of 20 h milling time and hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa. The optimal flexural strength and fracture toughness of the hot-pressed ZrB₂SiCZrO_2f ceramics reached 1084 MPa and 6.8 MPa m^1/2, respectively. The main toughening mechanisms were fiber debonding, fiber pull-out and transformation toughening. The results indicated that the ball milling technique was proposed as a potential and simple method to obtain usable quantities of ZrB₂SiCZrO_2f ceramic.     

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.     

Microstructure and mechanical behavior of the Cf/Ti3SiC2-SiC composites fabricated by compression molding and pressureless sintering

C_f/Ti₃SiC₂-SiC composites with different content of short carbon fibers were fabricated by the combination of compression molding and pressureless sintering. Microstructure and mechanical behavior of the composites were studied to evaluate the comprehensive performance of the material. In comparison, composites without carbon fibers were also fabricated in the same way. The results indicate that Ti₃SiC₂ phases were synthesized in each cases and exhibit typical laminated structure with smooth surface. With the increase of carbon fiber content, composites turn from brittle to toughness, and show obvious elastic and no-linear regions on the force-displacement curve. Moreover, composite with 30% (volume fraction) carbon fiber shows the highest flexural strength (284.03 MPa), open porosity (15.78%), and lowest density (2.37 g cm⁻³). There were chemical reactions occurred between carbon fibers and matrix which formed strong covalent bonds and interfaces. The micrographs also reveal that fiber bridging and pulling-out are the most important reinforcement mechanisms which contribute to the mechanical properties of the composites.     

Dynamic oxidation and protection of the PAN pre-oxidized fiber C/C composites

Pre-oxidized fibers as reinforcement are candidates for reducing the overall cost of C/C composites with superior properties. This study investigated the dynamic oxidation and protection of the pre-oxidized fiber C/C composites (Pr-Ox-C-C). According to the Arrhenius equation, the oxidation kinetics of the Pr-Ox-C-C consisted of two different oxidation mechanism with the transition point was at about 700 °C. Scanning electron microscopy investigation showed that oxidation initiated from the fiber/matrix interface of composites, whereas the matrix carbon was easily oxidized. To improve the anti-oxidant properties of Pr-Ox-C-C, a ceramic powder-modified organic silicone resin/ZrB₂-SiC coating was prepared by the slurry method. The coated samples were subjected to isothermal oxidation for 320 h at 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C with incurred weight losses of ? 1.6%, 0.77%, ? 1.28%, 0.68% and 1.19%, respectively. After 110 cycles of thermal shock between 1100 °C and room temperature, a weight loss of 1.30% was obtained. The Arrhenius curve presented four different phases and mechanisms for coating oxidation kinetics. The excellent oxidation resistance properties of the prepared coating could be attributed to the inner layer which was able to form B₂O₃-Cr₂O₃-SiO₂ glass to cure cracks, and the ZrB₂-SiC outer layer that could provide protective oxides to reduce oxygen infiltration and to seal bubbles.     

On the mechanical,thermophysical and dielectric properties of Nextel™ 440 fiber reinforced nitride matrix (N440/Nitride) composites

The Nextel? 440 fiber reinforced nitride matrix (N440/Nitride) composites were fabricated by precursor infiltration and pyrolysis (PIP) route. The results demonstrated that the original N440 fiber had a phase composition of amorphous SiO₂ and γ-Al₂O₃. Its single filament tensile strength was 3.03 GPa (at room temperature), while it dropped to 72.6% and 35.1% at 1200 °C and 1400 °C, respectively. The phase content of N440/Ntride composites was mainly γ-Al₂O₃ and amorphous BN, as well as mullite phase (formed at > 1100 °C). The composites owned a flexural strength up to 76.0 MPa at room temperature. The stair-stepping decrease in the load-displacement curve and fiber pull-outs in the fracture surface indicated a good fiber/matrix interface and toughness. By heating at 1400 °C, the composites still possessed 67.4% of original bending strength. It was found that the high temperatures caused strong fiber-matrix bonding and severe fiber degradation. The specific heat, CTE and thermal conductivity of the composites were 0.325–0.586 J g^?1 K^?1, (3.2–4.0) × 10^?6 K^?1 and 0.78–3.47 W m^?1 K^?1, respectively. The composites possessed a dielectric constant of 4.25–4.35 and loss tangent of 0.004–0.01 at 8–12 GHz. The good overall performances enabled the N440/Nitride composites advanced high-temperature wave-transparent applications.     

Fabrication of SiCw reinforced ZrB2-based ceramics

ZrB₂-based ceramics with SiC_w were produced by hot pressing at 1750 °C for 1 h from mixed powders after adding liquid polycarbosilane. The obtained ZrB₂-SiC_w composites had toughness up to 7.57 MPa m^1/2, which was much higher than those for monolithic ZrB₂, SiC particles reinforced ZrB₂ composites, and other ZrB₂–SiC_w composites directly sintered at high temperatures. The added liquid polycarbosilane could reduce the sintering temperatures and restrict the reaction of matrix with whisker, which led to fewer damages to the whisker and high fracture toughness.     

Fracture toughness of Al2O3 fibers with an artificial notch introduced by a focused-ion-beam

The present work was carried out to estimate the fracture toughness of two types of Al₂O₃ fibers (85Al₂O₃–15SiO₂, Altex^® (Sumitomo Chemical Co., Ltd) and α-Al₂O_3, Almax^® (Mitsui Mining Co., Ltd)) and to elucidate the transition from the intrinsic defects-induced fracture to introduced notch-induced one. With an application of the focused-ion (Ga⁺)-beam micromachining method, a mode I type straight-fronted edge notch with a notch-tip radius around 25 nm was introduced in fiber specimen. The fracture toughness K_Ic was estimated for each fiber specimen based on the fracture mechanical approach in which the measured values of notch depth, fiber diameter, fracture strength and calculated correction factor were substituted. The fracture toughness values of the 85Al₂O₃–15SiO₂ and α-Al₂O₃ fibers were estimated to be 1.86 ± 0.24 and 2.05 ± 0.13 MPa m^1/2, respectively. The fracture toughness value was almost independent of the fiber diameter and notch depth in both fibers tested. From the obtained fracture toughness value and the measured fracture strength of the original fiber, the notch depth at the transition from intrinsic defects-induced fracture to notch-induced one, corresponding to the equivalent size of the intrinsic defects that determines the strength of the original fiber, were estimated to be 0.3 and 0.8 μm for 85Al₂O₃–15SiO₂ and α-Al₂O₃ fibers, respectively.     

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.     

The fabrication and mechanical properties of SiC/ZrB2 laminated ceramic composite prepared by spark plasma sintering

Laminated SiC/ZrB₂ ceramic was fabricated by roll-compaction and spark plasma sintering at 1600 °C. A maximum fracture toughness of 12.3 ± 0.3 MPa m^1/2 was measured for the sintered SiC/ZrB₂ laminated ceramic. This significant improvement in fracture toughness can be attributed to the crack deflection along the interfacial layer and the presence of residual stresses in the sample. The effect of interlayer composition on the residual stresses was discussed in detail. It is observed that the residual thermal stress could be reduced by addition of ZrB₂ particles to the SiC interlayer. The bending strength can be increased to 388 ± 44 MPa with the addition of 20 vol% ZrB₂ to the SiC interlayer.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

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 mechanical properties of continuous carbon fiber-reinforced ZrB2-based composites via combined electrophoretic deposition and sintering

A process combining electrophoretic deposition (EPD) with hot pressing (HP) was developed to fabricate continuous carbon fiber-reinforced ZrB₂-based composites (C_f/ZrB₂-based composites). ZrB₂-based ultra-high temperature ceramic (UHTC) particles were uniformly pre-coated on continuous carbon fibers via EPD. Then, the UHTC-coated carbon fibers were stacked and hot pressed to prepare the C_f/ZrB₂-based composites. Microstructure observations revealed that almost no micro-pores were found in the inter-bundle and intra-bundle regions of fibers after HP. The flexural strength, fracture toughness and the work of fracture of the C_f/ZrB₂-based composite were measured as 199 ± 26 MPa, 6.71 ± 1.29 MPa·m^1/2, and 754 ± 58 J/m², respectively. Based on the observations of non-brittle fracture behavior, fractured morphology and crack propagation, the enhanced fracture properties were mainly attributed to the multiple toughening mechanisms, such as fiber pull-out, fiber bridging, crack deflection and branching along the interfaces.     

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.     

Ultra-high temperature ceramic coating for carbon/carbon composites against ablation above 2000 K

To improve the ablation resistance of carbon/carbon composites at the temperature above 2000 K, a ZrB₂-SiC-ZrC ultra-high temperature ceramic coating was prepared by combination of supersonic atmosphere plasma spray (SAPS) and reaction melt infiltration. The micro-holes in ZrB₂-Si-ZrC coating prepared by SAPS were effectively filled and the compactness and interface compatibility between the coating and C/C composites was improved through the reaction melt infiltration process. The ultra-high temperature ceramic coating exhibited good ablation resistance under oxyacetylene torch ablation above 2000 K. After ablation for 120 s, the mass and linear ablation rates of the ZrB₂-SiC-ZrC coated C/C samples were only ?0.016 × 10^?3 g/s and 1.30 µm/s, respectively. Good ablation resistance of the ultra-high temperature ceramic coating is mainly attributed to the dense coating structure and the improvement of interface compatibility between the coating and C/C composites.     

Mechanical behavior of two-step hot-pressed ZrB2-based composites with ZrSi2

In this study, fully dense ZrB₂-based composites containing ZrSi₂ were sintered using a two-step hot pressing process. The elastic moduli, fracture toughness and flexural strength of the hot-pressed composites were determined. The effects of ZrSi₂ content on densification behavior and properties of the composites were assessed. The results indicated that the ZrSi₂ improved the sinterability of ZrB₂ powders. Fully dense ZrB₂-based composites with ZrSi₂ were obtained at 1550 °C for 20–40 vol.% ZrSi₂-conatining ZrB₂ powders. The microstructure of the resulting composites was fine and homogeneous. The elastic moduli, fracture toughness and flexural strength of the obtained composites depended on ZrSi₂ content. The shear, Young's, and bulk moduli decreased with ZrSi₂ content. The range of fracture toughness values was measured to be 3.8–4.8 MPa m^1/2. The flexural strength, which was 556 MPa, was almost the constant for ZrSi₂ content of 30 vol.% or less. For 40 vol.% ZrSi₂, however, the strength lowered significantly to 382 MPa.