Effect of pyrolytic carbon coating on the microstructure and fracture behavior of the Cf/ZrB2-SiC composite

The high sintering temperature and interface interaction seriously degraded the toughening effects of continuous carbon fiber in ZrB₂-SiC ceramic. The pyrolytic carbon coated carbon fiber reinforced ZrB₂-SiC composite (C_f-PyC/ZrB₂-SiC) with desirable properties was successfully achieved via brushing nano ZrB₂-SiC slurry followed by spark plasma sintering at relatively low sintering temperature. The fabricated C_f-PyC/ZrB₂-SiC composite presented a non-brittle fracture feature and a remarkable enhancement in comparison with the ZrB₂-SiC composite reinforced by the as-received carbon fiber (C_f-AS/ZrB₂-SiC). The fracture toughness and critical crack size were increased from 5.97?±?0.18–7.66?±?0.24?MPa?m^1/2 and from 91.6 to 164.5?µm, respectively. A high work of fracture of 1915?J/m² for C_f-PyC/ZrB₂-SiC composite was achieved, almost four times higher than that of the C_f-AS/ZrB₂-SiC composite (463?J/m²). Multiple toughening mechanisms contributed to such enhancement, such as crack deflection, fiber bridging, fiber pull-out and crack branching. This work provides a feasible approach to fabricate high-performance fiber reinforced ceramic composites having a high work of fracture.     

Enhanced mechanical properties and thermal shock resistance of Cf/ZrB2-SiC composite via an efficient slurry injection combined with vibration-assisted vacuum infiltration

An efficient slurry injection combined with vibration-assisted vacuum infiltration process has been developed to fabricate 3D continuous carbon fiber reinforced ZrB₂-SiC ceramics. Homogenous distribution between carbon fiber and ceramic was achieved successfully, leading to an enhancement in mechanical properties. The C_f-PyC/ZrB₂-SiC composite exhibited a typical non-brittle fracture mode with a superior fracture toughness of 6.72 ± 0.21 MPa·m^1/2 and an extraordinary work of fracture of 2270 J/m², respectively, increasing by nearly 14.8 % and 36 % as compared with those of a parent composite fabricated by only slurry injection and slurry infiltration. The enhancement in fracture toughness and work of fracture were attributed to multiple toughening mechanism including crack deflection, PyC coated fiber bundles pull-out and fiber bridging. Moreover, a critical thermal shock temperature difference of 814 °C was achieved, higher than that of traditional ZrB₂-based ceramics. This work presents an efficient approach to fabricate high-performance C_f/UHTCs with uniform architecture.     

Characterization and mechanical properties of Cf/ZrB2-SiC composites fabricated by a hybrid technique based on slurry impregnation,polymer infiltration and pyrolysis and low-temperature hot pressing

The continuous carbon fiber reinforced ZrB₂-SiC composite was fabricated successfully via a hybrid technique based on nano ceramic slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing. The C_f/ZrB₂-SiC composites exhibited non-brittle fracture modes and the chemical interaction at the fiber/matrix interfaces was effectively inhibited owing to the low sintering temperature. The S2-C_f/ZrB₂-SiC composite presented the highest mechanical properties with fracture toughness of 4.47?±?0.15?MPa?m^1/2 and the work of fracture of 877?J/m², which was attributed to the multiple length-scale toughening mechanisms including the macroscopic toughening mechanisms of crack deflection and crack branching, the micro toughening mechanisms of fiber bridging and fiber pull-out. This work presented a novel and effective method to fabricate high-performance continuous carbon fiber reinforced ceramic matrix composites.     

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.     

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.     

Continuous carbon fiber reinforced ZrB2-SiC composites fabricated by direct ink writing combined with low-temperature hot-pressing

As one of additive manufacturing techniques, direct ink writing has significant advantages in the manufacture of ceramic matrix composites, nevertheless, the poor impregnability of ceramic slurry makes it difficult to fill the interior of fiber bundles, causing poor mechanical properties. Here, ultrasound-assisted fiber separation technique was introduced to impregnate ceramic slurry with a continuous carbon fiber bundle during direct ink writing of continuous carbon fiber/ceramic green body and subsequent low temperature hot-pressing was combined to improve its robustness. Suitable thickness of carbon coating could bring to high fracture resistance, whereas excessively thick carbon coating will adversely affect the mechanical properties. A carbon interface with thickness around 110 nm was incorporated, the flexural strength, fracture toughness and work of fracture of C_f/ZrB₂-SiC composite reached 388.3 MPa, 10.04 MPa·m^1/2 and 2380 J/m², respectively. Therefore, direct ink writing combined with low temperature hot-pressing, was effective to fabricate high-performance ceramic matrix composites.     

A novel vibration-assisted slurry impregnation to fabricate Cf/ZrB2-SiC composite with enhanced mechanical properties

The three dimensional needle-punched carbon fiber reinforced ZrB₂-SiC composite (C_f/ZrB₂-SiC) with highly uniform distribution was fabricated successfully via a novel vibration-assisted slurry impregnation and low-temperature (1450 °C) hot pressing technique using nanosized ZrB₂ powders. The carbon fiber/ceramic matrix interfaces were clear without obvious reaction products detected by the high resolution transmission electron microscopy (HR-TEM), indicating the degradation of carbon fiber was effectively inhibited. The C_f/ZrB₂-SiC composite exhibited a typical non-brittle fracture feature with a high work of fracture of 1104 J/m², which was approximately twice that of composite fabricated only by slurry impregnation and hot pressing. The enhancement in work of fracture was attributed to multiple toughening mechanisms of continuous carbon fibers such as extensive fiber bridging and pull-out accompanied by obvious crack deflection and branching. This work provides a valuable potential of preparing continuous carbon fiber reinforced ceramic composites with uniform component distribution and enhanced mechanical properties.     

Fabrication method and mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures

Herein, biomimetic C_f/ZrB₂-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C_f/ZrB₂-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C_f/ZrB₂-SiC ceramic films are coated with ZrB₂ and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C_f/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C_f/ZrB₂-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C_f/ZrB₂-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C_f/ZrB₂-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.     

Tough salami-inspired Cf/ZrB2 UHTCMCs produced by electrophoretic deposition

One of the biggest challenges of the materials science is the mutual exclusion of strength and toughness. This issue was minimized by mimicking the natural structural materials. To date, few efforts were done regarding materials that should be used in harsh environments. In this work we present novel continuous carbon fiber reinforced ultra-high-temperature ceramic matrix composites (UHTCMCs) for aerospace featuring optimized fiber/matrix interfaces and fibers distribution. The microstructures – produced by electrophoretic deposition of ZrB₂ on unidirectional carbon fibers followed by ZrB₂ infiltration and hot pressing – show a maximum flexural strength and fracture toughness of 330 MPa and 14 MPa m^1/2, respectively. Fracture surfaces are investigated to understand the mechanisms that affect strength and toughness. The EPD technique allows the achievement of a peculiar salami-inspired architecture alternating strong and weak interfaces.     

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.     

Microstructure and mechanical properties of reaction-hot-pressed zirconium diboride based ceramics

ZrB₂ ceramics were prepared by in-situ reaction hot pressing of ZrH₂ and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB₂-4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB₂-10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB₂ and ZrB₂–ZrC composition contained a residual oxide phase and ZrC inclusions, while the W-doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m^1/2 for the W-doped ZrB₂ compared to about 3.8 MPa m^½ for the ZrB₂ and ZrB₂–ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W-doped ZrB₂ compared with the other compositions. The study demonstrated that reaction-hot-pressing can be used to fabricate ZrB₂ based ceramics containing solid solution additives or second phases with comparable mechanical properties.     

Preparation and characterization of C/SiC–ZrB2 composites by precursor infiltration and pyrolysis process

Ultra-high temperature ceramic matrix composites (C/SiC–ZrB₂) are prepared by slurry and precursor infiltrations and pyrolysis method. C/SiC–ZrB₂ composites with ZrB₂ volume content from 10% to 24.6%, have balanced performance of fracture toughness (17.7–8.1 MPa m^1/2), flexural strength at room temperature (367–163 MPa) and at high temperature (strength retention 74% at 1800 °C and over 32% at 2000 °C), better oxidation and ablation resistance under oxyacetylene torch environment (recession rate 0.01 mm/s).     

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.     

Toughening and ablation mechanism of Si3N4 short fiber toughened ZrB2-based ceramics

ZrB₂-based ceramics with Si₃N₄ short fiber (ZSN) were prepared by wet-spinning extrusion and hot pressing. The toughness of ZSN was 5.6 MPa·m^1/2, which was 20% higher than that of monolithic ceramic (4.7 MPa·m^1/2). The ablation performance of ZSN was evaluated by air discharge plasma ablation platform with a heat flux of 8.04 MW/m² for 120 s. The mass and linear ablation rates of ZSN were − 0.19 mg/s and − 0.25 µm/s, respectively. The specimens of ZSN remained intact while monolithic ceramics exhibited destructive fracture. The better ablation performance of ZSN is attributed to the addition of Si₃N₄ short fiber which increased the fracture toughness, reduced the elastic modulus, and improved the thermal conductivity at high temperature.     

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).     

Microstructure and mechanical properties of multiwalled carbon nanotube toughened spark plasma sintered ZrB2 composites

ZrB₂ based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25?MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB₂-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2?MPa m^1/2 for the CNT toughened ZrB₂ ceramics, which is higher than the measured result for monolithic ZrB₂ (3.3?MPa m^1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms.     

Achieving synergy of load-carrying capability and damage tolerance in a ZrB2-SiC composite reinforced through discontinuous carbon fiber

Achieving synergy between load-carrying capability and excellent damage tolerance is one of the most concerned issues in the ultra-high temperature ceramic field. Herein, ZrB₂-SiC-C_f composite with homogenous architecture was constructed by a novel pressure filtration and slurry infiltration accompanied by low-temperature hot pressing. The composite exhibited a typical non-brittle fracture feature owing to crack deflection, crack bifurcation and fiber pull-out mechanisms, resulting in a reliable specific flexural strength of 75 MPa/(g/cm³) and superior work of fracture of 672 J/m².     

Enhanced fracture properties of ZrB2-based composites by in-situ grown SiC nanowires

To improve the fracture properties of ZrB₂-based composites, SiC nanowires (SiC_nw) were introduced into the powder mixtures via an in-situ growth method and the composites were fabricated by hot pressing. Microstructure observations found that the SiC_nw with a diameter of less than 100?nm presented twisted morphology and homogeneously distributed among ceramic particles. It was also found that the SiC_nw could inhibit the grain growth of ZrB₂-based composites. Compared to that of composites without SiC_nw, the fracture toughness and the work of fracture for the composites with SiC_nw were 7.17?MPa?m^1/2 and 205?J?m^?2, which increased by 61.1% and 91.6%, respectively. The main enhancing mechanisms could be associated with the obvious crack deflection, crack branching and pull out of SiC_nw. The SiC_nw could refrain the crack opening and reduce the stress intensity in front of crack tips, which absorbed more energy and improved the fracture properties.     

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.     

Experimental investigation on high strain rate compressive response of a ZrB2-SiC-G ceramic

The dynamic compression tests were conducted on a ZrB₂-SiC-graphite (ZrB₂-SiC-G) ceramic from the strain rate of 904–3136 s^–1 using the split Hopkinson pressure bar. The effects of strain rate on the compressive strength, critical strain, stress–strain relation, and fracture pattern were discussed from the experimental results. The results showed that the dynamic compressive response of this ZrB₂-SiC-G ceramic was obviously related to the strain rate at higher strain rates. At the strain rate of 3136 s^–1, the dynamic compressive strength, critical strain, and toughness of the ZrB₂-SiC-G ceramic increased to 1747 MPa, 0.0423, and 69.48 × 10⁶ J/m³, respectively. As the strain rate increased, the dynamic compressive strength and critical strain increased linearly, and the damage became more significant. Moreover, the energy absorption of the ZrB₂-SiC-G ceramic linearly increased with the strain rate, causing the ZrB₂-SiC-G ceramic fractured into numerous smaller fragments at higher strain rates.