Toughening effect of short carbon fibers in the ZrB2–ZrSi2 ceramic composites

The toughening effect of the short carbon fibers in the ZrB₂–ZrSi₂ ceramic composites were investigated, where the ZrB₂–ZrSi₂ ceramics without carbon fibers were used as the reference. The mechanical properties were evaluated by means of flexural and SENB tests, respectively. The microstructure was characterized by SEM equipped with EDS. The results found that the short carbon fibers were uniformly incorporated in the ZrB₂–ZrSi₂ matrix and the relative density was about 97.92%. The flexural strength of short carbon fiber-reinforced ZrB₂–ZrSi₂ composites is 437 MPa; the fracture toughness and the work of fracture are 6.89 MPa m^1/2 and 259 J/m², respectively, which increased significantly in comparing with composites without fibers. The microstructure analysis revealed that the improved fracture toughness could be attributed to the fiber bridging, the fiber–matrix interface debonding and the fiber pullout, which consumed more fracture energy during the fracture process.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Characterization of hot-pressed short ZrO2 fiber toughened ZrB2-based ultra-high temperature ceramics

Two different ZrB₂-based ultra-high temperature ceramics were produced by hot pressing: ZrB₂ + 20 vol.% SiC particle + 15 vol.% ZrO₂ fiber and ZrB₂ + 20 vol.% SiC whisker + 15 vol.% ZrO₂ fiber. The microstructures were analyzed by using transmission electron microscopy and high-resolution transmission electron microscopy. It was shown that a clean interface without any impurities was identified in ZrB₂-based hybrid ceramics with SiC whiskers and ZrO₂ fibers, which would significantly improve the toughening mechanism. The results of high-resolution transmission electron microscopy showed that stacking faults in SiC whiskers resulted from an insertion of a (111) layer, which would be one of the main reasons for material anisotropy. However, the interface between the SiC particle and ZrO₂ fiber was found to be ambiguous in ZrB₂-based hybrid ceramics with SiC particles and ZrO₂ fibers due to the slight reaction. The orientation relationship between t-ZrO₂ and m-ZrO₂ phases obeyed the classical correspondence: (100)_m//{100}_t and [001]_m//〈001〉_t, which further verified the feasibility of phase transformation toughening mechanism.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Effects of sintering velocity on the microstructure and mechanical properties of hot-pressed ZrB2–SiC–ZrO2f ceramics

The effects of cooling and heating velocity on the microstructure and mechanical properties of the hot-pressed ZrB₂–20 vol.% SiC ceramics added with 15 vol.% Zirconia fiber were investigated in details. It was indicated that the appropriate cooling velocity could reduce the residual stress concentration, and the proper heating velocity could allowed the green compacts to get densified, which was favorable to enhance the flexural strength and fracture toughness to 1117 ± 98 MPa and 7.1 ± 0.5 MPa m^1/2, respectively. Due to the introduction of the Zirconia fiber, the observed toughening mechanisms were attributed to the fiber debonding, fiber pull-out and phase transformation toughening. The results presented here point out a promising way for improving the mechanical properties of ZrB₂-based ultra-high temperature ceramics.     

Preparation of ZrB2 based composites by reactive melt infiltration at relative low temperature

ZrB₂ based composites were prepared by a novel reactive melt infiltration process. The porous boron bars were used as preforms and infiltrated by the low melting Zr₂Cu intermetallic compound. Thermodynamics calculations revealed that B could react with liquid Zr₂Cu to form ZrB₂ as low as 1100 °C. Composites were prepared by heating the two materials to 1200 °C for 3 h in vacuum. The resultant composites were studied with XRD and SEM. ZrB₂ was identified to be the main constituent. The composites had a flexural strength of 414.3 MPa, a flexural modulus of 183.6 GPa, and a fracture toughness of 5.5 MPa m^1/2.     

Effect of supercritical water applied to reactor on microstructure and flexural strength of ZrB2–SiC–Graphite ceramic

The ZrB₂–SiC–Graphite ceramic was immerged in the supercritical water for different times. The microcracks appeared on the surface of the specimen and the composition of the microcracks was confirmed by EDS analysis to be ZrO₂. The obvious corrosion of SiC and graphite flake on the surface of the specimen was not observed, which was attributed to the dissolution of the oxides of silicon and carbon (graphite) into the supercritical water. The corrosion of the specimen was accelerated as the pressure and temperature of the water increased. XPS analysis was carried out on the specimen corroded in water of 40 ± 1 MPa and 500 ± 10 °C for 75 min, and the significant peak of B 1s was also measured, indicating the presence of the ZrB₂ phase on the surface of the specimen. For the specimen immerged in all conditions, the Vickers’ hardness did not reduce, the fracture toughness was improved and the minimal strength of the immerged specimen was still higher than 90% of the original strength of 480 MPa, which indicated that the ZrB₂–SiC–Graphite ceramic has excellent resistance to corrosion of the supercritical water applied to reactor.     

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Effect of environment atmosphere on thermal shock resistance of the ZrB2–SiC–graphite composite

The thermal shock resistance of the ZrB₂–SiC–graphite composite was evaluated by measuring the retention of the flexural strength after the electrical resistance heating to the temperature ranging from 1000 °C up to 2500 °C. The experiment was operated in two different environment atmospheres (pure oxygen and low oxygen partial pressure which mixed O₂ and Ar with 1:9) at total pressure 2000 Pa. The residual strength for the specimen decreased gradually as the temperature increased up to 2200 °C, and it was slightly higher when heated in low oxygen partial pressure environment than in pure oxygen. In contrast to the specimen heated in low oxygen partial pressure environment, the residual strength for the specimen in pure oxygen increased steeply as the temperature increased from 1600 °C up to 1800 °C. The analysis of the SEM observations combined with EDS confirmed that the surface oxidation played a positive role in the thermal shock resistance of the ZrB₂–SiC–graphite composite with different environment atmospheres. The results here pointed out a potential method for charactering the effect of environment atmosphere on thermal shock resistance of the ZrB₂–SiC–graphite composite.     

Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route

Silicon carbide (SiC) interphase was introduced by chemical vapor deposition (CVD) process to prevent carbon fiber degradation and improve fiber–matrix interface bonding of C/ZrC composite prepared via precursor infiltration and pyrolysis (PIP) process. Moderate thickness of SiC interphase in fiber bundles could increase the density of the composite, but when the thickness of SiC interphase was over 0.5 μm, more close pores formed and the density of the composite decreased. The SiC interphase could protect carbon fiber effectively from carbo-thermal reduction, but could not enhance the mechanical properties of C/ZrC composite. The flexural strength and fracture toughness of C/ZrC composites with 0.05 μm thickness SiC layer were 252 MPa and 13.6 MPa m^1/2, and for those with 0.5 μm thickness SiC layer 240 MPa and 12.8 MPa m^1/2, both close to the value of the composite without SiC interphase (254 MPa and 14.5 MPa m^1/2), while those with 0.7 μm thickness SiC layer were only 191 MPa and 10.8 MPa m^1/2, respectively. Moderate content of SiC interphase could improve the ablation property of C/ZrC composites; however excessive content of SiC interphase would decrease the ablation property.     

Effects of polymer derived SiC interphase on the properties of C/ZrC composites

Polymer derived silicon carbide (SiC) interphase was introduced by precursor infiltration and pyrolysis (PIP) to prevent carbon fiber erosion and to improve the fiber–matrix interface bonding of C/ZrC composites prepared by PIP. Introducing SiC interphase increased the density of the composites. The SiC interphase not only protected carbon fibers effectively from erosion by carbo-thermal reduction, but also enhanced the mechanical properties of C/ZrC composites by strengthening the interface bond. The flexural strength and fracture toughness of C/ZrC composites with SiC interphase prepared by two PIP cycles were 319 MPa and 18.8 MPa m^1/2 respectively. The ablation properties of C/ZrC composites were with rising content of SiC interphase but then decreased when excessive. The mass loss rate and the linear recession rate of the C/ZrC composites with SiC interphase prepared by one PIP cycle were 0.0079 g/s and 0.0084 mm/s, respectively.     

Structure property relation of hybrid biocomposites based on jute,viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties

In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (K_IC) and the fracture energy (G_IC) of the PP–J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP–J30–V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa m^1/2 and 28.9 to 31.2 kJ/m², respectively. Similarly, the fatigue life increased 51% compared with the PP–J30–V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.     

Construction and characterization of stainless steel/polyurea/E-glass composite joints

This work was motivated by the desire to improve the long-term durability of E-glass/Hysol 9394 epoxy/Al-6XN stainless steel joints whose fracture energy in a previous study was shown to fall from ∼950 J/m² (ambient) to a mere ∼88 J/m² after only two days of exposure to a 90%RH, 50 °C environment. This paper reports a new polyurea chemistry to bond the E-glass and steel sections with very promising results. The fracture energy under ambient condition was measured to be 1232 ± 15 J/m² using a double cantilever beam experiment. This value degraded by only 13% to 1070 ± 35 J/m² in samples that were conditioned for 30 days at 90%RH and 50 °C. The failure in all samples was cohesive, within the first ply of the E-glass composite that neighbored the polyurea interface. The intrinsic fracture energy measurements, devoid of inelastic effects, were also carried out by submerging samples in a liquid nitrogen bath. Values of 590 ± 25 J/m² were obtained, which can be used by the designers to set the local failure condition in design simulations of large-scale structures.     

In situ synthesis and properties of Zr2Al3C4/ZrB2 composites

The Zr₂Al₃C₄/ZrB₂ composites are in situ synthesized by spark plasma sintering using Zr, Al, graphite, and B₄C powders as the initial materials. The introduction of ZrB₂ can not only evidently hinder the coarsening of Zr₂Al₃C₄ grains, but also benefit the densification and improve the hardness and Young’s modulus of the Zr₂Al₃C₄/ZrB₂ composites. When the ZrB₂ content is 20 vol.%, the composite shows an optimum fracture toughness value of 4.37 MPa m^1/2, about 20% higher than that of the monolithic Zr₂Al₃C₄. The unique mechanical properties can be mainly ascribed to the contribution of ZrB₂ as the reinforcing phase hindering the crack propagating. Compared with Zr₂Al₃C₄, the Zr₂Al₃C₄/20 vol.%ZrB₂ composite also exhibits a relatively higher thermal conductivity and better oxidation resistance.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Microstructures and fracture toughness of directionally solidified Mo–ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (Mo_SS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness K_Q evaluated by three point bending test using the single edge notched beam method is >13 MPa m^1/2 for AMC, 20 MPa m^1/2 for SMC and 9 MPa m^1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of Mo_SS phase in FZC leads to brittle {100} cleavage fracture associated with low fracture toughness.     

Mechanical properties and toughening mechanism of WC-doped ZrB2–ZrSi2 ceramic composites by hot pressing

WC-doped ZrB₂–ZrSi₂ ceramic composites were fabricated by hot pressing at temperatures ranging from 1450 °C to 1550 °C. The influence of ZrSi₂ content on the mechanical properties of the composites was investigated by means of three point bending test and single edge notched-beam test, respectively. The microstructure and phase composition were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The results revealed that: (i) the highest relative density was 99.5% for the composite fabricated at 1550 °C; (ii) the doping of WC refined the grain size and led to an anisotropic grain growth which was evidenced by the occurrence of elongated grains; (iii) the highest strength and fracture toughness were 585 MPa and 6.87 MPa m^1/2, respectively; (iv) the main toughening mechanism was considered as the pull out of elongated grains and the deflection of cracks.     

From random chopped to oriented continuous SiC fibers–ZrB2 composites

ZrB₂–continuous SiC fiber composites were prepared by vacuum-bag infiltration and hot pressing, using homemade 1D fabric preforms of Tyranno SA3 SiC fibers. Sintering behavior and microstructural features such as secondary phases and matrix/fiber interface were compared to those of chopped SiC fibers-reinforced composites. The infiltration process allowed the overall fiber content to be increased up to 40 vol%, because of the ordered arrangement of fibers. When the fiber preforms were properly infiltrated, the composites were nearly fully dense and the densification mechanisms were the same as those of unreinforced matrices. Different from composites containing short discontinuous fibers, the degree of chemical interaction at the fiber/matrix interface was very limited and this resulted in an easier pull out in the fractured surfaces, even in absence of fiber protective coating.