Slurry Impregnation Research Articles

Ablation behaviour of Cf–ZrC-SiC with and without rare earth metal oxide dopants

Continuous carbon fibre/carbon composites are candidates for ultra-high temperature applications due to their ability to retain their strength at elevated temperatures, however they easily oxidize and ablate in high temperature oxygen-based environments, a fact that seriously limits their applications in the aerospace industry. The introduction of ultra-high temperature ceramics (UHTCs) as a matrix in carbon fibre preforms effectively improves their oxyablation resistance at ultra-high temperatures in air atmospheres. In the present study, zirconium carbide-based ultra-high temperature ceramic matrix composites (UHTCMCs) simultaneously doped with silicon carbide and rare earth (RE) metal oxides were prepared by a slurry impregnation and pyrolysis method using phenolic resin as the bonding agent. The combination of injection and vacuum impregnation achieved a maximum density of up to 60% of theoretical. These composites were subsequently evaluated in terms of their ablation properties using an oxyacetylene (OAT) torch at temperatures of ∼2500 °C for 60 s. No significant surface damage was observed on any of the ablated samples. When ceria was used as the RE metal oxide dopant, the evidence suggests that cerium silicate was formed by the ceria dissolving in the molten silica; though this didn't appear to offer very much additional oxyablative protection. In contrast, when yttria was used, it improved the performance via stabilization of the cubic zirconia that formed on oxidation; this provided the greatest protection of all the samples tested. The combination of both ceria and yttria yielded the combined mechanism.

Ablation behavior of a C/C-ZrC-SiC composite based on high-solid-loading slurry impregnation under oxyacetylene torch

A C/C-ZrC-SiC composite was successfully prepared by high-solid-loading slurry impregnation combined with polymer infiltration and pyrolysis. The microstructure and ablation behavior of the C/C–ZrC–SiC composite were investigated. ZrC particles were uniformly distributed in the matrix, and the obtained C/C–ZrC–SiC composite had a high density of 2.74 g/cm3. After exposure to oxyacetylene flame with a heat flux of 3.86 MW/m2 for 120 s, the mass and linear ablation rates of the composite were 0.72 ± 0.11 mg/s and 0.52 ± 0.09 µm/s, respectively. The excellent ablation properties of the composite were attributed to the protection of the matrix by a three-layered oxide scale consisting of ZrO2/SiO2-rich/ZrO2-SiO2.

Thermal Qualification of the UHTCMCs Produced Using RF-CVI Technique with VMK Facility at DLR

Ultra high-temperature ceramic matrix composites (UHTCMCs) based on carbon fibre (Cf) have been shown to offer excellent temperature stability exceeding 2000 °C in highly corrosive environments, which are prime requirements for various aerospace applications. In C3Harme, a recent European Union-funded Horizon 2020 project, an experimental campaign has been carried out to assess and screen a range of UHTCMC materials for near-zero ablation rocket nozzle and thermal protection systems. Samples with ZrB2-impregnated pyrolytic carbon matrices and 2.5D woven continuous carbon fibre preforms, produced by slurry impregnation and radio frequency aided chemical vapour infiltration (RF-CVI), were tested using the vertical free jet facility at DLR, Cologne using solid propellants. When compared to standard CVI, RFCVI accelerates pyrolytic carbon densification, resulting in a much shorter manufacturing time. The samples survived the initial thermal shock and subsequent surface temperatures of >2000 °C with a minimal ablation rate. Post-test characterisation revealed a correlation between surface temperature and an accelerated catalytic activity, which lead to an understanding of the crucial role of preserving the bulk of the sample.

Effect of a Support on the Properties of Zinc Oxide Based Sorbents.

We present the comparative analysis of three Zn-based sorbents for the process of sulphur removal from hot coal gas. The sorbents were prepared by a slurry impregnation of TiO2, SiO2 and Al2O3, resulting in complex, multiphase materials, with the dominant phases of Zn2TiO4, Zn2SiO4 and ZnAl2O4, respectively. We have analyzed the effect of supports on the phase composition, texture, reducibility and H2S sorption. We have found that the phase composition significantly influences the susceptibility of the investigated materials to reduction by hydrogen. Zn2TiO4 have been found to be the easiest to reduce which correlates with its ability to adsorb the largest amount of hydrogen sulphide—up to 4.2 gS/100 g—compared to the other sorbents, which absorb up to 2.2 gS/100 g. In the case of Zn2SiO4 and ZnAl2O4, this effect also correlates with reducibility—these sorbents have been found to be highly resistant to reduction by hydrogen and to absorb much less hydrogen sulphide. In addition, the capacity of ZnAl2O4 for H2S adsorption decreases in the subsequent work cycles—from 2.2 gS/100 g in the first cycle to 0.8 gS/100 g in the third one. Computational analysis on the DFT level has shown that these materials show different thermodynamic stability of sulphur sites within the unit cells of the sorbents. For Zn2TiO4 and Zn2SiO4, the adsorption is favorable in both the first and second layers of the former and only the top layer of the latter, while for zinc aluminate it is not favorable, which is consistent with the experimental findings.

Fabrication and microstructure evolution of Csf/ZrB2-SiC composites via direct ink writing and reactive melt infiltration

Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites. In this work, we develop a novel processing route in fabrication of short carbon fiber reinforced ZrB2-SiC composites (Csf/ZrB2-SiC) overcoming the above two issues. At first, Csf preforms with oriented designation and uniform PyC/SiC interphase are fabricated via direct ink writing (DIW) of short carbon fiber paste followed by chemical vapor infiltration. After that, ZrB2 and SiC are introduced into the preforms by slurry impregnation and reactive melt infiltration, respectively. Microstructure evolution and optimization of the composites during fabrication are investigated in detail. The as-fabricated Csf/ZrB2-SiC composites have a bulk density of 2.47 g/cm3, with uniform weak interphase and without serious fiber damage. Consequently, non-brittle fracture occurs in the Csf/ZrB2-SiC composites with widespread toughening mechanisms such as crack deflection and bridging, interphase debonding, and fiber pull-out. This work provides a new opportunity to the material design and selection of short fiber reinforced composites.

Effect of heat treatment on the microstructure and strength of yttrium silicate matrix-modified SiCf/SiC composites

Yttrium silicate was introduced into the matrix of SiCf/SiC composites via the slurry impregnation and reactive chemical vapor infiltration (RCVI) methods to improve the water and oxygen corrosion resistance of the modified composite materials. The effects of heat treatment on the modified matrix and strength of the composites were systematically investigated. The results showed that the modified matrix was composed of a mixture of yttrium monosilicate, yttrium disilicate, and silicon carbide. The modified yttrium silicate matrix (named Y-Si-O matrix) and the silicon carbide matrix were laminated and well combined. After heat treatment, the amount of Y-Si-O in the mixed matrix increased. The modified composites with yttrium silicate had a similar flexural strength as SiCf/SiC composites (∼400 MPa). After treated at 1000 °C – 1300 °C, the strength of the modified composites increased by 17 %–26 %. The highest strength was measured for composites treated at 1200 °C.

Superior mechanical properties of lithium aluminosilicate composites by pyrolytic carbon intercalated carbon fibers/carbon nanotubes multi-scale reinforcements

Performance degradation always occurs in carbon fibers/carbon nanotubes (CFs/CNTs) multi-scale reinforced composites prepared by chemical vapor deposition (CVD) method. In this study, pyrolytic carbon (PyC) interlayers are introduced to overcome this problem, and the mechanism is studied in detail. The multi-scale reinforcements are combined with lithium aluminosilicate (LAS) glass-ceramic into ceramic matrix composites by slurry impregnation and hot pressing sintering. The results show that the PyC interlayers can protect the CFs from corrosion of the catalyst at high temperature, improve stress transfers and promote the synergy between various components. The CNTs and LAS matrix form a transition area, which causes deflection and shunting when cracks propagate. These factors have greatly increased the crack extension energy, so the mechanical properties have been greatly improved. The flexural strength, fracture toughness and work of fracture reach 602 ± 55 MPa, 10.7 ± 2 MPa m1/2, 4.6 ± 0.7 kJ m−2, respectively, which are 42.3%, 42.6% and 76.9% higher than CF/LAS. This work expands the study of the CFs/CNTs multi-scale reinforcements and the LAS composites, and provides a useful reference for the related research.

Ablation behavior and mechanisms of 3D Cf/ZrB2-SiC composite applied in long-term temperature up to 2400 °C

To evaluate the long-term ablation behavior and mechanism of carbon fiber reinforced ultra-high temperature ceramics, 3D Cf/ZrB2-SiC composite with homogeneous architecture was prepared via vibration-assisted slurry impregnation combined with precursor infiltration and pyrolysis. The intact structure of sample was remained owing to the triple oxide layers after ablation at Tmax = 2440 °C for 500 s, a dense ZrO2 layer owing to particle accumulation and sintering, followed by a dense SiO2+ZrO2 layer due to inhibition in diffusion of glassy SiO2 and next porous ZrO2 layer, leading to superior ablation resistance with linear and mass ablation rates of 1.8 × 10−4 mm/s and 1.08 × 10-5 g/mm2·s, respectively.

Mechanical and ablation properties of a C/C-HfB2-SiC composite prepared by high-solid-loading slurry impregnation combined with precursor infiltration and pyrolysis

An improved high-solid-loading slurry impregnation process was developed to introduce HfB2 particles into a low-density C/C preform efficiently, and precursor infiltration and pyrolysis process was used for densification to obtain a C/C-HfB2-SiC composite. The microstructure characterization revealed that HfB2 particles uniformly filled the pores in the C/C preform, and SiC well densified the interstices between HfB2 particles and the small pores in the carbon fiber bundles. After being tested, the C/C-HfB2-SiC composite had a density of 4.07 g/cm3 and a bending strength of 344.8 MPa, and exhibited a non-brittle fracture behavior. After ablation with oxyacetylene flame at 2500 ℃ for 120 s, the mass ablation rate and linear ablation rate of the C/C-HfB2-SiC composite were 0.5 mg/s and 0.415 μm/s, respectively. The good ablation performance is attributed to the hindering effect of the HfO2 scale on oxygen diffusion at high temperature.

Significant improvement of the self-protection capability of ultra-high temperature ceramic matrix composites

The oxidation behaviour of Cf/ZrB2-SiC with different fibre architectures, manufactured by slurry impregnation, polymer infiltration and mild pyrolysis, was investigated. Short term oxidation tests in air were performed for 1 min and 5 min at 1500 °C and 1650 °C in a bottom loading furnace. Microstructure, oxide scale thickness and composition were analysed by SEM/EDS/XRD. Results indicated that a good dispersion of ZrB2 particles in the polymer derived SiC(O) matrix promoted the formation of compact scales filling surface holes left by fibre oxidation. 20−30 vol% of ZrB2 in the material was found a good compromise between lightness and oxidation resistance.

The tensile damage behavior of SiCf/SiC–B4C after oxidation in wet atmosphere based on acoustic emission pattern recognition

AbstractIn this research, SiCf/SiC–B4C composites were prepared through slurry impregnation and reaction melting infiltration (RMI). To explore the damage evolution of this composites after oxidation in wet atmosphere, the changes of damage behavior of composites before and after oxidation were studied using acoustic emission (AE) pattern recognition (PR) technology. Results show that since the damage behavior of the composites changes after oxidation, the optimal number of clusters transforms from 3 to 5. In addition, as the oxidation time increases, the area of concentration of damage events is gradually broadened towards lower stress direction due to the decrease of damage threshold of composites. Meanwhile, because the BN interface is gradually oxidized, the density of C3‐cracks (axial yarns cracks) near the fracture and the proportion of Cluster‐4 (the signals of transverse yarns cracks) will increase, for which the rising of interfacial shear stress should be responsible. Through this research, not only can the damage evolution of SiCf/SiC–B4C composites after oxidation be understood, but also the method employed in this research can be as guide for studying other similar materials.

Influence of preparation method on microstructure and tribological behavior of C/C-BN composites

In this study, with 2.5D needle punched carbon felt as reinforcement, hexagonal boron nitride (h-BN) was introduced through chemical vapor infiltration (CVI) and slurry impregnation (SI) methods, and then pyrocarbon was deposited through CVI to convert the BN-containing preform into carbon fiber reinforced carbon-boron nitride binary matrix composites (C/C-BN). The microstructure and mechanical properties of C/C-BN(P) composites prepared by SI combined with CVI were compared with those of C/C-BN(CVI) composites prepared by CVI. The tribological behavior of C/C-BN(P) composites containing different contents of h-BN fillers and C/C-BN(CVI) composites with and without heat treatment was investigated by the ball-on-disk technique in dry sliding conditions using GCr15 steel balls and Si3N4 balls as sliding counterparts at two different sliding speeds (v = 0.2 and 0.8 m/s), and compared with the C/C composites used as a reference. C/C-BN(P) composites exhibit improved tribological behavior, which can be explained by the combined effect of SI and CVI on the microstructure and mechanical properties, leading to changes in the wear mechanism of the C/C-BN composites. The excellent reinforcement is attributed to the integrity structure of the C/C-BN(P), which can make the self-lubricating friction layer stably exist on the surface of the material and significantly reduce the friction coefficient and wear loss of the C/C-BN(P) composite. Furthermore, the mechanical and tribological properties of C/C-BN(P) composites can be adjusted by using different volume content of h-BN. The present work shows that C/C-BN(P) composites have wide application prospects as anti-friction and wear-resistant materials.

Summary of research on pretreatment methods of recycled aggregate

The recycled aggregate obtained by simple crushing and screening of waste concrete has a rough surface and many protruding edges and corners, and most of the aggregate surface is attached with old mortar, when directly used as aggregate in concrete, the water demand is large, the concrete strength and elastic modulus are low, and the durability is poor. Therefore, proper treatment of recycled aggregate is required.This article summarizes the research status of the pretreatment methods of mechanical strengthening, heating grinding, solution impregnation, cement slurry impregnation, carbonization treatment, nano-modification, microbial modification and water cleaning etc., and simply analyzes the advantages and disadvantages of various treatment methods, which provide a reference for the research of recycled aggregate processing methods.

Effect of silane grafted h-BN fillers on microstructure and mechanical properties of CVI-based C/C-BN composites

2.5D carbon fiber (Cf)-reinforced carbon and boron nitride dual matrix composites (C/C-BN) were prepared by a combined process of C/C silanization, hexagonal boron nitride (h-BN) slurry impregnation, and chemical vapor infiltration (CVI). The surface morphology of Cf after each step of modification, the fracture surface and microstructure of the C/C-BN composites were systematically characterized by scanning electron microscopy. Through the hydrolysis of silane, a polysiloxane network was successfully formed inside the porous C/C composites, which provided more attachment sites for the subsequent grafting of h-BN filler and the deposition of pyrocarbon (PyC). The microstructure and mechanical properties of the C/C-BN composites containing three different of h-BN fillers content were studied in comparison with pure C/C composites. The TEM characterization of the h-BN filler/PyC interface confirmed that the different crystal planes of the h-BN grains affect the structure of the PyC deposited thereon. The results show that the uniformly distributed h-BN filler reduces the size, thermal shrinkage anisotropy, and texture of the PyC deposited around it, resulting in the disappearance of the interlayer cracks in the matrix and the enhancement of the cohesion of the composite. The tight physical bond at the interface is due to the radial compressive stress caused by the thermal mismatch between the fiber, matrix, and h-BN fillers. Therefore, as the h-BN fillers content increases, the flexural strengths, compressive strength of the C/C-BN composites increase, which are both greater than that of C/C composite, and the compression properties gradually tended to be isotropic. However, when the h-BN content in the C/C composite is excessive (such as the volume content of 15 vol%), the densification efficiency of the subsequent CVI is reduced. As a result, the compactness of the transition interface and mixed matrix composed of h-BN and PyC decreased, which led to the decrease of mechanical properties of the composites.

Development of UHTCMCs via water based ZrB2 powder slurry infiltration and polymer infiltration and pyrolysis

Cf/ZrB2-SiC ultra-high temperature composites were manufactured via aqueous slurry impregnation coupled with polymer infiltration and pyrolysis, using a allylhydrido polycarbosilane precursor. For the first time we used ultra-high modulus pitch-based carbon fibres for the PIP process, investigating three different architectures, 0/0°, 0/90°, and 2D. Microstructure, mechanical properties and oxidation resistance in air at 1650 °C were investigated. As expected, the mechanical properties showed the tendency to decrease with increase of the preforms complexity, due to the higher amount of flaws and residual stresses. For instance, the flexural strength was approaching 500 MPa for 0/0°, 370 MPa for 0/90° and 190 MPa for 2D. The materials showed an optimal resistance to oxidation at 1650 °C thanks to formation of a viscous borosilicate glass that guaranteed a self-healing functionality.

Effects of preform pore structure on infiltration kinetics and microstructure evolution of RMI-derived Cf/ZrC-ZrB2-SiC composite

Reactive melt infiltration (RMI) is often used to fabricate highly dense ceramic matrix composite by infiltration of alloy melt into porous preform. Here, Cf/B4C-C preforms with various pore structures were prepared, and the effects of pore structure on the ZrSi2 melt infiltration and the as-received Cf/ZrC-ZrB2-SiC composites were investigated. Compared with the preform prepared by slurry impregnation (SI), the preform prepared by sol impregnation shows more uniform pore size distribution, which leads to more homogeneous melt infiltration, as well as more uniform formation of ZrC-ZrB2-SiC and better mechanical properties in the composites. The calculation results of infiltration kinetics indicate that the pore radius decreases quickly during the melt infiltration. As the time needed for pore closure in sol-preform is longer than that in SI-preform, it makes the infiltration kinetics more favorable in the former preform. This study can provide guidance for the pore structure regulation in the fabrication of RMI-composites.

Preparation and properties of C/C−ZrB2−SiC composites by high-solid-loading slurry impregnation and polymer infiltration and pyrolysis (PIP)

Abstract Ultrahigh-temperature ceramics were added to C/C composites to meet their application requirement in a high-temperature oxidizing environment. C/C−ZrB2−SiC composites were fabricated by high-solid-loading slurry impregnation with polymer infiltration and pyrolysis. The dispersion and rheological behavior of ZrB2 slurry and the microstructural, mechanical, and ablation properties of the C/C−ZrB2−SiC composites were investigated. Results indicated that a well-dispersed and low-viscosity ZrB2 slurry was obtained using 0.40 wt.% polyethyleneimine as a dispersant at pH 5. Ceramics were uniformly distributed in the short-cut fiber layer and needle-punched area. The flexural strength of the C/C−ZrB2−SiC composites was 309.30 MPa. The composites exhibited satisfactory ablation resistance under the oxyacetylene flame of 2500 °C, and the mass and linear ablation rates were 0.40 mg/s and 0.91 μm/s, respectively. A continuous and compact ZrO2 layer, which could effectively reduce the diffusion rate of oxygen and protect the composites from being ablated, was formed.

Co-enhancement of oxidation resistance and mechanical properties of Cf/LAS composites: The effects of h-BN addition

Carbon fiber reinforced lithium aluminosilicate glass-ceramic composites have been extensively studied and widely used in industry applications, because of its good mechanical properties and low thermal expansion coefficient. However, further investigation is also need to improve its oxidation resistance and mechanical performance to meet higher requirements. In this work, Cf/LAS glass-ceramic composites were fabricated by a slurry impregnation and hot-pressing method with different amounts of h-BN. Results indicate that composites with 2 wt.% h-BN addition exhibit excellent flexural strength and fracture toughness, reaching 910 ± 22 MPa and 21 ± 1 MPa·m1/2, respectively, and that after oxidation at 600, 800 and 1000 °C for 1 h, their strength residual ratio can reach 47%, 35% and 32%, respectively. TEM analyses suggest the improvement of oxidation resistance and mechanical properties should be attributed to the unique interface of composites caused by h-BN addition.

Oxidation behavior and ablation mechanism of Cf/ZrB2-SiC composite fabricated by vibration-assisted slurry impregnation combined with low-temperature hot pressing

The Cf/ZrB2-SiC composite was fabricated by vibration-assisted slurry impregnation and low-temperature hot pressing. The Cf/ZrB2-SiC composite exhibited an excellent thermal shock resistance and oxidation resistance with a dense SiO2-rich glassy layer exposing at 1500 °C/1 h condition for static oxidation. Moreover, the high temperature ablation resistance was investigated with an oxyacetylene ablative testing at temperature 1800 °C for 1000s. The mass loss rate and line loss rate were 7.32×10−4 mg/mm2·s and -1.70×10−4 mm/s, respectively. The enriched silica glass layer embedded with a small amount of zirconia particles was contributed mainly to the excellent ablation resistance of Cf/ZrB2-SiC composite.

Ablation Behavior of Zr–Al(Si)–C Layered Carbides Modified 3D Needled C/SiC Composites

3DN C/SiC composites modified through the combined method of slurry impregnation (SI) with reactive melt infiltration (RMI) can obtain obviously improved mechanical properties. However, when used under the oxyacetylene torch ablation environment, metal melt and low melting point oxides usually lead to the low ablative properties. In this paper, Al40Si60 alloy and Si melt are used to infiltrate 3DN C/SiC preforms impregnated with TiC or ZrC powders, respectively. Large amount of Ti3Si(Al)C2 or Zr3Al3C5 (and Zr3[Al(Si)]4C6) with nano‐laminated structure generated in the modification matrix, which not only increase the flexural strength of the composites, but also reduce the content of low melting point phases in the modification matrix. These layered carbides can keep steady under air flow, thus further improve material's ablative properties. Meanwhile, during the ablation of Zr–Al(Si)–C layered carbides modify 3D needled C/SiC, the high melting point layered carbides act synergistically with the high melting point ZrO2 generated. They can block the ZrO2 generated from migrating along with the air flow, so that ZrO2 can effectively act its pinning effect on the oxide melt and finally obtain desirable ablative properties.