Short fiber spraying process of all-oxide ceramic matrix composites: A parameter study

Fiber spraying processes have been established for polymer matrix composites for decades. In this study, we transferred an automated fiber spraying process to short fiber bundle-reinforced Nextel 610/ Al₂O₃-ZrO₂ oxide fiber composites (SF-OFC). The effect of the processing factors travel height, spray angle, and movement speed on the specimen strength was analyzed in a full factorial experimental design. As a result, the significance of the travel height as well as the interaction between travel height and movement speed was demonstrated. Furthermore, the influence of the fiber length (14, 28, 56, and 112 mm) on the bending stress and strain was investigated. Independent of the used fiber length, the SF-OFC exhibited an excellent quasi-ductile fracture behavior with bending strains in the range of .6% and in-plane isotropic material properties. The average bending strength increased from 133 ± 27 MPa with 14 mm fiber reinforcements to 163 ± 29 MPa with 112 mm fibers. The achieved bending strengths clearly exceeded the off-axis properties of currently used fabric-reinforced OFC. These properties, combined with the excellent drapability and cost effectiveness, make the novel material highly promising for industrial applications such as flame tubes, burner nozzles, kiln furnitures, or foundry components.     

Short fiber-reinforced oxide fiber composites

This paper presents a novel fiber spraying process for the manufacturing of short fiber bundle-reinforced Nextel™ 610/Al₂O₃-ZrO₂ oxide fiber composites (SF-OFC) and its characterization. First, the influence of varying fiber lengths (7, 14, and 28 mm, continuous fibers) and fiber orientations (unidirectional 0°, quasi-isotropic, ±45°) was investigated using hand-laid SF-OFC. Due to the weak matrix, the hand-laid material exhibited a strongly fiber-dominated material behavior, that is, variations in fiber length and orientation had a strong influence on the material properties. Second, the automated sprayed SF-OFC, however, exhibited a random orientation of the fiber bundles, which resulted in in-plane isotropic material properties. Average bending strengths of up to 177 MPa, strains of .39%, and a quasi-ductile fracture behavior were achieved. The strain was, therefore, in the range of fabric-reinforced OFC. While the bending strength of the SF-OFC was somewhat lower than that of fabric-reinforced OFC with the fiber orientation parallel to the loading direction, it was more than two times higher than the strength in 45° direction relative to the fabric reinforcement. Combined with good drapability and lower material costs compared to fabric-reinforced OFC, SF-OFC is, therefore, a promising material for industrial applications.     

Crack opening behavior in ceramic matrix composites

The evolution of matrix cracks in a melt‐infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time‐temperature‐environment‐stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross‐section. This calls into question the modeling assumption of through‐cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit.     

碳纤维增强SiC陶瓷复合材料的研究进展

碳纤维增强SiC陶瓷基复合材料具有良好的高温力学性能，是航空航天和能源等领域新的高温结构材料研究的热点之一．本文回顾了增强体碳纤维的发展，对材料的成型制备工艺，材料的抗氧化涂层研究进展和现有的一些应用做了综述，并展望了碳纤维增强SiC陶瓷基复合材料以后的研究重点及发展前景．     

Green State Joining of SiC without Applied Pressure

External pressure (uniaxial or isostatic) is usually necessary to form a thin and defect-free joint in green state joining of SiC ceramics. A successful method of joining SiC in the green state using a liquid polymer precursor, allylhydridopolycarbosilane (AHPCS), without applied pressure, is described. The thermal decomposition behavior of the polymer was examined, and defect formation during joint evolution was investigated by interrupting the heat treatment at various stages. Cracks and pores were observed in the joints formed by pure AHPCS during the pyrolysis of the polymer precursor. Adding SiC powder to the joining paste eliminated defect formation. Optimum SiC loading in the paste was determined to be in the range of 25–35 vol%. Joints formed by AHPCS + (SiC + 5 wt% B) paste were essentially indistinguishable from the matrix and had an average strength of 323 MPa, comparable to that of the control sample.     

Temperature dependent fracture toughness model for whisker-reinforced ceramic matrix composites

A temperature dependent fracture toughness model for whisker-reinforced ceramic matrix composites was developed in this study, which considers the effects of matrix fracture toughness, residual thermal stress, crack bridging, crack deflection, and their temperature dependence. Its predicted results were compared with the fracture toughness of six types of whisker-reinforced ceramic matrix composites at different temperatures, and good agreement between predicted results and experimental results is obtained. Furthermore, based on this model, we systematically analyzed the effects of the volume fraction and aspect ratio of whisker, Young's modulus of matrix and whisker, thermal expansion coefficient difference, stress-free temperature, the ratio between the fracture energy of matrix and that of interface, on their temperature dependent fracture toughness for the first time. Finally, insights and suggestions which could help to optimize and improve the composite fracture toughness at different temperatures are provided.     

A polymer infiltration and pyrolysis (PIP) process model for ceramic matrix composites (CMCs)

This work describes a physics-based model to simulate the polymer infiltration and pyrolysis (PIP) manufacturing process for ceramic matrix composites (CMCs). Models have been developed to characterize volumetric distribution of constituents and track porosity inside the composite at different PIP stages utilizing test data from TGA and DSC characterization of a commercial preceramic polymer. Laboratory experiments were done using C/SiC CMC specimens manufactured with a variable number of PIP cycles in order to obtain inputs for the models, and the analytical results have been shown to agree with porosity determined from physical measurements.     

Surface engineering of SiCf/SiC composites by selective thermal removal

Surface engineering based on the Selective Thermal Removal (STR) of SiC fibers from SiC_f/SiC composites was used to obtain a brush‐like surface in view of joining processes. As observed through 3D confocal microscopy, the thermal treatment led to a selective removal of the surface fibers so that the specific surface increased. Wetting tests were performed using a Ag–Cu–Ti brazing alloy. The STR led to a negligible increase in the contact angles, which ranged from 16° to 21° for as‐received composites and increased to 28° for composites after STR. Microstructural observations showed that the brazing alloy perfectly adhered to the brush‐like surfaces were giving a mechanical interlocking expected to enhance the strength of the joint.     

Damage modeling of oxide/oxide ceramic matrix composites under cyclic loading conditions

Ceramic matrix composites exhibit optimal high temperature property and complex nonlinear behaviors including irreversible deformations and stiffness degradation under different mechanical loading conditions. In the present work, the damage evolution of the material under multi-axial loads considering effects of loading-unloading cyclic was studied and a novel continuum damage constitutive model was proposed. The material degradation was decomposed into monotonic damages and cyclic damages. The anisotropic hardening behavior of the material was considered by taking orientation dependence into account. Compared to the experimental results, the constitutive model could accurately predict the stress-strain response and stiffness degradations of the oxide/oxide ceramic matrix composites for monotonic loading and cyclic loading.     

Increasing the tensile strength of oxide ceramic matrix mini-composites by two-step sintering

Adapting conventional sintering (CS) techniques of monolithic ceramics for the production of oxide ceramic matrix composites (Ox-CMCs) comes along with a few drawbacks, such as fiber degradation. Thus, the applicability of two-step sintering (TSS) for the production of Ox-CMCs based on Nextel™ 610 fibers and porous alumina matrix is investigated in this study for the first time. Uniaxial tensile tests were performed to evaluate the performance of mini-composites produced by TSS and compared with those produced by CS. Parameters known for influencing the mechanical behavior of the mini-composites, such as grain size, porosity, shrinkage, as well as matrix properties, were analyzed. Both sintering techniques resulted in similar grain size distributions, whereas TSS showed higher total porosity and lower amount of sintering-induced cracks. As a result, TSS samples showed a higher tensile strength of 230±27 MPa when compared to 133±8 MPa for CS. In general, it was observed that most of the densification happens during the first phase of TSS, while the matrix is slowly strengthened during the second step. Therefore, the reported TSS process is a very promising and easy-to-apply heat treatment for producing Ox-CMCs with controlled microstructure.     

Statistical analysis of the influence of microstructure on damage in fibrous ceramic matrix composites

The effect of microstructure on cracking was analyzed in a CMC using statistical methods. It was determined that the amounts of coating surrounding fibers and their dispersion within the matrix influenced where cracks evolved in transverse plies. Linear models predicted that maximum principal strains in transverse fiber coatings increased as (i) the fiber coating area increased and (ii) the length of matrix ligament between fibers decreased. Logistic models indicated that the likelihood of transverse fibers residing on a matrix crack increased as the (i) ratio of coating to filament decreased, (ii) distance between fibers decreased, or (iii) coating area increased.     

Mode I interlaminar fracture behavior of 2D woven ceramic matrix composites

Interlaminar fracture properties of melt-infiltrated woven SiC/SiC ceramic matrix composites were investigated using traditional and wedge-loaded double cantilever beam methods. The two methods produced comparable G_IC results for some specimens. The difference in boundary conditions between the two methods appeared to influence the crack propagation path. The DCB method, having free-end boundary condition, allowed more interaction between the crack and the composite microstructure than the wedge method did. The effect of fiber tow layout sequence had an effect on the interlaminar properties. Higher toughness was observed for the orientation where crack propagation occurs between planes with more transverse tows. Jump-arrest phenomenon was found to have higher significance on the rising R-curve behavior than fiber bridging.     

Enhancing thermal stability of oxide ceramic matrix composites via matrix doping

To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 °C and 1400 °C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.     

Mechanical and electromechanical properties of piezoelectric ceramic fibers drawn by the alginate gelation method

Piezoelectric ceramic fibers are widely used in piezocomposite devices. The various methods that are used to draw ceramic fibers differ in the way the fiber form is obtained, which in return closely affects the density, uniformity and the properties of the fibers that are obtained at the end. In this study, the processing-property relationship in the piezoceramic fibers drawn using the alginate gelation method is investigated, with a focus on the mechanical and electrical properties of individual fibers. Fibers with a Weibull strength of 65.3 MPa, remanent polarization of 22 μC/cm² and a total bipolar field induced strain of 0.25% under an electric field of 2.5 kV/mm, piezoelectric coefficient of 300 pm/V and dielectric constant of 3323 were produced. 1-3 piezocomposite devices prepared from these fibers had a 6 dB higher free-field voltage sensitivity and a 50% wider bandwidth compared to a solid disk transducer of the same dimensions.     

Direct ink writing of ceramic matrix composite structures

We present the development of an ink containing chopped fibers that is suitable for direct ink writing (DIW), enabling to obtain ceramic matrix composite (CMC) structures with complex shape. We take advantage of the unique formability opportunities provided by the use of a preceramic polymer as both polymeric binder and ceramic source. Inks suitable for the extrusion of fine filaments (<1 mm diameter) and containing a relatively high amount of fibers (>30 vol% for a nozzle diameter of 840 μm) were formulated. Despite some optimization of ink rheology still being needed, complex CMC structures with porosity of ~75% and compressive strength of ~4 MPa were successfully printed. The process is of particular interest for its ability to orient the fibers in the extrusion direction due to the shear stresses generated at the nozzle tip. This phenomenon was observed in the production of polymer matrix composites, but it is here employed for the first time for the production of ceramic matrix ones. The possibility to align high aspect ratio fillers using DIW opens the path to layer‐by‐layer design for optimizing the mechanical and microstructural properties within a printed object, and could potentially be extended to other types of fillers.     

Additive manufacturing of fiber reinforced ceramic matrix composites: Advances,challenges, and prospects

Fiber reinforced ceramic matrix composites (FRCMCs) have been used in various engineering fields. Additive manufacturing (AM) technologies provide new methods for fabricating FRCMCs and their structures. This review systematically reviews the additive manufacturing technologies of FRCMCs. In this review, the progress for additive manufacturing of FRCMCs were summarized firstly. The key scientific and technological challenges, and prospects were also discussed. This review aims to motivate the future research of the additive manufacturing of FRCMCs.     

A synergistic model of stress and oxidation induced damage and failure in silicon carbide-based ceramic matrix composites

A micromechanics-based modeling approach that allows for the simultaneous consideration of deformation, damage, and oxidation associated with each constituent of silicon carbide (SiC)-based ceramic matrix composites (CMC), including the fiber, fiber coating, and matrix, is described. Chemical kinetics models from the literature are combined with a progressive damage model. Rupture predictions of unnotched and notched stress-hold (creep) specimens are compared with experimental measurements from a SiC/SiC CMC to assess the efficacy of the modeling approach. Techniques of improving creep rupture life are explored using the model.     

New liquid processing of oxide/oxide 3D wowen ceramic matrix composites

In this work, a novel process named Flexible Injection Process (FIP) was developed to manufacture near-net shape oxide/oxide composites reinforced with 3D interlock fibers. This process uses a flexible membrane to apply pressure to promote transverse impregnation of the fibrous reinforcement by a slurry charged with sub-micron ceramic particles. Due to the through-thickness filtration and compaction, FIP process is much faster than typical in-plane impregnation and results in composites with lower residual porosity than those produced by traditional processes. In this study, a mathematical modeling of the impregnation in FIP was developed and compared to experimental infiltration experiments. Furthermore, ceramic matrix composites (CMCs) produced by FIP were compared to composites manufactured via an established RTM-like process. The two molding processes were compared to determine if the different flow behaviors have an impact on material densification, porosity formation, mechanical properties, and manufacturing time. CMCs produced by both methods resulted in similar microstructures, as determined by mercury intrusion porosimetry, even if FIP composites were marginally less porous. Finally, a comparison of mechanical properties resulting from the two manufacturing methods has shown a similar behavior. Thus, the main advantages of FIP molding were identified to be the shorter cycle time and the robustness of the impregnation compared to RTM-like processes.     

Tensile and fatigue behavior of oxide/oxide ceramic matrix composite with simulated foreign object damage in combustion environment

In this study, oxide/oxide ceramic matrix composite test coupons were quasi‐statically indented and tested for tensile strength and fatigue life in a combustion environment. The combustion environment simulated the gas turbine engine environment in an aircraft. Two different dent sizes were created on two different sets of test coupons with a blunt conical indentor. During mechanical testing, the combustion flame simultaneously impinged on the dent region resulting in a maximum test coupon surface temperature of 1250 ± 50°C. For a life of 90 000 cycles, the fatigue limit in the combustion environment was 85% of the postindentation degraded tensile strength. Microscopy images of the failed test coupons showed damage modes of fiber fracture and matrix cracking at the dent site. The run‐out test coupons which did not fail within 90 000 cycles showed residual strength that was not significantly different from that of their virgin counterparts.     

Shear properties of C/C-SiC sandwich structures

Carbon fiber reinforced carbon-silicon carbide (C/C-SiC) sandwich structures have been developed using the Liquid Silicon Infiltration process and the in situ joining method. They offer high mass-specific stiffness, low thermal expansion, and high environmental stability. Potential application areas are highly precise satellite structures, like optical benches. In this study, sandwich samples were manufactured using prepregs based on 2D carbon fibre fabrics and a phenolic resin precursor. Carbon fibre reinforced polymer preforms for folded and grid-cores, as well as for the skin panels were manufactured using autoclave technique. In the second step, the sandwich components were pyrolyzed, leading to C/C preforms. For the build-up of the sandwich samples, two skin panels were joined to a core structure and subsequently, the resulting C/C sandwich preform was siliconized. C/C-SiC sandwich samples were tested under shear load. Shear strength, modulus, and fracture strain were determined and compared to the results obtained by analytical calculation. The shear properties were dependent on the fiber orientation in the core structure as well as on the core type and orientation. The sandwich shear stiffness obtained in the tests was close to the expected theoretical values, calculated on the basis of the material properties and the core geometry.