Preparation,mechanical properties and long-term ablation resistance of Cf/SiBCN–ZrB2 composites

The C_f/SiBCN–ZrB₂ composites were prepared by dipping and winding combined with reactive hot pressing. The flexural strength and fracture toughness reached 261 MPa and 11.96 MPa • m^1/2, respectively, through continuous carbon fibers debonding, pulling and bridging mechanisms. Excellent mechanical properties ensured that the C_f/SiBCN–ZrB₂ composites remained intact after exposure to a plasma flame with a heat flux of 9.37 MW/m² for 300 s, with the mass and linear ablation rates of 1.78 mg/s, 1.01 μm/s, respectively. The excellent ablation resistance was due to the formation of dense oxide layers separating the matrix from the plasma flame. The SiO₂ formed in the low-temperature areas away from the center was the main ablation-resistant barrier, while the ZrO₂/SiO₂ double oxide layer formed in the high-temperature region at the center was the major ablation-resistant barrier.     

Fabricating thick-section carbon fiber/silicon carbide composites by machining-aided chemical vapor infiltration

Chemical vapor infiltration (CVI) is a prominent process for fabricating carbon fiber/silicon carbide (C/SiC) composites. However, the preparation of enclosed-structure or thick-section C/SiC composites/components with CVI remains a challenge, since the difficulty of densification increases. Here, machining-aided CVI (MACVI) is designed, in which infiltration-assisting holes are utilized (machined) to increase matrix deposition. To validate the approach, thick-section (10 mm thick) C/SiC composites were fabricated by MACVI. Porosity analysis and microstructure characterization were performed on the fabricated MACVI C/SiC composites and their CVI counterparts, showing a density increase up to 12.7% and a porosity decrease up to 32.1%. The mechanical behavior of the fabricated MACVI C/SiC composites was characterized, showing an increase of flexural strength by a factor of 1.72 at most. Besides, the toughness also largely increases. Both the porosity decrease and the strength and toughness increase brought by MACVI demonstrate its effectiveness for fabricating stronger and tougher enclosed-structure or thick-section ceramic matrix composites/components.     

Mechanical properties of SiCp/Al2O3 ceramic matrix composites prepared by directed oxidation of an aluminum alloy

Silicon carbide particulate reinforced alumina matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with appropriate dopants along with a preform of silicon carbide has led to the formation of alumina matrix surrounding silicon carbide particulates. SiC_p/Al₂O₃ ceramic matrix composites fabricated by the DIMOX process, possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. SiC_p/Al₂O₃ matrix composites were investigated for mechanical properties such as flexural strength, fracture toughness and hardness; the composite specimens were evaluated using standard procedures recommended by the ASTM. The SiC_p/Al₂O₃ ceramic matrix composites with SiC volume fractions from 0.35 to 0.43 were found to possess average bend strength in range 158-230 MPa and fracture toughness was found to be in range of 5.61-4.01 MPa√m. The specimen fractured under three-point loading as observed under scanning electron microscope was found to fail in brittle manner being the dominant mode. Further the composites were found to possess lower levels of porosity, among those prepared by DIMOX process.     

Mechanical and ablation properties of 2D-carbon/carbon composites pre-infiltrated with a SiC filler

In order to improve the mechanical and ablation properties of 2D-carbon/carbon composites, a SiC filler was added to a 2D-preform before isothermal chemical vapor infiltration densification by using a powder infiltration technique. Backscattered electron images showed that the SiC filler was mainly concentrated between the fiber bundles and between the layers. The tensile and flexural strengths of the composites were improved by the addition of the SiC filler because of the increase of interfacial surface areas between the bundles and between the layers, the less residual open porosity, and also the strong bonding between the SiC particles and the pyrocarbon matrix. The composites with filler experienced a 15.2% lower thickness erosion rate and a 51.7% lower mass erosion rate, compared to those C/C without filler. This was attributed to the low oxygen permeability of the SiO₂ shielding the exterior inter-bundle pores as well as to a thermal barrier effect.     

Tuning ceramic content,mechanical and ablative properties of C/C–ZrC-ZrxCuy composites via using various C/C skeletons

C/C–ZrC-Zr_xCu_y composites were prepared by pressure-assisted reactive melt infiltration with Zr₂Cu alloy as an infiltrator. The mechanical and ablation properties of C/C–ZrC-Zr_xCu_y composites were tuned by applying various C/C skeletons (1.28, 1.40, 1.58, and 1.73 g/cm³). When the C/C skeleton density increased from 1.28 to 1.73 g/cm³, the density of the composites decreased gradually. Additionally, due to the higher ceramic content, composites with low skeleton density (1.28, 1.40 g/cm³) exhibit a higher compressive and flexural strength. The fabricated composites with a skeleton density of 1.28 g/cm³ possess better mechanical properties and ablative properties at 2800 °C. After plasma flame ablation for 54 s, the linear and mass ablation rates of the composites are 33.70 ± 1.33 μm/s and 22.39 ± 2.99 mg/s, respectively.     

Effect of a diffusion-enhancing hole on the densification of a thick-section 2D C/SiC composite

Diffusion-enhancing holes (DEHs) have been used to mitigate the large density gradients that are formed in the thick-section ceramic matrix composites (CMCs) fabricated by chemical vapor infiltration (CVI). However, the densification characters of the thick-section CMCs with DEHs through vapor infiltration remain a concern. Here, the densifications of a10-mm-thick two dimensional (2D) C/SiC composite with or without DEHs are investigated by experiments and calculations. Results showed both the measured densities, the predicted final densities, and the density growth rates (DGRs) for the composite with DEHs (diameters of 2 or 4 mm) are higher than those of the counterpart without DEHs, due to the forming of dense rings (DRs) around DEHs and the increased infiltration in the large pores (diameter > 52 μm). In addition, the diffusion increase in infiltration with DEHs is attributed to the increase of Knudsen diffusion resulted from the reopening of the blocked/sealed pores by DEH-machining.     

HfB2 coating on C/C composites prepared by chemical vapor deposition: Thermodynamics and experimental investigation

A thermodynamic calculation on the HfB₂ coating prepared by chemical vapor deposition (CVD) through HfCl₄-BCl₃-H₂-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB₂ coating could be obtained above 900 °C with the ratios of BCl₃/HfCl₄ and H₂/HfCl₄ higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H₂ and BCl₃ flow rates had significant effects on the grain size, growth rate and phase composition of HfB₂ coatings. A dense and uniform HfB₂ coating was prepared at 1150 °C with a BCl₃/HfCl₄ ratio of 3 and a H₂/HfCl₄ ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame.     

Microstructure and ablation property of gradient ZrCSiC modified C/C composites prepared by chemical liquid vapor deposition

Chemical liquid vapor deposition was adopted to fabricate gradient ZrCSiC modified C/C composites, and the microstructure and ablation resistance were studied. Results displayed the content of SiC decreased from the composites edge to the center but that of ZrC increased, indicating SiC and ZrC ceramics have the gradient distribution in the composites. The gradient composites possessed a low CTE and high thermal conductivity. The low CTE restricted the formation and expansion of defects, which could slow the oxygen diffusion in the composites. The high thermal conductivity could transfer the heat quickly in ablation process, which reduced the heat accumulation on the ablation surface and weakened the thermal erosion. Therefore, the gradient composites possessed an outstanding anti-ablation property at two heat fluxes. Compared with the uniformed distribution composites, the linear and mass ablation rates of the gradient composites decreased by 60.9% and 66.7% at heat flux of 2.38 MW/m² and decreased by 55.9% and 67.2% at heat flux of 4.18 MW/m². Because of the gradient distribution, porous ZrO₂ coating, ZrO₂SiO₂ coating and SiO₂ coating with SiO₂ nanowires were generated on the ablation center, ablation transition zone and ablation edge, respectively. These coatings isolated the sample surface from the flame and inhibited the transport of oxygen into the sample inner.     

Microstructure and mechanical properties of TiB2-reinforced 7075 aluminum matrix composites fabricated by laser melting deposition

In this study, we adopt laser-melting deposition (LMD) technology to fabricate TiB₂/7075 aluminum matrix composites (AMCs), and we investigate in detail the effect of the TiB₂ content on the microstructure, nano-hardness, compressive properties, and wear performance. We prepare experimental samples by using a laser power of 800?W and a velocity of 0.01?m/s, and the results are evaluated. It was observed that the reinforcement particles dispersed irregularly throughout the Al matrix as the TiB₂ contents increased. The grain size of the fine-grain zone decreased appreciably by 31.9% from 8.41?µm (LMD sample without TiB₂ reinforcement) to 5.73?µm. Furthermore, the AMCs with 4?wt% reinforcement exhibited impressive mechanical properties, i.e., nanohardness of 1.939 Gpa, compressive strength of 734.8?MPa, and a wear rate of 1.889?×?10^?4 mm³/Nm. The wear resistance improved and the wear mechanism changed from adhesive wear to debris wear with the addition of TiB₂ reinforcement.     

C/SiC–ZrB2–ZrC composites fabricated by reactive melt infiltration with ZrSi2 alloy

C/SiC–ZrB₂–ZrC composites were prepared by reactive melt infiltration (RMI) combined with vacuum pressure impregnation (VPI) method. B₄C–C was first introduced into C/SiC composites with a porosity of about 30% by impregnating the mixture of B₄C and phenol formaldehyde resin, followed by pyrolysis at 900 °C. The molten ZrSi₂ alloy was then infiltrated into the porous C/SiC–B₄C–C to obtain C/SiC–ZrB₂–ZrC composites. The flexural strength was tested. The ablation behavior was investigated under an oxyacetylene torch flame. It has been found that the C/SiC–ZrB₂–ZrC showed a high flexural strength and an excellent ablation resistance. The reactions between ZrSi₂ alloy and B₄C–C were studied, and a model based on these reactions was built up to describe the formation mechanism of the matrix.     

Effect of CVI SiC content on ablation and mechanism of C/C-SiC-ZrC-Cu composites

C/C-SiC-ZrC-Cu composites were fabricated by chemical vapor infiltration, precursor infiltration-pyrolysis and vacuum-pressure infiltration methods. During Cu infiltration, the Cu_6·69Si and Cu₃Si new phases are generated through reaction between SiC and molten Cu. The formed Cu_6·69Si, Cu₃Si, ZrC and SiC phases can improve the wettability and interface combination between Cu and the doped carbon matrix. The ablation tests demonstrate that the CVI SiC content significantly affects the structure of protective oxide layer, and induces inverse effects in ablation center at 2500 ^°C and 3000 ^°C. The relatively high CVI SiC content enhances the ablation resistance of composites at 2500 ^°C, but increases the linear ablation rate at 3000 ^°C due to the excessive evaporation and mechanical denudation. During ablation, the formed Si-Zr-C-O layer underneath ablation center and the Si-Cu-C-O layer on transition or marginal areas can prevent carbon matrix from serious oxidation. After ablation for 20 s, the C/C-SiC-ZrC-Cu composites with high CVI SiC content display the best anti-ablation property at 2500 ^°C, and the ablation rates are 3.5 ± 0.1 μm/s and 3.4 ± 0.1 mg/s.     

Effects of silanization of C/C composites and grafting of h-BN fillers on the microstructure and interfacial properties of CVI-based C/C-BN composites

A low-density carbon/carbon (C/C) composite/silane coupling agent/hexagonal boron nitride (h-BN) hybrid reinforcement was prepared by grafting polyethyleneimine (PEI)-encapsulated modified h-BN fillers onto a carbon fiber surface using 3-aminopropyltriethoxysilane (APS) as the connection to improve the distribution uniformity of h-BN fillers in quasi-three-dimensional reinforcements and the interfacial properties between the fibers/pyrocarbon (PyC) in the C/C-BN composites obtained after densification by chemical vapor infiltration (CVI). The microstructure and chemical components of the hybrid reinforcement were investigated. The transmission electron microscopy (TEM) sample was prepared using a focused-ion beam (FIB) for the h-BN/PyC interfacial zone. The interlaminar shear strength (ILSS) and impact toughness were analyzed to inspect the composites’ interfacial properties. The results show that APS and h-BN are uniformly grafted on the fiber surface in the chopped fiber web inside the C/C composite without a density gradient, and agglomeration occurred and significantly increasing the fiber surface roughness. The highly ordered h-BN basal plane may affect the order degree of PyC near the h-BN/PyC interface. The addition of h-BN reduces the PyC texture near it, causing the annular cracks to disappear gradually. The lower PyC texture and the rougher fiber surface strengthen the interfacial bond of the fiber/matrix. Consequently, the ILSS strength of the C/C-BN composites first increases and then decreases as the h-BN filler content increases and is always higher than that of the C/C composite, while the addition of h-BN fillers weakens its impact toughness. When the h-BN content in the C/C-BN composite is 10 vol%, the ILSS of the C/C-BN composites was 15.6% higher than that of the C/C composites. However, when the h-BN content is excessive (15 vol%), the densely grafted h-BN will bridge each other, reducing the subsequent CVI densification efficiency to form a loose interface, causing a decrease in the shear strength.     

Microstructure and mechanical properties of SiC platelet reinforced BaOAl2O32SiO2 (BAS) composites

BaOAl₂O₃2SiO₂ (BAS) glass–ceramic powders were prepared by sol–gel technique. SiC platelet reinforced BAS glass–ceramic matrix composites with high density and uniform microstructure were fabricated by hot-pressing. The effect of additional crystalline seeds on hexagonal to monoclinic phase transformation of Barium aluminosilicate was studied. The effects of SiC platelet content on the microstructure and mechanical properties of the composites were also investigated. The results showed that the flexural strength and fracture toughness of the BAS glass–ceramic matrix composites can be effectively improved by the addition of silicon carbide platelets. The main toughening mechanism was crack deflection, platelets' pull-out and bridging. The increased value of flexural strength is contributed to the load transition from the matrix to SiC platelets.     

Effect of diffusion-assisting holes on the tensile properties of a thick-section two dimensional C/SiC composite fabricated by chemical vapor infiltration

The concept of diffusion-assisting holes (DAHs) has been developed to increase matrix deposition in the middle layers of the thick-section ceramic matrix composites (CMCs) that are fabricated by chemical vapor infiltration (CVI). However, the effect of DAHs on the tensile properties of CMCs has not been studied. Here, the tensile properties and the state of matrix deposition of a 10-mm-thick two dimensional (2D) C/SiC with DAHs are investigated. Results showed, with DAHs, a zone of increased deposition with a radius of ca. 1.4 mm around a hole was introduced and the net-section strength of the 10-mm-thick 2D C/SiC was increased by 48.1%. In addition, the tensile load bearing capacity was also increased by 34.1%, although the load bearing section decreased with DAHs. The increased net-section strength and tensile load bearing capacity of the C/SiC are attributed to the increased matrix deposition in the middle layers of the thick-section composite.     

Microstructure and mechanical properties of hot-pressed SiC/(W,Ti)C ceramic composites

SiC/(W, Ti)C ceramic composites with different content of (W, Ti)C solid-solution were produced by hot pressing. The effect of (W, Ti)C content on the microstructure and mechanical properties of SiC/(W, Ti)C ceramic composites has been studied. Densification rates of the SiC/(W, Ti)C ceramic composites were found to be affected by addition of (W, Ti)C. Increasing (W, Ti)C content led to increase the densification rates of the composites. The sintering temperature was lowered from 2100 °C for monolithic SiC to 1900 °C for the SiC/(W, Ti)C composites. Results show that additions of (W, Ti)C to SiC matrix resulted in improved mechanical properties compared to pure SiC ceramic. The fracture toughness and flexural strength continuously increased with increasing (W, Ti)C content up to 60 vol.%, while the hardness decreased with increasing (W, Ti)C content.     

Behavior of two-dimensional C/SiC composites subjected to thermal cycling in controlled environments

Thermal fatigue behavior of two-dimensional carbon fiber reinforced SiC matrix composites fabricated by chemical vapor infiltration technique was investigated using an on-line quench method in controlled environments which simulated an aero-engine gas. A system of damage information acquisition (SDIA) was developed to study changes in electrical resistance of the C/SiC composites during their damage in dynamic testing. Damage to composites was assessed by the ultimate tensile strength (UTS) and SEM characterization. The results showed that: (1) under different atmosphere, the 2D-C/SiC composites subjected to thermal cycling behaved very differently and the most sensitive atmosphere was the wet oxygen; (2) external load could accelerate the degradation of the composites and changed the oxidation regimes of fibers; (3) the electrical resistance of the specimen could be detected on-line, stored in real time and analyzed reliably by the newly-developed SDIA; (4) 2D-C/SiC composites had an excellent thermal fatigue resistance in different environments.     

Preparation,mechanical and ablation properties of a C/C-TaB2-SiC composite

A C/C-TaB₂-SiC composite was successfully prepared by high-solid-loading slurry impregnation combined with polymer infiltration and pyrolysis. The composite had a density of 3.46 g/cm³ and consisted of pyrolyzed carbon, carbon fibers, TaB₂ particles, and precursor-derived SiC with their mass fractions of 7.8 %, 13.0 %, 58.6 %, and 20.6 %, respectively. The C/C-TaB₂-SiC composite possessed a flexural strength of 248.2 ± 16.8 MPa and a fracture toughness of 13.7 ± 1.6 MPa·m^1/2, and demonstrated a non-brittle fracture behavior. After exposure to oxyacetylene flame with a heat flux of 4.18 MW/m² for 120 s, the ablation temperature of the sample surface reached a maximum of 2263 °C, and the mass and line ablation rates were 2.24 mg/s and 12.92 µm/s, respectively. The ablation resistance mainly comes from the hindrance of oxygen diffusion by the oxide layer composed of tantalum oxide and a small amount of SiO₂.     

Effect of thermal residual stress on the tensile properties and damage process of C/SiC composites at high temperatures

Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.     

Design of a C/SiC functionally graded coating for the oxidation protection of C/C composites

To reduce the residual thermal stress between the carbon fiber-reinforced carbon (C/C) composites and the SiC coating layer, functionally graded materials (FGM) consisting of a C/SiC compositionally graded layer (C/SiC interlayer) were adopted. After designing the compositional distribution of the C/SiC interlayer which can relieve the thermal stress effectively, the deposition conditions of the entire compositional range of the C/SiC composites were determined using a thermodynamic calculation. According to the design and calculation the C/SiC interlayer and the SiC outer layer were deposited on the C/C composites by a low pressure chemical vapor deposition (LPCVD) method at deposition temperatures of 1100 and 1300 °C. The stress calculation and the experimental results suggested that the SiC-rich compositional profile in the FGM layer is the most effective for relieving the thermal stress and increasing the oxidation resistance.     

Effect of additives on mechanical properties of oxidation-resistant carbon/carbon composite fabricated by rapid CVD method

The optimum composition of additives was obtained by the orthogonal design test method for oxidation-resistant carbon/carbon composites (C/C) fabricated by the rapid CVD method. The effect of additives on mechanical properties was examined. The additives used in this test included silicon carbide, silicon nitride, and metal borides. Additives doped into the matrix of C/C increase not only the initial oxidation temperature from 400 to 657°C (64%), but also its flexural strength from 121 to 254 MPa (110%), and flexural modulus from 25 to 45 GPa (96%). The increase of mechanical properties is considered to be due to the formation of a metallic carbon–boron compound in the microstructure.