Effect of SiC Location on the Ablation of C/C-SiC Composites in Two Heat Fluxes

How layer-segregated distribution of SiC affects the ablation of C/C-SiC composites was studied in the present work.A certain amount of SiC particles was deposited at the non-woven(C/C—SiC-1) and web(C/C-SiC-2) layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites,respectively.Ablation under oxyacetylene torch demonstrated that the two composites have similar ablation rates in heat flux of 2.38 MW/m~2 whereas ablation rates of C/C-SiC-2 were much higher than those of C/C—SiC-1 when heat flux was 4.18 MW/m~2.SiO_2 covered partially the defective surface of both composites in the lower heat flux.The different SiC locations induced distinct defects and then led to the two composites' dissimilar ablation rates in the higher heat flux.     

Ablative and Mechanical Properties of C/C—ZrC Composites Prepared by Precursor Infiltration and Pyrolysis Process

C/C-ZrC composites with continuous ZrC matrix were prepared by precursor infiltration and pyrolysis process using zirconium-containing polymer.Ablation properties of the composites were investigated by oxyacetylene flame with heat flux of 2380 and 4180 kW/m~2,respectively.The results showed that C/C-ZrC composites exhibited excellent ablation resistance under the heat flux of 2380 kW/m~2 for 120 s and a tree-coral-like ZrO_2protective layer formed after ablation.However,when the heat flux increased to 4180 kW/m~2,the maximum temperature of ablated surface reached 2500 ℃ and a strong degradation of ablation resistance was observed due to the weak bonding between the formed ZrO_2 layer and the composites.The flexural strength of C/C-ZrC composites was 110.7 ± 7.5 MPa.There were a large number of carbon fiber bundles pull-out,and the composites exhibited a pseudo-plastic fracture behavior.     

SiC Nanowires Toughed HfC Ablative Coating for C/C Composites

In order to improve ablation resistance of carbon/carbon(C/C) composites,SiC nanowires were prepared on C/C composites surface in prior through chemical vapor reaction before HfC coating.SiC nanowires grew randomly and had good combination with HfC coating.SiC nanowires toughed HfC coating had lower linear and mass ablation rates than original HfC coating.The surface was much flatter and exhibited smaller cracks in center region.The ablation mechanism of HfC coating has been changed by SiC nanowires.Thicker HfO2 fused layer was formed on the surface of the toughed HfC coating,which could provide efficient protection for C/C composites.Therefore,SiC nanowires toughed HfC coating behaved in better ablation resistance.     

Mechanical Property,Oxidation and Ablation Resistance of C/C–ZrB_2–ZrC–SiC Composite Fabricated by Polymer Infiltration and Pyrolysis with Preform of C_f/ZrB_2

C/C–ZrB_2–ZrC–SiC composites were fabricated by polymer infiltration and pyrolysis(PIP) with a preform of C_f/ZrB_2. The carbon fibers and the resin carbon were coated with ceramic layer after PIP in the composites. The composite presents a pseudo-plastic fracture due to deflection of cracks and pullout of fibers.The composite has a higher bending strength by this method in comparison with the conventional PIP process due to fewer heat treatment cycles. The static oxidation test shows that the mass loss of the composites is no more than 1% after 20 min oxidation at 1100 °C. The "core–shell" structure between ZrC–SiC ceramic and other phases plays a positive role in preventing the inward diffusion of oxygen. The ablation resistance of the C/C–ZrB_2–ZrC–SiC composite samples was tested using a plasma generator. After ablation for 120 s, the mass and linear ablation rates of the composites are 4.65 mg cm~(-2)s~(-1) and 2.46 μm s~(-1), respectively. The short carbon layer shows a better ablation resistance than the nonwoven carbon fabric layer after the ceramic coating is peeled off because of its higher ceramic content.     

W-Mo-Si/SiC Oxidation Protective Coating for Carbon/Carbon Composites

A W-Mo-Si/SiC double-layer oxidation protective coating for carbon/carbon （C/C） composites was prepared by a two-step pack cementation technique. XRD （X-ray diffraction） and SEM （scanning electron microscopy）results show that the coating obtained by the first step pack cementation was a thin inner buffer layer of SiC with some cracks and pores, and a new phase of （WxMo1-x）Si2 appeared after the second step pack cementation. Oxidation test shows that, after oxidation in air at 1773 K for 175 h and thermal cycling between 1773 K and room temperature for 18 times, the weight loss of the W-Mo-Si/SiC coated C/C composites was only 2.06%. The oxidation protective failure of the W-Mo-Si/SiC coating was attributed to the formation of some penetrable cracks in the coating.     

Interfacial Structure and Formation Mechanism of Ultrasonic-assisted Brazed Joint of SiC Ceramics with Al—12Si Filler Metals in Air

Ultrasonic-assisted brazing of SiC ceramics was performed by filling with an Al—12Si alloy at a low temperature of 620 °C in air. The interfacial characteristics and formation mechanism were investigated. The joint shear strength reached 84–94 MPa using the ultrasonic time of 2–16 s. The fracture morphology showed that the fracture path initiated and propagated in the joint alloy. The thin film of amorphous SiO 2 that formed on the SiC surface was non-uniformly decomposed and diffused into the liquid Al—12Si alloy under the cavitation erosion effect of ultrasound. Abnormal isolated blocks of Al_2 SiO_5 compounds formed at the interface between Al—12Si and a thicker SiO_2 layer formed during the thermal oxidation treatment of the Si C ceramic. The SiO_2 layer on the Si C ceramic did not hinder or impair the wetting and bonding process, and a stronger bond could form between Al—12Si and SiO_2 or Si C in ultrasonicassisted brazing.     

Ablation characteristics of mosaic structure ZrC-SiC coatings on low-density,porous C/C composites

Mosaic structure Zr C-SiC coatings were fabricated on low-density, porous C/C composites via thermal evaporation and an in-situ method. Zr C was packed in a typical lamellar mode, and the mosaic structure was formed by the deposition of Zr and Si atoms on the shallow surface of the porous C/C composites.Ablation analysis showed that the defects in the coatings originate from the boundary between the Zr C and holes created by the consumption of SiC at 2500?C. After ablation for 200 s at 3000?C, a dense ZrO_2 layer formed on the coating surface, and the defects were sealed owing to the continuous supply of ablative components. The mass and line ablation rates of the Zr C-SiC coatings were-0.46 ± 0.15 mg cm~(-2)·s~(-1) and-1.00± 0.04 μm s~(-1), respectively.     

Oxidation and Mechanical Properties of SiC/SiC-MoSi_2-ZrB_2 Coating for Carbon/Carbon Composites

To improve oxidation resistance of carbon/carbon(C/C) composites,a SiC/SiC-MoSi_2-ZrB_2 double-layer ceramic coating was prepared on C/C composites by two-step pack cementation.The phase compositions and microstructures of as-prepared multilayer coating were characterized by X-ray diffraction and scanning electron microscopy.The oxidation resistance at 1773 K and the effect of thermal shock between 1773 K and room temperature on mechanical performance of coated specimens were investigated.The results show that the SiC/SiC-MoSi_2-ZrB_2 coating exhibits dense structure and is composed of SiC,Si,MoSi_2 and ZrB_2·It can protect C/C composites from oxidation at 1773 K for more than 510 h with weight loss of 0.5%.The excellent anti-oxidation performance of the coating is due to the formation of SiO_2-ZrSiO_4 complex glassy film.The coating can also endure the thermal shocks between 1773 K and room temperature for 20 times with residual flexural strength of 86.1%.     

Design,Preparation and Properties of Carbon Fiber Reinforced Ultra-High Temperature Ceramic Composites for Aerospace Applications:A Review

Carbon fiber reinforced ultra-high temperature ceramic(UHTC) composites, consisting of carbon fibers embedded in a UHTC-matrix or a C–SiC–UHTC-matrix, are deemed as the most viable class of materials that can overcome the poor fracture toughness and thermal shock resistance of monolithic UHTC materials, and also improve the oxidation resistance and ablation resistance of C/C and C/SiC composites at ultra-high temperatures. In this review, we summarize the different processing routes of the composites based on the UHTC introducing methods, including chemical vapor infiltration/deposition(CVI/D),precursor infiltration and pyrolysis(PIP), reactive melt infiltration(RMI), slurry infiltration(SI), in-situ reaction, hot pressing(HP), etc; and the advantages and drawbacks of each method are briefly discussed. The carbon fiber reinforced UHTC composites can be highly tailorable materials in terms of fiber,interface, and matrix. From the perspective of service environmental applications for engine propulsions and hypersonic vehicles, the material designs(mainly focusing on the composition, quantity, structure of matrix, as well as the architecture of carbon fibers, UHTCs and pores), their relevant processing routes and properties(emphasizing on the mechanical and ablation properties) are discussed in this paper. In addition, we propose a material architecture to realize the multi-function through changing the distribution of carbon fibers, UHTCs and pores, which will be an important issue for future development of carbon fiber reinforced UHTC composites.     

Microstructure and Mechanical Properties of C/C-ZrC-SiC Composites Fabricated by Reactive Melt Infiltration with Zr,Si Mixed Powders

To meet the increasing demand for advanced materials capable of operation over 2000℃for future thermal protection systems application,C/C—ZrC—SiC composites were fabricated by reactive melt infiltration(RMI) with Zr,Si mixed powders as raw materials.The structural evolution and formation mechanism of the C/C—ZrC -SiC composites were discussed,and the mechanical property of the as-prepared material was investigated by compression test.The results showed that after the RMI process,a special structure with ZrC-SiC multi-coating as outer layer and ZrC-SiC-PyC ceramics as inner matrix was formed.ZrC and SiC rich areas were formed in the composites and on the coating surface due to the formation of Zr-Si intermetallic compounds in the RMI process.Mechanical tests showed that the average compression strength of the C/C-ZrC-SiC composites was 133.86 MPa,and the carbon fibers in the composites were not seriously damaged after the RMI process.     

Microstructure and tensile behavior of 2D-C_f/AZ91D composites fabricated by liquid–solid extrusion and vacuum pressure infiltration

2D carbon fiber reinforced AZ91 D matrix composites(2D-C_f/AZ91 D composites) were fabricated by liquid–solid extrusion and vacuum pressure infiltration technique(LSEVI). In order to modify the interface between fibers and matrix and protect the fiber, pyrolytic carbon(Py C) coating was deposited on the surface of T700 carbon fiber by chemical vapor deposition(CVD). Microstructure observation of the composites revealed that the composites were well fabricated by LSEVI. The segregation of aluminum at fiber surface led to the formation of Mg_(17)Al_(12) precipitates at the interface. The aluminum improved the infiltration of the alloy and Py C coating protected the fibers effectively. The ultimate tensile strength of 2D-C_f/AZ91 D composites was about 400 MPa. The fracture process of 2D-C_f/AZ91 D composites was transverse fiber interface cracking–matrix transferring load–longitudinal fibers bearing load–longitudinal fibers breaking.     

Mullite Oxidation Resistant Coating for SiC-coated Carbon/Carbon Composites by Supersonic Plasma Spraying

To improve the oxidation resistance of carbon/carbon (C/C) composites, mullite coating was prepared on the surface of SiC-coated C/C composites by supersonic plasma spraying. Phases and microstructures of mullite coating were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The coating primarily consists of a single phase of mullite (3Al2O3-2SiO2). The SEM results show that mullite coating was continuous and well bonded with the SiC inner layer without penetrating crack. Mullite coating exhibited good oxidation resistance. After 98.5 h oxidation at 1773 K and 9 thermal shock cycles between 1773 K and room temperature, the weight loss of the coated C/C composites was only 2.57%.     

High-efficiency Joining of C_f/Al Composites and TiAl Alloys under the Heat Effect of Laser-ignited Self-propagating High-temperature Synthesis

The aim of this study was to develop a high-efficiency joining method of Cf/Al composites and TiA l alloys under the heat effect of laser-ignited self-propagating high-temperature synthesis(SHS). The SHS reaction of Ni–Al–Zr interlayer was induced by laser beam and acted as local high-temperature heat source during the joining. Sound joint was obtained and verified the feasibility of this joining method. Effect of filler metals on the joint microstructure and shear strength was evaluated. When the joining pressure was 2 MPa with additive filler metals, joint shear strength reached the maximum of 41.01 MPa.     

Fabrication of barium-strontium aluminosilicate coatings on C/SiC composites via laser cladding

Barium-strontium aluminosilicate(BSAS) and Si/BSAS coatings were fabricated on the surface of C/SiC composites via a two-step laser cladding process. The microstructure, mechanical properties, and the water vapor corrosion behavior of the samples were investigated. The BSAS coating was found to be tightly bonded to the substrate and only a few pores and microcracks were observed. The introduction of a silicon middle layer was revealed to reduce thermal stress and promote the healing of defects formed during the laser cladding process. To evaluate the corrosion resistance, the BSAS and Si/BSAS-coated C/SiC composites were exposed to an atmosphere of 50% H_2O and 50% O_2 at 1250 °C. The resulting weight change and flexural strength were measured as a function of the corrosion time. The addition of the silicon middle layer below the BSAS top layer resulted in a better resistance to water vapor corrosion. Furthermore, the Si/BSAS-coated samples showed a lower weight loss and a smaller reduction in flexural strength than the BSAS-coated and the uncoated samples during water vapor corrosion. Thus, laser cladding is demonstrated to be an effective and feasible method to fabricate high-quality ceramic coatings on C/SiC composites. The introduction of a silicon middle layer can inhibit defect formation during the laser cladding process and protect the composite from water vapor corrosion.     

Oxidation and Mechanical Properties of SiC/SiC-MoSi2-ZrBa Coating for Carbon/Carbon Composites

To improve oxidation resistance of carbon/carbon （C/C） composites, a SiC/SiC-MoSi2-ZrB2 double-layer ceramic coating was prepared on C/C composites by two-step pack cementation. The phase compositions and microstructures of as-prepared multilayer coating were characterized by X-ray diffraction and scanning electron microscopy. The oxidation resistance at 1773 K and the effect of thermal shock between 1773 K and room temperature on mechanical performance of coated specimens were investigated. The results show that the SiC/SiC-MoSi2-ZrB2 coating exhibits dense structure and is composed of SiC, Si, MoSi2 and ZrB2. It can protect C/C composites from oxidation at 1773 K for more than 510 h with weight loss of 0.5%. The excellent anti-oxidation performance of the coating is due to the formation of SiO2-ZrSiO4 complex glassy film. The coating can also endure the thermal shocks between 1773 K and room temperature for 20 times with residual flexural strength of 86.1%.     

SiC-Si-ZrSiO_4 Multiphase Oxidation Protective Coating for Carbon/Carbon Composites

In order to improve the anti-oxidation property of carbon/carbon （C/C） composites, a novel SiC-Si-ZrSiO4 multiphase oxidation protective coating was produced on the surface of C/SiC coated carbon/carbon compo ites by a pack cementation technique. The phase composition and microstructure of the as-prepared coatings were characterized by XRD （X-ray diffraction）, SEM （scanning electron microscopy） and EDS （energy dispersive spectroscopy）. Oxidation behavior of the multiphase coated C/C composites was also investigated. It showed that the as-prepared coating characterized by excellent oxidation resistance and thermal shock re- sistance could effectively protect C/C composites from oxidation at 1773 K for 57 h in air and endure the thermal cycle between 1773 K and room temperature for 12 times, whereas the corresponding weight loss is only 1.47%. The excellent oxidation protective ability of the SiC-Si-ZrSiO4 coating could be attributed to the C/SiC gradient inner layer and the multiphase microstructure of the coating.     

Corrosion Behavior of SiCp／2024 Al Matrix Composites in 3.5 wt pct Sodium Chloride

The influences of volume fraction and particle size of SiC particulate reinforcements on the corrosion characteristics of SiCp/2024Al metal matrix composites in aerated 3.5wt pct NaCl aquecus solution were investigated.The electro-chemical behavior was investigated by prtentiodynamic polarization and electrochemical impedance spectroscopy, the general corrosion behavior of the composites was studied further by immersion tests.The results showed that pitting susceptibility was about the same for the composites and the alloy.The corrosion potentials were also independent of SiC phase.The corrosion resistance for the composites decreased as the volume fraction increased or particle size decreased.     

A Heat Flux and Erosion Experiment in the Scrape-off Layer of the HL-1 Tokamak

Under Ohmic heating (OH) discharges with a movable graphite limiter a heat flux and erosion experiment was carried out with a double-strip graphite probe located at the scrape-off layer in HL-1 tokamak. The typical parameters of OH discharges Were 2T of B_t, 120kA of I_p, 4E19m~(-3) of n_e and 1 sec of t_p. The integrated temperature rise of 150℃at the graphite strip located at a position of 2.5 cm outside the last closed flux surface and faced to the ion drift side was measured during an OH shot, and the relative integrated heat flux of 150 Wcm~(-2) deposited to the graphite strip was estimated. After 74 shots of OH discharge,the total mass erosion rate was 2.4% on the ion drift side and 4.95% on the electron drift side of the double strip graphite probe.Surface analysis of the exposured double strip probe showed that the deposited metal impurities of Ni, Cr and Fe in the SOL were reduced qreatly than while a SS movable limiter was used, and consequently the improved hydrogen plasma performance was found in OH discharges.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Prediction on Carbon/Carbon Composites Ablative Performance by Artificial Neutral Net

A preliminary estimation of ablation property for carbon-carbon composites by artificial neutral net （ANN） method was presented. It was found that the carbon-carbon composites＇ density, degree of graphitization and the sort of matrix are the key controlling factors for its ablative performance. Then, a brief fuzzy mathematical relationship was established between these factors and ablative performance. Through experiments, the performance of the ANN was evaluated, which was used in the ablative performance prediction of C/C composites. When the training set, the structure and the training parameter of the net change, the best match ratio of these parameters was achieved. Based on the match ratio, this paper forecasts and evaluates the carbon-carbon ablation performance. Through experiences, the ablative performance prediction of carbon-carbon using ANN can achieve the line ablation rate, which satisfies the need of precision of practical engineering fields.