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.     

Ablation Mechanism of Carbon/Carbon Composites Modified by HfC–SiC in Two Conditions Under Oxyacetylene Torch

C/C–HfC–SiC composites prepared by precursor infiltration and pyrolysis process were ablated by oxyacetylene torch under two different flame conditions.The ablation performance of the composites was investigated in the heat flux of 2.38 MW/m~2(HF-L) and 4.18 MW/m~2(HF-H) for 60 s.The mechanical denudation in 4.18 MW/m~2(HF-H) was higher than that in 2.38 MW/m~2(HF-L),while the results indicated that the composites had a similar and good ablation property under two different flame conditions.C/C–HfC–SiC composites can adapt the heat flux from 2.38 MW/m~2 to 4.18 MW/m~2.The HfO_2 was not melted completely in the heat flux of 2.38 MW/m~2(HF-L).So,both HfO_2 and SiO_2 layers acted as an effective barrier to the transfer of heat and oxidative gases into the underlying carbon substrate.SiO_2 was severely consumed in 4.18 MW/m~2(HF-H),where the HfO_2 molten layer played a more important role in protecting the inner composite.     

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.     

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.     

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%.     

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.     

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.     

Erosion Characteristics of Aluminum-based Metal Matrix Composites in Slurry Environments

The erosion resistance of the Al18B4O33 whisker reinforced AC4C Al composites in water and saline slurry were investigated using a jet-in-slit rig. Erosion tests were performed at slurry velocities between 6.4 m/s to 15.2 m/s and at normal impact angle. The detachment of flake and dislodgement of whisker were identified as the major mechanisms of material removal in slurry environments. The composites showed better erosion resistance due to the protection of the matrix by the whisker at low slurry velocities. Because of reduced fracture strain, the erosion rates of the composites were generally greater than that of the unreinforced alloy at high slurry velocities. Owing to interfacial reaction which resulted in decrease in hardness and fracture strain,the T6 treatment for the composites had a deleterious influence on the erosion resistance. By considering the material removal processes in the water slurry, a simple rationalization of the inverse dependence of slurry erosion rate on Hεf was obtained. In the saline slurry, there exists a strong synergistic effect between erosion and corrosion. The volume loss of the composites was enhanced through cracking of flakes and detaching of whisker induced by stress and corrosion.     

Microstructure and mechanical properties of hot extruded 6016 aluminum alloy/graphite composites

The incorporation of graphite particles into AA6016 aluminum alloy matrix to fabricate metal/ceramic composites is still a great challenge and various parameters should be considered. In this study, dense AA6016 aluminum alloy/(0-20 wt%) graphite composites have successfully been fabricated by powder metallurgy process. At first, the mixed aluminum and graphite powders were cold compacted at 200 MPa and then sintered at 500 ℃ for 1 h followed by hot extrusion at 450 ℃. The influence of ceramic phases(free graphite and in-situ formed carbides) on microstructure, physical and mechanical properties of the produced composites were finally investigated. The results show that the fabricated composites have a relative density of over 98%. SEM observations indicate that the graphite has a good dispersion in the alloy matrix even at high graphite content. Hardness of all the produced composites was higher than that of aluminum alloy matrix. No cracks were observed at strain less than 23% for all hot extruded materials.Compressive strength, reduction in height, ultimate tensile stress, fracture stress, yield stress, and fracture strain of all Al/graphite composites were determined by high precision second order equations. Both compressive and ultimate tensile strengths have been correlated to microstructure constituents with focusing on the in-situ formed ceramic phases, silicon carbide(SiC) and aluminum carbide(Al_4 C_3). The ductile fracture mode of the produced composites became less dominant with increasing free graphite content and in-situ formed carbides. Wear resistance of Al/graphite composites was increased with increasing graphite content. Aluminum/20 wt% graphite composite exhibited superior wear resistance over that of AA6016 aluminum alloy.     

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.     

Influence of ZrB_2 Nanoparticles on the Mechanical and Thermal Behaviors of Carbon Nanotube Reinforced Resol Composite

The present study focuses on the thermal response of carbon ?ber-reinforced phenolic composites, where the matrix has been modi?ed with different reinforcements. Two types of materials, multiwalled carbon nanotubes and zirconium diboride(ZrB_2), were used in a new design of mixture to produce the heatresistant ablative composite system. The CNT/ZrB_2/carbon/phenolic nanocomposite(Z/NT-CR) system corresponding to CNT/carbon/phenolic nanocomposite(NT-CR) showed a reasonable decrease in mass loss and the ablation rate as compared to carbon/phenolic composite(CR). However, substantial drop in two factors was found for Z/NT-CR as compared to carbon/phenolic and NT-CR. Ablation mechanisms for all three composites were investigated by thermal gravimetric analysis in conjunction with microstructural studies using a ?eld emission scanning electron microscope. The microstructural studies revealed that CNTs acted as an ablation resistant phase for protection against 2000 °C, and the conversion from ZrB_2 to Zr O2 played an important role as an insulator in the performance of char layer in the ablation resistance.     

Nature of Surface Layer and Electrochemical Behavior of NaOH Hydrothermally Treated NiTi Alloy

NiTi samples were hydrothermally treated in NaOH at 200℃ with different soaking times. The morphology of the surface layer formed was studied by scanning electron microscopy (SEM). The composition of the layer and the major phases present were determined by energy-dispersive spectroscopy (EDS) and X-ray diffractometry (XRD), respectively. In contrast to the results reported by some authors,the surface layer was essentially Ni(OH)2 instead of being TiO2. The electrochemical behavior of the samples was studied by electrochemical impedance spectroscopy (EIS) in 3.5% NaCI solution at 23℃,and analyzed using a simplified Randle circuit consisting of a resistance R and a capacitance C in parallel. After hydrothermal treatment, R was increased by a factor ranging from 1.5 to 5.0 times, depending on the treatment time. The value of R of all the samples became steady within a period of less than 15 h. Results of the present study indicate that alkaline treatment leads to the growth of an insulating layer on NiT     

High-Temperature Oxidation Behavior of a Ni-Cr-W-Al Alloy

A study was conducted to examine the isothermal oxidation behavior of a new Ni-Cr-W-Al alloy in air at 1250℃ with different time. Oxidation kinetics was determined from weight-change measurements. The microstructure and composition of the oxide scale were investigated by means of scanning electron microcopy and X-ray diffraction. The results showed that the oxide scales of the alloy were a compact and continuous outer Cr2O3 and NiCr2O4 layer and an inner Al2O3 layer that was in dendrite shape. Oxides scales with good adherence were formed on the surface of the alloy, which made the alloy perform excellent high-temperature oxidation resistance.     

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.     

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%.     

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%.     

Performance of Potassium Titanate Whisker Reinforced PPESK Composites

A group of poly (phthalazinone ether sulfone ketone)/potassium-titanate-whisker (PPSEK/whisker) composites was prepared by coprecipitation from solution. The whisker surface was modified using titanate coupling agent prior to blending. The tensile, impact, morphology and thermal properties of the moulded composites were investigated. The measurements showed that the tensile strength and impact strength of the composites increased with increasing whiskers content up to 10 to 20 phr, thereafter they showed a decrease in the whiskers content reached 40 phr. At the same time, the modulus of the composites increased with increasing whiskers content. Scanning electron microscopy studies revealed that the whiskers within the composites were dispersed uniformly by treated with coupling agent. Finally, thermogravimetric analysis showed that the heat resistance of the composites tended to increase with increasing whisker content. The results were analysed and discussed in terms of established models of the behavi     

Preparation and Microstructure of a Si-Mo Fused Slurry Coating on Carbon/Carbon Composites for Oxidation Protection

A coating of composition Si-40Mo (wt pct) was prepared by fused slurry coating method on the two-dimensional carbon/carbon (2D-C/C) composite to improve oxidation resistance. In the procedure of the fabrication, pure St slurry inner layer in the pre-coating was necessary to apply because of infiltration of liquid Si into the substrate during the sintering. The coating consists of Si continuous phase and MoSi2 particles. In addition, the infiltration of Si into the substrate and the SiC reaction layer between the coating and the C/C composite were observed. Oxidation resistance and the thermal shock resistance of the Si-Mo fused sluury coating were quite excellent at 1730℃     

Plasma Post Oxidation of Plasma Nitrocarburized SKD 61 Steel

Plasma nitrocarburizing and plasma oxidizing treatments were performed to improve the wear and corrosion resistance of SKD 61 steel.Plasma nitrocarburizing was conducted for 12 h at 540℃in the nitrogen, hydrogen and methane atmosphere to produce theε-Fe-（2-3）（N,C） phase.The compound layer produced by plasma nitrocarburising was predominantly composed ofε-phase,with a small proportion ofγ′-Fe-4 （N,C） phase. The thickness of the compound layer and the diffusion layer are about 10μm and about 200μm,respectively. Plasma post oxidation was performed on the nitrocarburized samples with various oxygen/hydrogen ratio at constant temperature of 500℃for 1 h.The very thin magnetite （Fe-3O-4） layer of 1-2μm in thickness on top of the compound layer was obtained.Anodic polarization test revealed that plasma nitrocarburizing process contributed a significant improvement of corrosion resistance of SKD 61 steel.However,the corrosion characteristics of the nitrocarburized compound layer was deteriorated by oxidation treatment.