Preparation and high‐temperature behavior of HfC–SiC nanocomposites derived from a non‐oxygen single‐source‐precursor

A single‐source precursor for the preparation of HfC‐SiC ceramics was synthesized via a Grignard reaction using bis(cyclopentadienyl)hafnium(IV) dichloride, trans‐1,4‐dibromo‐2‐butene, and (chloromethyl)trimethylsilane as raw materials. The composition, structure, pyrolysis process and high‐temperature behavior of the precursor were investigated. The results show that the precursor with a backbone comprising Hf–C, Si–C and CH=CH groups exhibits good solubility in common solvents, such as tetrahydrofuran, dimethylbenzene, and chloroform. Pyrolysis of the precursor at 1000°C yielded a microcrystalline HfC phase with a ceramic yield of 63.86 wt%. The pyrolytic products at 1600°C were HfC–SiC nanocomposite ceramics, which exhibited good thermal stability up to 2400°C. The formation of a (Hf,Si)C solid‐solution would be beneficial for densification during the sintering process. The non‐oxygen structure, high ceramic yield, homogeneous composition and excellent high‐temperature behavior of the pyrolytic products make the as‐prepared precursor a promising material for the preparation of high‐performance ultra‐high‐temperature ceramics.     

A gradient composite coating to protect SiC-coated C/C composites against oxidation at mid and high temperature for long-life service

To improve the oxidation resistance of carbon/carbon (C/C) composites at mid and high temperature, a gradient composite coating was designed and prepared on SiC-coated C/C composites by in situ formed-SiO₂ densifying the porous SiC-ZrSi₂ pre-coating. SiO₂ gradient distribution was conducive to inhibiting the cracking of the coating. A dual-layer structure with the outer dense layer and the inner microporous layer was formed in the coating during densifying. The dense layer had excellent oxygen diffusion resistance and the microporous layer alleviated CTE mismatch between SiC inner coating and dense layer. Moreover, ZrSiO₄ particles inhibited crack propagation and stabilized SiO₂ glass. Therefore, the coating can protect the C/C composites from oxidation at 1473 K, 1573 K and 1773 K for 810 h, 815 h and 901 h, respectively. The coated samples underwent 30 thermal cycles between room temperature and 1773 K without mass loss, exhibiting good thermal shock resistance.     

Ablation behavior of HfC coating with different thickness for carbon/carbon composites at ultra-high temperature

The thickness of the different HfC coatings from 20 μm to 50 μm were prepared on the surface of carbon/carbon (C/C) composites by low pressure chemical vapor deposition (LPCVD). The microstructure and thermal stress of the coatings after ablation were investigated, as well as the effect of thickness and thermal stress on the ablation resistance of the HfC coating was analyzed. After being ablated at a heat flux of 2.4 MW/m² for 60 s, the thermal stress gradually increased at first and then rapidly increased with the increasing thickness of coating. The results indicated that the moderate coating thickness can effectively release the thermal stress generated during the ablation process. The 40 μm-thick HfC coating showed the best ablation resistance with the mass ablation rate and line ablation rate were only 0.13 mg/s and 0.09 μm/s, respectively.     

SiC-Si coating with micro-pores to protect carbon/carbon composites against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperatures, a SiC-Si coating with micro-pores was prepared by slurry and heat-treatment on the surface of C/C composites with SiC-Si inner coating acquired by pack cementation (PC). The microstructure, phase composition, element distribution, and anti-oxidation properties of the dual-layer SiC-Si coating were investigated. The results show that a SiO₂-SiC inlay structure was formed during the oxidation process, due to a large amount of SiO₂ rapidly generated by the oxidation of SiC particles in the porous coating. The coating with this structure could inhibit the cracking of SiO₂ glass and had a good resistance to oxygen diffusion. Moreover, the crack propagation was blocked by the remaining micro-pores of the coating. The coating could protect C/C composites against oxidation for 846 h only with the mass loss of 0.16 % at 1773 K in air.     

SiC coating with high crack resistance property for carbon/carbon composites

A novel SiC coating with a relatively high crack resistance property (crack extension force (G_C): 12.0 J·m^?2) and outstanding thermal shock resistance was achieved merely by pack cementation. Compared with the conventional SiC coating with Al₂O₃ addition (AOSC2), SiC coating with Al–B–C additions (ABSC2) possesses refined and denser microstructure owing to different effects in promoting SiC densification under different additions. Therefore, the improvement in microstructures results in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (between 1500 °C and room temperature).     

Study of thermal stability and ablation behavior of carbon/epoxy‐novolac composites

The use of carbon/epoxy‐novolac composites as advanced ablative materials for insulation of exit cone of solid‐propellant rocket nozzles are studied. In this article, three types of carbon fabrics are used and their composites are prepared by use of impregnation and hand lay‐up methods. To study the thermal stability and ablation behavior, these composites are tested by thermal tests such as thermogravimetric analysis (TGA) and oxyacetylene standard flame tests; the latter test is one of the most important standard tests of ablative materials. The test apparatus is made according to American standard, ASTM‐E‐285‐80, and over 33 polymeric composites and 3 steel specimens were carried out according to its standards. It is found that the composites that are made up of C‐9750 fabric (high‐strength carbon fabric) in comparison with steel and the other types of carbon fabric specimens have the highest thermal stability and the best ablation behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2455–2461, 2003     

Ablation resistance of HfC-TaC/HfC-SiC alternate coating for SiC-coated carbon/carbon composites under cyclic ablation

HfC-TaC/HfC-SiC alternate coatings with different sublayer thicknesses were fabricated on SiC-coated carbon/carbon composites by supersonic atmosphere plasma spraying. Their ablation resistance was studied under oxyacetylene torch and compared with monolayered HfC-TaC coating. The alternate coating with 6 spray cycles of HfC-TaC and 3 spray cycles of HfC-SiC sublayers exhibited the best ablation performance as confirmed by the integral coating morphology and the lowest ablation rates. A dense oxide layer acting as an oxygen insulator and the release of thermal stress induced by the formation of dendritic cracks are thought to be responsible for its great ablation resistance. For the alternate coating with 4 spray cycles of HfC-TaC and 2 spray cycles of HfC-SiC sublayers, exfoliation occurred at the interface of two adjacent sublayers, leading to violent evaporation of exposed HfC-SiC sublayer.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Nickel‐multiwalled carbon nanotubes/polyvinylidene fluoride composites with high dielectric permittivity

Composites with nickel particles coated multiwalled carbon nanotubes (Ni‐MWNTs) embedded into polyvinylidene fluoride (PVDF) were prepared by solution blending and hot‐press processing. The morphology, structure, crystallization behavior, and dielectric properties of composites were studied. The results showed that the crystallization of PVDF was affected by Ni‐MWNTs. With the increment of Ni‐MWNTs, the content of β‐phase in PVDF increased. The dielectric permittivity was as high as 290 at 10³ Hz when the weight fraction of Ni‐MWNTs was 10%. The results can be explained by the space charge polarization at the interfaces between the insulator and the conductor, and the formation of microcapacitance structure. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3746–3752, 2013     

炭/炭复合材料抗氧化研究现状

总结了炭/炭复合材料抗氧化研究现状,重点介绍了抗氧化涂层的制备方法,包括包埋浸渍法、化学气相沉积(CVD)、溶胶-凝胶法(Sol-Gel)和水热电沉积法以及近几年针对抗氧化涂层开裂问题各国学者的最新研究成果。提出了炭/炭复合材料抗氧化研究今后努力的方向。     

Synthesis of fine zirconium carbide powder by carbothermal reaction of carbon‐coated zirconia particles

In this study, a novel method for the synthesis of fine ZrC powder was presented. It consists of chemical vapor deposition (CVD) of C from CH₄ on ZrO₂ particles followed by carbothermal reaction. Firstly, optimal CVD conditions (1300 K and 30 minutes) yielding the stoichiometric amount of C deposit (23 wt%) were determined. Carbothermal reaction behavior of the carbon‐coated oxide particles was then investigated in Ar flow at 1700‐1800 K for 0‐120 minutes. Mass measurements, XRD and SEM techniques were used to characterize the products at various stages of the process. Lattice constants and mass losses of the samples increased to the levels of ZrC with increasing temperature and time. Almost pure ZrC powder (oxygen content: 0.59 wt%) with a mean particle size of ~170 nm was synthesized at 1800 K within 120 minutes. The present study demonstrates that ZrC powder can be synthesized at lower temperatures and shorter reaction times using C‐coated ZrO₂ powders compared with the conventional method which uses a mixture of ZrO₂ and solid C particles.     

Thermal degradation of high‐temperature fluorinated polyimide and its carbon fiber composite

High‐temperature polymers are being used for a broad range of applications, such as composite matrices for structural applications (e.g., high speed aircraft). Polyimides are a special class of polymers that meet the thermal and oxidative stability requirements for high temperature composite aerospace applications. A weight loss study was performed on a fluorinated polyimide resin and its carbon fiber composite in an effort to determine its thermal stability and degradation mechanisms. Experiments were conducted using a preheated oven and thermogravimetric analysis to obtain the weight loss. Regardless of the method used, the resin and composite exhibited excellent thermal stability (less than 1% weight loss) below 430°C, regardless of 2–20 min of exposure. After 20 min of exposure at 510°C, the composite remained relatively stable with only 5.3% weight loss using the oven technique, whereas the neat polyimide sustained 12.6%. When degradation occurred, it was found to be the result of thermolysis and oxidation (to a lesser extent). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Preparation and properties of T300 carbon fiber‐reinforced thermoplastic polyimide composites

In this study, a series of T300 carbon fiber‐reinforced polyimide (CFRPI) composites were prepared by laminating premolding polyimide (PI) films with unidirectional carbon fiber (CF) layers. On the basis of PI systems design, the effect of CF volume fraction, processing conditions, and PI molecular structure on the properties of CFRPI composites was studied in detail. In addition, two kinds of nano‐particles, including carbon nano‐tube (CNT) and SiO₂ were filled into the premolding PI films with different concentrations. And the effect of nano‐particles on the properties of CFRPI composites was also investigated. The surface characteristic of T300 CF was measured by X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The properties of premolding PI film and CFRPI composites were measured by dynamic mechanical analysis (DMTA), SANS testing machine, scanning electron microscopy (SEM), and so forth. These experimental results showed that the properties of CFRPI composites were mainly affected by the premolding PI film and molding condition. The change of CF volume fraction from 55% to 65% took little effect on the mechanical properties of CFRPI composites. In addition, the incorporation of nano‐particle SiO₂ could further improve the properties of CFRPI composites, but CNT hardly improved the properties of CFRPI composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 646–654, 2006     

Phase control during synthesis of nanocrystalline ultrahigh temperature tantalum‐hafnium diboride powders

Ta_1?xHf_xB₂ material is attractive for various aerospace applications. In this study, 2 low‐cost approaches were adopted to synthesize nanocrystalline Ta_0.5Hf_0.5B₂ solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum‐hafnium‐boron oxide(s) and carbon obtained from aqueous solution processing of TaCl₅, HfCl₄, B₂O₃, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta₂O₅ and HfO₂ and the difference in reactivity of those 2 oxides with carbon (as well as B₂O₃), individual TaB₂ (‐rich) and HfB₂ phases always form separately. Those borides tend to remain phase separated due to the slow inter‐diffusion between them. However, it was observed that addition of copper “catalyst” noticeably accelerates the inter‐diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl₅ and HfCl₄ are directly reacted with sodium borohydride (NaBH₄). This method yields a single phase Ta_0.5Hf_0.5B₂ solid solution nanopowders in one step at much lower temperatures (e.g., 700°C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR‐based process.     

Internal friction behavior of carbon–carbon composites

The fundamental internal friction behavior of carbon–carbon composites is studied. Two internal friction mechanisms are proposed according to the special internal friction characteristics in carbon–carbon composites. A thermoelastic mechanism, which is independent of amplitude, mainly leads to the internal friction increase with increasing frequency. The other is a static hysteresis mechanism that internal friction depends on the amplitude but is independent of frequency. Moreover, it is very interesting that some abnormal internal friction phenomena can be observed. The variation characteristics of internal friction and dynamic modulus versus temperature in carbon–carbon composites are quite different from other materials. This special behavior may be a result of interfacial CTE effects, as well as the coordination effects of the individual response of the fibers, matrix and interface of carbon–carbon composites. Finally, the validity of internal friction analysis methods for densification process monitoring and non-destructive inspection of carbon–carbon composites is discussed for the first time. The results indicate that internal friction testing methods have great potential for monitoring process and inspecting components of carbon–carbon composites non-destructively.     

Interphase degradation of three‐dimensional Cf/SiC–ZrC–ZrB2 composites fabricated via reactive melt infiltration

Layer‐structured interphase, existing between carbon fiber and ultrahigh‐temperature ceramics (UHTCs) matrix, is an indispensable component for carbon fiber reinforced UHTCs matrix composites (C_f/UHTCs). For C_f/UHTCs fabricated by reactive melt infiltration (RMI), the interphase inevitably suffers degradation due to the interaction with the reactive melt. Here, C_f/SiC–ZrC–ZrB₂ composite was fabricated by reactive infiltration of ZrSi₂ melt into sol‐gel prepared C_f/B₄C–C preform. (PyC–SiC)₂ interphase was deposited on the fiber to investigate the degradation mechanism under ZrSi₂ melt. It was revealed that the degraded interphase exhibited typical features of Zr aggregation and SiC residuals. Moreover, the Zr species diffused across the interphase and formed nanosized ZrC phase inside the fiber. A hybrid mechanisms of chemical reaction and physical melting were proposed to reveal the degradation mechanism according to characterization results and heat conduction calculations. Based on the degradation mechanism, a potential solution to mitigate interphase degradation is also put forward.     

High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles

Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB₂‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.     

Thermal properties and performance of carbon fiber-based ultra-high temperature ceramic matrix composites (Cf-UHTCMCs)

The thermophysical properties of carbon fiber-based ultra-high temperature ceramic matrix composites have been determined to aid designers who need these properties when considering using the composites in ultra-high temperature aerospace applications. The coefficient of thermal expansion (CTE) and thermal diffusivity of the composites were measured parallel and perpendicular to the ply direction; the thermal conductivity was measured using the laser-flash method and the heat capacity calculated from the relationship between the thermal diffusivity, density, and thermal conductivity. Both the CTE and thermal conductivity showed higher values across the ply and increased with increasing temperature as expected, whilst the thermal diffusivity showed higher values parallel to the ply and increased smoothly with temperature. In addition, two different but related oxyfuel torch tests, based on oxyacetylene and oxypropane, were used to evaluate the thermo-ablation behavior of the composites. The tests showed how good the composites were at withstanding the ultra-high temperatures, high heat fluxes, and gas velocities involved.     

动态过程钢坯高温防氧化技术

概述了国内外钢坯高温防氧化涂料的研究现状。分析了动态过程钢坯高温防氧化涂料的技术要求。介绍了一种自主开发的动态过程钢坯高温防氧化涂料的应用状况。指出钢坯高温防氧化技术研究对减少钢材高温损耗、降低能源、提高生产效率和经济效益有重要的现实意义。     

Preparation of aluminum oxide coating on carbon/carbon composites using a new detonation sprayer

A new multi-chamber cumulative detonation sprayer (MCDS) was applied to fabricate an aluminum oxide coating on carbon/carbon composites. MCDS provides heating and acceleration of ceramic micropowders by means of combustible gas mixture detonation products with a frequency of 20 Hz and above. The ceramic aluminum oxide particle kinetic energy ensures the destruction of the weakened areas on the carbon-carbon composite material surface and the incorporation of these particles into the surface layer. The following powder particles decelerate on the already fixed particles and form a ceramic coating. The formed aluminum oxide coating is characterized by high hardness and low porosity (<1%). MCDS provides the formation of a high-quality ceramic layer, which can also serve as the basis for the formation of protective heat-resistant coatings.