Protective coatings for carbon bonded carbon fibre composites

Carbon bonded carbon fibre composites (CBCF) were modified by direct reaction with molten silicon in order to obtain a silicon carbide layer on the composite surface. Subsequently, the Si-infiltrated CBCF material was coated with a silica-based glass containing yttria and alumina by means of a slurry-dipping technique. On heat treatment the glass yielded a glass-ceramic layer thus giving a multi-layered oxidation and erosion protection system. The microstructural characterisation of the coating was conducted by standard microscopy techniques and by X-ray diffraction. The controlled crystallization of the glass-produced cristobalite, yttrium silicate (Y₂Si₂O₇, keiviite, β-form) and mullite as main crystalline phases. These are excellent ceramic materials for oxidation and erosion protection of SiC-coated carbon-based composites since their coefficients of thermal expansion (CTE) closely match that of SiC. The possibility of healing (closure) of micro cracks by a thermal treatment at 1375 °C, thus exploiting the viscous flow of the residual glass in the glass-ceramic, was explored in order to extend the service life of the protection system.     

Effect of precoated carbon layer on microstructure and anti‐erosion properties of SiC coating for 2D‐C/C composites

To improve the erosion resistant of carbon‐carbon composites, an SiC coating was synthesized on carbon‐carbon composites by the in situ reaction method. They are firstly coated with carbon layer by slurry, and then SiC coatings are obtained by chemical vapor reaction. The effects of precoated carbon layer on the microstructure and anti‐erosion properties of SiC‐coated C‐C composites were studied and characterized. The thickness of the SiC coating increased with the increase in the precoated carbon layer thickness. The different thickness of carbon layer affects hardness of the SiC coatings, resulting in diverse erosion resistance of the coatings. The SiC coating prepared with moderate thickness of precoated carbon layer exhibits the best erosion resistance, and show better resistance at an impact angle of 30° than 90°. The eroded surface revealed that coating cracking and brittle fracture, fiber‐matrix debonding, fiber breakage, and material removal, and the additional microcutting and microploughing at oblique impact angle are the major erosion mechanism of SiC coating for C/C composites.     

ZrC ablation protective coating for carbon/carbon composites

A zirconium carbide (ZrC) protective coating was deposited on carbon/carbon (C/C) composites by atmospheric pressure chemical vapor deposition. The phase compositions, surface and cross-section microstructures, and anti-ablative properties of the coatings were investigated. Results show that the method is an effective route to prepare a dense and thick ZrC coating on C/C composites. The coating can effectively protect C/C composites from ablation for 240 s in an oxy-acetylene torch system with a mass ablation rate of 1.1 × 10⁻⁴ g/cm² s and a linear ablation rate of 0.3 × 10⁻³ mm/s.     

Pulsed electrodeposition of carbon nanotubes-hydroxyapatite nanocomposites for carbon/carbon composites

Carbon nanotubes-hydroxyapatite (CNTs-HA) composite coatings, which behaved like single composites, were synthesized by a combined method composed of electrophoretic deposition and pulsed electrodeposition. The phase compositions and the microstructure of the composite coatings were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectrometry (FTIR). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies showed that the CNTs-HA composite coatings protected the bare carbon/carbon composites from corrosion in simulated body fluid (SBF) solution. The adhesion strength of CNTs-HA composite coating prepared by the combined method is 14.57±1.06 MPa achieved at the CNTs EPD time of 10 min. Compared to the other CNTs-HA composite coatings with different content of CNTs, the CNT-HA composite coating with the electrophoretic deposition of 10 min showed the best corrosion resistance. The morphology of CNTs-HA composite coatings immersed in SBF solution rendered the formation of HA crystallites. In addition, in vitro cellular responses to the CNTs-HA composite coatings were assessed to investigate the proliferation and morphology of mouse cells 3T3 cell line.     

Siliconization elimination for SiC coated C/C composites by a pyrolytic carbon coating and the consequent improvement of the mechanical property and oxidation resistances

Carbon/carbon (C/C) composites have a wide application as the thermal structure materials because of their excellent properties at high temperatures. However, C/C composites are easily oxidized in oxygen-containing environment, which limits their potential applications to a great degree. Silicon carbide (SiC) ceramic coating fabricated via pack cementation (PC) was considered as an effective way to protect C/C composites against oxidation. But the mechanical properties of C/C composites were severely damaged due to chemical reaction between the molten silicon and C/C substrate during the preparation of SiC coating by PC. In order to eliminate the siliconization erosion, a pyrolytic carbon (PyC) coating was pre-prepared on C/C composites by the chemical vapor infiltration (CVI) prior to the fabrication of SiC coating. Due to the retardation effect of PyC coating on siliconization erosion, the flexural strength retention of the SiC coated C/C composites with PyC coating increased from 46.27 % to 107.95 % compared with the specimen without PyC coating. Furthermore, the presence of homogeneous and defect-free PyC coating was beneficial to fabricate a compact SiC coating without silicon phase by sufficiently reacting with molten silicon during PC. Therefore, the SiC coated C/C composites with PyC coating had better oxidation resistances under dynamic (between room temperature and 1773 K) and static conditions in air at different temperatures (1773?1973 K).     

Oxidation Protection Coatings for C/SiC based on Yttrium Silicate

The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young’s modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.     

Organometallic polymer-derived interfacial coatings on carbon and SiC fibers

Organosilicon and organogermanium polymers containing unsaturated carbon–carbon bonds were used as precursors for the SiC-based interfacial coatings on commercially available carbon and silicon carbide fibers and fabrics. The approach based on usage of the organometallic polymer solutions allowed to obtain uniform, adherent, crack-free and non-bridging SiC-based interfacial coatings on carbon and SiC fibers. The coated fibers retain their tensile strength. The morphology, composition, structure of coated fibers were evaluated by various analytical techniques. The drop-like germanium-containing phase was detected in the organogermanium polymer-derived coating on carbon and SiC fibers.     

The chemistry, morphology, topography of titanium carbide modified carbon fibers

Titanium carbide coatings were successfully applied on carbon fibers using reactive chemical vapor deposition approach. Chemistry, morphology, and topography of the TiC modified carbon fibers have been studied by scanning electron microscopy/energy dispersive spectroscopy, atomic force microscopy, and X-ray diffraction analysis. Uniform, adherent, crack-free and non-bridging coatings were obtained. After application of the TiC coating on carbon fibers, relief becomes more smooth and uniform. This is confirmed by decrease of the roughness parameter and the average height value of relief, as well as by decrease of scatter of measured heights. The coating consists of radially oriented crystals with high aspect ratio. Crystals are aligned in rows which are parallel to fiber axis. An increase in the tensile strength has been achieved by introducing the TiC coating on carbon fiber, the average strength and the Weibull modulus parameters were 2.55 GPa and 4.36 GPa, respectively. The obtained results can be useful for the predictive design of the well-matched interphase zone for composite materials reinforced by the TiC-coated carbon fibers.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

Fiber Coatings for SiC-Fiber-Reinforced BMAS Glass-Ceramic Composites

Interfaces in SK-fiber-reinforced glass-ceramic matrix composites were modified by applying different coatings to the fibers and varying the coating thickness. Coatings of SiC/BN, Si₃N₄/BN, and BN were applied to the fibers by CVD prior to composite fabrication. Interfacial microstructures were characterized using high-resolution and analytical transmission electron microscopy. Oxidation of the SiC fibers during composite fabrication was suppressed by the fiber coatings, provided that the coatings were sufficiently thick to prevent oxygen diffusion from the matrix. The SiC/RN and BN coatings were stable during high-temperature exposures, while the Si₃N₄/BN coating underwent chemical reactions.     

Polysilazane‐based heat‐ and oxidation‐resistant coatings on carbon fibers

Carbon fibers must be protected from a high‐temperature oxidizing environment because, at approximately 500°C and above, the fibers exhibit reduced mass and strength stability. The fibers can be protected by the application of thermal coatings, which simultaneously improve the adhesive properties of the carbon fibers in the composite materials. Polysilazanes are a new family of heat‐resistant polymer coatings that are converted into silicone carbide or silicone nitride ceramic structures at high temperatures. The converted ceramics are resistant to the effects of high temperatures. In this research work, polysilazane‐based coatings were applied to carbon filament (CF) rovings with the dip‐coating method. Tensile testing at room temperature and under thermal stress was carried out to assess the mechanical and thermomechanical properties of both coated and uncoated rovings. Scanning electron microscopy and energy‐dispersive X‐ray analysis were performed to evaluate the surface topographical properties of the coated and uncoated rovings. Thermogravimetric analysis was executed to determine the thermal stability of the polymer coatings. The coating performance on the CF rovings was determined by assessment of the test results obtained. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

包埋法制备碳/碳复合材料碳化硅涂层缺陷的形成机制及控制

用包埋法在碳/碳(C/C)复合材料表面制备了碳化硅(SiC)涂层及改性涂层.用扫描电镜观察涂层的微观形貌.从理论上探讨了涂层缺陷的形成机制,分析了改性剂对SiC涂层形貌、晶粒尺寸的影响.结果表明:添加改性剂后,涂层晶粒变小,涂层致密,表面未出现裂纹,断面孔洞的数量减少,尺寸减小.在1 773K的抗氧化性比未添加改性剂涂层的显著提高.     

Electrostatic spray painting of carbon fibre-reinforced epoxy composites

Electrostatic spray deposition (ESD) of aesthetic and protective low curable transparent powder coatings onto carbon fibre-reinforced epoxy composites (i.e., carbon laminates) is the matter of the present investigation. An original environment friendly pretreatment of the substrate, based on fluidized bed peening of glass beads followed by a moderate temperature oven baking, has been proposed. Then, the influence of ESD operational parameters on coating performance has been looked into.Design of experiments (DOE) was used to schedule the experimental trials. Coating thickness and its uniformity over the coated substrates upon curing was systematically evaluated. Further, visual appearance of the coatings was analyzed by both optical and stereoscopic microscopy. Finally, analysis of variance (ANOVA) was performed to model the available experimental data. Detailed examinations of the experimental results allowed to define the best settings of ESD process as well as the maximum deposition time before the occurrence of severe electrical breakdown in the powder layer and/or the increase in the incidence of massive back-ionization phenomena. Accordingly, 3D process maps of the coating thickness versus the operational parameters, applied voltage, feeding and auxiliary pressure, were developed, thus supporting the practitioners in their choices and in the identification of processing windows wide enough for practical purposes.     

Ablation Resistance of Different Coating Structures for C/ZrB2–SiC Composites Under Oxyacetylene Torch Flame

C/ZrB₂–SiC composites were fabricated by polymer infiltration and pyrolysis combined with slurry impregnation method. Three kinds of coating structures for these composites were applied in order to improve their ablation resistance: pure silicon carbide coatings, ZrB₂–SiC mixture coatings, and ZrC–SiC alternating multilayer coatings. The ablation experiments were carried out on an oxyacetylene torch flame with a temperature of about 3000°C. The ZrC–SiC alternating multilayer showed the best ablation resistance. The linear erosion rate for ZrC–SiC alternating multilayer coatings is half of that for ZrB₂–SiC mixture and pure SiC coatings. A model was put forward to account for such a result.     

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.     

Microstructure and anti-oxidation properties of multi-composition ceramic coatings for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at elevated temperature, an effective WSi₂-CrSi₂-Si ceramic coating was deposited on the surface of SiC coated C/C composites by a simple and low-cost slurry method. The microstructures of the double-layer coatings were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy analyses. The coating exhibited excellent oxidation resistance and thermal shock resistance. It could protect C/C composites from oxidation in air at 1773 K for 300 h with only 0.1 wt.% mass gain and endure the thermal shock for 30 cycles between 1773 K and room temperature. The excellent anti-oxidation ability of the double-layer WSi₂-CrSi₂-Si/SiC coating is mainly attributed to the dense structure of the coating and the formation of stable vitreous composition including SiO₂ and Cr₂O₃ produced during oxidation.     

Appropriate thickness of pyrolytic carbon coating on SiC fiber reinforcement to secure reasonable quasi-ductility on NITE SiC/SiC composites

Pyrolytic carbon (PyC) coating of silicon carbide (SiC) fibers is an important technology that creates quasi-ductility to SiC/SiC composites. Nano-infiltration and transient eutectic-phase (NITE) process is appealing for the fabrication of SiC/SiC composites for use in high temperature system structures. However, the appropriate conditions for the PyC coating of the composites have not been sufficiently tested. In this research, SiC fibers, with several thick PyC coatings prepared using a chemical vapor infiltration continuous furnace, were used in the fabrication of NITE SiC/SiC composites. Three point bending tests of the composites revealed that the thickness of the PyC coating affected the quasi-ductility of the composites. The composites reinforced by 300?nm thick coated SiC fibers showed a brittle fracture behavior; the composites reinforced 500 and 1200?nm thick PyC coated SiC fibers exhibited a better quasi-ductility. Transmission electron microscope research revealed that the surface of the as-coated PyC coating on a SiC fiber was almost smooth, but the interface between the PyC coating and SiC matrix in a NITE SiC/SiC composite was very rough. The thickness of the PyC coating was considered to be reduced maximum 400?nm during the composite fabrication procedure. The interface was possibly damaged during the composite fabrication procedure, and therefore, the thickness of the PyC coating on the SiC fibers should be thicker than 500?nm to ensure quasi-ductility of the NITE SiC/SiC composites.     

Microstructure and ablation properties of zirconium carbide doped carbon/carbon composites

Carbon/carbon composites doped with zirconium carbide were prepared by a three-step process. Carbon fiber felts were first immersed in a zirconium oxychloride solution, followed by rapid densification using thermal gradient chemical vapor infiltration. The densified carbon/carbon composites were then graphitized at 2500 °C. The phase composition and morphology of the composites were investigated by X-ray diffraction and scanning electron microscopy. The ablation properties were tested in an oxyacetylene torch. The results show that the linear and mass ablation rates of the composites after doping with 4.14 wt.% zirconium carbide decreased by 83.0% and 77.0%, respectively. The ablated surface of the carbon matrix for pure carbon/carbon composites was very smooth and glossy, while that for doped carbon/carbon composites was honeycombed and dim. The bonding between carbon fibers and matrix decreased because of the formation of more zirconium dioxide, resulting in carbon fibers peeling off the matrix and the ablation resistance of carbon fibers could not be brought into play when the zirconium carbide contents achieved 4.14 wt.%. Although mechanical denudation does not seem to play a dominant role, the ablation was mainly controlled by heterogeneous mass transfer.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

Nanotribology of silver and silicon doped carbon coatings

Amorphous hydrogenated carbon coatings a-C:H become very popular materials mainly because of their excellent properties such as low coefficient of friction, high hardness, good anti-wear and corrosion properties. More and more often are carried works aimed at improvement of biocompatibility and adhesion of bacterial cells by doping diamond-like carbon (DLC) coating with third element. Among them recently a great majority is devoted to carbon coatings doped with silver or silicon. The presence of silver in the coating ensures protection of the implant against the disadvantageous influence of bacteria and fungi causing biofilm associated infections, local inflammation and other implant-tissue reactions. Incorporation of silicon promotes osteointegration and leads to the enhancement of mechanical and tribological properties of the coating, which is beneficial for biomedical applications.Silver and silicon incorporated DLC coatings were prepared by a hybrid Radio Frequency Plasma Assisted Chemical Vapor Deposition/Magnetron Sputtering deposition technique on AISI316L substrates. Obtained coatings were characterized in terms of morphology, surface topography and mechanical properties. Tribological properties of the coatings were measured by lateral force microscopy and reciprocating sliding test using nanoindenter.