Molten salt synthesis of in-situ TiC coating on graphite flakes

TiC coating was synthesized on graphite flakes (G_f) by molten salt synthesis (MSS) using metal Ti powder and alkali salts. Three different alkali chloride salts of KCl, NaCl, and NH₄Cl were selected as the molten salt media substrate. Two mass ratios of 1:3 and 1:5 were chosen for Ti: G_f ratio, and the mass ratio of the powder (Ti + G_f) to the salt was 1:1. The synthesis was carried out at a temperature of 1100°? for 4 h. XRD was used to study the effect of alkali chloride salts and the Ti: G_f mass ratio on the synthesized coating. FE-SEM and AFM were accomplished to investigate the carbide formation and microstructure of the samples. Results showed that TiC coating was formed at 1100 °C for 4 h with both mass ratios in all three alkali chloride salts, but KCl was found to be the optimum alkali chloride salt or reaction medium. FE-SEM results displayed the formation of uniform coatings, and results from AFM indicated that the surface roughness increased from 0.72 for G_f to 4.94 nm for TiC coated G_f.     

Optimization of C/TiCx duplex diffusion barrier coatings for SiCf/Ti composites based on interfacial structure evolution exploration

Introducing a carbon single coating is a popular method used to protect SiC_f/Ti composites from severe interface reactions. However, carbon coatings lose their protective effect on SiC fibres at high temperature, even after a short period time. As such, given the strong demand for high temperature applications in aeronautics and astronautics a more coating which is more effective at high temperatures is desirable. In order to improve the high temperature interfacial stability of SiC_f/Ti composites, a C/TiC_x duplex coating system with different C contents in TiC_x was introduced to explore the protection of fibres at 1200?°C for 1?h. The results show that the C/quasi-stoichiometric TiC coating system protects the SiC fibres most effectively. Based on insights from the evolution of the interface structure, TiC_x has been identified as an interfacial reaction product from the C single coating, exhibiting a gradient in C content and grain size, which is different from a deposited TiC layer with a well-distributed composition and structure. The different coating structure gives rise to different ability to resist C diffusion at high temperatures, in which poor resistance ability appears in TiC_x interfacial reaction layer coming from C single coating due to short-circuit diffusion in C-rich fine-grained TiC layer and fast intracrystalline diffusion trigged by amounts of vacancies in sub-stoichiometric coarse-grained TiC layer. Therefore, C/quasi-stoichiometric TiC duplex coatings with a thick, coarse-grained quasi-stoichiometric TiC layer could effectively inhibit C diffusion by comparison to C single coatings, and is more effective than C/rich-carbon TiC duplex coatings due to the existence of short-circuit diffusion in the latter. As such, C/quasi-stoichiometric TiC duplex coatings appear to be an optimal diffusion barrier for SiC_f/Ti composites at high temperature.     

Electrical double-layer characteristics of novel carbide-derived carbon materials

A variety of nanoporous carbide-derived carbon materials possessing improved pore size distributions were synthesised from a mixture of titanium carbide and titanium dioxide. It was observed that TiO₂ caused partial oxidation of the carbon particles created during high-temperature chlorination of the TiC/TiO₂ mixture. The resulting carbon powder is characterised by narrow pore size distribution with a peak pore size of around 8 Å and a noticeably smaller amount of pores below 6-7 Å compared to the carbon derived from pure TiC. Electrochemical and electrical double-layer characteristics of novel carbon materials in the acetonitrile solution of triethylmethylammonium tetrafluoroborate were obtained by using cyclic voltammetry and constant current methods. Carbon electrode materials of this study were tested over the temperature range from −10 °C to +60 °C. Results of this study affirmed a great potential of the synthesised advanced carbide-derived carbon, whose specific double-layer capacitance reaches approximately 90 F cm⁻³ and 125 F g⁻¹.     

Effect of iron content on the wear behavior and adhesion strength of TiC–Fe nanocomposite coatings on low carbon steel produced by air plasma spray

In this study, the effect of Fe content on the abrasion behavior of TiC–Fe nanocomposite coatings applied on the CK45 steel substrate by air plasma spray method was investigated. For this purpose, milled TiC powder was prepared at 1, 2, 3 and 4 h milled TiC powder for 4 h was selected as the suitable sample. In the next step, a suitable sample mixture with different iron powder concentrations of 5, 10, 15, 20 and 25% was prepared by mechanical milling. The granulated mixture was applied to the substrate using air plasma spray technique. Microstructural and phase analyzes were performed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). According to the results of Williamson-Hall calculations, the TiC crystallites' size decreased by 49 nm–29 nm, and network strain reached 0.16% by increasing milling time from 1 h to 4 h. Studies have shown that the coatings contain titanium carbide, iron oxide, and titanium oxide, with the number of phases formed depending on the amount of iron in the chemical composition. Investigation of the tribological properties of the coating layer showed that with increased iron content in the coating, the wear resistance of the samples is reduced. Hardness tests on coatings indicate that adding iron to nanocomposite from 5 to 25% reduces hardness from 1025 to 699 Hv. It can be argued that a slight increase in the adhesion strength of the coating to the substrate is due to increased wettability because of the formation of molten iron in the coating.     

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.     

Microstructure characteristics of silicon carbide coatings fabricated on C/C composites by plasma spraying technology

A functional gradient SiC coating on C/C composites has been developed using a novel process which is the combination of plasma spraying technology with reaction-formed heat-treatment. Microstructure observation and phase identification of the SiC coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Experimental results showed that a uniform silicon coating was deposited on C/C composite by plasma spraying technology. The reaction between the silicon coating and C/C substrate occurred during the heat-treatment at temperature of 1450 °C and 1600 °C in argon environment, respectively. A continuous SiC coating was formed on the surface of the C/C substrate. And a layer of SiC/C convention layer was formed on the near-surface area of the substrate, which was resulted from the molten silicon penetrating into the open pores and consequently reacting with the C/C composites. The thickness of the formed SiC coatings was closely related to the original silicon coatings.     

Comparative study of carbide-derived carbons obtained from biomorphic TiC and SiC structures

Biomorphic carbide ceramics, TiC and SiC, derived from paper performs by chemical vapor infiltration were converted into high porous carbon by carbide-derived carbon (CDC) approach using selective etching in chlorine or hydrogen/chlorine gas mixture in a temperature range of 400-1200 °C. A comparative study of both carbide precursors was performed regarding reaction kinetics, influence of hydrogen as well as microstructure of the resulting carbon. SiC showed lower reactivity than TiC. Temperatures below 650 °C are not sufficient to remove Si from SiC. Addition of hydrogen to the reactive gas inhibits the chlorination reaction. A linear decrease of etching rate with increasing hydrogen/chlorine ratio was observed for both carbide precursors. A critical ratio, where no etching takes place, was estimated to be 0.72 for TiC-CDC and 0.66 for SiC-CDC. The etching rate of TiC is independent from the temperature. In the case of SiC activation energy of the chlorination reaction of about 50 kJ/mol was estimated in the temperature range 650-800 °C. The structural ordering of CDC with increasing synthesis temperature affects also its oxidation resistance as shown by thermo gravimetric analysis.     

Fabrication of core-shell structured MWCNT-Ti(TiC) using a one-pot reaction from a mixture of TiCl3, TiH2, and MWCNTs

Multi-walled carbon nanotubes (MWCNTs) coated with metal titanium or carbide layers were prepared by heating a mixture of TiCl₃, TiH₂, and MWCNTs under vacuum. The resulting MWCNT-Ti(TiC) materials had a uniform and conformal core-shell structure. The thickness of the Ti(TiC) layer was determined by the deposition temperature and time. The amount of TiC in the shell depended primarily on the deposition temperature. Whether a metal or carbide coating was obtained was determined by controlling the deposition temperature. Ti(TiC)-coated MWCNTs showed better field emission properties than did pristine MWCNTs. This deposition method should prove to be a versatile route for fabricating other one-dimensional core-shell materials.     

Limiting withdrawal rate and maximum film thickness during dip-coating of titania sols onto a Si substrate

The article highlights new insights into production of thin titania films widely used as catalyst support in many modern reactors including capillary microreactors, microstructured fixed-bed reactors and falling film microreactors. Dip-coating of a titania sol onto a Si substrate has been studied in the range of the sol viscosities of 1.5-2.5 mPa s and the sol withdrawal rates of 0.2-18 mm/s. Different viscosities of sols were created by addition of desired amounts of nitric acid to the synthesis mixture of titanium isopropoxide and Pluronic F127 in ethanol which allowed to control the rate of the condensation reactions. Uniform mesoporous titania coatings were obtained at the solvent withdrawal rates below 10 mm/s at sol viscosities in the range from 1.6 mPa s to 2.5 mPa s. There exists a limiting withdrawal rate corresponding to a capillary number of ca. 0.01 beyond which uniform titania films cannot be obtained. Below the limiting withdrawal rate, the coating thickness is a power function of the sol viscosity and withdrawal rate, both with an exponent of 2/3. The limiting withdrawal rate increases as the solvent evaporation rate increases and it decreases as the sol viscosity increases.     

Synthesis of TiO2 scaffold by a 2 step bi-layer process using a molten salt synthesis technique

TiO₂ scaffolds of anisotropic rutile particles were grown from rutile seeds by using molten salt synthesis techniques. The rutile seeds were either in the form of a separate layer applied on a substrate or a sintered bulk pellet. Mixtures of amorphous titanium hydroxide and salt applied as coatings on the rutile seeds were heat treated. Depending on the morphology of the seed layer, heat treatment temperature, time and salt medium, rutile was grown with different morphologies and microstructures. For NaCl-KCl eutectic salt mixture and heat treatment at 700 °C for 5 h, nano-whiskers of 20-50 nm diameter and 0.5-1 μm length were obtained. For the NaCl salt sample treated at 850 ºC for 20 h, rutile platelets of 2-5 μm thick, 2-10 μm wide and 5-25 μm in length were produced. X-ray diffraction and Raman scattering were used to identify and characterize the rutile phase of the nanowhiskers.     

Chemical vapor deposition of carbon on particulate TiO2 from CH4 and subsequent carbothermal reduction for the synthesis of nanocrystalline TiC powders

The present study aims to investigate chemical vapor deposition of carbon from CH₄ on TiO₂ particles and to establish optimal conditions for the synthesis of nanocrystalline TiC powders. Mass measurements, XRD, HR-TEM and SEM were used to characterize the products at various stages of the reactions. Oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere and were coated with thin pyrolytic carbon layers of 10-20 nm. XRD analysis showed that the coated powders consisted of C, TiO₂ and titanium sub-oxides. The powders containing ∼33 ± 2 wt% carbon were used for the carbothermal reduction of TiO₂ to TiC at 1600-1800 K under Ar flow. Lattice constant and mass loss of the samples increased to the levels of TiC with temperature and time. Nearly pure TiC powders with a mean particle size of ∼125 nm were synthesized at 1750-1800 K within 30 min.     

Preparation of TiC/Ti2AlC coating on carbon fiber and investigation of the oxidation resistance properties

MAX phases were proposed as the interphase materials for carbon fiber reinforced ceramic matrix composites toward the applications in high‐dose irradiation and oxidation environments. A thickness‐controllable TiC/Ti₂AlC coating was fabricated on carbon fiber using an in situ reaction in a molten salt bath. The coating showed a multilayered structure, in which the inner layer was TiC and the outer layer was Ti₂AlC. The influence of the reaction conditions on the morphology, composition, and thickness of the coating was investigated. The oxidation resistance properties of the as‐prepared TiC/Ti₂AlC‐coated carbon fiber in static air and water vapor flow at elevated temperatures were investigated. The results showed that the as‐prepared TiC/Ti₂AlC coating could provide good protection to the carbon fiber in both static air and water vapor flow up to 800°C. As these TiC and Ti₂AlC materials have good irradiation resistance, the present work provides a potential way to develop an irradiation‐resistant interphase of carbon‐fiber‐reinforced ceramic matrix composites for nuclear applications. Furthermore, this work also provides a feasible way to prepare carbide/MAX phase coating on other carbon materials.     

Interface modification of carbon fibers with TiC/Ti2AlC coating and its effect on the tensile strength

A uniform TiC/Ti₂AlC gradient coating was obtained on carbon fibers via an in-situ reaction in molten salts. The results indicated that in-situ growth of TiC/Ti₂AlC coating caused strong interfacial bonding and surface defects. In this case, evident stress concentration was induced and cracks penetrated the fiber easily during tensile loading. Thus the tensile strength of carbon fibers was dramatically decreased to 78?±?13?MPa. In order to improve the performance of the as-prepared TiC/Ti₂AlC-coated carbon fibers, a pyrolytic carbon layer was pre-fabricated on carbon fibers. By introducing pyrolytic carbon layer, the interfacial bonding strength and surface defects were reduced accordingly. These improvements lead to a decrease of stress concentration and cracks propagation, and facilitate the interfacial debonding during tensile loading. As a result, the tensile strength of the fiber was significantly increased to 550?±?72?MPa. This fact indicates that pre-fabricating a pyrolytic carbon layer on carbon fibers is an effective method to improve the reliability of the TiC/Ti₂AlC-coated carbon fibers. The present work also provides a feasible way to fabricate TiC/Ti₂AlC interphase for high-performance Cf/SiC composites.     

Reactive synthesis for porous Ti3AlC2 ceramics through TiH2, Al and graphite powders

Porous Ti₃AlC₂ ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH₂ at 700 °C, which leads to the conversion of TiH₂ to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti₃AlC₂ at 1300 °C.     

Metallization of glass-ceramic technological shells and oxide materials in molten salts

Coherent titanium and niobium coatings on glass-ceramic shells are prepared using disproportionation reactions. Glass-ceramic shells with barrier coatings are used to fabricate articles from a titanium alloy powder by hot isostatic pressing. It is demonstrated that the compaction of the titanium alloy in an isostatic press at a temperature of 1173 K and a pressure of ～2000 kg/cm² for 4 h makes it possible to fabricate articles that can be easily separated from the glass-ceramic shells. After hot isostatic pressing, the thickness of the coatings decreases by 25–30%; however, the coating continuity is not disturbed. Articles with a density close to the theoretical value without visible defects and pores are fabricated in the glass-ceramic shells with titanium and niobium coatings. According to the character of the microstructure of these articles, they can be attributed to type III or IV α-titanium alloys. It is shown that various oxide materials can be metallized in molten salts.     

Nano-clay filled polyester coatings

Polyester composite coatings were produced by mixing un-saturated polyester (HP) resin and functionalized montmorillonite (MMT) powder. The effect of the mixing conditions on the final performances of the composite coatings was accurately evaluated: samples were produced with different MMT contents (up to 5 wt%) and stirring time (up to 1200 min). Rheological analyses of the un-catalyzed resin matrix were performed as well as tribological and scratch tests of coatings. Results show the strong effect of the mixing time on the performance of the MMT filled coatings. This effect is never negligible and can overcome the effect of the MMT content. After 20 h mixing, a 1 wt% filled mixture reaches a viscosity similar to a 5 wt% mixture after 30 min. By increasing the resin viscosity, the resulting coating thickness increases as well and this effect seems to dominate the tribological behavior of coatings rather than it's filling. Best results were obtained with low mixing times and filler contents.     

Purely inorganic coatings based on nanoparticles for magnesium alloys

The chemical nanotechnology is offering a chance to apply stable inorganic coatings onto magnesium alloys. The cast alloy AZ91 as well as the wrought alloy AZ31 could be dip-coated with aqueous dispersions based on commercially available silica particles and various additives. The high surface activity of the nanoparticles and appropriate additives, e.g. boron, aluminium or alkali salts, help to densify these coatings under moderate conditions even suitable for those thermally precarious magnesium alloys.Another coating technique is based on the electrophoretic deposition of nanoparticles already containing all sintering aids. These particles could be synthesised by a base-catalysed sol-gel process. Polydiethoxysiloxane can act as an adhesion promoter for these coatings. Additionally concentration gradients of different oxides within these particles can adjust the coating properties, too.Usually single coatings are very thin (200-500 nm). However, multiple coating applications as well as a process involving special particle mixtures lead to coatings with a thickness of up to several micrometers. Even after thermal treatment at 200 or 400 °C these coatings stay crack-free. The composition and texture of these coatings were studied using IR, atomic force microscopy (AFM), scanning electron microscopy (SEM) and other techniques. Electrochemical impedance measurements show an improvement of the corrosion performance by these coatings. The coating resistance is improving with the coating thickness.     

Deposition of titanium on SiC fibres from chloride melts

The present study examined the feasibility of producing titanium matrix composites based on a molten salt fibre coating process. It is demonstrated that sufficiently thick and coherent titanium coatings can be deposited on SiC fibres in LiCl–KCl–TiCl₂ at 700 K. One important aspect of the study was the behaviour of the substrate during electrolysis. Therefore, additional features related to carbon-coated SiC fibres and carbonaceous substrates in general are outlined. The morphologies and microstructures of electrolytically coated SiC fibres were investigated by scanning electron microscopy and compared to fibres coated by a physical vapour deposition technique. The cause and relevance of the growth failures that were observed in this study are also discussed.     

Evaluation of plasma sprayed alumina–40 wt% titania and partially stabilized zirconia coatings on high density graphite for uranium melting application

Ceramic coatings have been proposed on high density graphite crucibles for the application of uranium consolidation and distillation of molten salt in pyrochemical reprocessing of metallic fuels. Towards this, uranium melting experiments were carried out on plasma sprayed partially stabilized zirconia (PSZ) and Al₂O₃–40 wt% TiO₂ (A40T) coated high density graphite samples at 1350 °C using induction heating system for evaluating the compatibility of these coatings with molten uranium. The coated high density graphite samples were characterized before and after uranium melting test by scanning electron microscopy attached with energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. Microstructural observations revealed that no significant reaction layer or product was formed between uranium and PSZ coating, while uranium significantly adhered to A40T coating. PSZ coating offers better stability and protection to high density graphite crucibles from the chemical attack by molten uranium.     

Titanium carbide derived nanoporous carbon for energy-related applications

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200-1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp² bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600-800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm³/g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm³/g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.