Electrodeposition of nickel-BN composite coatings

Electrodeposition of nickel-boron nitride (Ni-BN) composites is carried out from a sulfamate bath containing up to 10 g/l of dispersed boron nitride particles with size 0.5 μm. Microhardness and wear resistance of the composites are investigated. Both the properties are influenced by the amount of incorporated boron nitride particles. Commercial surfactant containing alkyl-dimethyl-benzyl-ammonium saccharinate is used to stabilize the electrolyte: the effects on mechanical properties and structure of the electrodeposits are investigated. Morphology of the coatings and the effects of codeposited particles on metal matrix structure are reported.     

Electrodeposition of refractory carbide coatings from fluoride melts

Experimental studies relevant to the electrodeposition of the carbides of vanadium, niobium, titanium, and zirconium are reported. These focus on choices of suitable metal precursors, their solubility and reducibility. The state of knowledge in the field is summarized.     

Impedance of a rotating disc electrode with a reversible reaction

This paper presents a model of electrode impedance for the case of a fast reversible reaction. The various contributions of the impedance were analysed with particular emphasis on the charge transfer resistance: this resistance was shown to be also dependent on mass transfer phenomena. For the case of significant mass transfer control,the diameter of the high-frequency loop increases with the absolute value of the overpotential. The various physicochemical parameters involved in the expression for impedance were determined through previous measurements. The impedance model was validated by experimental measurements carried out with the hexacyanoferrate (II)–(III) couple ona Pt RDE.     

Tribological studies of electroless nickel/diamond composite coatings on steels

In this study, a commercially available process of electroless nickel plating with co-deposited diamond powders was applied to a steel substrate as an intermediate layer prior to diamond deposition by MPECVD. The diamond films show excellent adherence, since they are strongly bounded to the diamond particles, deeply anchored into the electroless plated nickel matrix. A synergism effect of electroless nickel plating and MPECVD diamond growth are discussed. The electroless nickel plate which can be hardened itself by the precipitated phosphide phases after the heat treatment is an efficient diffusion barrier against the inter-diffusion of iron from the steel substrate and carbon from CH₄. A more continuous and smoother diamond film can be formed on the outermost surface. The results of tribotesting indicated that each step in the process of composite formation significantly lowers the friction coefficient (μ), especially the secondary layer of electroless nickel plate (~ 1 μm) is particularly effective and possesses a steadily low value of μ, which has promise for tribological applications. The secondary nickel layer could enhance the adherence of diamonds in the metal matrix, and be responsible for the better continuity of the top diamond film.     

Orthogonal collocation simulation of the rotating disc electrode

The hydrodynamic problem of the rotating disc electrode is approximated using the weighted residual technique of orthogonal collocation. Results are given for a potential step applied to the electrode where simple reversible charge transfer to a single species takes place.     

Studies of the oxidation of ethanol on a reticulated nickel rotating cylinder electrode

Reticulated nickel rotating cylinder electrodes (RRCE) of different porosity grade have been developed and used in the study of the oxidation of ethanol in 1 M KOH. Well-formed waves have been obtained by voltammetry at 2 and 50 mV s^–1 and the RRCE seems to be a promising tool for electrochemical kinetic studies. As expected for a reaction not controlled by diffusion, the current does not depend on the rotation rate of the electrode.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Low-cost preparation and frictional behaviour of a three-dimensional needled carbon/silicon carbide composite

A low-cost carbon/silicon carbide (C/SiC) composite was manufactured by phenolic resin impregnation–pyrolysis combined with liquid silicon infiltration. The carbon fiber preform was prepared by three-dimensional needling. A carbon/carbon composite with a density of 1.22 g/cm³ after only one impregnation–pyrolysis cycle was achieved by using hot-pressing curing. The density of the final C/SiC was 2.10 g/cm³ with a porosity of 4.50% and SiC-content of 45.73%. The C/SiC composite had a high thermal conductivity of 48.72 W/(m K) perpendicular to the friction surface and demonstrated good friction and wear properties. The static and average dynamic friction coefficients were 0.68 and 0.32 (at a braking velocity of 28 m/s). The weight wear rates of the rotating disk and stationary disk were respectively 7.71 and 5.60 mg/cycle with linear wear rates, 1.67 and 1.22 μm/cycle, at a braking velocity of 28 m/s.     

High temperature oxidation of two- and three-dimensional hafnium carbide and silicon carbide coatings

The difficulty in using C/C composites as structural components above 2000 °C in an oxidizing atmosphere is their poor lifetime. The solution proposed here consisted in combining two refractory carbides, hafnium and silicon carbides, in coating with a complex architecture, named a three dimensional coating, over a C/C substrate. Such a coating protects the C/C composite at 2000 °C under air. The oxidation of the coating leads to the formation of a Si_xO_yHf_z hafnium-containing silicate liquid, combined with HfO_2(s). This liquid limits oxygen diffusion more than pure SiO₂ does, so it is a better protection against oxidation. Furthermore, HfO_2(s) acts as a frame holding Si_xO_yHf_z in place. From these results, an oxidation mechanism is proposed and discussed.     

Behavior of silicon carbide/cordierite composite material after cyclic thermal shock

In the present work Mg-exchanged zeolite and silicon carbide were used as starting materials for obtaining cordierite/SiC composite ceramics with weight ratio 30:70. Samples were exposed to the water quench test from 950 °C, applying various number of thermal cycles (shocks). Level of surface deterioration before and during quenching was monitored by image analysis. Ultrasonic measurements were used as non-destructive quantification of thermal shock damage in refractory specimens. When refractory samples are subjected to the rapid temperature changes crack nucleation and propagation occurs resulting in loss of strength and materials degradation. The formation of cracks decreases the density and elastic properties of material. Therefore by measuring these properties one can directly monitor the development of thermal shock damage level. Dynamic Young's modulus of elasticity and strength degradation were calculated using measured values. Level of degradation of the samples was monitored before and during testing using Image Pro Plus program for image analysis. The capability of non-destructive test methods such as: ultrasonic velocity technique and image analysis for simple, and reliable non-destructive characterization are presented.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Design of core-shell nickel oxide/silicon carbide whiskers towards excellent microwave absorption property

Toward the increasingly serious problem of electromagnetic wave pollution,the development of absorb-ing material with the properties of the light,thin,wide,strong,and multiple applications scenarios is still a huge challenge.Herein,the core-shell nickel oxide/silicon carbide whiskers (NiO/SiCw) with variational NiO morphologies (tulle-,flower,and rod-like) were designed and fabricated via a facile pre-hydrothermal method and post-annealing process.For the NiO shell morphology,it can effectively be controlled by the proportion of Ni(NO3)2·6H2O,NH4Cl,and CO(NH2)2.The structure of NiO/SiCw samples was investigated by XRD,SEM,XPS,TEM,and N2 absorption-desorption.Compared to other morphologies of NiO,the flower-like NiO/SiCw possess a porous structure and large surface area that can benefit from the multiple reflections and attenuation of the microwave.As an absorber,the composite with the flower-like NiO/SiCw filler loading of 50％ manifests a superior microwave attenuation capability due to its special porous structure,good impedance matching,and large dielectric loss induced from hetero-junction interfacial polarization.The minimum reflection loss of flower-like NiO/SiCw is up to-56.8 dB with a thickness of 1.9 mm,and the maximum effective absorption bandwidth (EAB) reaches 5.17 GHz.The as-prepared flower-like NiO/SiCw with strong absorption,thin thickness,and width EAB can meet the potential requirements for microwave absorption materials under oxidation environments.     

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.     

Corrosion resistance properties of electroless nickel composite coatings

Electroless nickel (EN) composite coatings incorporated with PTFE and/or SiC particles demonstrated significantly improved mechanical and tribological properties as well as low surface energy which are desired for anti-sticking and wear resistant applications. The corrosion resistance of these composite coatings, however, has not been systematically studied and compared. This work aimed to investigate the corrosion characteristics of EN composite coatings using electrochemical measurements which include open circuit potential (OCP), electrochemical impedance spectroscopy and potentiodynamic test. The effects of the co-deposited particles on corrosion behavior of the coatings in 1.0 N H₂SO₄ and 3% NaCl media were investigated. The surface autocatalytic properties and the post-heat-treatment on coating corrosion resistance were also discussed. The results showed that both EN and EN composite coatings demonstrated significant improvement of corrosion resistance in both acidic and salty atmosphere. Ni striking substantially enhanced the corrosion resistance due to the improvement of the surface autocatalytic properties and homogeneity. Proper post-heat-treatment significantly improves the coating density and structure, giving rise to enhanced corrosion resistance.     

Influence of oxidation on fatigue life of a carbon/silicon carbide composite in water vapor containing environments

The influence of oxidation on the fatigue life of two-dimensional carbon/silicon carbide composites in water vapor containing environments at 1300 °C was investigated. Tension–tension fatigue experiments were conducted at sinusoidal frequency of 3 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to peak fatigue stresses of 90, 120 and 150 MPa. The mean residual strength of the specimens after survived 100,000 cycles with a peak stress of 90 MPa was 83.9% of that of the as-received composite. The mean fatigue lives of the specimens subjected to peak fatigue stresses of 120 and 150 MPa were 42,048 and 13,514 cycles, respectively. Oxidation was the dominant damage mechanism, which remarkably decreased the fatigue life. Oxidizing species diffusion within the composite defects was discussed. The higher the applied stresses, the larger the equivalent radius of the defect and the shorter the fatigue life.     

Excellent ablation resistance and reusability of silicon carbide fiber reinforced silicon carbide composite in dissociated air plasmas

This work explores the potentials of SiC fiber reinforced SiC matrix composites (SiC_f/SiC) with SiC coating to resist aerodynamic ablations for thermal protection purpose. A plasma wind tunnel is employed to evaluate their anti-ablation property in dissociated air plasmas. The results suggest a critical ablation temperature of SiC coated SiC_f/SiC, ≈ 1910 °C, which is the highest ever reported in literatures. Benefited by ‘all-SiC’ microstructures and relative flat ablated surfaces, the SiC_f/SiC is still ablation-resistant up to ≈ 1820 °C after the occurrence of ablation. This implies an excellent ablation resistance and reusability property of SiC_f/SiC, which surpasses that of traditional carbon fiber reinforced composites. Finally, an ablation mechanism dominated by surface characteristic is proposed. For the SiC coated SiC_f/SiC, ablation is prone to take place at surface cracks formed by thermal mismatch; while for the ablated SiC_f/SiC, ablation is triggered at the exposed fiber bundles which is over-heated in the plasmas.     

Efficient fabrication of carbon/silicon carbide composite for electromagnetic interference shielding applications

Ceramic matrix composites are typically prepared by a costly, time-consuming process under severe conditions. Herein, a cost-effective C/SiC composite was fabricated from a silicon gel-derived source by Joule heating. The β-SiC phase was generated via carbothermal reduction, and the carbon fabric showed a well-developed graphitic structure, promoting its thermal and anti-oxidation stabilities. Owing to the excellent dielectric loss in carbon fabric, SiC and SiO₂ as well as the micropore structure of the ceramic matrix, the absolute electromagnetic interference shielding (EMI) effectiveness (SSE/t) reached 948.18 dB?cm²?g^-1 in the X-band, exhibiting an excellent EMI SE. After oxidation at 1000 °C for 10 h in the air, the SSE/t of the composite was only reduced to 846.02 dB?cm²?g^-1. The C/SiC composite promises the efficient fabrication of high-temperature resistant materials for electromagnetic shielding applications.     

Increasing the strength and toughness of a carbon fiber/silicon carbide composite by heat treatment

The effect of heat treatment on the strengthening and toughening of a carbon fiber/silicon carbide composite (C/SiC) with a thin pyrolytic carbon (PyC) interphase was investigated. Tensile strength and modulus were measured using tensile tests, and toughness was obtained by calculating the area under the stress–strain curves. Results show that with increasing heat treatment temperature both the strength and toughness of the C/SiC composite increased, but the modulus decreased. After heat treatment at 1900 °C the tensile strength and toughness increased by a maximum of 42% and 252%, respectively, and the modulus decreased by 48%. X-ray diffraction analysis and microstructural observation confirmed that the heat treatment mainly increased the graphitization of the amorphous PyC interphase, and this was responsible for the property changes observed because it decreased the interfacial sliding resistance associated with long fiber pull-out, relieved the thermal residual stress and lower stress concentrations on the fibers to uniformly share the load for improving the strength and toughness.