Electroless nickel coated short carbon fibres in aluminium matrix composites

Electroless nickel coated fibres have been used as reinforcement for the fabrication of aluminium matrix composites by liquid processing. Uniform, continuous and well-adhered nickel coatings are obtained with different phosphorus contents. Coated carbon fibres were mixed with AA6061 aluminium powders, compacted and heated at temperatures from 650 to 950 °C to study the reactivity, the nickel diffusion, and the resulting interfaces. Coatings improve the wetting behaviour of carbon fibres by molten aluminium because of the formation of a transient Al–Ni intermetallic at the matrix–fibre interface, limiting fibre segregation to obtain a homogeneous reinforcement distribution. Finally, the mechanical properties of the composite have been measured through nanoindentation tests.     

Fatigue behaviour of aluminium matrix composites reinforced with continuous alumina fibres

Abstract

Alumina (N610) reinforced pure Al (A9) or Al–2 wt-%Cu alloy (AU2) unidirectional (UD) composites which combine a highly ductile matrix with a strong interface bonding, present high static and dynamic mechanical performances. In the present paper, the fatigue behaviour of quasi­UD N610+A9 and N610+AU2 composites is investigated. Notwithstanding the presence of the transverse bundles, the longitudinal behaviour in tension and fatigue of these quasi-UD composites is nearly equivalent to that of the pure UD. Only in the case of the N610+AU2 quasi-UD composite, the fatigue limit is about 30% lower than that of the pure UD. Acoustic emission (AE) monitoring correlated with microfractography and microstuctural examinations has enabled identification of different stages in the evolution of damage and the association of these with damage and failure mechanisms. In fatigue, three main damage mechanisms are activated in a sequential and/or superposed mode during a three stage evolution.     

Copper coating on coir fibres

Conditions for obtaining continuous coatings of copper (thickness range 1.5 to 5μm) on coir fibres have been reported. Activation of the surface of coir fibres was achieved by treating the surface of the fibres with NaOH-HCHO/ammoniacal AgNO₃ solution. Copper was deposited on the activated surface of coir fibres from Fehling-formaldehyde solution. The effects of variation in formaldehyde and sodium hydroxide concentrations and pressures inside the coating vessel on deposition rates were determined. The minimum concentrations of NaOH and HCHO required for maintaining a maximum rate of deposition of copper from a solution contaning 10g l⁻¹ copper sulphate were found to be 6.6 g l⁻¹ and 2.5 to 3.5 g l⁻¹, respectively. Optical and scanning electron microscope studies show that relatively more uniform and non-porous copper coatings were obtained when deposition was carried out under reduced pressures. A 5μm thick copper coating on coir fibre prevents the propagation of flame as was shown by flammability tests. Copper coating on coir fibre decreases its electrical resistivity from 2.55×10⁶Ωcm to 4.68×10⁻³Ωcm with 1.5μm thick coating and 3.76×10⁻⁵Ωcm with 5μm thick coating. Reinforcement of polyester with copper-coated coir fibre leads to an increase of about 25% in tensile strength and flexural strength as compared to polyester reinforced with plain coir fibre.     

Studies on nickel coated carbon fibres and their composites

Uniform and continuous coating of nickel was given to the carbon fibres by cementation, electroless or electroplating techniques. The coating thickness was ranged between 0.2 and 0.6 m for all the three methods used. Coating thickness less than 0.2 m showed discontinuous coating of nickel over the fibre surface. Beyond 0.6 m thickness, nickel deposited in den-drite form over the continuous coating. For continuously coated fibres, the ultimate tensile properties of electroless coated fibres were near to uncoated carbon fibres suggesting adherent and defect free coating; while fibres coated by electrolytic and cementation process exhibited lower ultimate tensile strength (UTS) properties. The tensile fracture of the cementation coated fibres suggested degradation of the fibres. In composites, prepared by dispersing the coated fibres in pure aluminium matrix, no appreciable fibre-metal interaction was observed. NiAl₃ intermetallics were observed around and adjacent to the carbon fibres. Sometimes carbon fibres were found embedded in massive NiAl₃ intermetallics suggesting that fibre surface can also act as nucleating centre for these precipitates.     

Fabrication of aluminium composites reinforced with carbon fibres by a centrifugal infiltration process

Centrifugal-force infiltration was used for obtaining aluminium alloy composites reinforced with carbon fibre by the infiltration of preforms. The lost-wax-casting technique was used during the manufacturing process. Preforms fabricated with different percentages of reinforcement were heated to facilitate their filling with aluminium. Some samples were coated with nickel to favor the reinforcement wetting by the molten aluminium alloy. Composites with volume fraction of reinforcements above 7 vol.% and porosity values lower than 0.5 vol.% were obtained with this technique. The hardness of the composites increased with the volume fraction of reinforcement and the solution and the later precipitation of nickel coating caused an additional hardening effect.     

Polyamide coating on carbon fibres and potential application in carbon/kevlar/epoxy hybrid composites

Hybrid unidirectional composite materials, consisting of alternately laminated layers of Kevlar-49 fibres and carbon fibres in an epoxy resin, have been studied. Before embedding, the carbon fibres were coated with a Nylon 6,6 film by an interfacial in-situ polymerization technique. Emphasis is given to the mechanical properties of the hybrid composites based on coated carbon, with those based on uncoated carbon, for various values of partial volume fraction of the carbon fibres, V_cf, polyamide content deposited on the carbon fibres, C_N, and total fibre (Kevlar + carbon) volume fraction, V_f.     

Silver coating on carbon and SiC fibres

Electroless silver coating on carbon fibres using silver nitrate solutions has been studied. It was observed that the rate of silver coating depends on the degree of graphitization of carbon fibres. Fibres with a higher degree of graphitization were coated faster than those with a lower degree of graphitization. A physical model considering the number of nucleation sites on the carbon fibre surface as a function of the degree of graphitization is proposed for the silver coating process. The strength and modulus of coated and uncoated fibres have been determined using a high-sensitivity load cell with an Instron tensile testing machine. It was observed that silver coating did not alter the strength or modulus of the fibre. Aluminium matrix composites have been successfully fabricated with these fibres. The same coating technique was also used to coat silicon carbide fibres. Improvement in the infiltration during composite fabrication was observed when the fibres were silver-coated.     

Fibre and fibre-surface treatment effects in carbon/aluminium metal matrix composites

A systematic study of the relationship between the microstructure of the interface in C/Al composites and its dependence on variations in squeeze-casting parameters has been undertaken. This research has shown that the amount of Al₄C₃ reaction product at the interface is dependent on the surface structure of the reinforcing fibre and the surface treatment of the fibre. Additionally, the interface shear strength increases with an increase in the amount of reaction product at the interface. An increase in interface shear strength leads to a decrease in composite longitudinal strength. High-resolution electron microscopy and X-ray photoelectron spectroscopy analyses indicate that carbide formation is a conventional two-step process of nucleation and growth. Nucleation occurs preferentially at graphite edge planes on the carbon fibre surface, and growth is restricted along certain matrix planes and directions.     

Effects of coating treatment on mechanical properties of carbon fibres and reinforced silicon carbide composites

Abstract

Three-dimensionally braided carbon fibre reinforced SiC matrix composites have been fabricated and the effects of coating treatment on the mechanical properties have been investigated. It has been found that pyrocarbon coating can improve the strength of the heat treated carbon fibres. When the coating thickness was 0.5 m, the composites had better mechanical properties: a flexural strength of 643 MPa and a fracture toughness of 17.9 MPa m12. The composites also exhibited a toughening fracture mode.     

Microstructure and mechanical behaviour of aluminium matrix composites reinforced with graphene oxide and carbon nanotubes

Aluminium (Al) matrix composites reinforced with either 0.5 wt% graphene oxide (GO) or 0.5 wt% carbon nanotubes (CNTs) were hot extruded from ball-milled powders. A control, pure Al bar was also fabricated. Microstructural examination, including Raman mapping, showed a relatively poor dispersion of the carbon nanomaterials within the Al matrix, particularly in the case of the CNTs. Consequently, while the mean grain size of the Al matrix remains invariant with the addition of CNTs, the Al/GO composite exhibits reduced grain size compared to pure Al due to the pinning effect of the reinforcement. Moreover, the addition of both carbonaceous materials resulted in a slight decrease in the typical extrusion duplex <111> + <100> fibre texture intensity. This weakening of the texture was more pronounced in the Al/GO composite, partly due to the pinning effect of the reinforcement. In agreement with their relative mean grain sizes, the Al/GO composite shows an improved mechanical performance over pure Al. Despite the similarity of the mean grain sizes, the Al/CNT composite displays comparable hardness and a decreased compressive yield stress relative to the pure Al. In the absence of chemical reactions at the interfaces, this was attributed to a low efficiency of load transfer from the Al matrix to the reinforcement resulting from the large extent of agglomeration of CNTs.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

Corrosion behaviour of aluminium matrix composites

Samples of aluminium alloy 2014 reinforced with 20–40 vol % of alumina or silicon carbide particles were tested by the potentiodynamic polarization technique. The chosen medium was 0.1m lithium perchlorate which tends to cause localized corrosion. The measurements revealed no impairment of the corrosion performance of the matrix alloy as a result of the presence of the reinforcement phase.     

The effect of recycling on the mechanical response of carbon fibres and their composites

The development of processes for recycling carbon–fibre composite waste rises a question yet to be answered: how good can the performance of recycled composites be? This paper analyses fibres reclaimed in commercial facilities, and compares the performance of subsequently manufactured recycled woven composites to that of the virgin precursor (with the same fibre architecture). Different pyrolysis cycles resulted into different compromises between complete resin removal and full fibre strength retention. At the composite level, this paper shows how strength varies with the reclamation cycle, re-impregnation process and loading direction, while stiffness remains virtually unaffected. It is shown that composite tensile strength is favoured by gentle pyrolysis cycles generating little fibre damage, while compressive strength is fully retained after more aggressive cycles which completely remove the matrix. This work proves that the mechanical response of recycled composites can rival that of virgin precursors, while highlighting the benefits of application-driven optimisation of reclamation processes.     

Sintering behaviour of Copper/carbon nanotube composites and their characterization

The potential usage of Copper (Cu) is very limited, where combined mechanical and thermal properties are desirable, which can be overcome by using carbon nanotube (CNT) as a reinforcement. An attempt was made to synthesize Cu/CNT composites by varying CNT diameter and its concentration through a molecular level mixing technique followed by uniaxial compaction and conventional sintering. The sintering behaviour of Cu and Cu/CNT composites was studied to understand the influence of different parameters. The sintering duration of Copper was decreased with an increase of CNT diameter. The maximum enhancement of properties was achieved at 0.25 wt.% CNT irrespective of its diameter, where the thermal conductivity and hardness were obtained as 328 W/mK at 20–40 nm diameter CNT composites and 81.2 ± 2.9 VHN at 40–60 nm diameter CNT composites, respectively. The conventional method of synthesize can generate the desired characteristics of composites at par with high end techniques, such as SPS.