A Novel Powder Coated Fibre Pre‐processing Route to Metal Matrix Composites

Since the dawn of the transport, and in particular, the aerospace engineering industry, there has been an ever‐growing need for high performance and high temperature materials. Materials such as titanium/ silicon carbide fibre metal matrix composites (Ti/SiC MMCs) provide high strength, low density and operate at high temperature to their particular applications. Unfortunately, the maturing technology of Ti/SiC MMCs still suffers from extremely high fabrication costs. Reducing this and increasing component processing flexibility remain the priorities of current research. This article presents a Powder Coated Fibre pre‐processing technique to meet such industrial requirements. The technique is based on slurry powder metallurgy and presents itself as a cost‐effective alternative to current Ti/SiC MMCs processing methods. It involves firstly, mixing matrix powder particles with an appropriate organic binder and solvent to form a slurry, drawing a continuous silicon carbide fibre through the slurry onto a winding drum, drying the coated‐fibre and finally laying up the coated fibre into a composite pre‐form for subsequent consolidation. The organic component is removed from the pre‐form matrix via a binder burnout phase prior to composite consolidation.     

Towards cost effective manufacturing of Ti/SiC fibre composites and components

Abstract

Titanium/silicon carbide fibre composites offer an excellent combination of weight specific properties and are ideal for many components in gas and steam turbine engines. However, potential industrial applications are hampered by the relatively high cost of the materials. This paper critically examines the characteristics of various manufacturing or processing routes for such composites, including well developed processes, such as foil–fibre–foil, physical vapour deposition, and vacuum plasma spraying, and new slurry powder metallurgy processes currently being developed. For a given manufacturing route, composite properties are enhanced if the material possesses uniform fibre distribution and a low oxygen content and is free from fibre/matrix interface reactions, residual voids, and fibre damage. The capabilities of the above processes in satisfying these requirements are compared and discussed. Possibilities of reducing product costs are analysed. Several ways of improving the cost effectiveness of mamifacturing such composites are outlined.     

Analysis on effect of electroless coated SiCp on mechanical properties of polymer matrix composites

ABSTRACT

The versatility of polymer matrix composites in industrial applications has gained reputation and adaptability among advanced materials. Still, treatment of reinforcement for these composites has emerged as a vital domain to be explored. With a continuance to this fact, the present paper aims to analyze the effect of reinforced electroless coated silicon carbide particulates on mechanical properties of composites. The composite is developed using epoxy polymer as matrix and glass fibers as primary reinforcement. The electroless coated and uncoated silicon carbide particulates were used as secondary reinforcement. The phase identification of copper on secondary reinforcement was identified using X-ray powder diffraction technique. Fracture analysis during tensile testing and bonding behavior between matrix and reinforcement is examined using field emission scanning electron microscopy with energy dispersive spectroscopy. The presence of copper particles on secondary reinforcement results in improved interfacial bonding and resistance against fracture during loading.     

Finite Element Assisted Modelling of the Microscopic Impregnation Process in Thermoplastic Preforms

Fibre reinforced composite materials incorporating thermoplastic matrices are gaining increasing popularity in many industrial applications. One of the potential preforms for the manufacture of technical components is commingled yarn composed of reinforcement and matrix in fibre form. These are often employed in the pultrusion process. Another innovative preform consists of polymer powder preimpregnated sheath surrounding fibre bundles. To achieve adequate mechanical properties of the final product it is essential, when producing laminates by a process such as pultrusion with both types of preform, that sufficient matrix impregnation is achieved. The prevention of voids and dry-spots in the laminate requires a theoretical understanding of the mechanisms involved. On a microscopic scale, several finite element (FE) models can be used to simulate the progress of the matrix flow into the interstitial spaces between the single reinforcement fibres. In the present simulations, a hexagonal and a square arrangement account for two of the various fibre packings occurring in a laminate. It permits an estimation of the impregnation performance of commingled and powder impregnated yarns. For each preform the shear rate, to which the polymer matrix is subjected during the impregnation and consolidation process, can be predicted.     

Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si₃N₄ particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si₃N₄ powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si₃N₄ powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.     

C/SiC复合材料的制备及性能研究

本文对聚碳硅烷化学转化法制备碳纤维增强碳化硅复合材料的生产工艺进行了研究,探讨了工艺参数对材料性能的影响.研究结果表明,聚碳硅烷分子量是影响材料性能的重要参数,纤维和基体间热膨胀系数的不匹配可通过向基体内加少量二氧化锆进行改善.材料的力学和热学性能测试结果表明,C/SiC复合材料具有较好的综合性能,其断裂韧性比未增强的单一陶瓷提高较多.利用电镜分析了弯曲试样的断口形貌.     

Advances in the fabrication of titanium based composites

Titanium alloy metal matrix composites reinforced with silicon carbide fibre are being evaluated for a range of highly loaded aerospace applications. Although expensive, if used selectively they can have a dramatic effect on performance and weight. The SMC has one of the strongest capabilities in Europe for the development and production of titanium fibre reinforced MMCs. Consisting of the DRA Sigma silicon carbide fibre manufacturing facility, producing fibre on a commercial basis, and the manufacture of titanium alloy MMCs in commercial quantities by the foil fibre route, and at pilot scale using the alternative matrix coated fibre route. In the foil fibre route a filament winding and fugitive binder method is used to produce a range of component shapes with excellent fibre distribution and consistent properties. The matrix coated fibre process is seen to have advantages over the alternative methods and is likely to become an important manufacturing route for titanium MMCs, particularly for exotic high temperature titanium alloys and intermetallics, and for shapes such as rings, tubes and shafts. As no titanium MMCs components have yet reached full production, it is unclear which of these fabrication methods will become commercially viable, if any. But, the choice is likely to be based on cost, availability and product quality.     

Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder

A novel robust non-woven sisal fibre preform was manufactured using a papermaking process utilising nanosized bacterial cellulose (BC) as binder for the sisal fibres. It was found that BC provides significant mechanical strength to the sisal fibre preforms. This can be attributed to the high stiffness and strength of the BC network. Truly green non-woven fibre preform reinforced hierarchical composites were prepared by infusing the fibre preforms with acrylated epoxidised soybean oil (AESO) using vacuum assisted resin infusion, followed by thermal curing. Both the tensile and flexural properties of the hierarchical composites showed significant improvements over polyAESO and neat sisal fibre preform reinforced polyAESO. These results were corroborated by the thermo-mechanical behaviour of the (hierarchical) composites, which showed an increased storage modulus and enhanced fibre–matrix stress transfer. Micromechanical modelling was also performed on the (hierarchical) composites. By using BC as binder for short sisal fibres, added benefits such as the high Young’s modulus of BC, enhanced fibre–fibre and fibre–matrix stress transfer can be utilised in the resulting hierarchical composites.     

Processing of Ti-SiC metal matrix composites by tape casting

Abstract

Titanium-silicon carbide (Ti-SiC) continuous fibre composites are very attractive for aerospace applications. Although development of various components is under way, a cost effective method to manufacture the material still has to be identified. Here, a tape casting technique is investigated as a viable method of producing the composites. It involves relatively large inexpensive titanium powder and simple apparatus. Furthermore, the powder particles ensure good fibre distribution, reduced consolidation time, and little damage to the reinforcement. It is shown that uniform powder tapes with good packing density can be readily produced using appropriate casting parameters. Both thermogravimetry and mass spectrometry are used to analyse the burnout process of a fugitive binder system used to produce the tapes. Removal of the organics is found to take place in two stages, separated by over 100 K. composite materials processed by the tape casting route exhibit good fibre distribution and no signs of fibre damage.     

Interface and fracture of carbon fibre reinforced Al-7 wt% Si alloy

The interface structure in an aluminium-7 wt% silicon alloy reinforced with carbon fibres has been investigated using analytical electron microscopy. Crystals of aluminium carbide (Al₄C₃) have been identified in interface regions and their structure and growth are discussed. Mechanical properties of the composite have been measured and fracture behaviour studied using acoustic emission analysis in parallel with microstructural examination. The results indicated that the aluminium carbide interfacial reaction had produced a strong fibre matrix bond, but reduced the fibre strength and embrittled the matrix. Consequently, whole fibre bundles failed in a brittle manner in the longitudinal direction with limited pull-out of individual fibres. The findings are discussed in relation to the method used to manufacture the composite.     

The fabrication of ceramic-coated carbon fibre duplex elements

Attempts have been made to fabricate a bicomponent composite reinforcing element comprised of a central core of carbon fibre filaments surrounded by a cylindrical silicon carbide sheath. Such fibres are particularly attractive for composite reinforcement since they are potentially capable of exhibiting “duplex” type behaviour, thereby providing a possible means of minimizing anisotropy effects and increasing composite fracture toughness and ductility. Furthermore these elements should provide additional advantages such as eventually enabling multi-filament tows of high strength, low modulus carbon fibre to be formed into large compound fibres which combine high specific strength with a significantly improved overall Young's modulus arising from the stiffness of the ceramic sheath, which should also exhibit a high resistance to chemical attack. Methods of consolidating the multi-filament tow prior to coating have been investigated and suitable preliminary treatments evolved; tows have been coated with silicon carbide using a conventional vapour phase deposition technique to form elements basically conforming to “duplex” requirements. Initial tensile tests upon these elements are reasonably encouraging and reveal none of the side effects encountered previously with boron coatings; it is anticpated that much stronger silicon carbide tubes may be fabricated eventually by this technique using more closely controlled reaction conditions.     

Optimization of chemical reactions between alumina/silica fibres and aluminium-magnesium alloys during composite processing

An Al-Si-;Cu-Mg alloy reinforced with alumina/silica fibres (Fiberfrax®, alumina/silica ratio=45/55) has been extensively characterized in terms of microstructure, interfacial chemical reactions and mechanical properties. The composite was fabricated by squeeze casting. The above characteristics were measured as a function of (a) calcination temperature of the fibre preform before infiltration, and (b) subsequent composite heat treatment. The main reaction that occurs during the processing of aluminium alloy matrix composites is the reduction of silica in the binder and fibres by magnesium from the matrix. When calcined below 1000°C, the fibres remain amorphous with a coating of porous silica binder. In this condition, the reinforcement reacts strongly with the matrix during heat treatment of the composite. In contrast, at high calcination temperatures (1200°C), the fibres transform partially into mullite and the silica binder densifies; these fibres are somewhat less reactive with the matrix. In both cases, the matrix/reinforcement reactions are very strong during high-temperature heat treatment, leading to a complete reduction of silica in some cases. The degradation caused by chemical reactions adversely affects the mechanical properties of these composites. Therefore, in order to optimize the mechanical properties of this composite, the fibre preform first must be calcined at high temperature, then the composite heat treatment limited to low temperature.     

Improvement of the temperature resistance of aluminium-matrix composites using an acid phosphate binder

The use of phosphate binders instead of the widely used silica binder resulted in improved temperature resistance, increased tensile strength and decreased coefficient of thermal expansion. The effects were largest for the phosphate binder which contained the largest amount of phosphoric acid (P/Al atom ratio = 24 in the liquid binder). These effects were probably due to the protection of the SiC whiskers by the binder phases (aluminium metaphosphate or aluminium orthophosphate), the binder-SiC reaction product (SiP₂O₇) and the binder-aluminium reaction product (AIP) from further reaction between the SiC and aluminium. The tensile strength of the composite containing the SiC whisker preform made with the phosphate binder (P/Al atom ratio = 6 or 24 in the liquid binder) was increased after heating at up to 600 °C for 240 h. The silicon phosphate (SiP₂O₇) acted as an in situ binder and was primarily responsible for increasing the compressive strength of the preform and increasing the temperature resistance of the composite. The carbon fibre composite containing the preform made by using the phosphate binder (P/Al atom ratio = 24 in the liquid binder) with either water or acetone as the liquid carrier during wet forming of the preform had a higher tensile strength than the carbon fibre composite made by using the silica binder. After composite heat exposure to 600 °C for 14 h, the carbon fibre composite made by using this phosphate binder with acetone as the liquid carrier during wet forming of the preform showed the best temperature resistance, while the carbon fibre composites made by using this phosphate binder with water as the carrier showed the second best temperature resistance, and that made by using silica binder was the worst. The reason for the better effect of the phosphate binder than the silica binder is probably due to the ability of the phosphate binder and the binder-aluminium reaction product (AIP) to protect the carbon fibres from the undesirable reaction between the carbon fibres and aluminium. The lack of a binder-fibre reaction contributed to making the carbon fibre composites less temperature resistant than the SiC whisker composites. The use of a higher binder concentration is attractive for increasing the temperature resistance of the composites. The binder concentration in the preform can be increased by increasing the binder concentration in the slurry used in the wet forming of the preform.     

Initial-stage hot-pressing of SiC fibre/ Ti monotapes

The initial compaction under uniaxial hot-pressing conditions of titanium monotapes containing silicon carbide fibres is studied for variable matrix/fibre volume ratio, temperature (800–950°C) and compaction stress (10–40 MPa). The matrix compaction rate is predominantly affected by the latter two variables, less by temperature. The sample pore structure is strongly influenced by the preform packing characteristics including fibre-to-matrix volume ratio and irregularities in fibre arrangements.     

Optimisation of a GMT-based cold pressing technique for low cost textile reinforced thermoplastic composites

Much potential and interest exists for the fast processing of lightweight, inexpensive composite preforms in automotive and other transport applications. As a thermoplastic matrix is more suitable for mass production with short cycle times, a novel cost-effective glass fibre reinforced thermoplastic textile preform is developed. Weft-inserted warp knitting is used to produce this textile preform containing both the reinforcing fibres and the thermoplastic matrix material as split-film ribbons. The aim of the work is to establish a useful processing technique and to control those parameters which lead to the production of good quality composite parts. The current study is specifically directed at determining the feasibility of the GMT-based cold pressing technique for the manufacturing of this new type of thermoplastic composite. An experimental design method is used to develop a statistical model which gives response surfaces of the effects of the processing parameters on the mechanical performance of the final composite part. Processing variables are ranked in order of importance to determine the optimal processing window. An economical comparison with the use of long fibre reinforced GMT mats proves the cost-efficiency of this new continuous reinforced thermoplastic composite.     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Infiltration processing of fibre reinforced composites: governing phenomena

All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.     

Generalised constitutive equations for densification of metal matrix coated fibre composites

Abstract

The present paper completes a study of constitutive equations for the consolidation processing of continuous fibre reinforced metal matrix composite materials. It builds on an earlier paper in which physically based constitutive equations were derived for the case of symmetrical, isostatic loading. In the present paper, constitutive equations are developed for in plane, general stress states. The total deformation of the consolidating composite is expressed as the sum of a conventional deviatoric creep term, together with a dilatational term, which was derived using a variational method previously published. The equations contain only two material parameters, which are the conventional creep coefficient and exponent for the fibre coating material (in this case, Ti-6Al-4V). The resulting equations have been implemented into finite element software enabling the simulation of practical consolidation processes. The model has been verified by comparing predicted results with those obtained from independent micromechanical models. A number of experimental tests have been carried out, and the model is used to predict the rates of densification for a range of experimental pressure and temperature histories. Good comparisons have been achieved.     

TiO2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain TiO₂ coatings on three dimensional (3-D) SiC fibre (Nicalon ®-type) and carbon fibre substrates. Colloidal suspensions of commercially available TiO₂ nanoparticles in acetylaceton with addition of iodine were used. The EPD parameters, i.e., deposition time and voltage, were optimised for each fibre type. Strongly adhered TiO₂ deposits with high particle packing density were obtained. Scanning electron microscopy observations revealed high penetration of the titania nanoparticles into the fibre preforms. The TiO₂ deposits were sintered at 800°C for 1 h in order to produce relatively dense and uniform TiO₂ coatings covering completely the SiC or carbon fibres. For the carbon fibre/TiO₂ system, an effort was made to produce a 3-D titania matrix composite by further infiltration of the porous fibrous preform with TiO₂ by slurry dipping and subsequent pressureless sintering. The 3-D carbon fibre reinforced TiO₂ matrix composites fabricated contained residual porosity, indicating further infiltration and densification steps are required to produce dense composites of adequate structural integrity. For SiC fibre fabrics, oxidation tests in air established the effectiveness of the TiO₂ coating as oxidation protective barrier at 1000°C. After 120 h the increase of weight due to oxidation of coated fibres was more than twice lower than that of the uncoated fibres. TiO₂ coated SiC fibre preforms are attractive materials for manufacturing hot gas filters and as reinforcing elements for ceramic matrix composites.