Paul A. Trusty

Deposition of zirconia sols on woven fibre preforms using a dip-coating technique

Abstract The technique of depositing zirconia coatings onto woven fibre mats has been investigated in detail. The application of a coating to the fibre is essentially one of the easiest methods of providing a fibre–matrix interface with desired properties. Such coatings can act as reaction barriers or as fibre–matrix debond interfaces. This particular coating method, which is the dip-coating of fibre tows in zirconia sols, does not require sophisticated apparatus and, therefore, has great potential as a standard coating technique for use with woven fibre systems. In this work, an “in-house” produced zirconia sol, synthesised using hydrothermal processing, has been found to be the most successful coating material. Both alumina and alumina-silica woven fibre mats were used as the substrate materials. It has been shown that the zirconia sol employed can be deposited successfully in a single coating step, as a thin (1–2 μm) coating with minimal bridging of the fibre mats. The critical coating parameters when using this dip-coating technique have been discussed in detail.

Electrophoretic deposition infiltration of metallic fabrics with a boehmite sol for the preparation of ductile-phase-toughened ceramic composites

The infiltration of commercially available metallic fibre mats by a boehmite sol using the electrophoretic deposition (EPD) technique was investigated. The nanosized boehmite particles were positively charged in a colloidal suspension at pH 4 and migrated upon application of an electric field to the metallic fabric acting as the negative electrode. Three different type 316L stainless steel fabrics were considered and it was found that the quality of the infiltration depended on the fibre architecture. The EPD parameters, i.e., the applied voltage and deposition time, were optimized for obtaining a high solids loading in between the fibre tows and a firm adherent deposit. The infiltrated fibre mats, being of high quality, i.e., low macroporosity and absence of significant microcracking, serve as prepregs for the manufacture of alumina matrix composites reinforced with a two- or three-dimensional metallic phase.

Colloidal processing of a mullite matrix material suitable for infiltrating woven fibre preforms using electrophoretic deposition

Abstract Commercially available alumina and silica precursors for the preparation of mullite ceramic via colloidal processing and viscous transient sintering have been identified, including fumed nanosize powders and colloidal suspensions. These materials were chosen due to the fact that they can be used in the form of a sol, as mullite matrix precursors, to infiltrate woven fibre preforms using electrophoretic deposition. The sintered density of the mullite matrices sintered for 2 h, at the upper temperature for fabricating SiC-fibre reinforced composites (1300 °C) is only ≈ 90% of theoretical. However, by exploiting a viscous flow densification mechanism, it is envisaged that hot-pressing can be used to produce fully dense mullite matrix composites at the required temperatures. Additionally. using a simple pressureless sintering route, almost fully dense (98% of theoretical density) monolithic mullite has been obtained from the pre-mullite powders. A very homogeneous and fine microstructure was achieved by sintering for 5 h at a temperature of ≈ 1450 °C.

In situ characterization of the shrinkage behavior of ceramic powder compacts during sintering by using heating microscopy

Heating microscopy, used as optical dilatometry, provides an advantageous experimental method for characterizing in situ the sintering behavior of ceramic powder compacts. It enables the densification process to be monitored without the application of load, and thus, there is minimal interaction between the sintering sample and external constraints. In this study, the application of the technique in studing the constant-heating-rate sintering of different ceramic powder compacts is shown, focusing on assessing the isotropic or anisotropic shrinkage behavior of the powder compacts investigated. All compacts were made from powders having similar (irreguIar) particle shapes and were obtained by the same uniaxial compaction technique with the use of the same compacting pressures. However, both mullite and yttrium disilicate powder compacts showed isotropic sintering behavior, whereas barium magnesium aluminosilicate glass powder compacts showed shrinkage anisotropy. The complexities in predicting the occurrence of shrinkage anisotropy during sintering and its variation with the progress of densification are outlined briefly.

Processing and characterisation of 2-D woven metal fibre-reinforced multilayer silica matrix composites using electrophoretic deposition and pressure filtration

A novel, cost-effective and rapid processing route including electrophoretic deposition (EPD) and pressure filtration (PF) has been developed for the fabrication of 2-D woven metallic fibre mat reinforced multilayer silica matrix composites. Commercially available silica sol containing ultrafine ceramic particles (15 nm) was used as the matrix whilst 2-D woven metal stainless steel 316L fibre mat was used as the metal reinforcement to produce a composite having 2-D isotropic properties. The colloidal silica sol was modified with boria and boehmite in order to produce a silica matrix which could be sintered at 900°C (the maximum use temperature for the particular fibre architecture employed), and with densification taking place before crystallisation. An in-situ electrophoretic deposition (EPD) cell capable of measuring the weight gain in real time during deposition was designed. This technique enabled the woven fibre inter/intra tow regions to be infiltrated with the ultrafine silica particles in a very short time (2 min). Green bodies made from electrophoretically deposited fibre mats were further consolidated using pressure filtration. The EPD parameters were optimised in terms of time, voltage and deposition thickness as well as deposit formation rate. Microstructural observation indicated that the composites produced were dense and of high microstructural homogeneity.

Alternative Uses of Waste Glasses: Issues on the Fabrication of Metal Fibre Reinforced Glass Matrix Composites

In order to investigate a potential use for recycled speciality glasses (specifically those containing hazardous elements), a processing route has been developed for the fabrication of metallic fibre mat reinforced glass matrix composites. Commercially available 3-dimensional stainless steel 316L fibre mats were used as the metal reinforcement, and a borosilicate glass which had been used previously in radiation experiments was used as the glass matrix. The fibre mats were infiltrated with a commercially available silica sol using electrophoretic deposition (EPD), and the glass matrix was laid in between infiltrated fibre mats prior to consolidation using uniaxial cold pressing. It was found that composites with sufficient integrity could be obtained from this recycled waste glass after sintering in air at 850°C for 1 h. The deposited silica remained amorphous at the processing temperature, providing a porous interface between the metallic reinforcement and the waste glass matrix. The processing issues involved in composite fabrication, namely, the EPD infiltration parameters for the silica sol, the quantity and subsequent effect of the impurities present in the waste glass, and the densification of the composite material on sintering, have been discussed.

Novel Techniques for Manufacturing Woven Fiber Reinforced Ceramic Matrix Composites. I. Perform Fabrication

Abstract Further research and development of the processing science and technology for ceramic matrix composites (CMCs) is essential for the full industrial exploitation of these materials. This paper describes and discusses a novel, cost-effective processing technique for manufacturing CMCs containing 2-dimensional woven fiber reinforcements. The technique relies on the colloidal processing of ceramic sol precursors and the use of electrophoretic deposition (EPD) to impregnate these sols into woven fiber preforms. The fabrication routes for a woven SiC fiber reinforced mullite matrix and a woven alumina fiber reinforced alumina matrix composite have been investigated. The parameters necessary for the formation of dense green body compacts using EPD are described for the two composite systems. Micro structural examination of these compacts highlights the importance of the infiltration and drying stages to the fabrication of crack-free composite bodies. The laboratory-scale results achieved to date indicat...

Pressureless sintering and characterization of Al2O3-platelet-reinforced barium-magnesium aluminosilicate glass-ceramic composites

Abstract The fabrication of Al2O3-platelet-reinforced (BMAS) glass-ceramics via a low-cost pressureless sintering route has been considered. Two new compaction techniques were introduced for the fabrication of this type of composite: pressure filtration and rotary forging. For comparison, conventional processing using uniaxial and isostatic die cold-pressing was considered also. Pressure filtration did not yield improved results in comparison with dry compaction techniques in terms of final density. Rotary forging, on the contrary, was shown to be an attractive compaction technique since it allows the fabrication of nearly fully dense composites using forming pressures 10 times lower than those employed using conventional die-pressing. Composites containing up to 20 vol% of platelets could be densified fully under pressureless conditions at temperatures of 980°C by exploiting the viscous flow of the amorphous matrix.

Fabrication of woven metal fibre reinforced glass matrix composites

A processing route has been developed for the fabrication of metallic fibre mat reinforced glass matrix composites. For the model experiments reported here, a commercially available satin woven stainless steel 316L fibre mat was used as the reinforcement and soda-lime glass as the matrix. The process involves two steps: (1) the infiltration of the intra- and inter-tows regions of the fibre mats with silica sol using electrophoretic deposition and (2) the fabrication of a composite by cold uniaxial pressing and pressureless sintering of impregnated fibre mats sandwiched between layers of the matrix glass powder. The sintering took place in air at 670°C, and composite materials of sufficient integrity could be obtained without damaging the fibres. The deposited silica remained amorphous at the processing temperature providing a porous interface between the glass matrix and the metallic reinforcement. Obervation of fracture surfaces revealed that both fibre pull-out and fibre deformation occur, which should lead to a significant toughness enhancement. The presence of the interfacial silica layer, deposited using electrophoresis, is thought to be responsible for this behaviour.

Creep and densification during anisotropic sintering of glass powders

AbstractThe isothermal sintering behaviour of a barium magnesium aluminosilicate glass powder at 930°C was investigated using a heating microscope. The cylindrical samples exhibited a variable shrinkage anisotropy during sintering. The shrinkage anisotropy ratio, defined as the ratio of the relative change of height and diameter, varied linearly between ∼0.3 and ∼0.98 with the relative volume shrinkage during densification. Shrinkage anisotropy caused creep deformation of the samples. The creep rate varied exponentially with the densification rate and the ratio of creep to densification rates,