C.B. Ponton

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in “softening” particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min−1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.

Measurement of Young's modulus and internal friction of an in situ Al-Al3Ni functionally gradient material

An in situ Al-Al3Ni functionally gradient material (FGM) was produced by centrifugally casting an Al-20 mass% Ni alloy into a thick-walled tube. Four specimens, 90 mm long, with rectangular cross-sections (width × thickness) of 6 × 6, 6 × 5, 6 × 4 and 6 × 3 mm2 were machined from the tube such that the thickness direction of the specimens was in the radial direction of the tube. The microstructure of the FGM tube consisted of granular morphology Al3Ni as a second phase distributed within the aluminium matrix with an increasing volume fraction gradient from the inside to the outside of the tube. Thus, the thicker the specimen, the greater was the composition gradient and the thinner the specimen, the greater was the volume fraction of Al3Ni. The dependence of the Youngs modulus and internal friction on the composition gradient of the FGM was determined by a flexural forced-resonance technique from the resonant frequency and the resonance peak width, respectively, as a function of nominal specimen thickness. The Youngs modulus of the Al3Ni second phase was determined from a correlation plot of assumed Al3Ni Youngs modulus values against the calculated resonant frequency values corresponding to the associated FGM Youngs modulus values. The latter were calculated using a rule of mixtures with a fixed matrix Youngs modulus and a gradient volume fraction of Al3Ni for each specimen thickness. By plotting the experimental FGM specimen resonant frequencies on this plot, the average Al3Ni Youngs modulus was found to be 140 GPa. The Youngs modulus of the FGM was found to vary between 81.5 and 100.8 GPa across the 6 mm tube-wall thickness from the inner to outer surface, reflecting the 15.2 and 43.2 vol % Al3Ni second phase, respectively. The measured internal friction increased with the volume fraction of Al3Ni, and owing to the relatively large Al3Ni particle size, was thereby dependent on the resultant increase in the second phase-matrix interface number density rather than the dislocation density.

A TEM and HREM study of particle formation during barium titanate synthesis in aqueous solution

The formation mechanisms of barium titanate particles from an amorphous TiO2 gel during synthesis in aqueous solution at temperatures between 20 and 80°C have been investigated. It was found that barium ions diffuse into the gel almost immediately, with nanocrystalline BaTiO3 particles being formed after heating to just 40°C. These particles grew to dimensions of about 100 nm as the temperature was increased to 80°C, consuming the remaining TiO2 gel. Some remnants of gel were found on particle surfaces in a sample taken at this temperature but after holding the sol at 80°C for 2 or 4 h, the particle surfaces became “cleaner” and more rounded. It is proposed on the basis of these observations that the BaTiO3 particles were formed by an in-situ transformation of the amorphous TiO2 gel. The mechanism by which (i) the particles were then rounded off and (ii) the final gel fragments were incorporated into the main BaTiO3 particles was, however, less clear.

Densification and crystallization of glass powder compacts during constant heating rate sintering

Abstract Heating microscopy was used to study the interaction between the processes of densification and crystallization of glass powder compacts under constant heating rate sintering conditions without the application of external loads. For barium magnesium aluminosilicate (BMAS) glass powder compacts sintered between 800–1100 °C, it has been shown that the relative rates of crystallization and densification can be controlled by changing the heating rate. Samples sintered at a high heating rate of 15 K min−1 could be fully densified in the amorphous state, delaying the onset of crystallization. In the samples sintered at a low heating rate of 1 K min−1, the onset of crystallization coincided with the termination of densification at ∼ 1000 °C. Since the experiments were performed without an application of external loads, the results are applicable for the manufacturing of dense BMAS glass-ceramics via a pressureless sintering route.

Hydrothermal and electrophoretic deposition of lead zirconate titanate (PZT) films

Abstract Hydrothermal synthesis in conjunction with colloidal processing has been used to make PZT films in an aqueous solution. Fine PZT powders with a particle size of 200 nm and a narrow particle size distribution were synthesised hydrothermally at 300°C. After washing, a dispersant (di-ammonium citrate) was added to the slurry to make a stable suspension. Electrophoretic deposition (EPD) was then employed to deposit PZT films directly from the hydrothermal PZT suspensions. The effects of the synthesis conditions on the particle size and size distribution of the PZT powders are briefly discussed. Various stabilisation mechanisms for the hydrothermal slurries have been investigated. The effects of the slurry properties and deposition parameters on the microstructure of deposited films are discussed. It is shown that PZT films with a thickness 1000°C) in comparison with powder compacts. In addition, interaction with the substrate was observed when PZT films were deposited and sintered on Pt-coated alumina substrates.

Creek effects on the spallation of an alumina layer from a NiCrA1Y coating

Abstract A finite-element analysis has been performed of alumina spallation by wedge cracking from a flat Nil6Cr6A1Y coating substrate during cooling from 1100°C. The analysis allowed for creep deformation within both the coating and oxide but it was found that only creep within the coating influenced the growth of the interfacial wedge crack by relaxing stresses at the crack tip. As a result, crack growth was suppressed during the early stages of cooling. Computed values of the critical temperature drop to cause spallation are presented for various cooling rates and values of the coefficient of thermal expansion of the coating. The results show that the effective fracture energy for growth of the interfacial crack in the presence of creep is some 1–2 orders of magnitude greater than previously determined values for elastic fracture of the oxide and demonstrate the important role of coating creep. These results are used to produce a first spallation map for this system.

Low temperature hydrothermal synthesis of Ba(Mg1/3Ta2/3)O3 sol-derived powders

Powders of the microwave dielectric material barium magnesium tantalate Ba(Mg1/3Ta2/3)O3 have been produced by hydrothermal synthesis at moderately low temperatures (160 to 350°C). It was found that while it is relatively straightforward to produce the material in the desired perovskite phase at or below 200°C, the powder particles tend to be highly irregular in morphology with extremely small dimensions (of the order of 10 nm) and deficient in magnesium (with some precipitation of the excess magnesium as the hydroxide). The effects of both higher synthesis temperatures and different feedstock preparation were thus investigated with the aim of improving the precipitation of magnesium under hydrothermal conditions in order to produce a more homogeneous, stoichiometric powder and significant progress was made. It was found that when near-stoichiometric particles are formed, they adopt rounded morphologies and exhibit larger particle sizes (around 30–50 nm). These results show that the hydrothermal feedstock and the synthesis temperature used have a profound effect on particle stoichiometry, which in turn affects the growth morphology of the particles.

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 2-D woven SiC fibre mats with mixed sols of mullite composition

Electrophoretic deposition (EPD) has been used to infiltrate a dual-component mixed sol of mullite composition into woven, electrically conducting SiC (Nicalon) fibre preforms. Both silica and alumina precursors had a near-equiaxed particle shape and their controlled colloidal mixing at an optimised pH (3.1) has lead to their extensive heterocoagulation on a nanometre scale. The mechanisms of particle migration in the EPD cell are not fully clear at this stage. It is suggested that both sol species migrate simultaneously as a ‘composite’ sol particle to the fibre mat serving as one electrode in the cell. Owing to the small particle size of the silica and alumina used, both components were able to infiltrate the spaces within the fibre tows. Good particle packing and a high solids-loading were achieved, producing a firm matrix deposit which adhered to the fibres. The maintenance of the stoichiometric mullite composition in the deposited material was confirmed by XRD analysis of the deposited material heat-treated at 1350 °C for 5h. The high-quality infiltrated fibre mats serve as preforms for the fabrication of mullite matrix composites.

Behavior of Nicalon™-fiber-reinforced glass-matrix composites under thermal cycling conditions

Abstract The damage evolution of commercially available SiC Nicalon™-fiber-reinforced glass-matrix composites under thermal cycling conditions in oxidizing atmosphere has been investigated. The samples were alternated quickly between high-temperature ( T =700°C) and room-temperature air for different numbers of cycles. Thermal aging experiments were also conducted by exposing the samples in air at 700°C for long periods of up to 250xa0h. Both destructive and non-destructive measurement techniques were employed to characterize the samples and to detect differences in behavior for the various thermal loading conditions. The flexural strength and Youngs modulus decreased, while the internal friction increased with increasing numbers of cycles. Material degradation was attributed to phenomena related to viscous flow of the glass matrix, and to oxidation of the fiber, which occurred as a consequence of the extended exposures at high temperatures. The microstructural damage observed includes porosity formation (cavitation) within the matrix and at the fiber/matrix interfaces. The experimental results also suggest degradation of the in situ fiber strength due to fiber surface oxidation and damage, fiber displacement and consequent fiber-to-fiber contact.